Nonequilibrium equation of state in suspensions of active colloids

TL;DR

This study investigates the nonequilibrium equation of state in active colloids, revealing that activity modifies their behavior akin to effective temperature and adhesion, and providing a framework to understand their phase behavior.

Contribution

It introduces a novel approach to describe active colloids using equilibrium concepts, linking activity to effective temperature and adhesion, supported by experiments and simulations.

Findings

Active colloids behave as an ideal gas with activity-dependent effective temperature.

Increasing activity mimics increased adhesion between particles.

A scaling law relates activity to effective adhesion in both experiments and simulations.

Abstract

Active colloids constitute a novel class of materials composed of colloidal-scale particles locally converting chemical energy into motility, mimicking micro-organisms. Evolving far from equilibrium, these systems display structural organizations and dynamical properties distinct from thermalized colloidal assemblies. Harvesting the potential of this new class of systems requires the development of a conceptual framework to describe these intrinsically nonequilibrium systems. We use sedimentation experiments to probe the nonequilibrium equation of state of a bidimensional assembly of active Janus microspheres, and conduct computer simulations of a model of self-propelled hard disks. Self-propulsion profoundly affects the equation of state, but these changes can be rationalized using equilibrium concepts. We show that active colloids behave, in the dilute limit, as an ideal gas with an activity-dependent effective temperature. At finite density, increasing the activity is similar to increasing adhesion between equilibrium particles. We quantify this effective adhesion and obtain a unique scaling law relating activity and effective adhesion in both experiments and simulations. Our results provide a new and efficient way to understand the emergence of novel phases of matter in active colloidal suspensions.