Re-entrant bimodality in spheroidal chiral swimmers in shear flow

Hossein Nili,Ali Naji

doi:10.1038/s41598-018-26771-0

Hossein Nili, Ali Naji

Open Access

https://doi.org/10.1038/s41598-018-26771-0

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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.

Highlights

Self-propelled micro-/nano-swimmers have garnered increased interest over the past few decades[1,2,3,4]
The asymmetry leads to misalignment between the line of self-propulsion and the force dipole, a torque experienced by chiral swimmers that works as an extra factor affecting swimmer orientation
We study the steady-state behavior of a dilute suspension of chiral swimmers confined by the walls of a planar channel and subject to externally imposed shear with a linear profile across the channel width (Couette flow)

Summary

Introduction

Self-propelled micro-/nano-swimmers have garnered increased interest over the past few decades[1,2,3,4]. Many artificial swimmers have been realized that swim in fluid environments using different mechanisms[11,12,13]. Biological or artificial, the motion of small-scale swimmers in fluid media is governed by low-Reynolds-number hydrodynamics[15], given the small sizes and low self-propulsion speeds that are typical of these particles. Given the dynamic nature of fluid environments (especially biological/physiological), the swimmers would have to change orientation every while to adapt to, and optimally survive in, the changing environment. This pattern of motion is known as run-and-tumble[18,19], and is complemented by translational and rotational diffusion of the active particles. We report on how the population splitting of active particles into distinct oppositely swimming (downstream and upstream) sub-populations, arising–in the case of non-chiral swimmers42–from imposed shear rate surpassing a threshold, is altered qualitatively when the swimmers are chiral and exhibit finite thickness

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