Nate Cira was working in a lab as an undergraduate at University of Wisconsin–Madison when he saw something unusual. He was working with a mixture of food coloring and water, placing droplets on an ultra-clean slide—and the droplets seemed to be breathing. They were dancing, too: multiple droplets would race around and crash into one another in complicated patterns.
Now after three years of work as a graduate student at Stanford University, Cira and colleagues have derived the physics behind the strange behavior of those droplets. The mechanism behind their motion could help scientists understand how complex movement and interactions emerge from simple ingredients.
To figure out what was going on inside these droplets Cira and his collaborators, physical biology researcher Manu Prakash and postdoctoral student Adrien Benusiglio, placed tiny tracer beads in droplets of the food coloring–water mixture so that they could see the flow of fluids within the droplets. Then they did a series of experiments to see how multiple droplets with different concentrations would interact with one another. They described their findings, which can explain complex behavior in many different mixtures of two substances, in Nature March 11. (Scientific American is part of Nature Publishing Group.)
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On an extremely clean slide a droplet of water or of food coloring will form a flat pool instead of beading up. The water–food coloring combo, however, formed rounded beads. By watching the motion of the tracer particles, the researchers figured out why. The particles would cycle from the center of a droplet to the edges as water vapor evaporated from the droplet's surface and left the less-evaporative food coloring behind. Then, because water has higher surface tension than food coloring does, the more concentrated water at the center pulled the liquid and particles back in. This continuous flow, caused by the evaporation process and the difference in surface tension between the two fluids, holds the droplet in place so it cannot spread out. (The flow from lower to higher surface tension is called the Marangoni effect.) “It’s like an engine running in idle,” Prakash says. “There’s this internal flow running, but it’s perfectly symmetric, so the drop doesn’t go anywhere.”
Putting the droplet near another one breaks that symmetry and gets the droplet moving: The water vapor exuded by nearby droplets slows the evaporation on the side closest to them, so more water builds up on that side thereby pulling the droplet forward toward the vapor’s source, even from millimeters away. Droplets with a lower water concentration will race towards droplets with a higher one, push them forward and eventually fuse.
Once they had hammered out the physics behind the behavior of the droplets, they put them to work. Using a Sharpie, they drew a series of obstacle courses on glass slides. Because the water–food coloring mixture avoids the Sharpie’s hydrophobic ink, the droplets would follow the contours of the tracks. On a circular track, for example, a low-concentration droplet could chase a high-concentration one for minutes at a time. On other tracks, the droplets would self-assemble into lines or oscillate regularly up and down.
The researchers also set up a self-sorting system much like a coin-sorter. In it droplets would bounce along a series of Sharpie-drawn boxes holding different, multicolored mixes, propelled away by the high-water-content droplets until they found the box with their same concentration. In addition, using droplets on multiple slides, the researchers coaxed them into forming liquid lenses that aligned themselves automatically into focus.
And that “breathing” effect Cira spotted as an undergraduate? It was the water vapor in Cira’s own breath that caused the droplet to pulse in and out.
The physical process that Cira’s group uncovered could prove useful in many contexts. “New mechanisms are like bricks from which engineers can build houses,” says John Bush, a fluid dynamicist at Massachusetts Institute of Technology. “This is a beauty.” He says it is hard to predict the applications that stem from such a discovery but that they could be plentiful; using tiny amounts of fluids to transport substances is a growing field, and this solves a key problem of forces keeping fluids pinned in place.
Similar droplets form when any two nonreactive liquids mix, as long as one has higher surface tension and evaporates more quickly. (Just ask the lab members—at one point they tried every combination of chemicals in the lab, some of which had explosive results.) The group says that such mixes could eventually be used in microfluidic devices, to prepare surfaces for painting by cleaning and drying them, or even to rove around cleaning the surface of solar panels.
But the real draw is the demonstration of complexity from such commonplace ingredients and how it can inform our understanding of life and biology. “Clearly this is a nonliving system, and it’s very, very simple compared to what biological entities do, but it has sensing and motility combined together,” Prakash says. “And that’s very powerful.” The team posted video instructions here for producing the droplets at home.