Ripening of splashed4He crystals by acoustic waves with and without gravity

What is it about?

Ostwald ripening is a process in which smaller crystals melt and larger ones grow to minimize the overall surface energy. In classical materials, the process is very slow and the largest crystal is in the micrometer scale at most. 4He quantum crystals, however, grow from superfluid very quickly at sufficiently low temperatures and can be an exceptional material taking full advantage of the Ostwald ripening. Especially, a zero-gravity environment will allows one to observe Ostwald ripening of a gigantic size 4He crystals, unadulterated by competing gravitational forces.

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Why is it important?

By developing a refrigerator compatible with a parabolic flight of a small jet plane, 4He crystals in superfluid were obtained in a zero gravity condition at 0.6 K and their ripening took place after being splashed by an acoustic wave pulse. Only the largest crystal survived and all other smaller crystals melted and disappeared within a few seconds. Their final size was on the order of 10 mm, which is gigantic in view of the commonsense of Ostwald ripening of classical materials. 4He crystals in zero gravity were thus demonstrated to be a model system with which to study the surface-energy-driven dynamical phenomena to a large spatial scale within a short experimental observation time.

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