Dynamics of the spontaneously accelerative equatorial expansion of a droplet in a high-intensity acoustic standing wave field

This work reports an investigation of the acoustically induced accelerated deformation of drops in high-intensity acoustic standing wave fields generated by a single-axis acoustic levitator. The dynamic characteristics of droplet deformation are obtained and discussed based on high-speed visualization and in-house Python codes. Based on the actual physical characteristics, the finite element method numerical model has been developed for intercoupling the sound field and flow field, allowing for bidirectional feedback between the drop shape and the acoustic wave. The experimental results indicate that during the deformation process of droplets, their equatorial radius expands at an increasing speed without artificially increasing the sound field intensity. The simulation shows that the acoustic radiation suction acting on the equator dominates droplet deformation. Furthermore, there is a kind of positive feedback loop between the acoustic radiation pressure (p(r)) amplitude at the drop's equator and the aspect ratio (AR) during the deformation period. It is confirmed that this causes the spontaneous accelerated expansion of the droplet's equator. In addition, the functional relationship between p(r) at the drop's equator and the AR has been obtained through theoretical derivation, which is consistent with the simulation results. Finally, the critical Bond number (B-a,B-s) of the rim instability is also obtained. This work provides deeper insights into contactless liquid manipulation and ultrasonic atomization technology applications.