Experimental measurements of the collapse of a two-dimensional granular gas under gravity

We experimentally measure the decay of a quasi-two-dimensional granular gas under gravity. A granular gas is created by vibrofluidization, after which the energy input is halted, and the time-dependent statistical properties of the decaying gas are measured with video particle tracking. There are two distinct cooling stages separated by a high temperature settling shock. In the final stage, the temperature of a fluid packet decreases as a power law T proportional to(t(c)-t)(alpha) just before the system collapses to a static state. The measured value of alpha ranges from 3.3 to 6.1 depending on the height, significantly higher than the exponent of 2 found in theoretical work on this problem [D. Volfson, B. Meerson, and L. S. Tsimring, Phys Rev. E 73, 61305 (2006)]. We also address the question of whether the collapse occurs simultaneously at different heights in the system.