Surface acoustic wave induced instability and pattern formation in a liquid polymer film

Recently, standing surface acoustic waves (SAWs) have been demonstrated to stimulate wrinkles on polymer films. Here, we theoretically investigate the underlying mechanism of the acoustic-induced micro-structures on a thin liquid film. The standing SAWs leak into the liquid polymer and generate radiation potential gradients. The strong potential gradients produce acoustic radiation pressure and induce an instability that features the wrinkling structure. Surface stability of the liquid film is firstly investigated by a linear instability analysis and the critical conditions of instability are analytically derived. Theoretical prediction of the film thickness that is favorable for surface instability well agrees with the optimal value found in previous experiments. Balance between the radiation and gravity-capillary pressure at the fluid surface forms the equilibrium equation of the surface profiles. Analytic solution reveals that the component of radiation pressure varying spatially makes the wavelength of the generated surface patterns equal to half of the SAW wavelength, while the constant component of radiation pressure absorbs the film onto the substrate. The interplay between the acoustic waves and the surface morphology is also investigated. Effects of the surface deformation on the radiation pressure is negligible except for that the mean thickness of the liquid approaches the resonance mode where singularity of the radiation pressure occurs.(c) 2023 Elsevier Ltd. All rights reserved.