Active dipolar spheroids in shear flow and transverse field: Population splitting, cross-stream migration and orientational pinning

TL;DR

This paper investigates how active dipolar spheroids behave in shear flow and transverse magnetic fields, revealing diverse migration and orientation behaviors useful for particle separation in microfluidics.

Contribution

It introduces a continuum model analyzing the complex interplay of shape, activity, and external fields on swimmer dynamics, highlighting new migration regimes and population splitting phenomena.

Findings

Prolate swimmers migrate to the channel bottom in strong fields.

Oblate swimmers exhibit reverse migration at intermediate fields.

Swimmer populations split into oppositely polarized subgroups under shear and field effects.

Abstract

We study the steady-state behavior of active, dipolar, Brownian spheroids in a planar channel subjected to an imposed Couette flow and an external transverse field, applied in the 'downward' normal-to-flow direction. The field-induced torque on active spheroids (swimmers) is taken to be of magnetic form by assuming that they have a permanent magnetic dipole moment, pointing along their self-propulsion (swim) direction. Using a continuum approach, we show that a host of behaviors emerge over the parameter space spanned by the particle aspect ratio, self-propulsion and shear/field strengths, and the channel width. The cross-stream migration of the model swimmers is shown to involve a regime of linear response (quantified by a linear-response factor) in weak fields. For prolate swimmers, the weak-field behavior crosses over to a regime of full swimmer migration to the bottom half of the channel in strong fields. For oblate swimmers, a counterintuitive regime of reverse migration arises in intermediate fields, where a macroscopic fraction of swimmers reorient and swim to the top channel half at an acute `upward' angle relative to the field axis. The diverse behaviors reported here are analyzed based on the shear-induced population splitting (bimodality) of the swim orientation, giving two distinct, oppositely polarized, swimmer subpopulations (albeit very differently for prolate/oblate swimmers) in each channel half. In strong fields, swimmers of both types exhibit net upstream currents relative to the laboratory frame. The onsets of full migration and net upstream current depend on the aspect ratio, enabling efficient particle separation strategies in microfluidic setups.