Transport properties of circle microswimmers in heterogeneous media
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We simulate the dynamics of a single circle microswimmer exploring a disordered array of fixed obstacles. The microswimmer moves on circular orbits in the freely accessible space and follows the surface of an obstacle for a certain time upon collision. Two cases are considered, an ideal microswimmer[1] and a microswimmer subject to rotational diffusion[2]. Depending on the obstacle density and the radius of the circular orbits, the microswimmer displays either long-range transport or is localized in a finite region.
We show that for the ideal microswimmer there are transitions from two localized states to a diffusive state each driven by an underlying static percolation transition. We determine the non-equilibrium state diagram and calculate the mean-square displacements and diffusivities by computer simulations. Close to the transition lines transport becomes subdiffusive which is rationalized as a dynamic critical phenomenon.
The interplay of two different types of randomnesses, quenched disorder and time-dependent noise, is investigated to unravel their impact on the transport properties of the microswimmer subject to the rotational diffusion. We compute lines of isodiffusivity as a function of the rotational diffusion coefficient and the obstacle density. We find that increasing noise or disorder tends to amplify diffusion, yet for large randomness the competition 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