78th Annual Meeting of the APS Division of Fluid Dynamics (Nov 23 — 25, 2025)
P022: Spin & Split: The Dynamics of Levitated Droplets
We observe spontaneous spinning of a levitated submillimetric-sized water droplet trapped in mid-air at the nodes of an acoustic levitator and super impose six images across the page from a high-speed camera (65,000 fps). The acoustic field induces a secondary flow (aka acoustic streaming) that generates rotation. The centrifugal force (Fc) elongates the droplet until it splits into two smaller droplets when the angular velocity exceeds ~3000 rad/s. This phenomenon naturally occurs when the droplet charge is near zero and the rotational Bond number < 0.5 [1,2].When the water droplet carries an electrical charge (Q), it elongates due to electrostatic repulsion, promoting rotation around an axis perpendicular to the uncharged case. Eventually it exceeds the Rayleigh limit [3], causing the formation of a Taylor cone and ejects some nano-droplets that carry charges away. The equalized charge on the droplet returns to a spherical shape for a short time before the process begins again.1. S. Chandrasekhar, Proc. R. Soc. A 286, 1 (1965)2. R. A. Brown & L. E. Scriven, Proc. R. Soc. A. 371, 331 (1980)3. L. Rayleigh, Philos. Mag. 14, 184-186 (1882)
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