Re-entrant bimodality in spheroidal chiral swimmers in shear flow

TL;DR

This study uses a continuum model to analyze how dilute suspensions of chiral swimmers in shear flow exhibit phase transitions between unimodal and bimodal orientation distributions, revealing re-entrant behavior influenced by particle properties.

Contribution

It introduces a detailed analysis of population splitting and re-entrant phase behavior in chiral swimmers under shear, highlighting effects of chirality, aspect ratio, and propulsion speed.

Findings

Identification of unimodal and bimodal orientation phases.

Re-entrant transition behavior across chiral angular speeds.

Influence of particle aspect ratio and propulsion speed on phase behavior.

Abstract

We use a continuum model to report on the behavior of a dilute suspension of chiral swimmers subject to externally imposed shear in a planar channel. Swimmer orientation in response to the imposed shear can be characterized by two distinct phases of behavior, corresponding to unimodal or bimodal distribution functions for swimmer orientation along the channel. These phases indicate the occurrence (or not) of a population splitting phenomenon changing the swimming direction of a macroscopic fraction of active particles to the exact opposite of that dictated by the imposed flow. We present a detailed quantitative analysis elucidating the complexities added to the population splitting behavior of swimmers when they are chiral. In particular, the transition from unimodal to bimodal and vice versa are shown to display a re-entrant behavior across the parameter space spanned by varying the chiral angular speed. We also present the notable effects of particle aspect ratio and self-propulsion speed on system phase behavior and discuss potential implications of our results in applications such as swimmer separation/sorting.