Partial and complete wetting of droplets of active Brownian particles

Abstract

We study wetting droplets formed of active Brownian particles in contact with a repulsive potential barrier, in a wedge geometry. Our numerical results demonstrate a transition between partially wet and completely wet states, as a function of the barrier height, analogous to the corresponding surface phase transition in passive fluids. We analyse partially wet configurations characterised by a nonzero contact angle $\theta$ between the droplet surface and the barrier, including the average density profile and its fluctuations. These findings are compared with two equilibrium systems: a Lennard-Jones fluid and a simple contour model for a liquid-vapour interface. We locate the wetting transition where $\cos(\theta)=1$, and the neutral state where $\cos(\theta)=0$. We discuss the implications of these results for possible definitions of surface tensions in active fluids.