Propagating Jamming Fronts

We explore the initial moments of impact between two dense granular clusters in a two-dimensional geometry.

The particles are composed of solid CO2 and are levitated on a hot surface. Upon collision, the propagation of a dynamic “jamming front” produces a distinct regime for energy dissipation in a granular gas in which the translational kinetic energy decreases by over 90%.

Experiments and associated simulations show that the initial loss of kinetic energy obeys a power law in time ΔE=−Kt3/2, a form that can be predicted from kinetic arguments:

An anodized aluminum plate with tilted boundaries is heated to 100C. Two clusters of solid CO2 particles impact in the middle of the plate. Silicone rubber strips prevent the particles from falling off the plate edges. The collision is filmed with a high-speed camera from above(a). Sublimated gas from beneath a particle creates a high-pressure region which supports its weight. This leads to nearly frictionless translational motion (b). Ratio of final to initial kinetic energy versus relative initial velocity for single-particle collisions. There is a spread of values for the energy loss, even for similar relative velocities (c).

The initial area fraction inside each cluster is 0 ¼ 0:71. In both sets of images, the color indicates the magnitude of the velocity, as denoted by the scale bar on the bottom. Upon impact, a jamming front spreads quickly and eventually encompasses all of the particles when t=tjam ¼ 1.

Relative loss of kinetic energy immediately after impact for four experimental data sets (black dots) and the simulation of elliptical clusters (solid red line). The dashed line is the prediction [Eq. (4)] using parameters from the simulation. The inset compares five simulations using the same initial conditions but with different dissipative forces. Two have viscous forces that are 5 stronger and 5 weaker than in the main figure, and two others have friction coefficients of mu= 0.1 and mu= 0.9.

The results are virtually identical in all cases.