  Initially, the acoustic field forced the droplets into an oblate spheroid
shape, though the counteracting force of the cooling stream caused them to
circularize. Droplet geometry was thus the net result of streaming forces and
surface tension at the acoustic boundary layer/air-liquid interface. Nucleation
was determined to be neither homogeneous nor heterogeneous but secondary, and
thus dependent on the cooling rate and not on the degree of supercooling. It
was likely initiated by aerosolized ice particles from the air or from droplets
that had already nucleated and broken up. The latter secondary ice production
process resulted in multi-drop systems with statistically identical nucleation
times. Notably, this meant that the presence of interfacial rupture at an
adjacent droplet could influence the crystallization behaviour of another.
After the formation of an initial ice shell around the individual droplets,
dendritic protrusions grew from the droplet surface, likely seeded by the same
ice particles that caused nucleation, but at a quasi-liquid layer. When
freezing was complete, it was determined that the frozen core had undergone a
volumetric expansion of 30.75%, compared to 9% for pure, sessile water
expansion. This significantly greater expansion may have resulted from
entrained air bubbles at the inner solid-liquid interface and oscillations at
the moving phase boundary caused by changes in local acoustic forces. Soon
after melting began, acoustic streaming, the buoyancy of the remaining ice, and
convective currents caused by an inner thermal gradient and thermocapillary
effects along the air-liquid interface, all contributed to the droplet spinning
about the horizontal axis.
