Microswimmer suspensions in 2D random porous media*

Suspensions of motile microswimmers such as bacteria and active colloids often encounter porous environments both in nature and in industrial applications, but much remains unknown about their mechanistic behaviour and transport properties in the medium. Here, we explore microswimmer dynamics in a saturated 2D porous medium, at the scale of the constituent microscopic solid inclusions. To this end, we model the microswimmers as point-sized active Brownian particles in a large doubly-periodic domain containing randomly distributed polydisperse solid inclusions. The Brownian dynamics simulations reveal the influence of the randomness of the medium on the evolution of statistical quantities of the suspension such as the number density and the polarization near the surface of the inclusions and in the fluid channels. In particular, we study how the 'activity' of the microswimmers, together with the spatial constrictions and local heterogeneities of the surrounding medium, affect these statistics. We also examine the effects of an externally-imposed pressure-driven Stokes flow through the porous matrix, where advection and local shear-induced reorientation modify the dynamics and transport of the microswimmers.