Active particles crossing sharp viscosity gradients

TL;DR

This paper presents a theoretical model explaining how active particles, modeled as spherical squirmers, reorient when crossing sharp viscosity gradients, aligning well with recent experimental observations of microorganisms like algae.

Contribution

The authors develop a Snell's law-like model for active particle reorientation at viscosity interfaces, advancing understanding of mechanical taxis in inhomogeneous fluids.

Findings

Model accurately predicts particle reorientation at viscosity interfaces

Good agreement between theoretical predictions and experimental data

Provides mechanistic insight into viscosity-induced taxis

Abstract

Active particles (living or synthetic) often move through inhomogeneous environments, such as gradients in light, heat or nutrient concentration, that can lead to directed motion (or taxis). Recent research has explored inhomogeneity in the rheological properties of a suspending fluid, in particular viscosity, as a mechanical (rather than biological) mechanism for taxis. Theoretical and experimental studies have shown that gradients in viscosity can lead to reorientation due to asymmetric viscous forces. In particular, recent experiments with Chlamydomonas reinhardtii algae swimming across sharp viscosity gradients have observed that the microorganisms are redirected and scattered due to the viscosity change. Here we develop a simple theoretical model to explain these experiments. We model the swimmers as spherical squirmers and focus on small, but sharp, viscosity changes. We derive a law, analogous to Snell's law of refraction, that governs the orientation of active particles in the presence of a viscosity interface. Theoretical predictions show good agreement with experiments and provide a mechanistic understanding of the observed reorientation process.