We take for granted that we know what happens when a raindrop hits a puddle or cream gets poured into a cup of coffee. But while we imagine that liquids merge with other liquids seamlessly, like a pitcher of water poured into a pool, in these instances, droplets actually levitate before they fully submerge.

Scientists have actually known this for a long time, noting that raindrops first bounce across puddles, and cream first rests on coffee’s surface before merging with it. But on November 8, in a study published in the Journal of Fluid Mechanics, they explained for the first time why this happens.

In the paper, scientists from the Massachusetts Institute of Technology explain that whether or not a droplet resists another liquid surface or doesn’t all comes down to whether they have a difference in temperature.

Cold cream drops "levitate" on hot coffee.

Through a series of experiments,the MIT researchers determined that when a droplet of fluid is plopped onto another liquid, merging doesn’t happen until all the air, trapped between the droplet and the other liquid, is pushed out by the weight of the drop.

If both liquids are the same temperature, this process can happen in a matter of milliseconds. But if, say, the drop is colder than the other liquid, recirculating vortices — in fluid dynamics, a vortex is an area where liquid swirls around an axis — are generated in both liquid sources. The swirling in both movements increases the air drag in the gap between them, creating air resistance, which in turn sustains the weight of the drop.

Essentially the greater the temperature difference, the stronger the vortices — the flow that happens in liquids around its axis line. The stronger the vortices, the longer the levitation period will be.

What happens when the droplet resists.

“We found the force coming from the droplet’s weight and the force coming from the recirculation of the air will balance at a point, and to get that balance, you need a minimum, or critical temperature difference, in order for the droplet to levitate,” Michela Geri, an MIT graduate student and the study’s lead author, explained in a statement.

This knowledge doesn’t just explain a phenomenon that’s taken for granted. It can also be used to inform the design of new products and experiments that hinge this fundamental understanding of how fluids interact.

Geri tells MIT News that she hopes her work can help researchers understand how chemical substances are spread by rain, inform the design of microfluidic chips and help engineers use droplets as mechanical ball bearings in zero-gravity environments.

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