Confinement-induced alternating interactions between inclusions in an active fluid

TL;DR

This study investigates how confinement and activity levels of bath particles influence the effective interactions between colloidal inclusions in active fluids, revealing a transition from attraction to repulsion and the importance of particle layering.

Contribution

It provides new insights into how confinement and particle activity alter bath-mediated forces between colloids in active fluids, highlighting the role of particle layering and orientation.

Findings

Confinement significantly affects colloidal interactions.

Increased activity can switch interactions from attractive to repulsive.

Layering of active particles on boundaries influences interaction strength.

Abstract

In a system of colloidal inclusions suspended in a thermalized bath of smaller particles, the bath engenders an attractive force between the inclusions, arising mainly from entropic origins, known as the depletion force. In the case of active bath particles, the nature of the bath-mediated force changes dramatically from an attractive to a repulsive one, as the strength of particle activity is increased. We study such bath-mediated effective interactions between colloidal inclusions in a bath of self-propelled Brownian particles, being confined in a narrow planar channel. Confinement is found to have a strong effect on the interaction between colloidal particles, however, this mainly depends on the colloidal orientation inside the channel. Effect of the confinement on the interaction of colloidal disk is controlled by the layering of active particles on the surface boundaries. This can emerge as a competitive factor, involving the tendencies of the channel walls and the colloidal inclusions in accumulating the active particles in their own proximity.