Research

For most of my scientific life I worked on Granular Fluids. While real granular particles like sand, gravel, pills, cereals or coffee beans come in all sorts of shapes and are usually quite irregular, as a theoretician I exclusively use spherical (or disk shaped) particles. I prefer to solve problems with pen and paper (and the occasional help of Gradshteyn and Maple) but I also burn a lot of CPU time running and analyzing event driven molecular dynamics simulations.

Experiments by Durian et al.¹⁾ indicated that densely packed fluidized granular systems may be in a glassy state. I managed to generalize the so called Mode Coupling Theory²⁾ of the glass transition (that had been developed for equilibrium fluids) to the far from equilibrium stationary state of a driven granular fluid. The result is: It works, it makes nontrivial predictions and there is a granular glass transition

• see Granular Glass Transition

For simplicity, granular particles are most often assumed to be smooth. I.e., the slide past each other with zero friction. In reality this is certainly not the case. Frictional particles have a number of surprising features. E.g., there is no equipartition between rotational and translational degrees of freedom³⁾ and with an extremely long calculation, we could show that the axis of rotation and the direction of flight of a rough particle are correlated.

• see Rough Spheres

In contrast to, say, mixtures of different gases, mixtures of different granular particles do not evolve towards a state of energy equipartition. This surprising result can be explained as a competition between a drive to equilibrate the energy and different cooling rates. For highly polydisperse mixtures we analyzed the distribution of partial temperatures.

• see Granular Mixtures