This beautifully-made video about a beautifully-colored series of experiments from a Stanford research team showcases how a observing a few droplets of food coloring — made of water and propylene glycol — have led to a deeper study about how two-component fluids behave: chasing, avoiding, attracting… dancing in a “molecular minuet” and more.
Why do the fluids behave this way? Surface tension and evaporation. The two different chemical compounds within the fluids, co-mingling in different proportions, behave differently based on how surface tension and evaporation are affecting each of the compounds within the drops. From Stanford.edu:
Adding colors to the mixtures made it easier to tell how the droplets of different concentrations behaved and created some visually striking results.
In one experiment, a droplet with more propylene glycol seems to chase a droplet with more water. In actuality, the droplet with more water exerts a higher surface tension tug, pulling the propylene droplet along.
In another experiment, researchers showed how physically separated droplets could align themselves using ever-so-slight signals of evaporation.
In a third experiment they used Sharpie pens to draw black lines on glass slides. The lines changed the surface of the slide and created a series of catch basins. The researchers filled each basin with fluids of different concentrations to create a self-sorting mechanism. Droplets bounced from reservoir to reservoir until they sensed the fluid that matched their concentration and merged with that pool.