Like children playing a raucous game of tag, the brightly colored liquid drops shown in the video above chase each other wildly. They appear to know where they are going, but they're nothing more than water and food coloring. This mixture behaves unexpectedly, scientists have discovered, thanks to complicated vapor and surface tension interactions between drops. Such interactions arise because water has a higher surface tension and evaporates more easily than propylene glycol, a main ingredient of food coloring. As the drops evaporate, they feel the effect of each other's water vapor. Two drops sitting close together will evaporate less water from their adjacent sides, because of the increased humidity caused by their neighbor. This creates a higher water concentration—and therefore a higher surface tension—on the side closest to its neighbor, which pulls the drops together, scientists report online today in Nature. When drops with different propylene glycol concentrations get close enough to touch, this pattern changes, and one chases the other, thanks to the difference in surface tension between the two drops. The drop with the higher surface tension flees, as the other stays right on its heels. The researchers created an assortment of "fluidic machines" with this system, which include self-assembling fluid lenses and a rainbow droplet sorter that can arrange droplets according to their concentration. They also made tracks for the droplets by writing with permanent marker (which repels water), creating drops that will chase each other round and round a circular track until they merge, like a game of duck duck goose recreated with colorful droplets.
(Video credit: Nate Cira, Adrien Benusiglio, Manu Prakash)