Book Description

Dynamics of small particles in fluids has fascinated scientist for centuries. Phenomena such as Brownian motion and swimming of micro-organisms continue to inspire cutting-edge research and innovation. The fluid in which these particles move is typically isotropic, such as water or a dilute polymer solution. There is a growing interest in the dynamics of living and inanimate microparticles in crowded environments with some elements of the order. In this thesis, we demonstrate that the dynamics of microparticles is altered dramatically when the isotropic medium is replaced by an orientationally ordered fluid, the so-called nematic liquid crystal. The study is performed for both artificial (colloidal spheres) and biological (swimming bacteria and living cells) microparticles.The liquid crystal environment enables anomalous Brownian diffusion of colloidal spheres: the mean squared displacement (MSD) of the colloidal particle is a nonlinear function of the time step. This behavior is very different from the Brownian diffusion in an isotropic melt of a liquid crystal, in which MSD is increasing linearly with the time step. Both superdiffusive (superlinear growth of MSD with time) and subdiffusive (sublinear MSDs) regimes are observed. The new phenomenon of anomalous diffusion is explained by the viscoelastic response of the director perturbations around the inclusion that has a characteristic relaxation time. The observed anomalous diffusion is also anisotropic, as the MSD vs time dependence is different for the motion along the director and perpendicular to it. When the time steps are much longer than this characteristic time steps, the diffusion becomes normal. The study opens the opportunities to understand and regulate microtransport in complex systems with orientational order, such as living cell membranes. Previous studies revealed that a uniformly aligned nematic forces the bacteria to swim along the overall director orientation. In this thesis, we demonstrated that a prepatterned spatially varying director field of a water-based lyotropic chromonic liquid crystal can be used to control curvilinear trajectories of swimming bacteria, polarity of their motion and their concentration in space. The bacteria differentiate topological defects, heading toward defects of positive topological charge and avoiding negative charges. Moreover, we demonstrate that swimming bacteria recognize subtle differences in liquid crystal deformations, engaging in bipolar swimming in regions of pure splay and bend but switching to unipolar swimming in mixed splay-bend regions. These mixed splay-bend deformations can be used to trigger either circular or linear polar swimming of bacteria, depending on the design geometry. These types of motion can be used in the development of bacteria-powered micromachines and microcargo transporters.The approach to control active matter by prepatterned director field is extended to the case of living tissue-forming human dermal fibroblast (HDF) cells that adhere and proliferate on the surface of a patterned liquid crystal elastomer (LCE). Namely, prepatterned LCEs control the alignment of HDF cells, either uniform, or in the shape of topological defect arrays of integer (+1, -1) and semi-integer (+1/2, -1/2) strength. Microscopy observations prove that the HDF cells align along the director of the patterned LCE substrate. The patterns modulate cell density, as the cells accumulate near the cores of positive defects. The ability to create topological defects in populations of biological cells with the predetermined locations of the core is of importance in the development of controlled morphogenesis of biological tissues.