Mechanism of ultrasonic cavitation nucleation by quantized vortices

Cavitation nucleation, particularly in respect to liquid helium, and creation and properties of quantized vortices, are reviewed. The interaction between an ultrasonic field and one or a few quantized vortex lines in helium II was investigated using the audible cavitation threshold as a means of detection. Experiments have been conducted with dual shafts, with and without paddles immediately above the sound field, rotating at various speeds in the same and opposite senses to induce superfluid vorticity; rotation of a cylindrical bucket was also studied. Large samples of audible threshold measurements were taken and the values found to fluctuate with time in a cyclic manner both with and without rotation. Histograms of threshold events were not Gaussian but appeared to be bimodal. The higher modal value was most frequent without rotation and the lower was more or most frequent with rotation. These results are in agreement with those found by Finch and Chu in a plane wave system. With the present plane wave system two shafts with paddles rotating in opposite senses caused larger reductions in threshold than those with a single paddle; however, the shafts rotating in the same sense could not do this. With precision straight and polished shafts, without paddles attached, no significant effect was observed. Slight and gradual reductions of audible threshold were observed in the rotating bucket (R = 14.1 mm) usually beginning at speeds less than that predicted by the Arkhipov-Vinen formula. Reductions at speeds higher than the Arkhipov-Vinen value were very slight and apparently not proportional to the total number of vortices believed to be present. The results are consistent with single vortex nucleation of cavitation and are discussed accordingly. The possibility of growth of the core under ultrasonic stimulation is suggested as a mechanism by which such a process could occur.