Nucleation of Bubbles on Electrons in Liquid Helium

Cavitation in liquid helium has been studied for over forty years, beginning with the work of Misener and Hebert [1] and Beams [2]. The interest in helium stemmed from the expectation that because all other elements freeze at higher temperatures, liquid helium should have exceptionally high purity. It was expected that this would greatly reduce the chance of heterogeneous nucleation, and thus enable the study of homogenous nucleation under controlled conditions. However, the early experiments gave values of the cavitation strength that varied significantly from one measurement to the next [3-11], and were in all cases considerably less than the strength that was anticipated theoretically. Most of these experiments were conducted with large volumes of liquid, typically of the order of 1 cm, and used liquid that was part of the main bath of a helium cryostat which could easily contain small particles of frozen air. In 1989, Nissen et al. [12] used a hemispherical ultrasonic transducer to focus 566 kHz sound into a small volume of liquid helium (volume ~10 cm). Their measurements gave a cavitation strength that increased rapidly as the temperature decreased. At their lowest temperature of 1.6 K, they estimated that vapor bubbles did not appear in the liquid until the pressure reached about – 8 bars. The difference between the results of Nissen et al. and the earlier workers is presumably due to the presence of some centers for heterogeneous nucleation in the liquid. Suppose, for example, that the density of these objects is 100 cm and that one such object will result in bubble nucleation when the pressure becomes more negative than -0.1 bar. A measurement on a liquid sample of volume 1 cm will then almost certainly give a cavitation strength of 0.1 bars, whereas a measurement on a sample of volume 10 cm has a high probability of giving a result that is characteristic of the pure liquid.