Active Microrheology in an Emulsion Glass

Active microrheology can probe the non-linear properties of a colloidals on the microscale and thus provide insight into the microscopic origins of macroscopic viscoelasticity and yielding. Here we apply this method to uniform oil-in-water emulsions that show nearly hard-sphere behavior with an experimentally confirmed glass and a jamming transition at volume fractions of 59% and 64%, respectively. In our experiments, we use a laser tweezer to apply a constant force on a 2 micrometer-size polystyrene probe, with the aim to pull it through the emulsion host composed of oil droplets with the same diameter. We observed the trajectory of the probe, released from one cage and moving to the next, within the glassy phase, provided the applied force is high enough to break the cage and delocalize the particle. The force required for this transition can be defined as a critical force . We compare our results with theoretical work based on mode-coupling theory. We find agreement with MCT-results at packing fractions close to glass transition. However, as the volume fraction is increased beyond 61% our experimental results indicate much more pronounced increase of the critical force. Our results will provide critical input for the nonlinear viscoelastic response of dense colloidal systems.