Double flow reversal in thin liquid films driven by megahertz-order surface vibration

Arising from an interplay between capillary, acoustic and intermolecular forces, surface acoustic waves (SAWs) are observed to drive a unique and curious double flow reversal in the spreading of thin films. With a thickness at or less than the submicrometre viscous penetration depth, the film is seen to advance along the SAW propagation direction, and self-similarly over time t1/4 in the inertial limit. At intermediate film thicknesses, beyond one-fourth the sound wavelength λℓ in the liquid, the spreading direction reverses, and the film propagates against the direction of the SAW propagation. The film reverses yet again, once its depth is further increased beyond one SAW wavelength. An unstable thickness region, between λℓ/8 and λℓ/4, exists from which regions of the film either rapidly grow in thickness to exceed λℓ/4 and move against the SAW propagation, consistent with the intermediate thickness films, whereas other regions decrease in thickness below λℓ/8 to conserve mass and move along the SAW propagation direction, consistent with the thin submicrometre films.