Helium crystallization with sound waves

A pure liquid exposed to a powerful acoustic wave can “break” and produce bubbles (a process called cavitation) during the wave’s negative pressure swing (see Physics Today, February 2000, page 29)That liquid–gas transition has been known for some time. Now, physicists at Ecole Normale SupeArieure in Paris have observed an acoustically driven liquid–solid transition in He. They focused a short, powerful burst of 1-MHz ultrasound in a small region of liquid He near a clean glass plate and used a laser to monitor the He density at that spot. During the positive, overpressure swing of the ultrasound, they observed crystallites growing up to 15 µm in size—in a mere 150 ns—and an even-faster melting some 250 ns later when the negative pressure swing of the ultrasound wave passed through. The researchers argue that the crystallization process was pure: Nucleation took place at a clean wall but did not involve any impurities. Using a more powerful sound wave, they believe they can generate He crystals without the wall. (X. Chavanne, S. Balibar, F. Caupin, Phys. Rev. Lett.   86 , 5506, 2001  https://doi.org/10.1103/PhysRevLett.86.5506 )