Mechanism of capillary waves generation driven by high-frequency ultrasound*

High-frequency thickness mode ultrasound is an energy-efficient way to atomize high-viscosity fluid at a high flow rate into fine aerosol mists of micron-sized droplet distributions. However, the complex physics of the atomization process is not well understood. It is found that with low power the droplet vibrates at low frequency (10^2 Hz) when driven by high-frequency ultrasound (10^6 Hz and above). To study the mechanism of the energy transfer that spans these vastly different timescales, we measure the droplet's interfacial response to 6.6 MHz ultrasound excitation using high-speed digital holography---a revolutionary method for capturing three-dimensional surface dynamics at nanometer space and microsecond time resolutions. We show that the onset of low-frequency capillary waves is driven by feedback interplay between the acoustic radiation pressure distribution and the droplet surface. These dynamics are mediated by the Young-Laplace boundary between the droplet interior and the ambient environment. Numerical simulations are performed via global optimization against the rigorously defined interfacial physics. The proposed model is explicitly based on the pressure distribution hypothesis. For low-power acoustic excitation, the simulations reveal stable oscillatory feedback that induces capillary wave formation. The simulation results are confirmed with direct observations of the microscale droplet interface dynamics as provided by the high-resolution holographic measurements. The acoustic pressure interfacial feedback model accurately predicts the acoustic power required to initiate capillary waves, and interfacial oscillation amplitude and frequency. The radiation pressure distribution is likewise confirmed with particle migration observations. Viscous effects on wave attenuation are also studied by comparing experimental and simulated results for a pure water droplet and 90wt%-10wt% glycerol-water solution droplet.