Manu Prakash runs what he calls a curiosity-driven lab at the intersection of biology and physics.
His latest work involves, of all things, droplets of food coloring that chase one another, merge and do a kind of devilish dance on glass slides that makes them seem not only alive but possessed.
“It is almost as if each drop is sending a signal: ‘look where I am,’ ” said Dr. Prakash, an assistant professor of bioengineering at Stanford University. But there is no life, no intent in the activity, only the results of evaporation and surface tension. It is, he said, “the complexity that arises from a very simple system.”
The food-coloring project began in the Prakash lab when Nate Cira arrived as a graduate student from the University of Wisconsin. There, he had been puzzling over moving droplets of food coloring on a glass slide for a couple of years.
Dr. Prakash was interested in what he saw and brought in Adrien Benusiglio, who was doing postdoctoral research in Dr. Prakash’s lab. The three pursued the project for several years and just published their results in Nature.
The coloring turns out to be incidental. What is important is that food coloring contains propylene glycol. The combination of that fluid with water holds the droplets together and makes them move.
Water evaporates more quickly than propylene glycol and has greater surface tension. These differences result in continual movement inside a droplet. But the “little tornado inside” the drop reaches a balance that actually holds the droplet together, Dr. Prakash said.
The droplet moves when a change in relative humidity alters the tornado. Evaporated water from one droplet is a subtle but powerful signal, because it increases the humidity near another drop. That changes the second drop’s rate of evaporation, which disturbs the internal balance, and the dance begins.
By varying the percentages of the two fluids, the researchers were able to get droplets to move in ways that seemed mysterious — they sorted themselves according to their internal composition, formed a straight line and even climbed vertically.
The moving droplets could be useful. For instance, a mist might be used for cleaning surfaces, because the drops don’t leave any bit of themselves behind when they move.
And Dr. Prakash is excited about spreading the dancing droplets outside the laboratory. His lab is devoted not only to deep dives into everyday phenomena, but also to the democratization of science, moving scientific tools out of the lab and into the world, particularly to places and people with little money.
His group developed the Foldscope, an origami paper microscope that costs little and can be built and used anywhere.
The droplets offer another chance for outreach. Anybody can repeat the experiments, Dr. Prakash said, and he and his colleagues are making a video to show them how.
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