Researchers solve mystery of the dancing droplets

A puzzling observation, pursued through hundreds of experiments, has led Stanford researchers to a simple yet profound discovery: Under certain circumstances, droplets of fluid will move like performers in a dance choreographed by molecular physics.

“These droplets sense one another, they move and interact, almost like living cells,” said Manu Prakash, assistant professor of bioengineering.

The unexpected findings may prove useful in semiconductor manufacturing and self-cleaning solar panels, but what truly excites Prakash is that the discovery resulted from years of persistent effort to satisfy a scientific curiosity.

A paper describing the findings was published online March 11 in Nature. Prakash is the senior author, and Nate Cira, a graduate student in bioengineering, is the lead author.

The research began in 2009 when Cira, then an undergraduate at the University of Wisconsin, was doing an unrelated experiment. In the course of that experiment, Cira deposited several droplets of food coloring onto a sterilized glass slide and was astonished when they began to move.

Cira replicated and studied this phenomenon alone for two years until he became a graduate student at Stanford, where he shared this curious observation with Prakash. The professor soon became hooked by the puzzle, and recruited a third member to the team: Adrien Benusiglio, PhD, a postdoctoral scholar in the Prakash lab, who is a co-author of the paper.

Together they spent three years performing increasingly refined experiments to learn how these tiny droplets of food coloring sense one another and move. In living cells, these processes of sensing and motility are known as chemotaxis.

“We’ve discovered how droplets can exhibit behaviors akin to artificial chemotaxis,” Prakash said.

The critical fact turned out to be that in addition to water, food coloring contains propylene glycol. In such two-component fluids, two different chemical compounds coexist while retaining separate molecular identities. The researchers discovered how the dynamic interactions of these two molecular components enabled inanimate droplets to mimic some of the behaviors of living cells.

Surface tension and evaporation

Essentially, the droplets danced because of a delicate balance between surface tension and evaporation. On the surface of any liquid, some molecules convert to a gaseous state and float away. That’s evaporation.

Surface tension is what causes liquids to bead up. It arises from how tightly the molecules in a liquid bind together.

Water evaporates more quickly than propylene glycol. Water also has a higher surface tension. These differences cause a tornadolike flow within a droplet but also help it achieve a kind of internal equilibrium that holds it together.

It’s the proximity of multiple droplets evaporating that kicks off the dance. Each droplet sends aloft gaseous molecules of water. This evaporation increases the relative humidity near other droplets, decreasing their rates of water evaporation more on one side — the side closest to the increased humidity — which disturbs their internal equilibrium, causing them to move.

Rule for two-component fluids

The researchers experimented with varied proportions of water and propylene glycol. Droplets that were 1 percent propylene glycol to 99 percent water exhibited much the same behavior as droplets that were two-thirds propylene glycol to just one-third water.

Based on these experiments the paper describes a “universal rule” to identify any two-component fluids that will demonstrate sensing and motility.

Adding colors to the mixtures made it easier to tell how the droplets of different concentrations behaved and created some visually striking results.

In one experiment, researchers showed how physically separated droplets could align themselves using ever-so-slight signals of evaporation.

If necessity is the mother of invention, then curiosity is the father.

What started as a curiosity-driven project may also have many practical implications.

The deep physical understanding of two component fluids allows the researchers to predict which fluids and surfaces will show these unusual effects. The effect is present on a large number of common surfaces and can be replicated with a number of chemical compounds.

“If necessity is the mother of invention, then curiosity is the father,” Prakash said.

The work was supported by the National Science Foundation, The Pew Charitable Trusts, a Terman Fellowship, the Keck Foundation and the Gordon and Betty Moore Foundation.

Stanford’s Department of Bioengineering also supported the work. The department is jointly operated by the School of Engineering and the School of Medicine.