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.

No abstract

A droplet held in a single-axis ultrasonic levitator will principally sustain a certain external blowing along the levitation axis, which introduces the possibility of investigating heat and/or mass transfer from the droplet under conditions which are not too remote from those in spray systems. The focus of the present work is on the influence of the acoustic field on the external flow. More specifically, an axisymmetric submerged gas jet in an axial standing acoustic wave is examined, both in the absence and presence of a liquid droplet. Flow visualization is first presented to illustrate the global flow effects and the operating windows of jet velocities and acoustic powers which are suitable for further study. An analytic and numeric solution, based on the parabolic boundary layer equations are then given for the case of no levitated droplet, providing quantitative estimates of the acoustic field/flow interaction. Detailed velocity measurements using a laser Doppler anemometer verify the analytic results and extend these to the case of a levitated droplet. Some unresolved discrepancy remains in predicting the maximum velocity attainable before the droplet is blown out of the levitator. Two methods are developed to estimate the sound pressure level in the levitator by comparing flowfield patterns with analytic results. These results and observations are used to estimate to what extent acoustic aerodynamic levitators can be used in the future for investigating transport properties of individual droplets.

No abstract

No abstract