Computational rheology of active matter

Condensed Matter lunchtime seminar

Computational rheology of active matter

  • Event time: 1:00pm until 2:00pm
  • Event date: 12th November 2018
  • Speaker: (School of Physics & Astronomy, University of Edinburgh)
  • Location: Room 2511,

Event details

Active matter systems are comprised of active particles that dissipate energy into their surroundings. This can lead to, for example, self-motility, growth or replication of the particles. Examples of active matter include suspensions of swimming bacteria and collections of cytoskeletal filaments, such as actin or microtubules, and molecular motors [1]. In our simulations, we model active matter systems as nematic liquid crystals that exert a force along their primary axis, either outward or inwards. These are known as extensile and contractile systems respectively.

It is well known that when a flow is imposed in the direction of the helical axis cholesteric liquid crystals will exhibit the phenomenon of permeation. In this case, the viscosity of the fluid is seen to increase greatly and a plug-like flow profile is observed [2,3].

We present here results showing that the viscosity of 1D active nematic systems subject to a Poiseuille flow in a channel will scale with a dimensionless Ericksen number. This number has been found to be f Lz / ζ where f is the external pressure gradient applied to the fluid, Lz is the width of the channel and ζ is a parameter that controls the activity. In 2D the behaviour becomes more complex, we observe three distinct viscosity branches each corresponding to different flow and director profiles. Additionally, we show that for contractile active nemartics at low forcing, there is a vast perameter range where the rheology of the system resembles that asociated with permeation in cholesterics.

Furthermore we have investigated the behaviour of extensile active nematics subject to a shear flow. In experiments, it has recently been shown that the shear viscosity of dilute bacterial suspensions can become arbitrarily small indicating a superfluid-like state [4]. These observations are often explained using 1D active gel models that predict similar shear viscosities at low shear rates [5]. Here we show that such 1D predictions are incomplete since a sheared active gel exhibits flow instabilities at lower activity than the negative viscosity predicted by 1D models, resulting in complicated, often chaotic flow patterns. Thus we demonstrate that the viscosity of such an active fluid is not state variable and depends strongly on the confining geometry.

References:

[1]  D. Marenduzzo. The European Physical Journal Special Topics, 2016, 225(11-12):2065–2077.

[2]  W. Helfrich, Phys. Rev. Lett., 1969, 23:372–374.

[3]  D. Marenduzzo, E. Orlandini, & J. M. Yeomans, Physical review letters, 2004, 92(18), 188301.

[4] H. M. López, J. Gachelin, C. Douarche, H. Auradou, & E. Clément, Physical review letters, 2015,

115(2), 028301.

[5] G. Foffano et al., The European Physical Journal E, 2012, 35.10, 1-11.

About Condensed Matter lunchtime seminars

This is a weekly series of informal talks given primarily by members of the soft condensed matter and statistical mechanics groups, but is also open to members of other groups and external visitors. The aim of the series is to promote discussion and learning of various topics at a level suitable to the broad background of the group. Everyone is welcome to attend..

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