Underwater ultrasound causes various physical phenomena in megasonic cleaning baths, e.g. cavitation inception, bubble translation and free-surface deformation (FSD) due to acoustic radiation pressure. Because FSD is especially noticeable in the case of high frequency ultrasound due to its high directivity, it is essential to investigate the interaction between FSD and bubble translation in megasonic cleaning bath. In our present experiments, we construct a typical setup for megasonic cleaning and irradiate water with 1 MHz ultrasound vertically upwards. We visualize FSD and bubbles and analyze the height of FSD and the translational velocity in frequency space. The bubbles translate in both short and long time scales caused by bubble-bubble interaction and periodic FSD, respectively, and the latter has periodicity. The most dominant frequency component in FSD shows good agreement with that in the translational velocity of the bubbles and does not depend on whether cavitation occurs or not. Therefore, it is suggested that FSD causes periodicity of bubble translation.

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