On the hydrodynamics of active particles in viscosity gradients

TL;DR

This study investigates how active spherical particles, modeled as squirmers, behave in viscosity gradients, revealing their tendency to align down the gradient and highlighting the importance of both local and nonlocal effects on their motion.

Contribution

It provides a detailed analysis of active particle dynamics in viscosity gradients, emphasizing the role of boundary conditions and disturbance effects on motion and alignment.

Findings

Active squirmers tend to align down viscosity gradients.

The rate of rotation and swimming speed depend on particle-viscosity interactions.

Both local and nonlocal viscosity effects significantly influence particle dynamics.

Abstract

In this work, we analyze the motion of an active particle, modeled as a spherical squirmer, in linearly varying viscosity fields. In general, the presence of a particle will disturb a background viscosity field and the disturbance generated depends on the boundary conditions imposed by the particle on the viscosity field. We find that, irrespective of the details of the disturbance, active squirmer-type particle tend to align down viscosity gradients (negative viscotaxis). However, the rate of rotation and the swimming speed along the gradient do depend on the details of the interaction of the particle and the background viscosity field. In addition, we explore the relative importance on the dynamics of the local viscosity changes on the surface of active particles versus the (nonlocal) changes in the flow field due to spatially varying viscosity (from that of a homogeneous fluid). We show that the relative importance of local versus nonlocal effects depends crucially on the boundary conditions imposed by the particle on the field. This work demonstrates the dangers in neglecting the disturbance of the background viscosity caused by the particle as well as in using the local effects alone to capture the particle motion.