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.