(402b) Non-Equilibrium Forces in Two-Point Active Microrheology

Using optical tweezers, the forces acting on two micrometer-diameter probe particles are measured as they are driven at a fixed velocity in a direction orthogonal to their line of centers through a quiescent suspension ("bath") of colloidal particles. Direct visualization of the suspension microstructure is achieved using concurrent high-speed confocal microscopy. Far from equilibrium (at Peclet number, Pe >> 1), and in the limit in which the bath particles are nearly equal in diameter to the probe particles, a characteristic non-equilibrium bath microstructure develops around each probe, consisting of a boundary layer upstream of the probe, and a downstream wake depleted of bath particles. As the non-equilibrium microstructures interact at decreasing probe separations, an effective attraction is induced between the probes. This force is a function of probe separation and Peclet number. We will discuss the relationship between the attraction and the combined non-equilibrium bath microstructure for two probe particles in close proximity.