  High frequency thickness mode ultrasound is an energy-efficient way to
atomize high-viscosity fluid at 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 (O[100 Hz]) when driven by high-frequency
ultrasound (O[1 MHz] 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. 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 ambient environment. Numerical simulations are
performed via global optimization against the rigorously defined interfacial
physics. The proposed pressure-interface feedback model is explicitly based on
the pressure distribution hypothesis. For low power acoustic excitation, the
simulations reveal a 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 pressure-interface feedback model accurately
predicts the on-source vibration amplitude 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 90% wt.- 10%
wt. glycerol-water solution droplet.
