Active Microrheology in Emulsion Glass

Microscopic observations of probe particles under passive or force driven motion provides unique insights into the dynamics of colloidal dispersions. Here, we study experimentally the motion of polystyrene probe particles seeded in a micron scale oil-in-water emulsion system. We apply a well-defined constant force on the probe particles via a gradient intensity laser line trap and determine the displacements and probability distributions at various forces in the fluid and glass. Over the range studied, our emulsion droplets acts like hard spheres displaying a jamming and glass transition at 64% and 59% packing fractions, respectively. Both PS particles and emulsion droplets are sterically stabilized and identical in size. The crossover from localized to delocalized behavior happens at a threshold force which highly depends on the composition and corresponding cage strength (in the glass) and cage relaxation (in the fluid). Our experiments reveal intermittent dynamics and bimodal van Hove distribution functions around a depinning transition at a threshold force. For smaller forces, linear response connects the mean displacement and the quiescent mean squared displacement. We compare our observations to Mode coupling theory and find qualitative and semi-quantitative agreement.