Ripening of splashed 4He crystals by acoustic waves with and without gravity

By developing a refrigerator compatible with the parabolic flight of a small jet plane, He-4 crystals in a superfluid were obtained in zero-gravity conditions at 0.6 K. We report Ostwald ripening of these crystals in the superfluid after being splashed by acoustic waves. Ostwald ripening is a process in which smaller crystals melt and larger ones grow to minimize the overall surface energy. Under gravity on the ground, this ripening was not apparent because it stopped growing at a capillary length of about 1 mm; crystals at lower positions grew to minimize the gravitational energy. Without gravity, however, Ostwald ripening was observed up to the order of 10 mm, a much greater length than the 1 mm due to the infinitely long capillary length, exhibiting a novel evolution of the crystal shape driven solely by the surface energy. S Online supplementary data available from stacks.iop.org/NJP/14/123023/mmedia