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Watch: the beautiful science of cream hitting coffee

Something nearly magical happens when you add one drop of cream to a hot coffee.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

I’m going to show you a magic trick.

Get out a hot cup of coffee and an eye dropper filled with cold milk or cream. Now add single drop to the coffee, and watch. Something mesmerizing happens: The cream appears to hover over the surface of the coffee, just for a moment. Like this:

M. Geri, B. Keshavarz, G. McKinley, and J. Bush/MIT

Scientists at MIT recently solved the mystery of why this happens in a study published Wednesday in the Journal of Fluid Mechanics. Turns out it has to do with the temperature difference between the hot coffee and the cold cream.

In the video below, MIT scientists explain that when the drop of cream first hits the surface of the coffee, a bit of air becomes trapped between the drop and the rest of the liquid. That air acts as a buffer, and it has to be pushed out before the drop can continue to merge with the surface.

If the cream and coffee are the same temperature, they’ll merge within a few milliseconds. But if there’s a big temperature difference, tiny vortexes of air form along the gradient, keeping the trapped air in place.

M. Geri, B. Keshavarz, G. McKinley, and J. Bush/MIT

When the MIT scientists performed the experiment, they used silicone oil, which, depending on preparation, can mimic the properties of water as well as more viscous liquids. In these experiments, the scientists were able to make a drop of the oil float on the surface of a basin of silicone oil for as long as 10 seconds if the temperature difference was 86 degrees Fahrenheit.

Anyone who has admired the swirling patterns cream makes in coffee can appreciate this finding. But it could actually help advance medicine too: Researchers say the insight — and the mathematical models they built to predict how long the drops hover — could aid in engineering new “labs on chips,” where tiny droplets of chemicals could run diagnostic medical tests or mimic reactions in body cells.

In the meantime, it’s something cool to look at.

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