DYNAMIC MOTION OF SUPERFLUID BUBBLE IN SOLID HE-4

We have observed an interesting motion of a superfluid bubble in solid He-4 with the solid/liquid interface through the video camera. The ho st hcp crystal was formed at around 0.8 K with c-axis oriented approxi mately 45 degrees from the horizontal solid/liquid interface. When a f airly strong power of 10 MHz pulsed sound was applied to a transducer in the solid, a small rounded liquid droplet was nucleated at one spot on the transducer. The droplet became larger as the sound pulse conti nued to be applied, developing over the entire transducer, and finally began to grow upwards. As it increased in size, the c-facet was obser ved in the bubble. At this time the shape of the bubble became somethi ng like a pyramid. When a top of the pyramidal shape of the bubble tou ched the interface, it changed its shape drastically, rapidly merged i nto the superfluid phase. The detailed motion was determined by carefu l observation of the video records. When the bubble size became pretty large, the horizontal interface moved upward to compensate the lower density of the bubble in order that total number of atoms in the cell should be retained.