We investigate the dynamics of the freely cooling granular gas. For this purpose we diamagnetically levitate the grains providing a terrestrial milligravity environment. At early times we find good agreement with Haff's law, where the time scale for particle collisions can be determined from independent measurements. At late times, clustering of particles occurs. This can be included in a Haff-like description taking into account the decreasing number of free particles. At very late times, only a single particle determines the dynamics, which is again described by a version of Haff's law. With this a good description of the data is possible over the whole time range.
We study diamagnetically levitated foams with widely different liquid fractions. Due to the levitation, drainage is effectively suppressed and the dynamics is driven by the coarsening of the foam bubbles. For dry foams, the bubble size increases as the square root of foam age, as expected from a generalized von Neumann law. At higher liquid content, the behavior changes to that of Ostwald ripening where the bubbles grow with the 1/3 power of the age. Using Diffusing Wave Spectroscopy we study the local dynamics in the different regimes and find diffusive behavior for dry foams and kinetic behavior for wet foams.
In the usual description of the granular Maxwell's demon experiment, where phase separation occurs due to an instability in the densities, the control parameter scales linearly with gravity. In this paper we investigate this scaling experimentally using the properties of diamagnetic particles in strong magnetic-field gradients to reduce and even balance gravitation. We find that phase separation occurs even at vanishingly small gravitational accelerations as is predicted by other theories. This is due to the fact that granular samples tend to form clusters as a result of the inelasticity of the particle collisions. Combining the heat balance of the driven granular gas with the cooling rate and thus the appearance of clustering, we are able to describe the crossover between the limiting cases.
h i g h l i g h t s• Coarsening dynamics of foams studied by multi-speckle diffusing wave spectroscopy.• Magnetic levitation allows a large range of liquid fractions to be studied for hours.• Cross over from Neumann to Ostwald regime at liquid fraction around 30%.• In dry foams intermittent bursts of activity signal collective bubble rearrangements.• In wet foams bubbles move ballistically with random collisions.
t r a c tWe use diffusing wave spectroscopy to study the microscopic dynamics of foams. These foams are levitated diamagnetically, such that very high liquid fractions can be achieved. We find that at low liquid fraction the dynamics is dominated by local rearrangements, whereas at high liquid fraction the movement of bubbles is ballistic and large scale rearrangements are absent. This change in the microscopic dynamics coincides with a change in the scaling of coarsening on increasing the liquid fraction that we have found earlier.
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