Publication List

Scientific Papers

95) Nanostructures and Thin Films of Poly(Ethylene Glycol)-Based Surfactants and Polystyrene Nanocolloid Particles on Mica: An Atomic Force Microscopy Study
COATINGS 13 pages 1187-1199 (2023)
Walker J, Schofield AB, Koutsos V
Abstract We studied the nanostructures and ultrathin films resulting from the deposition and adsorption of polystyrene nanocolloidal particles and methoxy poly(ethylene glycol) methacrylate surfactants on mica surfaces from mixed suspensions in water. The samples were prepared by droplet evaporation and dip coating and imaged with atomic force microscopy. Topography and phase imaging revealed a significant richness in morphological features of the deposited/adsorbed films. We observed uniform ultrathin films and extended islands of the surfactant oligomers indicating their self-assembly in monolayers and multilayers, while the polystyrene nanocolloids were embedded within the surfactant structures. Droplet evaporation resulted in the migration of particles towards the edges of the droplet leaving an intricate network of imprints within the surfactant film. Dip coating induced the formation of extended nanocolloid clusters with colloidal crystalline structuring.
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94) Contactless interfacial rheology: Probing shear at liquid-liquid interfaces without an interfacial geometry via fluorescence microscopy
JOURNAL OF RHEOLOGY 67 pages 67-80 (2023)
Muntz I, Richards JA, Brown S, Schofield AB, Rey M, Thijssen JHJ
Abstract Interfacial rheology is important for understanding properties such as Pickering emulsion or foam stability. Currently, the response is measured using a probe directly attached to the interface. This can both disturb the interface and is coupled to flow in the bulk phase, limiting its sensitivity. We have developed a contactless interfacial method to perform interfacial shear rheology on liquid/liquid interfaces with no tool attached directly to the interface. This is achieved by shearing one of the liquid phases and measuring the interfacial response via confocal microscopy. Using this method, we have measured steady shear material parameters such as interfacial elastic moduli for interfaces with solid-like behavior and interfacial viscosities for fluidlike interfaces. The accuracy of this method has been verified relative to a double-wall ring geometry. Moreover, using our contactless method, we are able to measure lower interfacial viscosities than those that have previously been reported using a double-wall ring geometry. A further advantage is the simultaneous combination of macroscopic rheological analysis with microscopic structural analysis. Our analysis directly visualizes how the interfacial response is strongly correlated to the particle surface coverage and their interfacial assembly. Furthermore, we capture the evolution and irreversible changes in the particle assembly that correspond with the rheological response to steady shear.
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93) Structure And Flow Conditions Through A Colloidal Packed Bed Formed Under Flow And Confinement
SOFT MATTER 18 pages 8995-9007 (2022)
Delouche N, Dersoir B, Schofield AB, Tabuteau H
Abstract
When a colloidal suspension flows in a constriction, particles deposit and are able to clog it entirely, this fouling process being followed by the accumulation of particles. The knowledge of the dynamics of formation of such a dense particle assembly behind the clog head and its structural features is of primary importance in many industrial and environmental processes and especially during filtration. While most studies concentrate on the conditions under which pore clogging occurs, i.e., the pore narrowing up to its complete obstruction, this paper focuses on the accumulation of particles that follows pore obstruction. We determine the relative contribution of the confinement dimensions, the ionic strength and the flow conditions on the permeability and particle volume fraction of the resultant accumulation. In high confinement the irreversible deposition of particles on the channel surfaces controls the structure of the accumulation and the flow through it, irrespective of the other conditions, leading to a Darcy flow. Finally, we show that contrarily to the clog head, in which there is cohesion between particles, those in the subsequent accumulation are held together by the fluid and form a dense suspension of repulsive hard spheres.
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92) Versatile Strategy For Homogeneous Drying Patterns Of Dispersed Particles
NATURE COMMUNICATIONS 13 article number 2840 (2022)
Rey M, Walter J, Harrer J, Perez CM, Chiera S, Nair S, Ickler M, Fuchs A, Michaud M, Uttinger MJ, Schofield AB, Thijssen JHJ, Distaso M, Peukert W, Vogel N
Abstract
After spilling coffee, a tell-tale stain is left by the drying droplet. This universal phenomenon, known as the coffee ring effect, is observed independent of the dispersed material. However, for many technological processes such as coating techniques and ink-jet printing a uniform particle deposition is required and the coffee ring effect is a major drawback. Here, we present a simple and versatile strategy to achieve homogeneous drying patterns using surface-modified particle dispersions. High-molecular weight surface-active polymers that physisorb onto the particle surfaces provide enhanced steric stabilization and prevent accumulation and pinning at the droplet edge. In addition, in the absence of free polymer in the dispersion, the surface modification strongly enhances the particle adsorption to the air/ liquid interface, where they experience a thermal Marangoni backflow towards the apex of the drop, leading to uniform particle deposition after drying. The method is independent of particle shape and applicable to a variety of commercial pigment particles and different dispersion media, demonstrating the practicality of this work for everyday processes.
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91) Bicontinuous Soft Solids with a Gradient in Channel Size
ADVANCED MATERIALS INTERFACES 9 article number 2102307 (2022)
French DJ, Schofield AB, Thijssen JHJ
Abstract
This paper presents examples of bicontinuous interfacially jammed emulsion gels ("bijels") with a designed gradient in the channel size along the sample. These samples are created by quenching binary fluids which have a gradient in particle concentration along the sample, since the channel size is determined by the local particle concentration. A gradient in local particle concentration is achieved using a two-stage loading process, with different particle volume fractions in each stage. Confocal microscopy and image analysis are used to quantitatively measure the channel size of the bijels. Bijels with a gradient in channel size of up to 2.8% mm^-1 have been created. Such tailored soft materials could act as templates for energy materials optimized for both high ionic transport rates (high power) and high interfacial area (high energy density), potentially making them useful in novel energy applications.
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supplimentary information.

