  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.
