Nonlinear response to probe vitrification


Project Sperl/Zippelius (Köln/Göttingen)

Slow dynamics in homogeneously driven granular systems


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.

P6 Sperl/Zippelius, DLR Köln und Universität Göttingen

Rheology and microrheology of homogeneously driven granular matter

The description of driven granular matter close to its glass transition shall be extended from
homogeneously driven quiescent states to the regime of microscopically and macroscopically
sheared media. On the theory side, the method of integration through transients shall be used
together with mode-coupling approximations to derive microscopic equations of motion for the
dissipative Newtonian dynamics in granular matter under steady-state shear and for microrheology.
Theoretical results shall be applied to describe simulations and experiments. Experimentally,
the microrheology for a pulled granular intruder shall be investigated in two dimensions on
a vibration table and in three dimensions in a fluidized bed using X-ray radiography.