A Novel Measurement Method to Investigate Dynamics of Single Acoustic Bubble Near a Rigid Wall in the Water

This paper proposes a non-invasive method to research on the dynamic behaviors of a micrometer scale air-bubble near a rigid boundary in ultrasound field. As the rapid change and tiny size of the acoustic bubble near rigid wall, it is very hard to record the dynamic behaviors of acoustic bubbles, which is one of the key fundamental problems in the application of ultrasonic cavitation. In this paper, a new method combining microscope, high speed photography and synchronous technology is proposed to noninvasively and accurately record the dynamic behaviors of a single bubble in ultrasound field. Using a bespoke experimental setup, a single bubble of air, with almost the same internal composition of an acoustic cavitation bubble, was created in water with little interference of surrounding pressure and temperature. The experiments were conducted to investigate the dynamics of the generated single bubble in the vicinity of a rigid wall under the driving of ultrasound. In the experiment, the temporal evolution of the bubble was recorded by a high-speed camera at 300,000 frames per second along with the corresponding data including the time characteristics of single bubble and the velocity of the farthermost point on the bubble from the rigid wall. Results are presented for a single bubble generated near the rigid wall with the normalized standoff distance γ=1.85 under a wide range of ultrasonic power from 7.98 W to 53.6W. The results show that the dynamics of a single bubble near the rigid wall in ultrasonic field can be divided into four parts: oscillation, movement, collapse and rebound. With the increase of the ultrasonic power, the oscillation time, collapse time, the velocity of the jet, and the rebound of the bubble vary evidently under the different ultrasonic conditions. The oscillation time and the collapse time have clear trends to decrease with the increasing of the ultrasonic power. When the power of the applied ultrasound is large enough, the oscillation and the collapse time stabilize at a certain value. In addition, as the ultrasonic power increases, the velocity of the bubble wall and the average speed of the farthermost point on the bubble also increase. Furthermore, the maximum velocity of the bubble wall which is resulted from the formation of the high-speed liquid jet always increases with the increasing of the ultrasonic power.