Ultrasonic atomization is employed to generate size-controllable droplets for
a variety of applications. Here, we minimize the number of parameters dictating
the process by studying the atomization of a single drop pending from an
ultrasonic horn. Spatiotemporally resolved X-ray phase-contrast imaging
measurements show that the number-median sizes of the ejected droplets can be
predicted by the linear Navier-Stokes equations, signifying that the size
distribution is controlled by the fluid properties and the driving frequency.
Experiments with larger pendant water drops indicate that the fluid-structure
interaction plays a pivotal role in determining the ejection onset of the
pendant drop. The atomization of viscoelastic drops is dictated by extended
ligament formation, entrainment of air, and ejection of drop-encapsulated
bubbles. Existing scaling laws are used to explain the required higher input
amplitudes for the complete atomization of viscoelastic drops as compared to
inviscid drops. Finally, we elucidate the differences between capillary
wave-based and cavitation-based atomization and show that inducing cavitation
and strong bubble oscillations quickens the onset of daughter drop ejection but
impedes their size control.Comment: 36 pages, 11 figure