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

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.

Enhanced propagation of motile bacteria on surfaces due to forward scattering

“…Soft-lithography techniques can also be employed to fabricate obstacles on surfaces with improved control over their size and distribution, thus enabling a quantitative study of how these parameters influence the position and the width of the experimentally observed peak in effective velocity with obstacle density. For example, in the presence of high densities of periodic obstacles (ρ > 12%), forward-scattering events could amplify cell propagation if the spacing between the obstacles became comparable to the cells' characteristic run length due to cells being channeled by the periodic lattice [38,39,47].…”

Topotaxis of active Brownian particles

“…How sensitive is the performance of topotaxis with respect to the obstacles' shape [18,37,38], the type of motion (e.g., persistent random walk, run-and-tumble, Lévy walk, etc. [18,21,26,[54][55][56]), and the details of particle-obstacle interactions [29,37,[57][58][59]? Another interesting setting of the problem could be obtained by considering random arrangements of obstacles, where, unlike in the lattices studied here, particles can be trapped into convex-shaped features that can significantly alter their motion [26,28].…”

Random motion of a circle microswimmer in a random environment

“…Transport properties of active particles [1,2] change significantly when they are exposed to a strongly heterogeneous medium [3,4]. Both amplification [5,6,7] and suppression of diffusion [8,9,10,11,12] with an increase of introduced obstacle density has been found in various scenarios, in experiments and in computer simulations. For active particles, not only is diffusion affected, but ratchet effects [4], negative differential mobility [13,14,15], and clogging [16] emerge.…”

“…When these active particles undergo scattering from the inhomogeneities in the environment, diffusion is usually suppressed [9,7,22]. Yet, transport may be also enhanced, particularly, for circle microswimmers [23].…”