Tensile strength and visible ultrasonic cavitation of superfluid4He

Abstract

Motion picture photographs of cavitation in He II revealed new characteristics pertinent to the liquid's tensile strength and bubble dynamics. A cylindrical acoustic standing wave with a frequency of 50.58 kHz induced the cavitation in He II at a temperature of 2.09 K. Analysis of light diffracted by the sound gave measurements of the acoustic pressure amplitude which were used both for selecting the best drive frequency and for obtaining the tensile strength. Bubbles appeared to originate on pressure antinodes, expanded to a diameter of 0.5–1.0 mm in about 0.3 msec, and eventually fragmented into smaller bubbles. They originated where the negative pressure extremum was as small as −0.6 bar (+0, −50%), a tensile strength much smaller than the predictions of theories developed for the homogeneous nucleation of bubbles in classical liquids. The bubble fragments were frequently nonspherical and had widths of 0.1–0.2 mm. Small bubbles also displayed an unexpected preference to originate on the surface of a stainless steel tube inserted in the sound field. Subsequent to nucleation, bubbles were frequently attracted to acoustic pressure nodes in agreement with a theory of vibrations and forces originally developed for bubbles in normal liquids. Attempts to detect first and second sound radiated by cavitation are described.