Living Crystals of Light-Activated Colloidal Surfers

TLDR

Self‑organization is observed in active living matter, exemplified by bacterial colonies and bird flocks. The study demonstrates self‑organization driven by nonequilibrium forces in photoactivated colloidal suspensions. The assembly arises from competition between light‑activated osmotic/phoretic attraction and particle self‑propulsion. The experiments reveal two‑dimensional living crystals that form, break, and re‑form, exhibit giant‑number fluctuations, match simple simulations, and depend on out‑of‑equilibrium collisions.

Abstract

Spontaneous formation of colonies of bacteria or flocks of birds are examples of self-organization in active living matter. Here, we demonstrate a form of self-organization from nonequilibrium driving forces in a suspension of synthetic photoactivated colloidal particles. They lead to two-dimensional "living crystals," which form, break, explode, and re-form elsewhere. The dynamic assembly results from a competition between self-propulsion of particles and an attractive interaction induced respectively by osmotic and phoretic effects and activated by light. We measured a transition from normal to giant-number fluctuations. Our experiments are quantitatively described by simple numerical simulations. We show that the existence of the living crystals is intrinsically related to the out-of-equilibrium collisions of the self-propelled particles.