Abstract
Surface acoustic waves (SAWs) are used in droplet microfluidics for operations on sessile droplets, such as internal mixing, directional motion, or nebulization. An unsolved problem has been to understand how high frequency (MegaHz) SAWs can counterintuitively cause low frequency (10-500 Hz) oscillations on the drop free surface, which is known to enhance drop mobility. Using simultaneous measurements of the free-surface dynamics and internal acoustic pressure, we find that the drop oscillation instability results from coupling between an intracavity acoustic mode excited by the SAW, and an inertia-capillary surface eigenmode, through amplitude modulation and delayed radiation pressure feedback.
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