Surface anchoring controls orientation of a microswimmer in nematic liquid crystal

Abstract

Microscopic swimmers, both living and synthetic, often dwell in anisotropic viscoelastic environments. The most representative realization of such an environment is water-soluble liquid crystals. Here, we study how the local orientation order of liquid crystal affects the motion of a prototypical elliptical microswimmer. In the framework of well-validated Beris-Edwards model, we show that the microswimmer’s shape and its surface anchoring strength affect the swimming direction and can lead to reorientation transition. Furthermore, there exists a critical surface anchoring strength for non-spherical bacteria-like microswimmers, such that swimming occurs perpendicular in a sub-critical case and parallel in super-critical case. Finally, we demonstrate that for large propulsion speeds active microswimmers generate topological defects in the bulk of the liquid crystal. We show that the location of these defects elucidates how a microswimmer chooses its swimming direction. Our results can guide experimental works on control of bacteria transport in complex anisotropic environments.

Interaction of Microswimmers in Viscoelastic Liquid Crystals

“…If W > W crit , the pusher prefers to swim perpendicular to n ∞ . Similar results were obtained in [31] for the stable swimming direction of a single microswimmer in classical LC described by the Beris-Edwards model without conformation tensor C. Compared to the swimmer dynamics in non-viscoelastic nematics, the new finding here is that viscoelasticity affects the convergence rate of the swimmer's orientation to its stable swimming direction if χ < 0. A puller with weak planar anchoring whose preferred swimming direction is perpendicular to n ∞ rotates to its preferred swimming direction faster (see Figure 2(b)).…”

“…In general, for a swimmer in LC, there are two preferred orientations: (i) a swimming direction favored due to the type of swimmer; (ii) another swimming direction favored due to the elongated shape of the swimmer together with the anchoring type. In the case when these two favored directions are different, critical behavior is found [31].…”

“…and N 2 slow down the dissipation of θ Q and θ C to 0 and reduce the aligning torque T ela exerted on the swimmer due to substrates aligning force F exter and the surface anchoring force F anc . As discussed in [31], the convergence rate of swimming direction α to the stable direction for an elongated swimmer is a competition between the torque T hydro due to hydrodynamic interactions and the torque T ela due to elastic interactions induced by F exter and F anc . This reducing of the aligning torque T ela makes the convergence of swimming direction α to the direction favored by the aligning force and surface anchoring less pronounced.…”

Multiparticle collision dynamics simulations of a squirmer in a nematic fluid

“…Recently, Lintuvuori et al [ 16 ] have studied the reorientation dynamics of a spherical squirmer in nematic LC and found that at steady state pushers (pullers) swim parallel (perpendicular) to the nematic director. The analytical calculation showed that the reorientation dynamics of a spherical squirmer is governed by a nematodynamic toque associate with the squirmer’s flow field and anisotropic viscosity of the suspending medium [ 16 – 18 ]. This unique behaviour of the microswimmers was utilized to transport cargo of cells/particles along prescribed pathways determined by the local nematic director field over long distances [ 14 , 19 , 20 ].…”

“…A closer look into the theoretical literature shows that dynamics of microswimmers in LCs have been investigated using either the lattice Boltzmann method [ 16 ] or finite element method [ 18 ]. These methods solve the nematodynamics but do not include thermal fluctuations.…”