90) Flow Decline During Pore Clogging By Colloidal Particles
PHYSICAL REVIEW FLUIDS 7 034304 (2022)
Delouche N, Dersoir B, Schofield AB, Tabuteau H
Abstract
The flow of colloidal suspensions through porous media often leads to the deposition of particles inside the pores which increases the local hydrodynamic resistance by narrowing the pore space available and modifying the flow path of the transported particles. There is a significant flow decline in the extreme case when the entire porous medium becomes clogged. However, there are no experimental studies that determine directly the amplitude of this flow decline when compared with the dynamics of the formation of the particle deposit. This is mainly due to the great challenge of gaining experimental access to the features of the internal structure of the deposit as it grows and thus the ability to determine the flow inside it. In this paper, we show that is possible to monitor the flow decline corresponding to the successive deposition of colloidal particles inside a constriction (pore), ending by its complete blocking. The variation of the flow is determined by the measurement of the velocity of the particles through our channel. Such a technique coupled to the precise knowledge of what is deposited inside the pore, thanks to image analysis, enables us to determine the different contributions to the flow decline. We also use numerical simulations to access the flow inside the porous structure of the deposit as it grows. Together, experiments and simulations demonstrate that the obstruction process and the subsequent limited growth of the clog, corresponding to a few layers of accumulated particles, have a higher impact on the amplitude of the flow decline than the extra growth of the clog.
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89) The Yielding Of Defect-Entangled Dispersions In A Nematic Solvent
JOURNAL OF RHEOLOGY 65 1297 (2021)
Katyan N, Schofield AB, Wood TA
Abstract
Oscillatory rheology, at both small and large (LAOS) amplitudes, was performed to measure the dynamic response of a soft solid, formed on dispersing colloids into a thermotropic nematic liquid crystal at volume fractions of φ > 18%. Due to weak homeotropic anchoring of nematogens at colloid surfaces, a Saturn-ring defect line, known as a "disclination", encircles each particle and entangles with neighboring Saturn-ring disclinations [Wood et al., Science 334, 79-83 (2011)]. We present the first experimental investigation of the yielding behavior of the resulting gel to reveal the underpinning physics. Results reveal that the frequency response of the composite is independent of volume fraction φ, an indication that the dispersed phase simply increases the density of disclinations spanning the composite without further effect. Beyond the linear viscoelastic regime (LVR), LAOS experiments indicate the composite is an elastoplastic fluid exhibiting both strain-hardening and shear-thinning behaviors with Chebyshev coefficients e₃ > 0 and v₃ < 0, respectively. We deduce that the disclination number density n_d is constant until the strain amplitude is sufficient to break disclinations leading to shear-thinning behavior beyond the LVR. A simple theory is introduced revealing that the Ericksen number E_r determines the onset of flow, when E_r > 1, generating a strain-hardening response since Frank elasticity resists reorientation of molecular alignment within confined nematic domains. Above a critical frequency ω_c, the loss modulus G'' increases slowly due to enhanced viscosity within confined nematic domains, G'' ∝ ω^1/2 [Larson, R. G., The Structure and Rheology of Complex Fluids (Oxford University, 1999), p. 463]. Observation of this behavior in a small-molecule nematic solvent provides insights into the physics of flow behavior in other, more complex, defect-mediated liquid crystalline structures exhibiting similar properties [Colby, Europhys. Lett. 54, 269-274 (2001); Sahoo and Dhara, Liq. Cryst. 44, 1582-1591 (2017); and Romo-Uribe, Polym. Adv. Technol. 32, 651-662 (2021)].
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88) Stress Versus Strain Controlled Shear: Yielding And Relaxation Of Concentrated Colloidal Suspensions
JOURNAL OF RHEOLOGY 65 1219 (2021)
Pamvouxoglou A, Schofield AB, Petekidis G, Egelhaaf SU
Abstract
In rheological experiments, the relationship between stress and strain is determined. In the transient regime, this relationship may depend on which of these properties is applied and which is measured. In general, data collected using one or the other as the control parameter are not necessarily equivalent. Moreover, the assumed steady state and the relaxation following this state might depend on whether stress or strain has been applied. We examined colloidal suspensions with concentrations around the glass transition and compared their response to stress and strain, in particular, their transient response after the start-up of shear, their steady state, and their relaxation after cessation of shear. After the start-up of shear, the transient behavior was found to significantly depend on whether the sample is exposed to a constant shear rate or a constant stress. Nevertheless, the transients lead to a rheological steady state that is independent of how it is reached, as long as yielding occurred and the corresponding shear rate or stress is applied. After cessation of shear, the relaxation under strain and stress control shows both similarities and differences. This is quantified based on, e.g., the hydrodynamic, Brownian, and residual stress as well as the recovered strain. In addition, the responses of the rheometers to the abrupt changes have been characterized. The corresponding technical data are presented and taken into account in the data interpretation.
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87) Complex High-Internal Phase Emulsions That Can Form Interfacial Films With Tunable Morphologies
LANGMUIR 37 9802-9808 (2021)
Li T, Xie R, Chen W, Schofield AB, Clegg PS
Abstract
High-internal phase emulsions (HIPEs) were considered as an important functional material and have been the focus of intense development effort, but their fundamental attributes have hardly been altered at either the microcosmic or macroscopic level, which severely limits their practical applications in various areas. In this work, we report a general strategy for creating complex HIPEs that can form interfacial films at liquid interfaces. Double HIPEs and Janus HIPEs are both realized for the first time. They feature complex microscopic patterns with short-range anisotropy and exhibit non-Newtonian pseudoplastic flow behavior. By taking advantage of their response to a high-pH subphase, interfacial films can be successfully obtained, which are tunable in thickness and morphologies under compression. Complex HIPEs can greatly expand the applications of liquid materials, and the interfacial films of droplets represent an important step toward producing 2D soft materials with a unique functionality that can be broadly applied to biological processes.
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86) The Contribution Of Colloidal Aggregates To The Clogging Dynamics At The Pore Scale
JOURNAL OF MEMBRANE SCIENCE 635 119509 (2021)
Delouche N, van Doorn JM, Kodger TE, Schofield AB, Sprakel J, Tabuteau H
Abstract
During the filtration of colloidal dispersions by a membrane, pores often get clogged by the suspended particles. Knowing the shape and size of the particles that cause this clog would be a great help to membrane users since they could then choose the ideal filtering device. Microfluidic technology enables the fabrication of model membranes or filters that are transparent, which allows for measuring the particle geometrical features that deposit either at the surface of the pores or on top of the fouling layer that has already formed. However, the use of microfluidic filters have been confined to the study of clog formation at the pore scale, overlooking the influence of the dynamics of the particle deposition on the clogging process. We have recently shown that looking precisely at what is deposited and how this is captured inside the pore provides new insight into the clogging process. In particular, we have found that a minute concentration of aggregates in a supposedly monodisperse dispersions are mainly responsible for pore fouling. In this paper, we use the same imaging technique to determine the entire clogging process for different types of monodisperses dispersions under various flow conditions, DLVO interactions with the pore walls, and confinements. We show that the way clogs form is appear complex but is also quite systematic in the fact that aggregates are the building blocks of the clog. Pores are clogged by progressive accumulation of aggregates with the average size of the aggregate required to cause the blockage increasing with increasing flow velocity. This work demonstrates that particle size and shape distributions of the feeding dispersion must be determined to understand which physical mechanisms are at play during the clogging process.
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85) Yielding and Resolidification of Colloidal Gels Under Constant Stress
JOURNAL OF PHYSICS: CONDENSED MATTER 33 284002 (2021)
Moghimi E, Schofied AB, Petekidis G
Abstract
We examine the macroscopic deformation of a colloidal depletion gel subjected to a step shear stress. Three regimes are identified depending on the magnitude of the applied stress: (i) for stresses below yield stress, the gel undergoes a weak creep in which the bulk deformation grows sublinearly with time similar to crystalline and amorphous solids. For stresses above yield stress, when the bulk deformation exceeds approximately the attraction range, the sublinear increase of deformation turns into a superlinear growth which signals the onset of non-linear rearrangements and yielding of the gel. However, the long-time creep after such superlinear growth shows two distinct behaviors: (ii) under strong stresses, a viscous flow is reached in which the strain increases linearly with time. This indicates a complete yielding and flow of the gel. In stark contrast, (iii) for weak stresses, the gel after yielding starts to resolidify. More homogenous gels that are produced through enhancement of either interparticle attraction strength or strain amplitude of the oscillatory preshear, resolidify gradually. In contrast, in gels that are more heterogeneous resolidification occurs abruptly.We also find that heterogenous gels produced by oscillatory preshear at intermediate strain amplitude yield in a two-step process. Finally, the characteristic time for the onset of delayed yielding is found to follow a two-step decrease with increasing stress. This is comprised of an exponential decrease at low stresses, during which bond reformation is decisive and resolidification is detected, and a power law decrease at higher stresses where bond breaking and particle rearrangements dominate.
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84) Dynamics Of Progressive Pore Clogging By Colloidal Aggregates
SOFT MATTER 16 9899-9907 (2020)
Delouche N, Schofield AB, Tabuteau H
Abstract
The flow of a suspension through a bottleneck often leads to its obstruction. Such a continuous flow to clogging transition has been well characterized when the constriction width to particle size ratio, W/D, is smaller than 3-4. In such cases, the constriction is either blocked by a single particle that is larger than the constriction width (W/D < 1), or there is an arch formed by several particles that try to enter it together (2 < W/D < 4). For larger W/D ratios, 4 < W/D < 10, the blockage of the constriction is presumed to be due to the successive accumulations of particles. Such a clogging mechanism may also apply to wider pores. The dynamics of this progressive obstruction remains largely unexplored since it is difficult to see through the forming clog and we still do not know how particles accumulate inside the constriction. In this paper, we use particle tracking and image analysis to study the clogging of a constriction/pore by stable colloidal particles. These techniques allow us to determine the shape and the size of all the objects, be they single particles or aggregates, captured inside the pore. We show that even with the rather monodisperse colloidal suspension we used individual particles cannot clog a pore alone. These individual particles can only partially cover the pore surface whilst it is the very small fraction of aggregates present in the suspension that can pile up and clog the pore. We analyzed the dynamics of aggregate motion up to the point of capture within the pore, which helps us to elucidate why the probability of aggregate capture inside the pore is high.
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83) Interaction Between Nearly Hard Colloidal Spheres At An Oil-Water Interface
PHYSICAL REVIEW RESEARCH 2 023388 (2020)
Muntz I, Waggett F, Hunter M, Schofield AB, Bartlett P, Marenduzzo D, Thijssen JHJ
Abstract
We show that the interaction potential between sterically stabilized, nearly hard-sphere [poly(methyl methacrylate)-poly(lauryl methacrylate) (PMMA-PLMA)] colloids at a water-oil interface has a negligible unscreened-dipole contribution, suggesting that models previously developed for charged particles at liquid interfaces are not necessarily applicable to sterically stabilized particles. Interparticle potentials, U(r), are extracted from radial distribution functions [g(r), measured by fluorescence microscopy] via Ornstein-Zernike inversion and via a reverse Monte Carlo scheme. The results are then validated by particle tracking in a blinking optical trap. Using a Bayesian model comparison, we find that our PMMA-PLMA data is better described by a screened monopole only rather than a functional form having a screened monopole plus an unscreened dipole term. We postulate that the long range repulsion we observe arises mainly through interactions between neutral holes on a charged interface, i.e., the charge of the liquid interface cannot, in general, be ignored. In agreement with this interpretation, we find that the interaction can be tuned by varying salt concentration in the aqueous phase. Inspired by recent theoretical work on point charges at dielectric interfaces, which we explain is relevant here, we show that a screened 1/r² term can also be used to fit our data. Finally, we present measurements for poly(methyl methacrylate)-poly(12-hydroxystearic acid) (PMMA-PHSA) particles at a water-oil interface. These suggest that, for PMMA-PHSA particles, there is an additional contribution to the interaction potential. This is in line with our optical-tweezer measurements for PMMA-PHSA colloids in bulk oil, which indicate that they are slightly charged.
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82) Precise Self-Positioning of Colloidal Particles on Liquid Emulsion Droplets
LANGMUIR 35 pages 13053-13061 (2019)
Liber SR, Butenko AV, Caspi M, Guttman S, Schultz M, Schofield AB, Deutsch M, Sloutskin E
Abstract
Decorating emulsion droplets by particles stabilizes foodstuff and pharmaceuticals. Interfacial particles also influence aerosol formation, thus impacting atmospheric CO₂ exchange. While studies of particles at disordered droplet interfaces abound in the literature, such studies for ubiquitous ordered interfaces are not available. Here, we report such an experimental study, showing that particles residing at crystalline interfaces of liquid droplets spontaneously self-position to specific surface locations, identified as structural topological defects in the crystalline surface monolayer. This monolayer forms at temperature T = T_s, leaving the droplet liquid and driving at T_d < T_s a spontaneous shape-change transition of the droplet from spherical to icosahedral. The particle's surface position remains unchanged in the transition, demonstrating these positions to coincide with the vertices of the sphere-inscribed icosahedron. Upon further cooling, droplet shape-changes to other polyhedra occur, with the particles remaining invariably at the polyhedra's vertices. At still lower temperatures, the particles are spontaneously expelled from the droplets. Our results probe the molecular-scale elasticity of quasi-two-dimensional curved crystals, impacting also other fields, such as protein positioning on cell membranes, controlling essential biological functions. Using ligand-decorated particles, and the precise temperature-tunable surface position control found here, may also allow using these droplets for directed supra-droplet self-assembly into larger structures, with a possible post-assembly structure fixation by UV polymerization of the droplet's liquid.
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81) Periodic Buckling And Grain Boundary Slips In A Colloidal Model Of Solid Friction
SOFT MATTER 15 pages 5227-5233 (2019)
Janai E, Butenko AV, Schofield AB, Sloutskin E
Abstract
The intermittent 'stick-slip' dynamics in frictional sliding of solid bodies is common in everyday life and technology. This dynamics has been widely studied on a macroscopic scale, where the thermal motion can usually be neglected. However, the microscopic mechanisms behind the periodic stick-slip events are yet unclear. We employ confocal microscopy of colloidal spheres, to study the frictional dynamics at the boundary between two quasi-two-dimensional (2D) crystalline grains, with a single particle resolution. Such unprecedentedly-detailed observations of the microscopic-scale frictional solid-on-solid sliding have never been previously carried out. At this scale, the particles undergo an intense thermal motion, which masks the avalanche-like nature of the underlying frictional dynamics. We demonstrate that the underlying sliding dynamics involving out-of-plane buckling events, is intermittent and periodic, like in macroscopic friction. However, unlike in the common models of friction, the observed periodic frictional dynamics is promoted, rather than just suppressed, by the thermal noise, which maximizes the entropy of the system.
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80) Particle-Stabilized Janus Emulsions That Exhibit pH-Tunable Stability
CHEMICAL COMMUNICATIONS 55 pages 5773-5776 (2019)
Li T, Schofield AB, Chen K, Thijssen JHJ, Dobnikar J, Clegg PS
Abstract
By developing a deeper understanding of the formation mechanism and the origin of the stability, we report a simple and large-scale fabrication approach to create Janus emulsions that can be controlled in size, geometry and stability.
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79) Dynamics Of Pore Fouling By Colloidal Particles At The Particle Level
JOURNAL OF MEMBRANE SCIENCE 573 pages 411-424 (2019)
Dersoir B, Schofield AB, Robert de Saint Vincent M, Tabuteau H
Abstract
Particle filtration occurs whenever particles flow through porous media such as membrane. Progressive capture or deposition of particles inside porous structure often leads to complete, and generally unwanted, fouling of the pores. Previously there has been no experimental work that has determined the particle dynamics of such a process at the pore level, since imaging the particles individually within the pores remains a challenge. Here, we overcome this issue by flowing fluorescently dyed particles through a model membrane, a microfluidic filter, imaged by a confocal microscope. This setup allows us to determine the temporal evolution of pore fouling at the particle level, from the first captured particle up to complete blocking of the pore. We show that from the very beginning of pore fouling the immobile particles inside the pore significantly participate in the capture of other flowing particles. For the first time it is determined how particles deposit inside the pore and form aggregates that eventually merge and block the pore.
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78) Bacteria As Living Patchy Colloids: Phenotypic Heterogeneity In Surface Adhesion
SCIENCE ADVANCES 4, article number eaao1170 (2018)
Vissers T, Brown AT, Koumakis N, Dawson A, Hermes M, Schwarz-Linek J, Schofield AB, French JM, Koutsos V, Arlt J, Martinez VA, Poon WCK
Abstract
Understanding and controlling the surface adhesion of pathogenic bacteria is of urgent biomedical importance. However, many aspects of this process remain unclear (for example, microscopic details of the initial adhesion and possible variations between individual cells). Using a new high-throughput method, we identify and follow many single cells within a clonal population of Escherichia coli near a glass surface. We find strong phenotypic heterogeneities: A fraction of the cells remain in the free (planktonic) state, whereas others adhere with an adhesion strength that itself exhibits phenotypic heterogeneity. We explain our observations using a patchy colloid model; cells bind with localized, adhesive patches, and the strength of adhesion is determined by the number of patches: Nonadherers have no patches, weak adherers bind with a single patch only, and strong adherers bind via a single or multiple patches. We discuss possible implications of our results for controlling bacterial adhesion in biomedical and other applications
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77) Axial Confocal Tomography of Capillary-Contained Colloidal Structures
LANGMUIR 33 pages 13343-13349 (2017)
Liber SR, Indech G, van der Wee EB, Butenko AV, Kodger TE, Lu PJ, Schofield AB, Weitz DA, van Blaaderen A, Sloutskin E
Abstract
Confocal microscopy is widely used for threedimensional (3D) sample reconstructions. Arguably, the most significant challenge in such reconstructions is posed by the resolution along the optical axis being significantly lower than in the lateral directions. In addition, the imaging rate is lower along the optical axis in most confocal architectures, prohibiting reliable 3D reconstruction of dynamic samples. Here, we demonstrate a very simple, cheap, and generic method of multiangle microscopy, allowing high-resolution high-rate confocal slice collection to be carried out with capillary-contained colloidal samples in a wide range of slice orientations. This method, realizable with any common confocal architecture and recently implemented with macroscopic specimens enclosed in rotatable cylindrical capillaries, allows 3D reconstructions of colloidal structures to be verified by direct experiments and provides a solid testing ground for complex reconstruction algorithms. In this paper, we focus on the implementation of this method for dense nonrotatable colloidal samples, contained in complex-shaped capillaries. Additionally, we discuss strategies to minimize potential pitfalls of this method, such as the artificial appearance of chain-like particle structures.
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76) Interfacial Rheology of Sterically Stabilized Colloids at Liquid Interfaces and Its Effect on the Stability of Pickering Emulsions
LANGMUIR 33 pages 4107-4118 (2017)
Van Hooghten R, Blair VE, Vananroye A, Schofield AB, Vermant J, Thijssen JHJ
Abstract
Particle-laden interfaces can be used to stabilize a variety of high-interface systems, from foams over emulsions to polymer blends. The relation between the particle interactions, the structure and rheology of the interface, and the stability of the system remains unclear. In the present work, we experimentally investigate how micron-sized, nearhard- sphere-like particles affect the mechanical properties of liquid interfaces. In particular, by comparing dried and undried samples, we investigate the effect of aggregation state on the properties of the particle-laden liquid interface and its relation to the stability of the corresponding Pickering emulsions. Partially aggregated suspensions give rise to a soft-solid-like response under shear, whereas for stable PMMA particulate layers a liquid-like behavior is observed. For interfacial creep-recovery measurements, we present an empirical method to correct for the combined effect of the subphase drag and the compliance of the double-wall ring geometry, which makes a significant contribution to the apparent elasticity of weak interfaces. We further demonstrate that both undried and dried PMMA particles can stabilize emulsions for months, dispelling the notion that particle aggregation, in bulk or at the interface, is required to create stable Pickering emulsions. Our results indicate that shear rheology is a sensitive probe of colloidal interactions but is not necessarily a predictor of the stability of interfaces, e.g., in quiescent Pickering emulsions, as in the latter the response to dilatational deformations can be of prime importance.
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75) Clogging Transition Induced by Self Filtration in a Slit Pore
SOFT MATTER 13 pages 2054-2066 (2017)
Dersoir B, Schofield AB, Tabuteau H
Abstract
Particles that flow through porous environments like soils, inside a filter or within our arteries, often lead to pore clogging. Even though tremendous efforts have been made in analysing this to circumvent this issue, the clog formation and its dynamics remain poorly understood. Coupling two experimental techniques, we elucidate the clogging mechanism at the particle scale of a slit pore with its height slightly larger than the particle diameter. We identify all the particle deposition modes during the clog formation and accurately predict the corresponding deposition rate. We show how the geometrical features of the pores and the competition between deposition modes can profoundly change the clog morphology. We find that the direct capture of particles by the pore wall is rather limited. The clog formation is more closely related to the short range hydrodynamic interaction between flowing particles and those which are already immobilized within the pore. Finally we demonstrate that all the clogging regimes can be gathered on a single phase diagram based on the flow conditions and the filter design.
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supplimentary information

