Focused acoustic beams for nozzleless droplet formation

The authors report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity from a free liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency range 5 to 300 MHz, with corresponding droplet diameters from 300 to 5 μm. The physics of droplet formation is essentially unchanged over this frequency range. For acoustic focusing elements have similar geometries, droplet diameter has been found to scale inversely with the acoustic frequency. The authors summarize the results of a simple model that is used to obtain analytical expressions for important parameters and their scaling with acoustic frequency. The authors also briefly describe a numerical model that successfully predicts the key features of droplet formation