Nanometer-scale surface acoustic waves traveling through a piezoelectric substrate can push a thin film of silicone oil over topographical obstacles — bumps, ridges, steps — that the film would otherwise be unable to cross. The mechanism is acoustic streaming: the wave generates a net body force in the fluid that competes with surface tension and gravity. Whether the film successfully coats the obstacle depends on the balance among all three, a balance that shifts with wave amplitude, obstacle geometry, and film thickness.

Li and colleagues build a 2D model that directly incorporates obstacle shape into the thin-film equations rather than treating it as a perturbation. The representation of acoustic streaming over uneven substrates is novel — previous models assumed flat surfaces where the wave-induced force is uniform. Over obstacles, the force field deforms, creating regions where streaming reinforces capillary flow and regions where it opposes it.

Coating is usually passive — gravity or capillarity does the work. Adding an acoustic drive changes the problem from “will the film reach equilibrium?” to “can the film be pushed through a configuration it would never reach on its own?” The obstacle doesn’t just resist the film; it reshapes the force that drives it. Control of thin-film flow is inseparable from control of the force field’s interaction with the geometry it encounters.

(arXiv:2603.00308)