Fractal aggregation of active particles

TL;DR

This paper investigates how active particles with motile and non-motile states aggregate into fractal or homogeneous structures depending on density and tumbling rate, revealing tunable porosity in the aggregates.

Contribution

It introduces a model of active run-and-tumble particles with state-changing collisions and analyzes the resulting aggregation patterns and their dependence on system parameters.

Findings

Fractal aggregates form at low density.

Homogeneous structures emerge at high density.

Tumbling rate influences aggregate porosity.

Abstract

We study active run-and-tumble particles with an additional two-state internal variable characterizing their motile or non-motile state. Motile particles change irreversibly into non-motile ones upon collision with a non-motile particle. The system evolves towards an absorbing state where all particles are non-motile. We initialize the system with one non-motile particles in a bath of motile ones and study numerically the kinetics of relaxation to absorbing state and its structure as function of the density of the initial bath of motile particles and of their tumbling rate. We find a crossover from fractal aggregates at low density to homogeneous ones at high density. The persistence of single-particle dynamics as quantified by the tumbling rate pushes this crossover to higher density and can be used to tune the porosity of the aggregate. At the lowest density the fractal dimension of the aggregate approaches that obtained in single-particle diffusion limited aggregation. Our results could be exploited for the design of structures of desired porosity. The model is a first step towards the study of the collective dynamics of active particles that can exchange biological information.