Free surface shape for an ultrasonic nebuliser

Nebulisers are used for the delivery of fine droplets of drugs to the lungs or smaller bronchii. An ultrasonic nebuliser uses the oscillations caused by a piezoelectric crystal to produce a ``fountain'' in a liquid, which leads to the formation of a mist of fine particles. This work describes a model of an ultrasonic nebuliser, with the aim of demonstrating the mechanism of the fountain formation. It is shown that the oscillating ultrasound causes a mean flow in the liquid due to acoustic streaming, which in turn creates a deformation of the free surface. Cavitation within the liquid, which is an energy intensive process, leads to the dissipation necessary for acoustic streaming to occur. Finally, a mechanism for explaining the location of the mist on the free surface is proposed, based upon the results