74) Photo-Crosslinkable Colloids: From Fluid Structure and Dynamics of Spheres to Suspensions of Ellipsoids
GELS 2 article number 29 (2016)
Cohen AP, Alesker M, Schofield AB, Zitoun D, Sloutskin E
Abstract
Recently-developed photo-crosslinkable PMMA (polymethylmethacrylate) colloidal spheres are a highly promising system for fundamental studies in colloidal physics and may have a wide range of future technological applications. We synthesize these colloids and characterize their size distribution. Their swelling in a density- and index- matching organic solvent system is demonstrated and we employ dynamic light scattering (DLS), as also the recently-developed confocal differential dynamic microscopy (ConDDM), to characterize the structure and the dynamics of a fluid bulk suspension of such colloids at different particle densities, detecting significant particle charging effects. We stretch these photo-crosslinkable spheres into ellipsoids. The fact that the ellipsoids are cross-linked allows them to be fluorescently stained, permitting a dense suspension of ellipsoids, a simple model of fluid matter, to be imaged by direct confocal microscopy.
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73) The Secret Life of Pickering Emulsions: Particle Exchange Revealed Using Two Colours of Particle
SCIENTIFIC REPORTS 6 article 31401 (2016)
French DJ, Brown AT, Schofield AB, Fowler J, Taylor P, Clegg PS
Abstract
Emulsion droplets stabilised by colloidal particles (Pickering emulsions) can be highly stable, so it is unsurprising that they are beginning to be exploited industrially. The individual colloidal particles have interfacial attachment energies that are vastly larger than the thermal energy, hence they are usually thought of as being irreversibly adsorbed. Here we show, for the first time, particles being exchanged between droplets in a Pickering emulsion. This occurs when the emulsion contains droplets that share particles, often called bridging. By starting with two emulsions showing bridging, each stabilised by a different colour of particle, the dynamics can be studied as they are gently mixed together on a roller bank. We find that particle exchange occurs by two routes: firstly, during a period of unbridging and rebridging whose duration can be tuned by varying the wettability of the particles, and secondly, during very rare events when particles are ejected from one droplet and re-adsorbed onto another.
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72) Dipolar colloids in apolar media: direct microscopy of two-dimensional suspensions
SCIENTIFIC REPORTS 6 article 28578 (2016)
Janai E, Cohen AP, Butenko AV, Schofield AB, Schultz M, Sloutskin E
Abstract
Spherical colloids, in an absence of external fields, are commonly assumed to interact solely through rotationally-invariant potentials, u(r). While the presence of permanent dipoles in aqueous suspensions has been previously suggested by some experiments, the rotational degrees of freedom of spherical colloids are typically neglected. We prove, by direct experiments, the presence of permanent dipoles in commonly used spherical poly(methyl methacrylate) (PMMA) colloids, suspended in an apolar organic medium. We study, by a combination of direct confocal microscopy, computer simulations, and theory, the structure and other thermodynamical properties of organic suspensions of colloidal spheres, confined to a two-dimensional (2D) monolayer. Our studies reveal the effects of the dipolar interactions on the structure and the osmotic pressure of these fluids. These observations have farreaching consequences for the fundamental colloidal science, opening new directions in self-assembly of complex colloidal clusters.
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71) Start-up shear of concentrated colloidal hard spheres: Stresses, dynamics, and structure
JOURNAL OF RHEOLOGY 60 pages 603-623 (2016)
Koumakis N, Laurati M, Jacob AR, Mutch KJ, Abdellali A, Schofield AB, Egelhaaf SU, Brady JF, Petekidis G
Abstract
The transient response of model hard sphere glasses is examined during the application of steady rate start-up shear using Brownian dynamics simulations, experimental rheology and confocal microscopy. With increasing strain, the glass initially exhibits an almost linear elastic stress increase, a stress peak at the yield point and then reaches a constant steady state. The stress overshoot has a nonmonotonic dependence with Peclet number, Pe, and volume fraction, φ, determined by the available free volume and a competition between structural relaxation and shear advection. Examination of the structural properties under shear revealed an increasing anisotropic radial distribution function, g(r), mostly in the velocity-gradient (xy) plane, which decreases after the stress peak with considerable anisotropy remaining in the steady-state. Low rates minimally distort the structure, while high rates show distortion with signatures of transient elongation. As a mechanism of storing energy, particles are trapped within a cage distorted more than Brownian relaxation allows, while at larger strains, stresses are relaxed as particles are forced out of the cage due to advection. Even in the steady state, intermediate super diffusion is observed at high rates and is a signature of the continuous breaking and reformation of cages under shear.
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70) Direct Imaging of Vibrations in Colloidal Crystals: In Equilibrium and in a Steady Drift
JOURNAL OF PHYSICAL CHEMISTRY C 120 pages 8392-8398 (2016)
Janai E, Butenko AV, Schofield AB, Sloutskin E
Abstract
Crystals of colloids, micron-size particles in a solvent, typically contain high concentrations of structural defects, limiting their applicability in self-assembly of metamaterials. Defects and grain boundaries play an important role for most properties of these crystals. Most previous research of colloidal crystals, by experiment and theory, focused on spatially averaged vibrational spectra: the differences in local environment between the bulk crystal particles and those at a grain boundary were typically neglected. We employ direct confocal microscopy and recent more accurate particle tracking algorithms to study the potential wells of individual particles in thermally vibrating quasitwo- dimensional colloidal crystals. We demonstrate that the energy landscape probed by a particle sensitively depends on its local environment. Furthermore, we emphasize the commonly neglected role of slight out-of-equilibrium drift of colloidal crystals, demonstrating that particle vibrations depend significantly on the drift velocity, so that the drifting crystals are softer, allowing an effective "drift temperature" to be defined.
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69) Compressing a spinodal surface at fixed area: bijels in a centrifuge
SOFT MATTER 12 pages 4375-4383 (2016)
Rumble KA, Thijssen JHJ, Schofield AB, Clegg PS
Abstract
Bicontinuous interfacially jammed emulsion gels (bijels) are solid-stabilised emulsions with two interpenetrating continuous phases. Employing the method of centrifugal compression we find that macroscopically the bijel yields at relatively low angular acceleration. Both continuous phases escape from the top of the structure, making any compression immediately irreversible. Microscopically, the bijel becomes anisotropic with the domains aligned perpendicular to the compression direction which inhibits further liquid expulsion; this contrasts strongly with the sedimentation behaviour of colloidal gels. The original structure can, however, be preserved close to the top of the sample and thus the change to an anisotropic structure suggests internal yielding. Any air bubbles trapped in the bijel are found to aid compression by forming channels aligned parallel to the compression direction which provide a route for liquid to escape.
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68) Structural Transition in a Fluid of Spheroids: A Low-Density Vestige of Jamming
PHYSICAL REVIEW LETTERS 116 article 098001 (2016)
Cohen AP, Dorosz S, Schofield AB, Schilling T, Sloutskin E
Abstract
A thermodynamically equilibrated fluid of hard spheroids is a simple model of liquid matter. In this model, the coupling between the rotational degrees of freedom of the constituent particles and their translations may be switched off by a continuous deformation of a spheroid of aspect ratio t into a sphere (t=1). We demonstrate, by experiments, theory, and computer simulations, that dramatic nonanalytic changes in structure and thermodynamics of the fluids take place, as the coupling between rotations and translations is made to vanish. This nonanalyticity, reminiscent of a second-order liquid-liquid phase transition, is not a trivial consequence of the shape of an individual particle. Rather, free volume considerations relate the observed transition to a similar nonanalyticity at t=1 in structural properties of jammed granular ellipsoids. This observation suggests a deep connection to exist between the physics of jamming and the thermodynamics of simple fluids.
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67) Particle-Size Effects In The Formation Of Bicontinuous Pickering Emulsions
PHYSICAL REVIEW E 92, 032308, (2015)
Reeves M, Brown AT, Schofield AB, Cates ME, Thijssen JHJ
Abstract
We demonstrate that the formation of bicontinuous emulsions stabilized by interfacial particles (bijels) is more robust when nanoparticles rather than microparticles are used. Emulsification via spinodal demixing in the presence of nearly neutrally wetting particles is induced by rapid heating. Using confocal microscopy, we show that nanospheres allow successful bijel formation at heating rates two orders of magnitude slower than is possible with microspheres. In order to explain our results, we introduce the concept of mechanical leeway, i.e., nanoparticles benefit from a smaller driving force towards disruptive curvature. Finally, we suggest that leeway mechanisms may benefit any formulation in which challenges arise due to tight restrictions on a pivotal parameter, but where the restrictions can be relaxed by rationally changing the value of a more accessible parameter.
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66) Crystallization And Reentrant Melting Of Charged Colloids In Nonpolar Solvents
PHYSICAL REVIEW E 91, 030301(R) (2015)
Kanai T, Boon N, Lu PJ, Sloutskin E, Schofield AB, Smallenburg F, van Roij R, Dijkstra M, Weitz DA
Abstract
We explore the crystallization of charged colloidal particles in a nonpolar solvent mixture.We simultaneously charge the particles and add counterions to the solution with aerosol-OT (AOT) reverse micelles. At low AOT concentrations, the charged particles crystallize into body-centered-cubic (bcc) or face-centered-cubic (fcc) Wigner crystals; at high AOT concentrations, the increased screening drives a thus far unobserved reentrant melting transition. We observe an unexpected scaling of the data with particle size, and account for all behavior with a model that quantitatively predicts both the reentrant melting and the data collapse.
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65) Different Mechanisms For Dynamical Arrest In Largely Asymmetric Binary Mixtures
PHYSICAL REVIEW E 91, 032308 (2015)
Hendricks J, Capellmann R, Schofield AB, Egelhaaf SU, Laurati M
Abstract
Using confocal microscopy we investigate binary colloidal mixtures with large size asymmetry, in particular the formation of dynamically arrested states of the large spheres. The volume fraction of the system is kept constant, and as the concentration of small spheres is increased we observe a series of transitions of the large spheres to different arrested states: an attractive glass, a gel, and an asymmetric glass. These states are distinguished by the degree of dynamical arrest and the amount of structural and dynamical heterogeneity. The transitions between two different arrested states occur through melting and the formation of a fluid state. While a space-spanning network of bonded particles is found in both arrested and fluid states, only arrested states are characterized by the presence of a space-spanning network of dynamically arrested particles.
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64) Inter-particle correlations in a hard-sphere colloidal suspension with polymer additives investigated by Spin Echo Small Angle Neutron Scattering (SESANS)
SOFT MATTER 10 pages 3016-3026 (2014)
Washington AL, Li X, Schofield AB, Hong K, Fitzsimmons MR, Dalgliesh R, Pynn R
Abstract
Using a neutron scattering technique that measures a statistically-averaged density correlation function in real space rather than the conventional reciprocal-space structure factor, we have measured correlations between poly(methyl-methacrylate) (PMMA) colloidal particles of several sizes suspended in decalin. The new method, called Spin Echo Small Angle Neutron Scattering (SESANS) provides accurate information about particle composition, including the degree of solvent penetration into the polymer brush grafted on to the PMMA spheres to prevent aggregation. It confirms for particles, between 85 nm and 150 nm in radius that inter-particle correlations closely follow the Percus-Yevick hard-sphere model when the colloidal volume-fraction is between 30% and 50% provided the volume-fraction is used as a fitted parameter. No particle aggregation occurs in these systems. When small amounts of polystyrene are added as a depletant to a concentrated suspension of PMMA particles, short-range clustering of the particles occurs and there is an increase in the frequency of near-neighbor contacts. Within a small range of depletant concentration, near-neighbor correlations saturate and large aggregates with power law density correlations are formed. SESANS clearly separates the short- and long-range correlations and shows that, in this case, the power-law correlations are visible for inter-particle distances larger than roughly two particle diameters. In some cases, aggregate sizes are within our measurement window, which can extend out to 16 microns in favorable cases. We discuss the advantages of SESANS for measurements of the structure of concentrated colloidal systems and conclude that the method offers several important advantages.
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63) Heterogeneous Crystallization Of Hard And Soft Spheres Near Flat And Curved Walls
THE EUROPEAN PHYSICAL JOURNAL SPECIAL TOPICS 233 pages 439-454 (2014)
Sandomirski K, Walta S, Dubbert J, Allahyarov E, Schofield AB, Löwen H, Richtering W, Egelhaaf SU
Abstract
Crystallization represents a long-standing problem in statistical physics and is of great relevance for many practical and industrial applications. It often occurs in the presence of container walls or impurities, which are usually unavoidable or might even be desirable to facilitate crystallization by exploiting heterogeneous nucleation. Heterogeneous nucleation relies on a seed. Here we discuss the role of the seed and concentrate on a very generic situation, namely crystallization of hard and soft colloidal spheres in the presence of flat or curved hard walls. Curvature serves as a simple means to introduce a tunable mismatch between the seed-induced crystal lattice and the thermodynamically-favoured lattice. The mismatch induces distortions and elastic stress, which accumulate while the crystallite grows. This has an important consequence: once the crystallite reaches a critical size, it detaches from the seed allowing it to relax. The relaxed crystal continues to grow in the bulk, but crystallization ceases before reaching the seed, which now represents an impurity. Therefore, while seeds favour nucleation, any mismatch, like the seed curvature or an incommensurate structure, induces unfavourable distortions and can lead to the detachment of the crystallite. An additional mechanism to relax distortions is available to soft spheres, which can exploit their interaction potential and possibly deform. The different multi-step processes have been investigated by confocal microscopy, which provides particle-level information, and compared to computer simulations and theoretical results.
