Microrheology with Optical Tweezers: Principles and Applications

ince their first appearance in the 1970s [1], optical tweezers (OT) have been extensively developed and have proved to be an invaluable tool for a variety of applications throughout the natural sciences.

From a mechanical point of view, OT can be considered as exceptionally sensitive transducers able to resolve pN forces and nm displacements, with high temporal resolution (down to μsec). The physics underpinning the working principles of the OT relies on the ability of a focused laser beam to trap, in 3D, micron-sized dielectric particles suspended in a fluid. Accessing the time-dependent trajectory of a micron-sphere, to high spatial and temporal resolution, is one of the basic principles behind microrheology techniques.

Microrheology is a branch of rheology, but it works at micron length scales and with micro-litre sample volumes. Therefore, microrheology techniques are revealed to be very useful tools for all those rheological studies where rare or precious materials are employed; e.g., in biomedical studies [2].

In the case of microrheology with OT of ‘complex fluids’, Tassieri et al. [3] have provided the solution to a long-standing issue: i.e., the evaluation of the fluids’ linear viscoelastic properties from the analysis of a finite set of experimental data, describing (for instance) the time-dependent mean-square displacement of suspended probe particles experiencing Brownian fluctuations. In particular, they showed, for the first time in the literature, the linear viscoelastic response of an optically trapped bead suspended in a Newtonian fluid, over the entire range of experimentally accessible frequencies; both for synthetic and real experimental data. In addition, the new method has been validated by direct comparison with conventional bulk rheology methods, and has been applied both to characterise synthetic linear polyelectrolytes solutions and to study important biomedical samples.

Notably, the general validity of the proposed method makes it transferable to the majority of microrheology and rheology techniques.

[1] Ashkin A., Phys. Rev. Lett., 24 (1970).

[2] Robertson E.J., et al., J. Infect. Dis. (2013).

[3] Tassieri M. et al., New J. Phys. 14, (2012).