Dense granular flows
If the volume fraction ranges between, the granular material behaves like a liquid (Fig. 1(b)), where complicated rearrangements of granular particles determine their flow property [8, 11].
Approach: To understand the dense flows of granular materials, we clarify microscopic structures of the rearrangements from a point of view of statistical mechanics. Here, we have experimentally studied dense granular particles driven by water convection [7]. Because the particles move with convective flows, their motions are ballistic in real experiments. However, if we subtract the convective transport from the total displacements of granular particles, we can observe their glassy dynamics hidden behind the mean flows. Then, we find a cross-over from sub-diffusion to diffusion as a sign of caging of granular particles. In addition, we apply a prescription of the dynamic heterogeneity to the experimental data, where we find that a characteristic time for the structural relaxation diverges with a typical length scale for the 4-point correlation function in the limit of jamming transition. That is an analogue of the dynamic criticality in granular fluids.
Method overview: The applicant made all the numerical programs for the experimental data analyses, where the coarse-graining (CG) method is used to calculate velocity fields of granular particles. After subtracting the convective mean flows from the experimental data by using the CG method, the diffusion and dynamic heterogeneity of granular particles are numerically calculated.