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62) Squeezing Particle-Stabilized Emulsions Into Biliquid Foams - Equation Of State
SOFT MATTER 9 pages 7757-7765 (2013)
Maurice L, Maguire R, Schofield AB, Cates ME, Clegg PS, Thijssen JHJ
Abstract
Using a centrifuge, we measure the (pressure vs. density) equation of state of Pickering emulsions stabilized by hard-sphere colloids, in order to elucidate the particle contribution to their mechanical properties. Moreover, we have developed a transparent Pickering emulsion, allowing us to determine local volume fraction as a function of distance within the sediment using confocal microscopy, thus extracting an entire equation of state from one centrifugation cycle. We can explain and predict trends in our data using a quantitative model incorporating interdroplet films with a thickness on the scale of the (micron-sized) particles and repulsive interactions across these films. We suggest that the effective repulsion between droplets is due to the deformation of the liquid-liquid interface between particles on one droplet due to compression against a neighbouring droplet.
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61) Dense Colloidal Fluids Form Denser Amorphous Sediments
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES 110 pages 5769-5773 (2013)
Liber SR, Borohovich S, Butenkoa AV, Schofield AB, Sloutskin E
Abstract
We relate, by simple analytical centrifugation experiments, the density of colloidal fluids with the nature of their randomly packed solid sediments. We demonstrate that the most dilute fluids of colloidal hard spheres form loosely packed sediments, where the volume fraction of the particles approaches in frictional systems the randomloose packing limit, φ_RLP=0.55. The dense fluids of the same spheres form denser sediments, approaching the so-called random close packing limit, φ_RCP = 0.64. Our experiments, where particle sedimentation in a centrifuge is sufficiently rapid to avoid crystallization, demonstrate that the density of the sediments varies monotonically with the volume fraction of the initial suspension. We reproduce our experimental data by simple computer simulations, where structural reorganizations are prohibited, such that the rate of sedimentation is irrelevant. This suggests that in colloidal systems, where viscous forces dominate, the structure of randomly close-packed and randomly loose-packed sediments is determined by the well-known structure of the initial fluids of simple hard spheres, provided that the crystallization is fully suppressed.
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60) Eliminating Cracking During Drying
THE EUROPEAN PHYSICAL JOURNAL E 36 article 28 (2013)
Jin Q, Tan P, Schofield AB, Xu L
Abstract
When colloidal suspensions dry, stresses build up and cracks often occur - a phenomenon undesirable for important industries such as paint and ceramics. We demonstrate an effective method which can completely eliminate cracking during drying: by adding emulsion droplets into colloidal suspensions, we can systematically decrease the amount of cracking, and eliminate it completely above a critical droplet concentration. Since the emulsion droplets eventually also evaporate, our technique achieves an effective function while making little changes to the component of final product, and may therefore serve as a promising approach for cracking elimination. Furthermore, adding droplets also varies the speed of air invasion and provides a powerful method to adjust drying rate. With the effective control over cracking and drying rate, our study may find important applications in many drying- and cracking-related industrial processes.
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59) Structural Studies Of Mixed Nano-Spheres And Polymers
THE CUBAN PHYSICS JOURNAL 29 pages 1E20-1E22 (2012)
Washington A, Li X, Schofield A, Hong K, Fitzsimmons MR, Pynn R
Abstract
A newly developed neutron scattering technique known as Spin Echo Small Angle Neutron Scattering (SESANS) allows real-space density correlations to be probed in bulk samples over distances ranging from ~20 nm to up to several microns. We have applied this technique to study correlations between polymer-stabilized poly(methyl methacrylate) (PMMA) spheres suspended in either dodecane or decalin. As expected, the data show that for colloid volume factions below about 40%, correlations between PMMA spheres are accurately described by the Percus-Yevick hard-sphere model. When a small amount of polymer is added to the colloidal suspension and when the carrier fluid is a good solvent for the polymer, short-range correlations between PMMA spheres are increased by the presence of the polymer depletant and are in reasonably agreement with calculations using an integral equation model. When higher concentrations of polymers are added, we find that long-range, power-law correlations develop between spheres, even though the sample flows freely. When the solvent is not a good solvent for the polymer depletant, correlations between PMMA spheres are unaffected by the addition of small quantities of polymer.
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58) Structure and Interactions in Fluids of Prolate Colloidal Ellipsoids: Comparison Between Experiment, Theory, and Simulation
THE JOURNAL OF CHEMICAL PHYSICS 137 article 184505 (2012)
Cohen AP, Janai E, Rapaport DC, Schofield AB, Sloutskin E
Abstract
The microscopic structure of fluids of simple spheres is well known. However, the constituents of most real-life fluids are non-spherical, leading to a coupling between the rotational and translational degrees of freedom. The structure of simple dense fluids of spheroids - ellipsoids of revolution - was only recently determined by direct experimental techniques [A. P. Cohen, E. Janai, E.Mogilko, A. B. Schofield, and E. Sloutskin, Phys. Rev. Lett. 107, 238301 (2011)]. Using confocal microscopy, it was demonstrated that the structure of these simple fluids cannot be described by hard particle models based on the widely used Percus-Yevick approximation. In this paper, we describe a new protocol for determining the shape of the experimental spheroids, which allows us to expand our previous microscopy measurements of these fluids. To avoid the approximations in the theoretical approach, we have also used molecular dynamics simulations to reproduce the experimental radial distribution functions g(r) and estimate the contribution of charge effects to the interactions. Accounting for these charge effects within the Percus-Yevick framework leads to similar agreement with the experiment.
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57) Transient Dynamics in Dense Colloidal Suspensions Under Shear: Shear Rate Dependence
JOURNAL OF PHYSICS: CONDENSED MATTER 24 article 464104 (2012)
Laurati M, Mutch KJ, Koumakis N, Zausch J, Amann CP, Schofield AB, Petekidis G, Brady JF, Horbach J, Fuchs M, Egelhaaf SU
Abstract
A combination of confocal microscopy and rheology experiments, Brownian dynamics (BD) and molecular dynamics (MD) simulations and mode coupling theory (MCT) have been applied in order to investigate the effect of shear rate on the transient dynamics and stress-strain relations in supercooled and glassy systems under shear. Immediately after shear is switched on, the microscopic dynamics display super-diffusion and the macroscopic rheology a stress overshoot, which become more pronounced with increasing shear rate. MCT relates both to negative sections of the generalized shear modulus, which grow with increasing shear rate. When the inverse shear rate becomes much smaller than the structural relaxation time of the quiescent system, relaxation through Brownian motion becomes less important. In this regime, larger stresses are accumulated before the system yields and the transition from localization to flow occurs earlier and more abruptly.
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56) Characterizing Concentrated, Multiply Scattering, and Actively Driven Fluorescent Systems with Confocal Differential Dynamic Microscopy
PHYSICAL REVIEW LETTERS 108 article 218103 (2012)
Lu PJ, Giavazzi F, Angelini TE, Zaccarelli E, Jargstorff F, Schofield AB, Wilking JN, Romanowsky MB, Weitz DA, Cerbino R
Abstract
We introduce confocal differential dynamic microscopy (ConDDM), a new technique yielding information comparable to that given by light scattering but in dense, opaque, fluorescent samples of micron-sized objects that cannot be probed easily with other existing techniques. We measure the correct wave vector q-dependent structure and hydrodynamic factors of concentrated hard-sphere-like colloids. We characterize concentrated swimming bacteria, observing ballistic motion in the bulk and a new compressed-exponential scaling of dynamics, and determine the velocity distribution; by contrast, near the coverslip, dynamics scale differently, suggesting that bacterial motion near surfaces fundamentally differs from that of freely swimming organisms.
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55)Understanding the Low-Frequency Quasilocalized Modes in Disordered Colloidal Systems
PHYSICAL REVIEW LETTERS 108 095501 (2012)
Tan P, Xu N, Schofield AB, Xu L
Abstract
In disordered colloidal systems, we experimentally measure the normal modes with the covariance matrix method and clarify the origin of low-frequency quasilocalization at the single-particle level. We observe important features from both jamming and glass simulations: There is a plateau in the density of states [D(ω)] which is suppressed upon compression, as predicted by jamming; within the same systems, we also find that the low-frequency quasilocalization originates from the large vibrations of defective structures coupled with transverse excitations, consistent with a recent glass simulation. The coexistence of these features demonstrates an experimental link between jamming and glass. Extensive simulations further show that such a structural origin of quasilocalization is universally valid for various temperatures and volume fractions.
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54)Non-crystalline colloidal clusters in two dimensions: size distributions and shapes
SOFT MATTER 8 2924-2929 (2012)
Janai E, Schofield AB, Sloutskin E
Abstract
Cluster formation in many-body systems is very common, yet still not fully understood. We employ direct confocal microscopy to measure the size distribution and reconstruct the shapes of permanent gel clusters formed by sticky colloidal spheres in a two-dimensional (2D) suspension; the linear dimensions of the clusters are then measured by their radii of gyration R_g. We compare these non-ergodic clusters with the short-lived clusters, which reversibly form and deform, in a thermodynamically-equilibrated system of spherical colloids which interact solely by repulsions. Surprisingly, a similar behavior is observed for both types of clusters. In both cases, the average R_g of large clusters consisting of M particles scales as 〈R_g〉~M^1/2, which indicates that these clusters are solid, while the smaller clusters are much more ramified. A simple lattice model with a single free parameter quantitatively describes this complex behavior of 〈R_g(M)〉. The experimental size distribution P(M) of our clusters is a (truncated) power law M^~α, where the index α scales with colloid density and depends on the interparticle interactions. Strikingly, the observed behavior cannot be described by the common theoretical models which predict shorter correlation lengths and a density-independent value of a; thus, further theoretical efforts are necessary to fully understand the physics of clustering in this simple and fundamental system.
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53)Relationship between cooperative motion and the confinement length scale in confined colloidal liquids
SOFT MATTER 8 814-818 (2012)
Sarangapani PS, Schofield AB, Zhu Y
Abstract
The "confinement length scale", defined as the critical spacing where deviations from bulk behaviors begin, is widely examined with confined molecular and colloidal liquids, yet its origin and relationship to cooperative motion remain an open question. In this work, we examine the correlation between the sizes of cooperatively rearranging regions (CRRs) to the shift of "hard-sphere" colloidal glass transition in confined domains. We find that the confinement length scale observed in our prior work can be viewed as a regime where CRRs reach a finite size and sets the range for cooperative motion. Additionally, string-like motions within these mobile regions are enhanced at film thicknesses below the confinement length scale and reach maximal at the smallest thickness examined, suggesting an increase in the fragility of confined suspensions.
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52) Fluid Suspensions of Colloidal Ellipsoids: Direct Structural Measurements
PHYSICAL REVIEW LETTERS 107 article 238301 (2011)
Cohen AP, Janai E, Mogilko E, Schofield AB, Sloutskin E
Abstract
A fluid of spheroids, ellipsoids of revolution, is among the simplest models of the disordered matter, where positional and rotational degrees of freedom of the constituent particles are coupled. However, while highly anisometric rods, and hard spheres, were intensively studied in the last decades, the structure of a fluid of spheroids is still unknown.We reconstruct the structure of a simple fluid of spheroids, employing direct confocal imaging of colloids, in three dimensions. The ratio t between the polar axis and the equatorial diameter for both our prolate and oblate spheroids is not far from unity, which gives rise to a delicate interplay between rotations and translations. Strikingly, the measured positional interparticle correlations are significantly stronger than theoretically predicted, indicating that further theoretical attention is required, to fully understand the coupling between translations and rotations in these fundamental fluids.
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51) Hindered Coarsening of a Phase-Separating Microemulsion Due to Dispersed Colloidal Particles
LANGMUIR 27 Pages 13436-13443 (2011)
van't Zand DD, Schofield AB, Thijssen JHJ, Clegg PS
Abstract The addition of sterically stabilized colloidal particles to a phase-separating microemulsion leads to dramatic changes in its demixing behavior, especially during the later stages. Our microemulsion is composed of reverse micelles of sodium dodecyl sulfate, pentanol, and water in a dodecane continuous phase which separates into micelle-rich and micelle poor phases above a lower critical solution temperature. The poly(methyl methacrylate) particles preferentially partition into the less structured, micelle-poor phase. Nucleation of the minority phase or spinodal decomposition close to criticality continue to occur in the presence of particles, albeit with pronounced pretransitional clustering of particles when the micelle-poor phase is in the minority. The coalescence of micelle-poor droplets and the coarsening of micelle-rich domains are both strongly modified due to the presence of colloidal particles. We use our observations of the early stages of phase separation to understand these late stage changes.
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50) A Self-Quenched Defect Glass in a Colloid-Nematic Liquid Crystal Composite
SCIENCE 334 Pgs 79-83 (2011)
Wood TA, Lintuvuori JS, Schofield AB, Marenduzzo D, Poon WCK
Abstract Colloidal particles immersed in liquid crystals frustrate orientational order. This generates defect lines known as disclinations. At the core of these defects, the orientational order drops sharply. We have discovered a class of soft solids, with shear moduli up to 104 pascals, containing high concentrations of colloidal particles (volume fraction φ≥20%) directly dispersed into a nematic liquid crystal. Confocal microscopy and computer simulations show that the mechanical strength derives from a percolated network of defect lines entangled with the particles in three dimensions. Such a "self-quenched glass" of defect lines and particles can be considered a self-organized analog of the "vortex glass" state in type II superconductors.
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49) How do (fluorescent) surfactants affect particle-stabilized emulsions?
