0313ctt droplets dance lo res

[Image above] Credit: Stanford; YouTube 

Stanford University researchers have solved the science behind an incredible yet simple phenomenon—food coloring droplets, when plopped onto a clean glass slide, move and dance as if they’re alive.

Don’t believe it? Watch the captivating choreography below.

Credit: Stanford; YouTube

Although the dance is a seemingly simple phenomenon, getting at the complex science behind the choreography was no simple task.

According to a Stanford press release, lead author Nate Cira first witnessed the droplets’ dance back in 2009. Five years, countless experiments, and two additional curious colleagues later, Cira’s research detailing the science behind their movement is published in Nature.

“These droplets sense one another. They move and interact, almost like living cells,” says Manu Prakash, a bioengineering professor and senior author of the published research, in the release.

The trio of scientists used precise experiments and careful analysis to decipher the droplets’ dance moves, including plotting paths of fluid flow within single droplets using 1-μm-diameter, fluorescently-labeled tracer beads.

Food coloring is a two component liquid made from a mixture of water and propylene glycol. The scientists’ painstakingly collected results show that differing rates of evaporation and differing surface tensions between the two components create a complex interplay within the liquid, leading to the droplets’ autonomous movements.

0313ctt droplets dance researchers lo res

(L to R) Adrien Benusiglio, Nate Cira, and Manu Prakash are happy that food coloring and glass slides got them a publication in Nature. Credit: Linda A. Cicero; Stanford News Service

Water molecules within the food coloring evaporate more quickly than propylene glycol molecules, and, because they also have higher surface tension, they tend to do so from the lower edges of the droplet. This preferential evaporation creates an imbalanced composition within the droplet—more propylene glycol on bottom, more water on top.

Differing surface tensions between these separated molecules initiates a molecular game of tug-of-war, creating turbulent flow within the droplet that propels it forward. According to the release, the droplets can sense one another’s location from the presence of local evaporated water molecules, making the droplets seem as if they’re chasing one another in an epic game of tag.

In addition to solving a scientific curiosity, the results provide insight into how to control a liquid’s wetting behavior, an important consideration for also controlling materials’ surface properties. According to the release, the research may also have implications for semiconductor manufacturing, self-cleaning solar panels, and other industrial applications.

The article is “Vapour-mediated sensing and motility in two-component droplets” (DOI: 10.1038/nature14272).