Shape evolution and bubble formation of acoustically levitated drops

In this study, we investigated the shape evolution and bubble formation of acoustically levitated drops upon increasing the sound intensity. Here, a levitated liquid drop evolves progressively from an oblate spheroidal shape to a flattened film to a thin bowl-shaped film, eventually forming a closed bubble. Through systematic experiments, numerical simulation, and scaling analysis, we demonstrate that the buckled geometry of the liquid film can drastically enhance the suction effect of acoustic radiation pressure at its rim, forming a significant pressure gradient inside the film which causes an abrupt area expansion and bubble formation. Our results provide the mechanical origin responsible for the shape evolution and bubble formation of acoustically levitated drops, and highlight the role of buckled geometry in the levitation and manipulation of liquid films in an ultrasound field.

在这项研究中，我们研究了在声强增加时声悬浮液滴的形状演变和气泡形成。在此，一个悬浮的液滴逐渐从扁球体形状演变为扁平薄膜，再到薄碗状薄膜，最终形成一个封闭的气泡。通过系统的实验、数值模拟和标度分析，我们证明了液膜的褶皱几何形状能够极大地增强其边缘处声辐射压力的抽吸效应，在膜内形成显著的压力梯度，从而导致面积突然扩大和气泡形成。我们的研究结果揭示了声悬浮液滴形状演变和气泡形成的力学根源，并强调了褶皱几何形状在超声场中对液膜的悬浮和操控所起的作用。