SOFT MATTER 7 Pgs 7965-7968 (2011)
Thijssen JHJ, Schofield AB, Clegg P
Abstract
We present the first confocal-microscopy study of synergistic effects in emulsions stabilized by both colloidal particles and a common fluorescent dye that acts as a surfactant. In situ microscopic imaging reveals surfactant adsorption onto the liquid-liquid interface and onto the colloidal particles, which changes the interfacial tension and the particle contact angle. This leads to emulsions that are more stable, more polydisperse and can incorporate more of the dispersed phase.
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48) Novel, Robust, and Versatile Bijels of Nitromethane, Ethanediol, and Colloidal Silica: Capsules, Sub-Ten-Micrometer Domains, and Mechanical Properties
ADVANCED FUNCTIONAL MATERIALS 21 Pgs 2020-2027 (2011)
Tavacoli JW, Thijssen JHJ, Schofield AB, Clegg PS
Abstract
Bicontinuous, interfacially jammed emulsion gels (bijels) are a class of soft solid materials in which interpenetrating domains of two immiscible fluids are stabilized by an interfacial colloidal monolayer. Such structures form through the arrest of the spinodal decomposition of an initially single-phase liquid mixture containing a colloidal suspension. With the use of hexalmethyldisilazane, the wetting character of silica colloids, ranging in size and dye content, can be modified for fabricating a novel bijel system comprising the binary liquid ethanediol-nitromethane. Unlike the preceding water-lutidine based system, this bijel is stable at room temperature and its fabrication and resultant manipulation are comparatively straightforward. The new system has facilitated three advancements: firstly, we use sub 100 nm silica particles to stabilize the first bijel made from low molecular weight liquids that has domains smaller than ten micrometers. Secondly, our new and robust bijel permits qualitative rheological work which reveals the bijel to be significantly elastic and self healing whilst its domains are able to break, reform and locally rearrange. Thirdly, we encapsulate the ethanediol-nitromethane bijel in Pickering drops to form novel particle-stabilized bicontinuous multiple emulsions that we christen bijel capsules. These emulsions are stimuli responsive - they liberate their contained materials in response to changes in temperature and solvency, and hence they show potential for controlled release applications.
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47) Inversion of particle-stabilized emulsions of partially miscible liquids by mild drying of modified silica particles
JOURNAL OF COLLOID AND INTERFACE SCIENCE 359 Pgs 126-135 (2011)
White KA, Schofield AB, Wormald P, Tavacoli JW, Binks BP, Clegg PS
Abstract
Using a system of modified silica particles and mixtures of water and 2,6-lutidine to form particle-stabilized emulsions, we show that subtle alterations to the hydration of the particle surface can cause major shifts in emulsion structure. We use fluorescence confocal microscopy, solid state nuclear magnetic resonance (NMR) and thermo-gravimetric analysis (TGA) to explore this sensitivity, along with other shifts caused by modifications to the silica surface chemistry. The silica particles are prepared by a variant of the Stober procedure and are modified by the inclusion of 3-(aminopropyl)triethoxysilane and the dye fluorescein isothiocyanate. Treatment prior to emulsification consists of gently drying the particles under carefully controlled conditions. In mixtures of water and 2,6-lutidine of critical composition, the particles stabilize droplet emulsions and bijels. Decreasing particle hydration yields an inversion of the emulsions from lutidine-in-water (L/W) to water-in-lutidine (W/L), with bijels forming around inversion. So dependent is the emulsion behavior on particle hydration that microscopic differences in drying within a particle sample can cause differences in the wetting behavior of that sample, which helps to stabilize multiple emulsions. The formation of bijels at emulsion inversion is also crucially dependent on the surface modification of the silica.
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46) Direct Experimental Evidence Of Growing Dynamic Length Scales In Confined Colloidal Liquids
Sarangapani PS, Schofield AB, and Zhu Y
PHYSICAL REVIEW E 83 article number 030502 (2011)
Abstract
The modification of the glass transition in confined domains, particularly the length scales associated with cooperative motion, remains a mystery. Hard-sphere suspensions are confined between two surfaces to progressively smaller dimensions to probe the confinement effect on the growth of dynamic heterogeneities via confocal microscopy. The confinement length scale is defined as the critical spacing where deviations from bulk behaviors begin and is observed to occur at progressively larger gap spacings as the volume fraction is increased. However, dynamic length scales extracted from the four-point correlation function are on average smaller than the confinement length scale.
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45) Synthesis and Directed Self-Assembly of Patterned Anisometric Polymeric Particles
Zhang Z, Pfleiderer P, Schofield AB, Clasen C, Vermant J
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 133 392-395 (2011)
Abstract
A simple and versatile method for making chemically patterned anisotropic colloidal particles is proposed and demonstrated for two different types of patterning. Using a combination of thermo/mechanical stretching followed by a wet chemical treatment of a sterically stabilized latex, both patchy ellipsoidal particles with sticky interactions near the tips as well as particles with tunable fluorescent patterns could be easily produced. The potential of such model colloidal particles is demonstrated, specifically for the case of directed self-assembly.
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44) Quantitative Imaging Of Concentrated Suspensions Under Flow
Isa L, Besseling R, Schofield AB, Poon WCK
ADVANCES IN POLYMER SCIENCE 236 Pgs 163-202 (2010)
Abstract
We review recent advances in imaging the flow of concentrated suspensions, focussing on the use of confocal microscopy to obtain time-resolved information on the single-particle level in these systems. After motivating the need for quantitative (confocal) imaging in suspension rheology, we briefly describe the particles, sample environments, microscopy tools and analysis algorithms needed to perform this kind of experiments. The second part of the review focusses on microscopic aspects of the flow of concentrated model hard-sphere-like suspensions, and the relation to non-linear rheological phenomena such as yielding, shear localization, wall slip and shear-induced ordering. Both Brownian and non-Brownian systems will be described. We show how quantitative imaging can improve our understanding of the connection between microscopic dynamics and bulk flow.
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43) Orders-Of-Magnitude Performance Increases In GPU-Accelerated Correlation Of Images From The International Space Station
Lu PJ, Oki H, Frey CA, Chamitoff GE, Chiao L, Fincke EM, Foale CM, Magnus SH, McArthur WS, Tani DM, Whitson PA, Williams JN, Meyer WV, Sicker RJ, Au BJ, Christiansen M, Schofield AB, Weitz DA
JOURNAL OF REAL-TIME IMAGE PROCESSING 5 Pgs 179-193 (2010)
Abstract
We implement image correlation, a fundamental component of many real-time imaging and tracking systems, on a graphics processing unit (GPU) using NVIDIA's CUDA platform. We use our code to analyze images of liquid-gas phase separation in a model colloid-polymer system, photographed in the absence of gravity aboard the International Space Station (ISS). Our GPU code is 4,000 times faster than simple MATLAB code performing the same calculation on a central processing unit (CPU), 130 times faster than simple C code, and 30 times faster than optimized C++ code using single-instruction, multipledata (SIMD) extensions. The speed increases from these parallel algorithms enable us to analyze images downlinked from the ISS in a rapid fashion and send feedback to astronauts on orbit while the experiments are still being run.
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42) Arrested fluid-fluid phase separation in depletion systems: Implications of the characteristic length on gel formation and rheology
Conrad JC, Wyss HM, Trappe V, Manley S, Miyazaki K, Kaufman LJ, Schofield AB, Reichman DR, Weitz DA
JOURNAL OF RHEOLOGY 54 Pgs 421-438 (2010)
Abstract
We investigate the structural, dynamical, and rheological properties of colloid-polymer mixtures in a volume fraction range of φ=0.15-0.35. Our systems are density-matched, residual charges are screened, and the polymer-colloid size ratio is ∼ 0.37. For these systems, the transition to kinetically arrested states, including disconnected clusters and gels, coincides with the fluid-fluid phase separation boundary. Structural investigations reveal that the characteristic length, L, of the networks is a strong function of the quench depth: for shallow quenches, L is significantly larger than that obtained for deep quenches. By contrast, L is for a given quench depth almost independent of φ this indicates that the strand thickness increases with φ. The strand thickness determines the linear rheology: the final relaxation time exhibits a strong dependence on φ, whereas the high frequency modulus does not. We present a simple model based on estimates of the strand breaking time and shear modulus that semiquantitatively describes the observed behavior.
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41) Drying of Complex Suspensions
Xu L, Bergès A, Lu PJ, Studart AR, Schofield AB, Oki H, Davies S, Weitz DA
PHYSICAL REVIEW LETTERS 104 article 128303 (2010)
Abstract
We investigate the 3D structure and drying dynamics of complex mixtures of emulsion droplets and colloidal particles, using confocal microscopy. Air invades and rapidly collapses large emulsion droplets, forcing their contents into the surrounding porous particle pack at a rate proportional to the square of the droplet radius. By contrast, small droplets do not collapse, but remain intact and are merely deformed. A simple model coupling the Laplace pressure to Darcy's law correctly estimates both the threshold radius separating these two behaviors, and the rate of large-droplet evacuation. Finally, we use these systems to make novel hierarchical structures.
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40) Dynamic Light Scattering Measurements In The Activated Regime Of Dense Colloidal Hard Spheres
El Masri D, Brambilla G, Pierno M, Petekidis G, Schofield AB, Berthier L, Cipelletti L
JOURNAL OF STATISTICAL MECHANICS: THEORY AND EXPERIMENT P07015 (2009)
Abstract
We use dynamic light scattering and numerical simulations to study the approach to equilibrium and the equilibrium dynamics of systems of colloidal hard spheres over a broad range of densities, from dilute systems up to very concentrated suspensions undergoing glassy dynamics. We discuss several experimental issues (sedimentation, thermal control, non-equilibrium ageing effects, dynamic heterogeneity) arising when very large relaxation times are measured. When analyzed over more than seven decades in time, we find that the equilibrium relaxation time, τ_α, of our system is described by the algebraic divergence predicted by mode-coupling theory over a window of about three decades. At higher density, τ_α increases exponentially with distance to a critical volume fraction φ₀, which is much larger than the mode-coupling singularity. This is reminiscent of the behavior of molecular glass-formers in the activated regime. We compare these results to previous work, carefully discussing crystallization and size polydispersity effects. Our results suggest the absence of a genuine algebraic divergence of τ_α in colloidal hard spheres.
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39) Particle Dynamics In Colloidal Suspensions Above And Below The Glass-Liquid Re-entrance Transition
Latka A, Han Y, Alsayed AM, Schofield AB, Yodh AG, Habdas P
EPL 86 article 58001 (2009)
Abstract
We study colloidal particle dynamics of a model glass system using confocal and fluorescence microscopy as the sample evolves from a hard-sphere glass to a liquid with attractive interparticle interactions. The transition from hard-sphere glass to attractive liquid is induced by short-range depletion forces. The development of liquid-like structure is indicated by particle dynamics. We identify particles which exhibit substantial motional events and characterize the transition using the properties of these motional events. As samples enter the attractive liquid region, particle speed during these motional events increases by about one order of magnitude, and the particles move more cooperatively. Interestingly, colloidal particles in the attractive liquid phase do not exhibit significantly larger displacements than particles in the hard-sphere glass.
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38) Passive And Active Microrheology Of Hard-Sphere Colloids
Wilson LG, Harrison AW, Schofield AB, Arlt J, Poon WCK
THE JOURNAL OF PHYSICAL CHEMISTRY B 113(12) pgs 3806-3012 (2009)
Abstract
We performed passive and active microrheology using probe particles in a bath of well-characterized, model hard-sphere colloids in the fluid state over the whole range of volume fractions below the glass transition. The probe and bath particles have nearly the same size. Passive tracking of probe particles yields short-time self-diffusion coefficients. Comparison with literature data demonstrates that the interaction between probe and bath particles is hard-sphere-like. The short-time diffusivities yield one set of microviscosities as a function of volume fraction, which agrees with previous macrorheological measurements of the high-frequency viscosity of hard-sphere colloids. Using optical tweezers, we measure the force on a trapped probe particle as the rest of the sample is translated at constant velocity. This yields a second set of microviscosities at high Péclet numbers. These agree with previous macrorheological measurements of the high-shear viscosity of similar colloids, at shear-rates below the onset of shear-thickening.
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37) Structure, Dynamics And Rheology Of Colloid-Polymer Mixtures: From Liquids To Gels
Laurati M, Petekidis G, Koumakis N, Cardinaux F, Schofield AB, Brader JM, Fuchs M, Egelhaaf SU
THE JOURNAL OF CHEMICAL PHYSICS 130, 134907 (2009)
Abstract
We investigate the structural, dynamical, and viscoelastic properties of colloid-polymer mixtures at intermediate colloid volume fraction and varying polymer concentrations, thereby tuning the attractive interactions. Within the examined range of polymer concentrations, the samples varied from fluids to gels. In the liquid phase, an increasing correlation length of the density fluctuations when approaching the gelation boundary was observed by static light scattering and microscopy, indicating clustering and formation of space-spanning networks. Simultaneously, the correlation function determined by dynamic light scattering decays completely, indicating the absence of dynamical arrest. Clustering and formation of transient networks when approaching the gelation boundary is supported by significant changes in the viscoelastic properties of the samples. Upon increasing the polymer concentration beyond the gelation boundary, the rheological properties changed qualitatively again, now they are consistent with the formation of colloidal gels. Our experimental results, namely, the location of the gelation boundary as well as the elastic (storage) and viscous (loss) moduli, are compared to different theoretical models. These include consideration of the escape time as well as predictions for the viscoelastic moduli based on scaling relations and mode coupling theories.
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Copyright (2009) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics The following article appeared in The Journal Of Chemical Physics 130, 134907 (2009) and may be found at Laurati et al

