Distortion and destruction of colloidal flocks in disordered environments

TL;DR

This paper investigates how collective motion in flocks and swarms is affected by disordered environments, combining experiments and theory to reveal mechanisms that enable or suppress flocking amidst obstacles.

Contribution

It provides the first combined experimental and theoretical analysis of flocking behavior in disordered media, revealing how disorder influences collective motion and phase transitions.

Findings

Disorder channels flock flow along sparse river networks.

Increasing disorder leads to a first-order phase transition suppressing collective motion.

Experiments with motile colloids confirm theoretical predictions.

Abstract

How do flocks, herds and swarms proceed through disordered environments? This question is not only crucial to animal groups in the wild, but also to virtually all applications of collective robotics, and active materials composed of synthetic motile units. In stark contrast, appart from very rare exceptions, our physical understanding of flocking has been hitherto limited to homogeneous media. Here we explain how collective motion survives to geometrical disorder. To do so, we combine experiments on motile colloids cruising through random microfabricated obstacles, and analytical theory. We explain how disorder and bending elasticity compete to channel the flow of polar flocks along sparse river networks akin those found beyond plastic depinning in driven condensed matter. Further increasing disorder, we demonstrate that collective motion is suppressed in the form of a first-order phase transition generic to all polar active materials.