Rheology and microrheology of homogeneously driven granular matter

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We propose the investigation of the slow off-equilibrium dynamics in dense granular systems. To overcome the dissipation caused by the collisions among the particles, the granular particles shall be driven homogeneously. Some features of the dynamics observed in such systems are reminiscent of glassy dynamics known from equilibrium systems. First, this steady state shall be characterized in theory, computer simulation, and experiment. Second, a single particle within the driven steady state shall be pulled by an additional external force. In contrast to thermal systems, vibrated granular matter is out-of-equilibrium already in the steady state. Also, while the response regime of low forces might be considered equivalent to the classical linear response regime, in the granular case low forces already probe non-equilibrium dynamics. We want to derive effective fluctuation-dissipation relations in the linear regime and extend the investigation to the nonlinear regime with higher pulling forces. The experimental realization is planned for a two-dimensional system on a vibrating table where the probe particle can be pulled both with constant velocity and constant force. The computer simulations shall be done using event-driven algorithms adapted for granular collisions. The theoretical description will rest on a combination of projection operator techniques and granular kinetic theory.