Sound pressure threshold of non-spherical oscillation of an attached bubble evaluated by a laser Doppler vibrometer

This paper demonstrates characteristics of an oscillation of an attached bubble in ultrasonic field, compared with a free bubble. First, we experimentally investigated the resonant frequency of the attached bubble with radius of 50 - 250 μm, based on the analysis of steady or transient oscillation. The incident frequency f_d of the ultrasound with sound pressure amplitude smaller than several kilo Pascal was 28 or 38 kHz. The displacement amplitude of bubble oscillation was measured by using laser Doppler vibrometer. It was found that the frequency of the attached bubble was smaller than that of a free bubble which was calculated by using theoretical model. Second, we estimated the sound pressure threshold where the non-spherical oscillation mode appeared. We measured sub-harmonic component (f_d/2) in the frequency spectrum of the oscillation which generated by parametric instability. Based on the sub-harmonic generation, the appearance of the non-spherical oscillation mode was evaluated. As a result, the threshold pressure was one tenth of that in case of a free bubble.