36) Probing The Equilibrium Dynamics Of Colloidal Hard Spheres Above The Mode-Coupling Glass Transition
Brambilla G, El Masri D, Pierno M, Berthier L, Cipelletti L, Petekidis G, Schofield AB
PHYSICAL REVIEW LETTERS 102 article 085703 (2009)
Abstract We use dynamic light scattering and computer simulations to study equilibrium dynamics and dynamic heterogeneity in concentrated suspensions of colloidal hard spheres. Our study covers an unprecedented density range and spans seven decades in structural relaxation time, τ_α, including equilibrium measurements above φ_c, the location of the glass transition deduced from fitting our data to mode-coupling theory. Instead of falling out of equilibrium, the system remains ergodic above φ_c and enters a new dynamical regime where τ_α increases with a functional form that was not anticipated by previous experiments, while the amplitude of dynamic heterogeneity grows slower than a power law with τ_α, as found in molecular glass formers close to the glass transition.
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This paper has since generated some debate about its results and there has been published one comment upon them and a subsequent reply.

35) The Effect Of Curvature And Topology On Membrane Hydrodynamics
Henle Ml, McGorty R, Schofield AB, Dinsmore AD, Levine AJ
EPL 84(4) article 48001 (2008)
Abstract We study the mobilities of point-like and extended objects (rods) on a spherical membrane to show how these quantities are modified in a striking manner by the curvature and topology of the membrane. We also present theoretical calculations and experimental measurements of the membrane fluid velocity field around a moving rod bound to the crowded interface of a water-in-oil droplet. By using different droplet sizes, membrane viscosities, and rod lengths, we show that the viscosity mismatch between the interior and exterior fluids leads to a suppression of the fluid flow on small droplets that cannot be captured by the flat-membrane predictions.
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34) Influence Of Particle Composition And Thermal Cycling On Bijel Formation
White KA, Schofield AB, Binks BP, Clegg P.S.
JOURNAL OF PHYSICS: CONDENSED MATTER 20 article 494223 (2008)
Abstract
Colloidal particles with appropriate wetting properties can become very strongly trapped at an interface between two immiscible fluids. We have harnessed this phenomenon to create a new class of soft materials with intriguing and potentially useful characteristics. The material is known as a bijel: bicontinuous interfacially-jammed emulsion gel. It is a colloid-stabilized emulsion with fluid-bicontinuous domains. The potential to create these gels was first predicted using computer simulations. Experimentally we use mixtures of water and 2,6-lutidine at the composition for which the system undergoes a critical demixing transition on warming. Colloidal silica, with appropriate surface chemistry, is dispersed while the system is in the single-fluid phase; the composite sample is then slowly warmed well beyond the critical temperature. The liquids phase separate via spinodal decomposition and the particles become swept up on the newly created interfaces. As the domains coarsen the interfacial area decreases and the particles eventually become jammed together. The resulting structures have a significant yield stress and are stable for many months. Here we begin to explore the complex wetting properties of fluorescently tagged silica surfaces in water-lutidine mixtures, showing how they can be tuned to allow bijel creation. Additionally we demonstrate how the particle properties change with time while they are immersed in the solvents.
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33)Effects Of Shear Induced Crystallization On The Rheology And Ageing Of Hard Sphere Glasses
Koumakis N, Schofield AB, Petekidis G
SOFT MATTER 4 Pg 2008-2018 (2008)
Abstract
The rheological properties of highly concentrated suspensions of hard sphere particles are studied with particular reference to the rheological response of shear induced crystals. Using practically monodisperse hard spheres, we prepare shear induced crystals under oscillatory shear and examine their linear and non-linear mechanical responses in comparison with their glassy counterparts at the same volume fraction. It is evident, that shear induced crystallization causes a significant drop in the elastic and viscous moduli due to structural rearrangements that ease flow. For the same reason the critical (peak of G'') and crossover (overlap of G' and G'') strain are smaller in the crystal compared to the glass at the same volume fraction. However, when the distance from the maximum packing in each state is taken into account the elastic modulus of the crystal is found to be larger than the glass at the same free volume, suggesting a strengthened material due to long range order. Finally, shear induced crystals counter-intuitively exhibit similar rheological ageing to the glass (with a logarithmic increase of G'), indicating that the shear induced structure is not at thermodynamic equilibrium.
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32)Dynamics Of Drying In 3D Porous Media
Xu L, Davies S, Schofield AB, Weitz DA
PHYSICAL REVIEW LETTERS 101 Article 094502 (2008)
Abstract
The drying dynamics in three dimensional porous media are studied with confocal microscopy. We observe abrupt air invasions in size from single particle to hundreds of particles. We show that these result from the strong flow from menisci in large pores to menisci in small pores during drying. This flow causes air invasions to start in large menisci and subsequently spread throughout the entire system. We measure the size and structure of the air invasions and show that they are in accord with invasion percolation. By varying the particle size and contact angle we unambiguously demonstrate that capillary pressure dominates the drying process.
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31)Gelation Of Particles With Short-Range Attraction
Lu PJ, Zaccarelli E, Ciulla F, Schofield AB, Sciortino F, Weitz DA
NATURE 453 pg 499-503 (2008)
Abstract
Nanoscale or colloidal particles are important in many realms of science and technology. They can dramatically change the properties of materials, imparting solid-like behaviour to a wide variety of complex fluids. This behaviour arises when particles aggregate to form mesoscopic clusters and networks. The essential component leading to aggregation is an interparticle attraction, which can be generated by many physical and chemical mechanisms. In the limit of irreversible aggregation, infinitely strong interparticle bonds lead to diffusion-limited cluster aggregation (DLCA). This is understood as a purely kinetic phenomenon that can form solid-like gels at arbitrarily low particle volume fraction. Far more important technologically are systems with weaker attractions, where gel formation requires higher volume fractions. Numerous scenarios for gelation have been proposed, including DLCA, kinetic or dynamic arrest, phase separation, percolation and jamming. No consensus has emerged and, despite its ubiquity and significance, gelation is far from understood-even the location of the gelation phase boundary is not agreed on. Here we report experiments showing that gelation of spherical particles with isotropic, short-range attractions is initiated by spinodal decomposition; this thermodynamic instability triggers the formation of density fluctuations, leading to spanning clusters that dynamically arrest to create a gel. This simple picture of gelation does not depend on microscopic system-specific details, and should thus apply broadly to any particle system with short-range attractions. Our results suggest that gelation-often considered a purely kinetic phenomenon is in fact a direct consequence of equilibrium liquid-gas phase separation. Without exception, we observe gelation in all of our samples predicted by theory and simulation to phase separate; this suggests that it is phase separation, not percolation, that corresponds to gelation in models for attractive spheres.
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30)Bicontinuous Emulsions Stabilized Solely By Colloidal Particles
Herzig EM, White KA, Schofield AB, Poon WCK, Clegg PS
NATURE MATERIALS 6 (12) Pg 966-971 (2007)
Abstract
Recent large-scale computer simulations suggest that it may be possible to create a new class of soft solids, called 'bijels', by stabilizing and arresting the bicontinuous interface in a binary liquid demixing via spinodal decomposition using particles that are neutrally wetted by both liquids. The interfacial layer of particles is expected to be semi-permeable; hence, if realized, these new materials would have many potential applications, for example, as micro-reaction media. However, the creation of bijels in the laboratory faces serious obstacles. In general, fast quench rates are necessary to bypass nucleation, so that only samples with limited thickness can be produced, which destroys the three dimensionality of the putative bicontinuous network. Moreover, even a small degree of unequal wettability of the particles by the two liquids can lead to ill-characterized, 'lumpy' interfacial layers and therefore irreproducible material properties. Here, we report a reproducible protocol for creating three-dimensional samples of bijel in which the interfaces are stabilized by essentially a single layer of particles. We demonstrate how to tune the mean interfacial separation in these bijels, and show that mechanically, they indeed behave as soft solids. These characteristics and their tunability will be of great value for microfluidic applications.
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29)Spinodal Decomposition in a Model Colloid-Polymer Mixture in Microgravity
Bailey AE, Poon WCK, Christianson RJ, Schofield AB, Gasser U, Prasad V, Manley S, Segre PN, Cipelletti L, Meyer WV, Doherty MP, Sankaran S, Jankovsky AL, Shiley WL, Bowen JP, Eggers JC, Kurta C, Lorik T Jr, Pusey PN, Weitz DA
PHYSICAL REVIEW LETTERS 99 (20) Article 205701 (2007)
Abstract
We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.
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28)Three-dimensional imaging of colloidal glasses under steady shear
Besseling R, Weeks ER, Schofield AB, Poon WCK
PHYSICAL REVIEW LETTERS 99 (2): Art. No. 028301 (2007)
Abstract
Using fast confocal microscopy we image the three-dimensional dynamics of particles in a yielded hard-sphere colloidal glass under steady shear. The structural relaxation, observed in regions with uniform shear, is nearly isotropic but is distinctly different from that of quiescent metastable colloidal fluids. The inverse relaxation time and diffusion constant D, as functions of the local shear rate , show marked shear thinning with ∝ D ∝ over more than two decades in . In contrast, the global rheology of the system displays Herschel-Bulkley behavior. We discuss the possible role of large scale shear localization and other mechanisms in generating this difference.
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27)Slow Dynamics and Aging in Colloidal Gels Studied by X-ray Photon Correlation Spectroscopy
Fluerasu A, Moussaïd A, Madsen A, Schofield AB
PHYSICAL REVIEW E 76 article 010401(R) (2007)
Abstract
Slow, nonequilibrium dynamics during delayed sedimentation in a colloidal depletion gel was studied by x-ray photon correlation spectroscopy. The intermediate scattering functions change during the process from stretched to compressed exponential decays, indicating a jamming transition toward full aging. A complex aging behavior follows this process; it is proposed that large-scale network deformations trigger an unjamming, leading to the final collapse of the gel.
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26)Emulsification of Partially Miscible Liquids Using Colloidal Particles: Nonspherical and Extended Domain Structures
Clegg PS, Herzig EM, Schofield AB, Egelhaaf SU, Horozov TS, Binks BP, Cates ME, Poon WCK
LANGMUIR 23 Pg 5984-5994 (2007)
Abstract
We present microscopy studies of particle-stabilized emulsions with unconventional morphologies. The emulsions comprise pairs of partially miscible fluids and are stabilized by colloids. Alcohol-oil mixtures are employed; silica colloids are chemically modified so that they have partial wettability. We create our morphologies by two distinct routes: starting with a conventional colloid-stabilized emulsion or starting in the single-fluid phase with the colloids dispersed. In the first case temperature cycling leads to the creation of extended fluid domains built around some of the initial fluid droplets. In the second case quenching into the demixed region leads to the formation of domains which reflect the demixing kinetics. The structures are stable due to a jammed, semisolid, multilayer of colloids on the liquid-liquid interface. The differing morphologies reflect the roles in formation of the arrested state of heterogeneous and homogeneous nucleation and spinodal decomposition. The latter results in metastable, bicontinuous emulsions with frozen interfaces, at least for the thin-slab samples, investigated here.
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25)Noncentral Forces In Crystals Of Charged Colloids
Reinke D, Stark H, von Grünberg HH, Schofield AB, Maret G, Gasser U
PHYSICAL REVIEW LETTERS 98 article 038301 (2007)
Abstract
The elastic properties of fcc crystals consisting of charge stabilized colloidal particles are determined from real space imaging experiments using confocal microscopy. The normal modes and the force constants of the crystal are obtained from the fluctuations of the particles around their lattice sites using the equipartition theorem. We show that the Cauchy relation is not fulfilled and that only noncentral many-body forces can account for the elastic properties.
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24)Hypersonic Acoustic Excitations in Binary Colloidal Crystals: Big Versus Small Hard Sphere Control
Tommaseo G, Petekidis G, Steffen W, Fytas G, Schofield AB, Stefanou N
JOURNAL OF CHEMICAL PHYSICS 126 article 014707 (2007)
Abstract
The phononic band structure of two binary colloidal crystals, at hypersonic frequencies, is studied by means of Brillouin light scattering and analyzed in conjunction with corresponding dispersion diagrams of the single colloidal crystals of the constituent particles. Besides the acoustic band of the average medium, the authors' results show the existence of narrow bands originating from resonant multiple modes of the individual particles as well as Bragg-type modes due to the (short-range) periodicity. Strong interactions, leading to the occurrence of hybridization gaps, is observed between the acoustic band and the band of quadrupole modes of the particles that occupy the largest fractional volume of the mixed crystal; the effective radius is either that of the large (in the symmetric NaCl-type crystalline phase) or the small (in the asymmetric NaZn₁₃-type crystalline phase) particles. The possibility to reveal a universal behavior of the phononic band structure for different single and binary colloidal crystalline suspensions, by representing in the dispersion diagrams reduced quantities using an appropriate length scale, is discussed.
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23)Slip, Yield and Bands in Colloidal Crystals Under Oscillatory Shear
Cohen I, Davidovitch B, Schofield AB, Brenner MP, Weitz DA
PHYSICAL REVIEW LETTERS 97 article 215502 (2006)
Abstract
We study dense colloidal crystals under oscillatory shear using a confocal microscope. At large strains the crystals yield and the suspension forms shear bands. The pure harmonic response exhibited by the suspension rules out the applicability of non linear rheology models typically used to describe shear banding in other types of complex fluids. Instead, we show that a model based on the coexistence of linerally responding phases of the colloidal suspension accounts for the observed flows. These results highlight a new use of oscillatory measurements in distinguishing the contribution of linear and nonlinear local rheology to a globally nonlinear material response.
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22)Stable Jets Of Viscoelastic Fluids and Self-Assembled Cylindrical Capsules By Hydrodynamic Focusing
Edmond KV, Schofield AB, Marquez M, Rothstein JP, Dinsmore AD
LANGMUIR 22 Pg 9052-9056 (2006)

