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The ‘beautiful’ science that explains why liquid droplets dance with each other

A puzzling observation, pursued through hundreds of experiments, has led Stanford researchers to a discovery: Under certain circumstances, droplets of fluid will move dancers. (Video: Kurt Hickman/Stanford News)
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Back in 2009, Nate Cira was an undergraduate student running an experiment when he just happened to notice something odd: Tiny drops of water, dyed with food color and placed on glass, began moving.

And these droplets didn't just move sporadically; it was a synchronized motion. It looked as if they were dancing with each other. Cira spent years trying to understand why.

Cira is now a graduate student and, with two other researchers at Stanford University, has finally figured out the physical principles behind this dancing motion.

Cira, Manu Prakash and Adrien Benusiglio described their findings in a paper published Wednesday in the journal Nature and say that the rule that governs the movement could have useful applications, such as helping develop a self-cleaning system for solar panels.

"All three of us really wanted to know the answer, and it's kind of a puzzle you get obsessed with, and that's what led to this work," said Prakash, the paper's senior author and an assistant professor of bioengineering.

The drops won't dance when they are just made up of water; the researchers added a second fluid, making the drops binary or two-compound fluids.

The most striking-looking examples of this movement occur when the drops are mixed with food coloring, which contains the chemical propylene glycol. But the researchers also tested out 40 other chemicals. Prakash said this principle holds true for any two chemicals that mix, but don't react, with each other.

"The aesthetics of the system was definitely one thing that kept us going," he said. "It's just so beautiful -- how could we not understand it?"

Think of a droplet's shape: it is a dome, with a peak and a periphery. Water evaporates much faster than propylene glycol and has a higher surface tension. So as the water evaporates from the droplet's edge, that leaves more of the chemical behind. The water's higher surface tension toward the top pulls, and drives an inward flow.

"It's almost like a tornado inside the drop," Prakash said. "Now the engine is running like a car, but the clutch is not engaged. The drop doesn't know where to go."

Add another droplet, and the evaporation from one is like a signal, telling the other where to move. It's as if they are communicating with each other, Prakash said.

The researchers found that applying permanent marker also changed the surface area, and thus acted like a guide for the drops.

The movement and the principle behind it resembles a biological process called chemotaxis, in which molecules move in relation to chemical stimulus. For instance, neutrophils move about inside the human body and respond to bacterial infections via signaling pathways.

Having one droplet send a signal out, and another finding the signal and moving in relation to that droplet, captures the physics of chemotaxis, Prakash said.

Recreating artificial chemotaxis could help researchers figure out new ways to clean glass and silicon surfaces, or even a mechanism by which the surfaces of solar panels could self-clean.

But that's not why the researchers spent years studying the droplets. It was driven out of sheer curiosity.

"Sometimes you see something, and you know all the laws of physics that you know but it doesn't explain it," Prakash said. 'The whole beauty of trying to uncover how things happen, why they happen in nature, is really driven by the moment you see something that nobody can explain."

The researchers even created a guide to show others how to create their own dancing droplets. You'll need food coloring, water and a glass slide or another piece of glass:

Researchers at Stanford have discovered that, under certain circumstances, droplets of fluid will move like performers in a dance choreographed by molecular physics. Here's how to observe the dancing using every day materials. (Video: Nate Cira, Adrien Benusiglio and Manu Prakash/Prakash Lab, Stanford University)