# Population splitting of rodlike swimmers in Couette flow

## TL;DR

This paper analyzes how self-propelled rod-like swimmers in a shear flow undergo a population splitting transition, reversing swimming directions and forming two distinct groups, driven by shear rate and characterized by a phase diagram.

## Contribution

It introduces the concept of population splitting in active suspensions under shear flow and maps the transition between regimes using a phase diagram based on Peclet numbers.

## Key findings

- Swimmers split into two populations with opposite orientations at high shear rates.
- Transition from self-propulsion to shear dominance is discontinuous.
- Explains non-monotonic orientation behavior of swimmers near walls.

## Abstract

We present a quantitative analysis on the response of a dilute active suspension of self-propelled rods (swimmers) in a planar channel subjected to an imposed shear flow. To best capture the salient features of shear-induced effects, we consider the case of an imposed Couette flow, providing a constant shear rate across the channel. We argue that the steady-state behavior of swimmers can be understood in the light of a population splitting phenomenon, occurring as the shear rate exceeds a certain threshold, initiating the reversal of swimming direction for a finite fraction of swimmers from down- to upstream or vice versa, depending on swimmer position within the channel. Swimmers thus split into two distinct, statistically significant and oppositely swimming majority and minority populations. The onset of population splitting translates into a transition from a self-propulsion-dominated regime to a shear-dominated regime, corresponding to a unimodal-to-bimodal change in the probability distribution function of the swimmer orientation. We present a phase diagram in terms of the swim and flow Peclet numbers showing the separation of these two regimes by a discontinuous transition line. Our results shed further light on the behavior of swimmers in a shear flow and provide an explanation for the previously reported non-monotonic behavior of the mean, near-wall, parallel-to-flow orientation of swimmers with increasing shear strength.