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
t
1/4
in the inertial limit. At intermediate film thicknesses, beyond one-fourth the sound wavelength λ
ℓ
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 λ
ℓ
/8 and λ
ℓ
/4, exists from which regions of the film either rapidly grow in thickness to exceed λ
ℓ
/4 and move
against
the SAW propagation, consistent with the intermediate thickness films, whereas other regions decrease in thickness below λ
ℓ
/8 to conserve mass and move
along
the SAW propagation direction, consistent with the thin submicrometre films.
Double flow reversal in thin liquid films driven by megahertz-order surface vibration
