On the acoustic levitation of droplets

This paper deals with the theoretical and experimental investigation of acoustically levitated droplets. A method of calculation of the acoustic radiation pressure based on the boundary element method (BEM) is presented. It is applied to predict shapes of droplets levitated in an acoustic field (and as a result, deformed by it). The method was compared with several known exact and approximate analytical results for rigid spheres and shown to be accurate (and a widely used approximate formula for the acoustic levitation force acting on a rigid sphere was found to be inaccurate for sound wavelengths comparable with the sphere radius). The method was also compared with some other theoretical approaches known from the literature. Displacement of the droplet centre relative to the pressure node is accounted for and shown to be significant. The results for droplet shapes and displacements are compared with experimental data, and the agreement is found to be rather good. Furthermore, the experimental investigations reveal a unique relationship between the aspect ratio of an oblate droplet and the sound pressure level in the levitator. This relationship agrees well with the predicted shapes. A practical link between droplet shape or droplet displacement and sound pressure level in a levitator is therefore now available.