Non-equilibrium dynamics of an active colloidal "chucker"

TL;DR

This paper uses Monte Carlo simulations to study the non-equilibrium behavior of a colloidal particle that emits solutes, revealing how its diffusion and mobility are affected by emission rate and surface interactions, with implications for biological and synthetic active particles.

Contribution

It introduces a simulation framework for active colloids emitting solutes, analyzing diffusion, mobility, and steady-state distributions, bridging theoretical predictions and experimental observations.

Findings

Diffusion constant increases with small emission rates

At high emission rates, diffusion slows due to crowding

Mobility estimates violate fluctuation-dissipation theorem

Abstract

We report Monte Carlo simulations of the dynamics of a "chucker": a colloidal particle which emits smaller solute particles from its surface, isotropically and at a constant rate k_c. We find that the diffusion constant of the chucker increases for small k_c, as recently predicted theoretically. At large k_c the chucker diffuses more slowly due to crowding effects. We compare our simulation results to those of a "point particle" Langevin dynamics scheme in which the solute concentration field is calculated analytically, and in which hydrodynamic effects can be included albeit in an approximate way. By simulating the dragging of a chucker, we obtain an estimate of its apparent mobility coefficient which violates the fluctuation-dissipation theorem. We also characterise the probability density profile for a chucker which sediments onto a surface which either repels or absorbs the solute particles, and find that the steady state distributions are very different in the two cases. Our simulations are inspired by the biological example of exopolysaccharide-producing bacteria, as well as by recent experimental, simulation and theoretical work on phoretic colloidal "swimmers".