Emergence of Anti-chemotactic Flocking in Active Biomimetic Colloids

TL;DR

This study uncovers how resource competition in active colloids driven by actin treadmilling leads to emergent behaviors like anti-chemotaxis and flocking, revealing a generic mechanism for self-organization in active matter systems.

Contribution

It demonstrates that symmetry-breaking, self-propulsion, and flocking arise from local competition for actin monomers, combining experiments and theory to uncover a new active matter mechanism.

Findings

Beads exhibit anti-chemotaxis and asymmetric actin gradients.

Flocking depends on actin polymerization rate and monomer diffusivity.

Active stress and reaction-diffusion lead to complex collective behaviors.

Abstract

Competition for resources is a fundamental constraint that guides the self-organization of natural, biological, and human systems, ranging from urban planning and ecosystem development to intracellular pattern formation. Here, we reveal that competition for resources is at the origin of the collective dynamics that emerge in a population of colloids propelled by actin treadmilling, an out-of-equilibrium process where filaments grow from one end while shrinking from the other. Using a combination of experiments and theory, we show that symmetry-breaking, self-propulsion, and flocking emerge from the local competition for actin monomers. We demonstrate that beads propelled by actin treadmilling are anti-chemotactic and spontaneously generate asymmetric actin gradients that trigger and sustain directed motility. Flocking emerges from the combined effects of anti-chemotaxis and local competition for monomers. The flocking transition depends on the actin polymerization rate, actin monomer diffusivity, and the bead's motility, whose interplay controls the emergence of short-range attractive interactions between the colloids. Our findings demonstrate that active stress generation coupled to reaction-diffusion is a generic mechanism that can lead to a multiscale cascade of behaviors when active agents remodel their environment. Actin treadmilling offers a platform to study how motile agents that interact through a field self-organize in novel dynamical phases, with potential applications in non-reciprocal and trainable active matter.