Dynamical steady-states of active colloids interacting via chemical fields

TL;DR

This study uses Brownian dynamics simulations to explore how chemically active colloids form various steady states, revealing the influence of chemical interactions and activity levels on phase behavior and clustering.

Contribution

It identifies four distinct dynamical states of active colloids and elucidates how chemical field-induced interactions affect phase separation and clustering transitions.

Findings

Long-range chemical interactions lower the threshold for phase separation.

The largest cluster fraction effectively characterizes phase transitions.

Transitions vary from discontinuous to continuous depending on activity levels.

Abstract

We study the dynamical steady-states of a monolayer of chemically active self-phoretic colloids as a function of packing fraction and self-propulsion speed by means of Brownian dynamics simulations. We focus on the case that a chemical field induces competing attractive positional and repulsive orientational interactions. Analyzing the distribution of cluster size and local density as well as the hexatic order parameter, we distinguish four distinct dynamical states which include collapsed, active gas, dynamical clustering, and motility-induced phase-separated states. The long-range chemical field-induced interactions shift the onset of motility-induced phase separation (MIPS) to very low packing fractions at intermediate self-propulsion speeds. We also find that the fraction of particles in the largest clusters is a suitable order parameter characterizing the dynamical phase transitions from an active gas or dynamical clustering steady-state to a phase-separated state upon increase of the packing fraction. The order parameter changes discontinuously when going from an active gas to a MIPS-like state at intermediate self-propulsion speeds, whereas it changes continuously at larger activities where the system undergoes a transition from a dynamical clustering state to MIPS-like state.