Physics for your next shower

There’s a classic elementary school experiment that gives you the inkling that fluids have more to them than meets the eye. You’re handed a penny, a glass of water, and an eyedropper. Your task: fit as many drops of water as you can onto that penny.

As the droplet grows, the experiment acquires the dramatic tension of a game of Jenga. Will this drop burst the droplet? Will the next? The task delights and fascinates schoolchildren. Some of those schoolchildren grow up into physicists, and a good fraction of physicists, for whom the delights of the penny experiment perhaps never fade, devote their entire careers to probing the weird, often counterintuitive behavior of fluids.

The following video, filmed by researchers at the University of Twente in the Netherlands, shows the unexpected consequences of squirting shampoo out of a bottle:

The mysterious Kaye effect was first seen in the 1960s, and has fascinated scientists ever since. As the video describes, the falling shampoo piles up until a dimple is formed. Then the jet continues to pull air with it down into the dimple, eventually gliding on this air pocket and ramping out again into the air. You can see the effect with oil too.

The University of Twente’s “Gallery of Fluid Motion” features videos that have won awards at the past meetings of the American Physical Society’s Division of Fluid Dynamics. The videos show us strange phenomena, and the unexpected reasons behind them. For instance, one video on snapping shrimp reveals that the shrimp’s claw doesn’t directly stun its prey. Instead, a very different mechanism is at work.

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