Non-Contact Mode-Selective Excitation of Liquid Motion in a Container using Acoustic Radiation Force of Airborne Convergent Ultrasound

We demonstrate a non-contact method for mode-selective excitation of liquid in a container with its spatial tract of several tens of millimeters by focusing airborne ultrasound on the antinodes of the liquid surface in a spatiotemporally synchronized manner. The key technology in our method is pinpoint acoustic radiation force exerted on the liquid surface by converged ultrasound, which has been used to manipulate a various kind of objects. However, the target objects have been limited to extremely lightweight ones due to the very weak net force exerted to the objects. In this study, we utilized the resonance of the target system and succeeded in liquid motion excitation with an amplitude that has not been realized in previous midair radiation-force related studies. In addition, we experimentally verified that rotational liquid motions that resulted in non-contact liquid agitation was also possible with our method. Our achievements will lead to a new approach of noncontact liquid handling with an improved degree of freedom, requiring no mechanical components that might be eroded during contact with the liquid.