Flowing active liquids in a pipe: Hysteretic response of polar flocks to external fields

TL;DR

This paper studies how colloidal flocks respond to external fields, revealing hysteresis and robustness in their collective motion, and demonstrates their potential for self-sustained microfluidic oscillators.

Contribution

It provides experimental and theoretical insights into the hysteretic behavior of polar active liquids under external fields, highlighting mechanisms of collective robustness and bistability.

Findings

Colloidal flocks align with external flows similar to spins in magnetic fields.

Confined colloidal flocks can move against external flows due to collective effects.

Intrinsic bistability enables the creation of self-sustained microfluidic oscillators.

Abstract

We investigate the response of colloidal flocks to external fields. We first show that individual colloidal rollers align with external flows as would a classical spin with magnetic fields. Assembling polar active liquids from colloidal rollers, we experimentally demonstrate their hysteretic response: confined colloidal flocks can proceed against external flows. We theoretically explain this collective robustness, using an active hydrodynamic description, and show how orientational elasticity and confinement protect the direction of collective motion. Finally, we exploit the intrinsic bistability of confined active flows to devise self-sustained microfluidic oscillators.