(624c) Out-of-Equilibrium Forces Between Colloids

Two colloidal probe particles are held with optical traps orthogonal to a uniformly flowing suspension of colloidal bath particles.  Using confocal microscopy, we characterize the local bath suspension microstructure as a function of the probe separation and flow velocity. We find that at sufficiently close separations, bath particles are excluded from passing between the probes, resulting in an asymmetric, non-equilibrium microstructure in which the major features are a depleted region between the probes and dense boundary layers along the surfaces that face away from the neighboring probe. Consequently, the drag force acting on the probes is lower than that acting on a single probe and a net force pushes the probes together along their line of centers. The strength of the latter mutual force increases with increasing flow velocity. These experiments demonstrate that depletion-like forces can be induced between two particles by a non-equilibrium microstructure in a strongly driven suspension.