ON THE ACOUSTIC LEVITATION OF DROPLETS

This paper deals with the theoretical and experimental investigation o f acoustically levitated droplets. A method of calculation of the acou stic 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 comp ared with several known exact and approximate analytical results for r igid spheres and shown to be accurate (and a widely used approximate f ormula for the acoustic levitation force acting on a rigid sphere was found to be inaccurate for sound wavelengths comparable with the spher e radius). The method was also compared with some other theoretical ap proaches known from the literature. Displacement of the droplet centre relative to the pressure node is accounted for and shown to be signif icant. The results for droplet shapes and displacements are compared w ith experimental data, and the agreement is found to be rather good. F urthermore, the experimental investigations reveal a unique relationsh ip between the aspect ratio of an oblate droplet and the sound pressur e level in the levitator. This relationship agrees well with the predi cted shapes. A practical link between droplet shape or droplet displac ement and sound pressure level in a levitator is therefore now availab le.