Surfing droplets on nanoscopic films driven by surface acoustic waves

Formation of micron-sized droplets on open surfaces continues to remain a challenge in microfluidics. The problem is even stiffer for highly spreading liquids. Here, we report the formation of microdroplets from a nanoscopically thick film of low surface tension and low-viscosity liquid following its spreading under high-frequency nanoscale acoustic wave excitation. Uniquely, these droplets are observed to "surf" on a thin air layer atop the liquid film along the direction of the acoustic wave. Using theoretical scaling and numerical simulations we explore this remarkable behavior and show that the droplet generates via shear-driven pinch off from spatially periodic fluid protrusions in the film, specified by the acoustic excitation wavelength in the solid (λ_{SAW}). We also predict the drop size (d_{d}) and noncoalesced drop velocity (U_{d}) from theory and find an excellent match with experiments. Further, the drop "surfing" dynamics is found to be a consequence of the acoustic radiation pressure imposed on the droplets as the wave is transmitted through the film and into the overlying air phase.