Using surface acoustic waves to drive thin film flow over an obstacle

Source abstract

Abstract We study a new paradigm for ultrasonic-driven object coating by using a model system where MHz-level surface acoustic waves (SAWs) drive the spreading of a silicone oil film atop topographical obstacles. We use experiments to show that nanometre-amplitude SAWs, propagating in the substrate of a piezoelectric actuator, propel macroscopic oil films to climb and traverse solid obstacles placed on the actuator. The oil dynamics reveal rich coupling between ultrasonic forcing, capillarity and gravity, the balance of which determines coating. In addition to the experiment, we formulate a simplified two-dimensional theoretical model that incorporates obstacle geometry directly in the oil thin-film evolution equation, introducing a new representation of acoustic streaming in the presence of substrate height variations. Despite the simplifications inherent in the modelling, simulations show qualitative agreement with experiments, providing evidence that the model captures the key physics of the coating flows observed.