Substrate dependent drop deformation and wetting under high frequency vibration

We explore the peculiar steady component of a sessile drop response to MHz order vibration, found to be dependent on its initial wettability. Placed on a vibrating hydrophobic substrate, the drop elongates vertically in the direction of the incident sound wave while remaining hydrophobic. In contrast, the drop is seen to spread on a slightly hydrophilic substrate. We elucidate this discrepancy by revealing the competing effects between the radiation pressure exerted at the bulk air/water interface and the acoustic streaming force on the contact line, revealing the critical role of the flow in the viscous boundary layer.