Capillary condensation in an active bath

TL;DR

This study investigates how active Brownian particles induce capillary condensation and forces between fixed rods, revealing attraction or repulsion depending on particle density and capillary width, without observing long-range Casimir effects.

Contribution

It provides a detailed analysis of capillary forces in active baths near MIPS, highlighting the dependence on system parameters and the absence of Casimir interactions.

Findings

Force can be attractive or repulsive depending on parameters

Damped oscillations of force with increasing capillary width

No long-range Casimir interactions observed

Abstract

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.