Dynamics of a simple model microswimmer in an anisotropic fluid: implications for alignment behavior and active transport in a nematic liquid crystal

TL;DR

This paper develops a theoretical model to understand how microswimmers behave in anisotropic fluids like nematic liquid crystals, revealing alignment behaviors and guiding principles for controlled active transport.

Contribution

It derives an analytical Green's function for anisotropic fluids and analyzes microswimmer dynamics, including alignment mechanisms, supported by stability analysis and experimental comparisons.

Findings

Swimmers align parallel or perpendicular to anisotropy axis depending on propulsion and viscosities.

Hydrodynamic coupling causes reorientation and distortion of flow fields.

Predictions agree qualitatively with experimental observations on bacteria in nematic liquid crystals.

Abstract

Several recent experiments investigate the orientational and transport behavior of self-driven bacteria and colloidal particles in nematic liquid crystals. Correspondingly, we study theoretically the dynamics of a minimal model microswimmer in a uniaxially anisotropic fluid. As a first step, the hydrodynamic Green's function providing the resulting fluid flow in response to a localized force acting on the anisotropic fluid is derived analytically. On this basis, the behavior of both puller- and pusher-type microswimmers in the anisotropic fluid is analyzed. Depending on the propulsion mechanism and the relative magnitude of different involved viscosities, we find alignment of the swimmers parallel or perpendicular to the anisotropy axis. Particularly, also an oblique alignment is identified under certain circumstances. The observed swimmer reorientation results from the hydrodynamic coupling between the self-induced fluid flow and the anisotropy of the surrounding fluid, which distorts the self-generated flow field. We support parts of our results by a simplified linear stability analysis. Our theoretical predictions are in qualitative agreement with recent experimental observations on swimming bacteria in nematic liquid crystals. They support the objective of utilizing the, possibly switchable, anisotropy of a host fluid to guide individual microswimmers and active particles along a requested path, enabling controlled active transport.