Diffusion of self-propelled particles in complex media

TL;DR

This paper studies how self-propelled particles, like microorganisms, diffuse in complex media such as soil, revealing a significant increase in rotational diffusion and potential self-trapping effects, supported by analytical and computational analysis.

Contribution

It introduces a theoretical and computational framework to understand the diffusion behavior of active particles in heterogeneous environments, highlighting environmental effects on their dynamics.

Findings

Rotational relaxation time decreases with swimming velocity.

Effective rotational diffusion coefficient increases dramatically in complex media.

Analytical model accurately predicts computational results.

Abstract

The diffusion of active microscopic organisms in complex environments plays an important role in a wide range of biological phenomena from cell colony growth to single organism transport. Here, we investigate theoretically and computationally the diffusion of a self-propelled particle (the organism) embedded in a complex medium comprised of a collection of non-motile solid particles that mimic soil or other cells. Under such conditions we find that the rotational relaxation time of the swimming direction depends on the swimming velocity and is drastically reduced compared to a pure Newtonian fluid. This leads to a dramatic increase (of several orders of magnitude) in the effective rotational diffusion coefficient of the self-propelled particles, which can lead to "self-trapping" of the active particles in such complex media. An analytical model is put forward that quantitatively captures the computational results. Our work sheds light on the role that the environment plays in the behavior of active systems and can be generalized in a straightforward fashion to understand other synthetic and biological active systems in heterogenous environments.