6th Warsaw School of Statistical Physics

Driving a colloidal tracer through a complex material provides unique insights into the internal dynamics of the host system. We use Langevin dynamics simulations and mode-coupling theory (MCT) to study the motion of a tracer pulled by a constant force through a colloidal glass of hard spheres. On the MCT side, this work extends an older theoretical approach [1] to the regime where the applied force is strong enough to drive the probe out of the cage. An important feature of the system is the critical force, above which the probe delocalizes. Friction coefficients of delocalized probes decrease with force, indicating force-thinning behavior. Furthermore, the probe motion is strongly anisotropic. Transverse to the force it exhibits force-enhanced diffusion. Along the force we observe heterogeneous dynamics, which manifests itself in exponential tails in the probe van Hove function. Theoretical predictions are in general qualitative agreement with simulations.