Abstract
We demonstrate formation of long-lived cylindrical jets of a viscoelastic fluid using hydrodynamic focusing. A solution of polyacrylamide in water is driven coaxially with immisible oil and subjected to strong extensional flow. At high flow rates, the aqueous phase forms jets that are 4-90µm in diameter and several centimeters long. The liquid surfaces of these jets are then used as templates for assembly of microspheres into novel rigid and hollow cylinders.

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21)Fluids of Clusters in Attractive Colloids
Lu PJ, Conrad JC, Wyss HM, Schofield AB, Weitz DA
PHYSICAL REVIEW LETTERS 96 article 028306 (2006)

Abstract
We show that colloidal particles with attractive interactions induced by a non-adsorbing polymer exhibit a stable phase consisting of a fluid of clusters of particles. This phase persists even in the absence of any long-range repulsion due to charge, contrary to expectations based on simulation and theory. Cluster morphology depends strongly on the range of the interparticle attraction. With a shorter range, clusters are tenuous and branched; with a longer range they are more compact.

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20)Colloid-stabilized emulsions: behaviour as the interfacial tension is reduced
Clegg PS, Herzig EM, Schofield AB, Horozov TS, Binks BP, Cates ME, Poon WCK
JOURNAL OF PHYSICS-CONDENSED MATTER 17(45): S3433-S3438 (2005)

Abstract
We present confocal microscopy studies of novel particle-stabilized emulsions. The novelty arises because the immiscible fluids have an accessible upper critical solution temperature. The emulsions have been created by beginning with particles dispersed in the single-fluid phase. On cooling. regions of the minority phase nucleate. While coarsening, these nuclei become coated with particles due to the associated reduction in interfacial energy. The resulting emulsion is arrested, and the particle-coated interfaces have intriguing properties. Having made use of the binary-fluid phase diagram to create the emulsion we then make use of it to study the properties of the interfaces. As the emulsion is re-heated toward the single-fluid phase the interfacial tension falls and the volume of the dispersed phase drops. Crumpling, fracture or coalescence can follow. The results show that the elasticity of the interfaces has a controlling influence over the emulsion behaviour.

