Dynamics of Colloids in Liquid Crystals
Oleg D. Lavrentovich
Liquid Crystal Institute, Kent State University, Kent, Ohio, USA
Behavior of small particles in fluids has fascinated scientist for centuries. Phenomena such as Brownian motion, sedimentation, and electrophoresis continue to inspire cutting-edge research and innovation. The fluid in which the colloidal particles move is typically isotropic, such as water or a dilute polymer solution. There is a growing interest to the dynamics of colloids in crowded environments with some elements of order. As a model system, we explore colloidal dynamics in a nematic liquid crystal. Orientational order of the liquid crystal leads to long-range elastic interactions that dramatically alter static behavior of the colloids (1) and support their levitation (2). Equally strong changes are seen in the dynamics. The liquid crystalline environment enables anomalous Brownian diffusion (3) and new mechanisms of electrokinetics with nonlinear (quadratic) dependence of flow velocities on the applied electric field (4). Addition of swimming bacteria to a non-toxic liquid crystal creates an experimentally controlled active system, which exhibits a cascade of transitions from equilibrium to non-equilibrium orientational patterns triggered by an increase of activity (5). Patterned director fields (6) including those with topological defects (7) control the threshold and polarity of active flows and spatial distribution of microswimmers (8). The new phenomena are rooted in anisotropy of the liquid crystals properties, such as surface tension, viscosity and elasticity, dielectric permittivity and conductivity (2).
1. P. Poulin, H. Stark, T. C. Lubensky, D. A. Weitz, Novel colloidal interactions in anisotropic fluids. Science 275, 1770 (1997).
2. O. D. Lavrentovich, Active colloids in liquid crystals. Curr Opin Colloid In 21, 97 (2016).
3. T. Turiv et al., Effect of Collective Molecular Reorientations on Brownian Motion of Colloids in Nematic Liquid Crystal. Science 342, 1351 (2013).
4. O. D. Lavrentovich, I. Lazo, O. P. Pishnyak, Nonlinear electrophoresis of dielectric and metal spheres in a nematic liquid crystal. Nature 467, 947 (2010).
5. S. Zhou et al, Living liquid crystals. P Natl Acad Sci USA 111, 1265 (2014).
6. R. Green, J. Toner, V. Vitelli, The geometry of threshold-less active flow in nematic microfluidics. Arxiv, 1602.00561 (2016).
7. L. Giomi et al, Defect dynamics in active nematics. Philos T R Soc A 372, 20130365 (2014).
8. C. Peng et al, Command of active matter by topological defects and patterns. Science 354, 882 (2016).