Structure of bubble cluster adjacent to the water surface in the ultrasonic field.

The generation and evolution of bubble clusters in ultrasound fields were studied using high-speed photography. The transition of a spherical bubble cluster to a layer-like bubble cluster was demonstrated in detail. At a distance of half a wavelength to the water surface, the rising spherical cluster oscillated strongly and its equilibrium size grew. The speed was about 0.4 m/s and had a tendency to decrease. A jet caused by the last collapse of the spherical cluster rushed to the water surface, creating a bulge on the surface. Subsequently, due to the primary acoustic field, bubbles accumulated again below the bulge, and a layer-like bubble cluster gradually formed. The effects of acoustic frequency and intensity on the layer-like cluster were considered. It was found that the clusters located at a distance-to-wavelength ratio of about 0.08 to 0.13, very close to the water surface. The flickering bubble clusters were easy to be observed at 28 kHz and 40 kHz, while the accumulation of bubbles and their flicker were relatively weak at 80 kHz. The higher the frequency, the shorter the wavelength, the closer the structure to the water surface. However, at 80 kHz, the cavitation threshold is supposed to be higher and the resonance size of the bubbles is smaller, so the bubble oscillations and their interactions were weaker, and the phenomenon was different from the cases of 28 kHz and 40 kHz. Multiple structures mainly exist at 40 kHz. The formation and evolution of the layer-like cluster are closely dependent on the adequate supply of bubble nuclei from the water surface and the surrounding liquid. A Y-shaped bifurcation was used to model the branch streamers, which provided a path of bubbles accumulate into the clusters. The secondary Bjerknes forces between bubbles were adapted to analyze the interactions, and the results proved that it plays an important role in the appearance and evolution of the substructures.