High speed imaging of acoustic vaporization of single droplets

Abstract

This paper will show the vaporization of single droplets by the application of single tone bursts. A high speed video system on a microscope was used to monitor droplets in a flow tube. A focused 3.5 MHz transducer was aimed at the intersection of tube and optical beam. A highly dilute droplet emulsion was injected and a special syringe setup was used to position individual droplets in the focus. Single 10 cycle bursts were sufficient to phase transition droplets. During the acoustic irradiation, the droplets showed dipole type oscillations along the acoustic axis (average amplitude 1.3 /spl mu/m, independent of droplet diameter). The onset of vaporization was monitored as either spot-like within the droplet or homogeneous through-out the droplet's imaged cross-section. The spot-like centers of nucleation were solely observed along the direction of oscillation. Smaller droplets required more acoustic energy for vaporization than larger droplets, which is consistent with earlier experiments on emulsions. Simulations of the free field scattering theory of rigid spheres have been found to be only a small fraction of the dipole motion in the experimental observations.