Diffusion of active particles in a complex environment: Role of surface scattering

TL;DR

This paper investigates how boundary conditions, especially sliding along surfaces, influence the diffusion of active particles in obstacle environments, revealing significant effects on diffusivity and lattice-guided dynamics.

Contribution

It introduces a model based on Run-and-Tumble particles with obstacle-induced reorientations and highlights the importance of non-classical surface scattering in complex environments.

Findings

Sliding boundary conditions lead to high diffusivity at dense obstacles.

Run-and-Tumble model captures large-scale diffusivity but misses fine structure.

Lattice-guided particle dynamics emerge at high obstacle densities.

Abstract

Experiments have shown that self-propelled particles can slide along the surface of a circular obstacle without becoming trapped over long times. Using simulations and theory, we study the impact of boundary conditions on the diffusive transport of active particles in an obstacle lattice. We find that particle dynamics with sliding boundary conditions result in large diffusivities even at high obstacle density, unlike classical specular reflection. These dynamics are very well described by a model based on Run-and-Tumble particles with microscopically derived reorientation functions arising from obstacle-induced tumbles. This model, however, fails to describe fine structure in the diffusivity at high obstacle density predicted by simulations. Using a simple deterministic model, we show that this structure results from particles being guided by the lattice. Our results thus show how non-classical surface scattering introduces a dependence on the lattice geometry at high densities. We discuss implications for the study of bacteria in complex environments.