Tunable long range forces mediated by self-propelled colloidal hard spheres

TL;DR

This study uses simulations to show how the effective forces between walls in a suspension of active colloids can be tuned from repulsive to attractive by adjusting particle density or confinement height, revealing new ways to control microscopic assembly.

Contribution

It demonstrates how active colloidal particles can dynamically alter the effective interactions between surfaces, introducing tunable long-range forces through density and confinement modifications.

Findings

High-density active spheres form a dynamic crystalline bridge causing oscillating repulsion.

Decreasing density leads to a dynamic depletion attraction.

Adjusting confinement height in quasi-2D systems also tunes the effective forces.

Abstract

Using Brownian dynamics simulations, we systematically study the effective interaction between two parallel hard walls in a 2D suspension of self-propelled (active) colloidal hard spheres, and we find that the effective force between two hard walls can be tuned from a long range repulsion into a long range attraction by changing the density of active particles. At relatively high densities, the active hard spheres can form a dynamic crystalline bridge, which induces a strong oscillating long range dynamic wetting repulsion between the walls. With decreasing density, the dynamic bridge gradually breaks, and an intriguing long range dynamic depletion attraction arises. A similar effect occurs in a quasi-2D suspension of self-propelled colloidal hard spheres by changing the height of the confinement. Our results open up new possibilities to manipulate the motion and assembly of microscopic objects by using active matter.