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, an assistant professor of bioengineering at Stanford University.
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.
The research began in 2009 when Nate 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, a postdoctoral scholar in the Prakash Lab.
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.
As they explain in Nature, the critical fact was that food coloring is a two-component fluid. In such fluids, two different chemical compounds coexist while retaining separate molecular identities.
The droplets in this experiment consisted of two molecular compounds found naturally in food coloring: water and propylene glycol.
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.
Evaporation is easily understood. On the surface of any liquid, some molecules convert to a gaseous state and float away.
