(5ck) Towards a Microrheological Description of Granular Segregation

Microrheology has emerged as a useful paradigm for understanding the dynamics of granular segregation: recent experiments suggest that microviscosities in granular flows obey an Einstein relationship, with observed segregation fluxes related to an effective temperature, T_eff. Here, we explore the granular microrheology paradigm both theoretically and computationally, considering the motion of a single forced probe in a dilute, rapidly sheared granular bath, and by extension, fluxes in bi-disperse granular flows. In this dilute limit, T_eff scales as the ?granular temperature?, T_k, and particle mobility is characterized by a competition between forcing and collision-induced diffusion ? a viscous plateau in the passive forcing regime gives way to order Pe² force-thickening and a sharp increase in the relative probe temperature in the active regime. Aided by particle dynamics simulations, we explore the behavior of the ratio T_eff/T_k as a function of particle fill ratio, volume fraction, and forcing. We touch briefly upon the formidable challenge of extending this work to more dense granular flows.