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19) Stability of the Binary Colloidal Crystals AB₂ and AB₁₃
Schofield A.B., Pusey P.N., Radcliffe P.
PHYSICAL REVIEW E 72 article 031407 (2005)

Abstract
Suspensions of binary mixtures of hard-sphere poly-methylmethacrylate colloidal particles were studied at six different size ratios, a. The main aim was to determine the range of size ratios over which the binary colloidal crystals AB₂ and AB₁₃ are stable. Combining these results with those of earlier work, we found stability of AB₂ for 0.60≥a≥0.425., in good agreement with theoretical predictions by computer simulations and cell model methods. AB₁₃ was observed for 0.62≥a≥0.485, the lower limit being significantly smaller than the theoretical prediction of about 0.525. Rough measurements of crystallization rates showed that AB₂ tended to crystallize fastest at small size ration, whereas the opposite was true for AB₁₃. These findings should provide a guide to the optimum conditions for materials applications of these binary colloidal crystals.

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18) Formation of Self-Supporting Reversible Cellular Networks in Suspensions of Colloids and Liquid Crystals
Vollmer D., Hinze G., Ullrich B., Poon W.C.K., Cates M.E., Schofield A.B
LANGMUIR 21 Pg 4921-4930 (2005)

Abstract
In mixtures of thermotropic liquid crystals with spherical poly (methyl methacrylate) particles, self-supporting networklike structures are formed during slow cooling past the isotropic-to-nematic phase transformation. To characterize the process of network formation in terms of morphology, phase transformation kinetics, and mechanical properties, we have combined data from polarization and laser scanning confocal microscopy with calorimetric, NMR, and rheological results. Our data suggest that the mechanism of network formation is dominated by a broadened temperature and time interval of phase transformation rather than by particle size and concentration. The observation that the width of the transformation interval strongly depends on sample preparation supports the hypothesis that a third component, most likey alkane remnants slowly liberated from the particles, plays a crucial role. In addition, calorimetric findings for liquid crystal/colloid mixtures, heated and cooled up to 13 times, point to separation of the liquid crystal into two compartments with different phase transformation kinetics. This could be explained by redistribution and enrichment of alkane in the particle-composed network walls. A further increase in the storage modulus, G', and incomplete dissolution of the networks in the isotropic state indicate that the formation of two compartments during repeated temperature cycles stabilizes the network and confers strong memory effects.

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17) Network Formation in Suspensions of Colloids and Liquid Crystal: Reversibility - Memory Effects
Vollmer D., Schofield A.B., Hinze G.
PROGRESS IN COLLOID AND POLYMER SCIENCE 129 Pg 76-81 (2004)

Abstract
Suspensions of colloids, thermotropic low molecular weight liquid crystal, and small amounts of alkane are investigated by laser scanning confocal microscopy and calorimetry. While cooling the suspension through the isotropic-nematic phase transition the colloid particles are expelled by the liquid crystal, causing the formation of a three-dimensional particle network. Upon reheating back into the isotropic phase the network may break up. However, taking the sample a number of times through the colling/heating cycle leads to annealing of the network, accompanied by an increase of the loss modulus of the system by several orders of magnitude. Eventually it hardly breaks up, even after resting in the isotropic phase for several hours. When this happens calorimetric studies reveal that the temperature dependent heat capacity of the system contains two peaks.

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16) Phonons in Suspensions of Hard Sphere Colloids: Volume Fraction Dependence
Kriegs H., Petekidis G., Fytas G., Penciu R.S., Economou E.N., Schofield A.B.
JOURNAL OF CHEMICAL PHYSICS 121(16) Pg 7849-7854 (2004)

Abstract
The propagation of sound waves in suspensions of hard sphere colloids is studied as a function of their volume fraction up to random close packing using Brillouin light scattering. The rich experimental phonon spectra of up to five phonon modes are successfully described by theoretical calculations based on the multiple scattering method. Two main types of phonon modes are revealed: Type A modes are acoustic excitations which set up deformations in both the solid (particles) and the liquid (solvent) phases; for type B modes the stress and strain are predominantly localized near the interface between the solid particles and the surrounding liquid (interface waves). While the former become harder (increase their effective sound velocity) as the particle volume fraction increases the latter become softer (the corresponding sound velocity decreases).

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15) Local Order in a Supercooled Colloidal Fluid Observed by Confocal Microscopy
Gasser U, Schofield A, Weitz DA
JOURNAL OF PHYSICS: CONDENSED MATTER 15(1) Pg S375-S380 (2003)

Abstract
The local order in a supercooled monodisperse colloidal fluid is studied by direct imaging of the particles with a laser scanning confocal microscope. The local structure is analysed with a bond order parameter method, which allows one to discern simple structures that are relevant in this system. As expected for samples that crystallize eventually, a large fraction of the particles are found to sit in surroundings with dominant face-centred cubic or hexagonally close-packed character. Evidence for local structures that contain fragments of icosahedra is found, and, moreover, the icosahedral character increases with volume fraction f, which indicates that it might play an important role at volume fractions near the glass transition.

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Abstract
Addition of polymer to colloidal suspensions induces an attractive part to the colloidal pair potential, which is of purely entropic origin ("depletion interaction"). We investigated the influence of polymer branching on depletion forces by studying mixtures of hard sphere colloids and star polymers with increasing arm number f = 2-32 but constant r_g = 500A. We found a pronounced effect of branching on the position of the gas/liquid demixing transition. Using small angle neutron scattering (SANS) we were able to measure partial structure factors in star polymer/colloid mixtures. The relative distance to the demixing transitions is reflected in our scattering data.

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Abstract
We report a detailed experimental study of the aging of the (initial) random hexagonal close-packed (rhcp) crystals formed in suspensions of hard-sphere colloids near the melting point. By suspending the same colloidal particles in two different mixtures of solvents we are able to tune the strength of the gravitational forces acting on the particles. The crystal structure is deduced from diffraction patterns measured by the light scattering equivalent of powder x-ray crystallography. A spontaneous aging of the structure is observed over long periods of time, consisting of a fraction of pure face-centered cubic (fcc) crystals growing at the expense of the randomly stacked crystallites. The rate of growth of the new crystals is small and consistent with the predictions by Pronk and Frenkel. Gravity is also revealed to affect the crystals and favor fcc order but through a slow gradual rearrangement of the random stacking. An important new observation is that small mechanical perturbations can strongly affect the structure of the colloidal crystals, promoting fcc growth and interfering with the spontaneous aging process. Previous experimental results are also discussed in the light of these new findings.

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Abstract
Experiments, theory, and simulation were used to study glass formation in a simple model system composed of hard spheres with short-range attraction ("sticky hard spheres"). The experiments using well-characterised colloids, revealed a reentrant glass transition line. Mode-coupling theory calculations and molecular dynamics simulations suggest that the reentrance is due to the existence of two qualitatively different glassy states: one dominated by repulsion (with structural arrest due to caging) and the other by attraction (with structural arrest due to bonding). This picture is consistent with a study of the particle dynamics in the colloid using dynamic light scattering.

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Abstract
The possibility of binary hard-sphere colloids crystallising with the cesium chloride (CsCl) structure was examined experimentally using poly (methyl methacrylate) particles dispersed in organic solvents. Towards this end, two dispersions were prepared which contained particles with a radius ratio a=R_B/R_A, where A refers to the large particles and B the small, of 0.736. This is close to the optimum ratio of 0.732 at which this structure is predicted to form as determined by space-filling calculations. The phases found within the binary mixture were examined using laser light crystallography and scanning electronmicroscopy and some were shown to have the CsCl structure. Over a period of time some of the CsCl crystals disappeared indicating that they were metastable and that this structure may not be the most enduring phase at this size ratio.

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Abstract
We examine the demixing transition in star-polymer-colloid mixtures for star arm numbers f = 2,6,16,32 and different star-polymer-colloid size ratios 0.18 less than or equal to less than or equal to 0.50. Theoretically, we solve the thermodynamically self-consistent Rogers-Young integral equations for binary mixtures using three effective pair potentials obtained from direct molecular computer simulations. The numerical results show a spinodal instability. The demixing binodals are approximately calculated and found to be consistent with experimental observations.

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9) Glasslike Kinetic Arrest At The Colloidal-Gelation Transition
Segre PN, Prasad V, Schofield AB, Weitz DA
PHYSICAL REVIEW LETTERS 86(26) Pg 6042-6045 JUN 25 2001

Abstract
We show that gelation of weakly attractive colloids is remarkably similar to the colloidal glass transition. Like the glass transition, dynamic light scattering functions near gelation scale with scattering vector, and exhibits a two-step decay with a power-law divergence of the final decay time. Like the glass transition, static light scattering does not change upon gelation. These results suggest that, like the glass transition, gelation results from kinetic arrest due to crowding of clusters, and that both gelation and the glass transition are manifestations of a more general jamming transition.

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Abstract
Significant progress has been made in the last decade in understanding mixtures of hard-sphere colloids and (smaller) non-adsorbing, ideal, linear polymers. We introduce extra complexity into this simple model system by replacing the linear polymers with star-branched polymers with increasing functionality but constant radius of gyration. The observed phase diagrams, interpreted in light of what is known about hard-sphere colloid plus linear polymer and binary-hard-sphere mixtures, suggest that 32-arm stars are close to behaving hard-sphere-like in colloid-star mixtures at this size ratio.

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7) Real-Space Imaging Of Nucleation And Growth In Colloidal Crystallization
Gasser U, Weeks ER, Schofield A, Pusey PN, Weitz DA
SCIENCE 292(5515) Pg 258-262 APR 13 2001

Abstract
Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.

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6) Entropically Driven Colloidal Crystallization On Patterned Surfaces
Lin KH, Crocker JC, Prasad V, Schofield A, Weitz DA, Lubensky TC, Yodh AG
PHYSICAL REVIEW LETTERS 85(8) Pg 1770-1773 AUG 21 2000

Abstract
We investigate the self-assembly of colloidal spheres on periodically patterned templates. The surface potentials and the surface phases are induced entropically by the presence of dissolved, nonadsorbing polymers. A rich variety of two-dimensional fluidlike and solidlike phases was observed to form on template potentials with both one- and two-dimensional symmetry. The same methodology was then used to nucleate an oriented single fee crystal more than 30 layers thick. The general approach provides a new route for directed self-assembly of novel mesoscopic structures.

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5) Three-Dimensional Direct Imaging Of Structural Relaxation Near The Colloidal Glass Transition
Weeks ER, Crocker JC, Levitt AC, Schofield A, Weitz DA
SCIENCE 287(5453) Pg 627-631 JAN 28 2000

Abstract
Confocal microscopy was used to directly observe three-dimensional dynamics of particles in colloidal supercooled fluids and colloidal glasses. The fastest particles moved cooperatively; connected clusters of these mobile particles could be identified: and the cluster size distribution, structure, and dynamics were investigated. The characteristic cluster size grew markedly in the supercooled fluid as the glass transition was approached, in agreement with computer simulations; at the glass transition, however, there was a sudden drop in their size. The clusters of fast-moving particles were largest near the alpha-relaxation time scale for supercooled colloidal fluids, but were also present, albeit with a markedly different nature, at shorter beta-relaxation time scales, in both supercooled fluid and glass colloidal phases.

pdf version of paper

Fiction

The Quest For The Holy Ale

Synopsis

What is the best ale in the world?

Would you risk all on a quest to gain it?

Hops was a desperate man. His beloved inn lay in ruins, thanks to sabotage by a rival. But would he dare all in the wilds to claim a prize that would restore his fortunes? No! Not a chance! He'd got underlings for that type of thing...

Barbarian, troubleshooter Terry Testosterone and junior barman Roffo are plucked from their comfortable lives and sent with a mysterious monk on a quest to find this paramount brew. But can they survive the strange peoples and cultures that lie in their path? Will they be able to elude the attempts of their rival's minions to stop them? And should they do so, will they be able to best The Ale's bearded guardians and master The Ten Trials Of The Wise? One thing is sure: succeed in all this, and they will truly have earned a good drink!

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