Tracer motion in complex media

Trajectories of a forced probe particle in a micro-emulsion glass.
Image taken from: Senbil, et al (2019).

The motion of a probe particle immersed in a complex environment reveals unique local insights into the properties of the surroundings.
Open questions concern the (de-)coupling of different dynamical degrees of freedom under strong driving and the non-Markovian fluctuation spectra. What forces are required to detach a pinned probe particle from its initial environment in a complex fluid, in a soft amorphous solid, or in a granular system?  Which mechanisms determine field dependent velocity-force relations and their statistics seen in different ensembles?