High-speed photography and image analysis of orbital motion of gas bubbles in ultrasonic field

Abstract

Bubbles in the fluid have a great influence on the macroscopic physical properties and flow state of the fluid. The study of bubble motion in fluid is of great significance to ultrasonic cleaning, sonochemistry, flood discharge and energy dissipation, aeration and cavitation reduction. Bubbles in the fluid may also exhibit a special translational motion in an ultrasonic field—orbital motion. The orbital motions of gas bubbles attached to a boundary in ultrasonic field are investigated experimentally by high-speed photography and image analysis. The present study focuses on the trajectories of wandering bubbles, the arrangement of gas bubble array, the source of gas bubbles and the surface fluctuation details of gas bubbles. It is found that the circular trajectory of the wandering gas bubble is unstable, irregular and unsmooth. The holding gas bubbles in the bubble array originate from wandering bubbles. The transformation of wandering bubbles into array bubbles is mainly realized by merging and increasing the volume, thereby reducing the Bjerknes force. The wandering bubbles are produced by the merging of a large number of cavitation bubbles in the ALF (acoustic lichtenberg figure) structure, which makes the radial vibration bubbles gradually transform into the surface fluctuating bubbles. Under the action of Bjerknes force, the array bubbles show regular surface fluctuations, while the smaller ones are completely unstable, showing extremely irregular morphology, and have an influence on the surface fluctuation of nearby array bubbles. The array bubbles show a very regular arrangement, and the adjacent array bubbles have opposite vibration phases and repel each other. The orbital motion of gas bubbles attached to a boundary is significantly different from that of the bubbles suspended in liquid. First of all, the attached bubbles move along the solid wall, while the suspended bubbles move completely away from the vessel wall in the liquid. Secondly, the attached bubbles move around a regular array of bubbles, while the suspended bubbles move orbitally alone. In addition, the attached bubble is nearly hemispherical, and its surface fluctuates violently, and its motion track is extremely unsmooth, which is different from the circular motion of spherical suspended bubble. Finally, there is a strong interaction between the attached wandering bubble and the array bubble, which has a great influence on the vibration and trajectory of the attached bubble. In contrast, the circular motion of the suspended bubble does not have such a complex effect.