Quick Crystallization With Sound Waves

Physicists at the Ecole Normale Superieure (ENS) in Paris have, for the first time, used sound waves to solidify a liquid. This was made possible by focusing a short burst of 1 MHZ ultrasound in a small region of liquid helium near a clean glass plate. They produced very high sound powers (200dB), about 100,000 times higher than that produced by a jet engine. They observed crystallites growing up to 15 microns in size, and at a speed of 100 m/s during the positive swing of the sound wave, and a subsequent even-faster melting some 250 nanoseconds later when the negative pressure swing of the ultrasound wave passed through.

It is the ability of helium crystals to grow extremely fast at low temperature which made this first observation possible. Indeed, at a tenth of a Kelvin, solid helium grows about 10 billion times faster than usual solids such as ice near 0 C. The main goal of this research is to gain a better understanding of the stability of liquids by seeing how they behave when being supercooled or overpressurized. (Chavanne, Balibar, Caupin, Physical Review Letters, 4 June 2001; contact Sebastien Balibar, sebastien.balibar@lps.ens.fr, 33-1-4432-3499; text at Physics News Select)