Active Partitioning in Porous Media

Active Brownian Particles (ABPs) distribute non-homogeneously near surfaces, and understanding how this depends on the particle properties — size, shape, activity level etc. — is essential for predicting behavior in active matter systems. In contrast, passive Brownian particles are insensitive to the geometric details of confining barriers and will distribute homogeneously. Furthermore, the effective diffusivity of passive Brownian particles in porous media depends only on the porosity or solids volume fraction, while the ABPs' effective diffusivity is also sensitive to the pore size. We investigate the behavior of active particles in a porous medium, where the porosity is achieved by having arrays of obstacles. The partitioning of ABPs between a lattice of spherical obstacles and a bulk reservoir is predicted analytically from the particles' accumulation at obstacle surfaces, and the results are compared to Brownian dynamics simulations. ABPs' effective diffusivity increases with increasing obstacle size for a fixed volume fraction and the reduction in diffusivity due to obstacles is less than for passive particles.