Nucleation pressure threshold in acoustic droplet vaporization

Abstract

Acoustic droplet vaporization (ADV) has been studied by several laboratories around the world, including the parametric dependence of its nucleation. In this abstract, we will present our approach of combining classical homogeneous nucleation theory with superharmonic focusing to predict necessary pressures to induce nucleation in ADV. We will show that linear acoustics is a valid approximation to leading order when particle displacements in the sound field are small relative to the radius of the exposed droplet. This is done by perturbation analysis of an axisymmetric compressible inviscid flow about a micrometer-sized droplet with small surface perturbations relative to the mean droplet radius when subjected to an incoming ultrasonic wave. A calibrated single element spherical focus transducer (Olympus A321S 7.5 MHz) was positioned in a water tank above an inverted microscope (Leica DM IL) to emit single ten-cycle bursts. Single droplets (3–30 μm) were placed on ultrasound gel and exposed quasi free-field. Theoretical derivations and predictions will be compared to experimental findings. The ADV nucleation pressure threshold inside the droplet is calculated to be −9.33±0.30 MPa for typical experimental parameters. As a result we are able to predict if a given incident pressure waveform will induce nucleation in an exposed perfluorocarbon droplet.