Closing of density wake in concentrated colloidal suspensions: two regimes governed by compressibility, friction, and self-propulsion

Abstract

When an obstacle moves through a compressible medium, it creates a density perturbation. Theoretical arguments regarding the shape of this density wake in Brownian suspensions either suppose infinite dilution or describe only the far field. Here, we study via experiments and numerical simulations the closing of the density perturbation just behind a circular obstacle in dense 2D suspensions of particles that are thermal or self-propelled. In the thermal case, we show that the characteristic length of the wake is a non-monotonic function of density, decreasing then increasing. We rationalise the scalings of both regimes by analytical arguments that highlight the role of the compressibility factor of the suspension. Based on this new understanding of the passive case, we show that the influence of self-propulsion observed in experiments and simulations can be understood as if the obstacle is effectively larger in an active bath, with non-trivial and distinct roles of the self-propulsion velocity and of the persistence time.