Viscotaxis: microswimmer navigation in viscosity gradients

TL;DR

This paper develops a theoretical framework explaining how microswimmers navigate viscosity gradients, revealing that body shape asymmetry enables viscotaxis, which may be exploited for designing targeted drug delivery systems.

Contribution

It introduces a new model linking body shape asymmetry to viscotaxis in microswimmers, providing insights into microorganism behavior and guiding synthetic swimmer design.

Findings

Body shape asymmetry induces viscotaxis in microswimmers.

Microorganisms may use shape changes to avoid low-viscosity regions.

The theory can inform the design of synthetic viscotactic swimmers.

Abstract

The survival of many microorganisms, like \textit{Leptospira} or \textit{Spiroplasma} bacteria, can depend on their ability to navigate towards regions of favorable viscosity. While this ability, called viscotaxis, has been observed in several bacterial experiments, the underlying mechanism remains unclear. Here, we provide a framework to study viscotaxis of self-propelled swimmers in slowly varying viscosity fields and show that suitable body shapes create viscotaxis based on a systematic asymmetry of viscous forces acting on a microswimmer. Our results shed new light on viscotaxis in \textit{Spiroplasma} and \textit{Leptospira} and suggest that dynamic body shape changes exhibited by both types of microorganisms may have an unrecognized functionality: to prevent them from drifting to low viscosity regions where they swim poorly. The present theory classifies microswimmers regarding their ability to show viscotaxis and can be used to design synthetic viscotactic swimmers, e.g.\ for delivering drugs to a target region distinguished by viscosity.