Fifteen years ago, researchers used high speed video to show that when a drop of water coalesces into a layer of the same liquid, it does so not instantaneously but in a matryoshka-like cascade, with each step generating a smaller drop. Now, a newly published study finds that soap bubbles do something similar.

“When gently deposited onto a horizontal film, a bubble may interact with the underlying film in such a way as to decrease in size, leaving behind a smaller daughter bubble with approximately half the radius of its progenitor,” write MIT applied mathematician John Bush and his colleagues in a recent issue of Physics of Fluids. FYFD’s Nicole Sharpe elaborates: “If the the soap film beneath the bubble ruptures, some air from the inside of the bubble can escape. Part of the bubble coalesces with the soap film and a smaller daughter bubble is left behind.”

“Alternatively,” writes Sharpe, explaining the second half of the above footage, “if the soap film did not rupture, the air inside the bubble had no escape, and the bubble would coalesce into a hemispherical lens atop the soap film.”


Bush and his colleagues write that they observed the progenitor—>daughter process repeat up to three times before the bubbles coalesced completely. When researchers first observed this phenomenon in ordinary water droplets (i.e. sans soap), they reported witnessing as many as six consecutive cascade events before the droplet coalesced completely. The highest number of events I’ve been able to find recorded on video is five (or maybe six, depending on how you count), though Bush has previously reported witnessing as many as seven in a row.

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[Physics of Fluid via FYFD]

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