Microscale locomotion in a nematic liquid crystal

TL;DR

This paper investigates how the anisotropic and elastic properties of nematic liquid crystals influence the swimming speed of microorganisms using a modified Taylor sheet model, revealing significant effects depending on fluid properties.

Contribution

It extends classical swimming models to anisotropic nematic liquids, analyzing the impact of elasticity and anisotropy on microorganism locomotion and proposing a new swimming method in such fluids.

Findings

Swimming speed varies with liquid crystal properties.

Hexatic liquid crystals approximate behavior well.

Anisotropic effects dominate near flow-aligning transition.

Abstract

Microorganisms often encounter anisotropy, for example in mucus and biofilms. We study how anisotropy and elasticity of the ambient fluid affects the speed of a swimming microorganism with a prescribed stroke. Motivated by recent experiments on swimming bacteria in anisotropic environments, we extend a classical model for swimming microorganisms, the Taylor swimming sheet, actuated either by transverse or longitudinal traveling waves in a three-dimensional nematic liquid crystal without twist. We calculate the swimming speed and entrained volumetric flux as a function of the swimmer's stroke properties as well as the elastic and rheological properties of the liquid crystal. The behavior is quantitatively and qualitatively well-approximated by a hexatic liquid crystal except in the cases of small Ericksen number and in a nematic fluid with tumbling parameter near the transition to a flow-aligning nematic, where anisotropic effects dominate. We also propose a novel method of swimming or pumping in a nematic fluid by passing a traveling wave of director oscillation along a rigid wall.