Capillary condensation in an active bath

We study capillary condensation in a bath consisting of active Brownian particles (ABPs) and the forces acting on the capillary close to the motility-induced phase separation (MIPS). The capillary is modelled as two parallel rods of high aspect ratio which are fixed in space. We consider a bath of ABPs having a self-propulsion speed much larger than the critical speed necessary for MIPS to occur. For a given particle speed, we gradually increase the packing fraction of ABPs, starting from a homogeneous dilute phase of ABPs and going towards the binodal of MIPS. Depending on the packing fraction of ABPs and capillary width, we find that the effective force between the capillary rods can be either attractive or repulsive. In fact, with increasing width it shows damped oscillations as long as capillary condensation occurs. We analyze them in detail by studying the distribution of particle distances from the inner and outer wall of the capillary, respectively. In addition, we examine the capillary in the active bath prepared under conditions close to the critical point. We do not observe signs of the presence of long-range Casimir interactions.
7 pages, 8 figures