Seminar über aktuelle Fragen zur Dynamik komplexer Fluide: Active microrheology in an emulsion glass

Microscopic observations of probe particles under passive or force driven motion provides unique insights into the structural dynamics of colloidal dispersions. Here, we experimentally observe the trajectories of polystyrene(PS) probe particles embedded in oil-in-water emulsion system with 2.05 micrometers in diameter and 12% polydispersity under well-defined constant force applied via laser line trap, and determine the probability distributions at various forces in fluid and glass. Our emulsion system acts like hard sphere system displaying jamming and glass transition at 64% and 59% packing fractions, respectively(1). Both PS and emulsion particles are sterically stabilized and identical in size. Constant force is induced only on the probe particle due to the refractive index mismatch with the emulsion host particles. We apply high enough forces to break the cage and delocalize the probe. The crossover from localized to delocalized behavior happens at a threshold force which highly depends on the cage strength(2). 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 mean displacement and quiescent mean squared displacement. As the force approaches to critical value, Van-Hove function exhibits an exponential tail in force direction. We compare our observations to Mode coupling theory (MCT). Our experimental observations show qualitative and semi-quantitave agreement with MCT calculations. Our results are important to shed light on the mechanical properties of complex and living systems.

1. Zhang C, Gnan N, Mason TG, Zaccarelli E, Scheffold F. Dynamical and structural signatures of the glass transition in emulsions. J Stat Mech-Theory Exp. 2016:26.
2. Gruber M, Abade GC, Puertas AM, Fuchs M. Active microrheology in a colloidal glass. Physical Review E. 2016;94(4):19.