Water Atop Oil

At first glance, this image looks much like the impact of any drop on a pool of the same liquid, but that’s not what you’re seeing. This is the impact of a water droplet on a thin film of oil, and the immiscibility of those two fluids has important effects on the collision. When the water drop impacts, it spreads and forms a compound crown that rises out of the fluid. Eventually, that momentum runs out and the crown falls into the liquid.

Water’s intermolecular forces are strong enough to pull the remains of the droplet back in on itself. As that fluid collides at the center, it gets forced up into a central jet with enough energy to eject a droplet or two at its tip. Even though this looks like a Worthington jet, it’s not. Worthington jets form after the collapse of a cavity in the impacted liquid – in other words, they form on pools, not on films. Despite the visual similarity, this central jet is formed entirely differently! (Image and research credit: Z. Che and O. Matar, source; submitted by O. Matar)

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