Shaping and Levitating Droplets

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Opposing ultrasonic speakers can be used to trap and levitate droplets against gravity using acoustic pressure. Changes to field strength can do things like bring separate objects together or flatten droplets. The squished shape of the droplet is the result of a balance between acoustic pressure trying to flatten the drop and surface tension, which tries to pull the drop into a sphere. If the acoustic field strength changes with a frequency that is a harmonic of the drop’s resonant frequency, the drop will oscillate in a star-like shape dependent on the harmonic. The video above demonstrates this for many harmonic frequencies. It also shows how alterations to the drop’s surface tension (by adding water at 2:19) can trigger the instability. Finally, if the field strength is increased even further, the drop’s behavior becomes chaotic as the acoustic pressure overwhelms surface tension’s ability to hold the drop together. Like all of this week’s videos, this video is a submission to the 2103 Gallery of Fluid Motion. (Video credit: W. Ran and S. Fredericks)

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