Active-Particle Polarization Without Alignment Forces

TL;DR

This paper demonstrates that active particles can exhibit polarization patterns without alignment forces, using experiments and theory to explain the phenomena in microswimmer suspensions.

Contribution

It provides the first experimental evidence and analytical explanation of polarization without alignment forces in active particles.

Findings

Polarization occurs without particle alignment forces.

The interfacial layer does not exert pressure on the bulk.

Analytical theory matches experimental observations.

Abstract

Active-particle suspensions exhibit distinct polarization-density patterns in activity landscapes, even without anisotropic particle interactions. Such polarization without alignment forces is at work in motility-induced phase separation and betrays intrinsic microscopic activity to mesoscale observers. Using stable long-term confinement of a single hot microswimmer in a dedicated force-free particle trap, we examine the polarized interfacial layer at a motility step and confirm that it does not exert pressure onto the bulk. Our observations are quantitatively explained by an analytical theory that can also guide the analysis of more complex geometries and many-body effects.