Transport properties of circle microswimmers in heterogeneous media

Microswimmers are exposed in nature to crowded media and their transport properties depend in a subtle way on the interaction with obstacles. Here, we investigate a model for a single circle swimmer exploring a two-dimensional disordered array of impenetrable obstacles. The microswimmer follows the surface of an obstacle for a certain time upon collision. An ideal microswimmer [1] can display long-range transport or be localized in a finite region depending on the obstacle density and the radius of circular orbits. Close to the transition lines from two localized states to a diffusive state the transport becomes subdiffusive, which is rationalized as a dynamic critical phenomenon. We determine the non-equilibrium state diagram and evaluate the diffusivities. For the microswimmer subjected to angular noise [2] increasing the noise tends to amplify diffusion, yet large randomness leads to a strong suppression of transport. We rationalize the suppression and amplification of transport by comparing the relevant time scales of the free motion to the mean-free path time between collisions with obstacles.

1. O. Chepizhko, T. Franosch, Soft Matter, 2019, 15, 452
2. O. Chepizhko, T. Franosch, submitted