A trio of researchers at Stanford recently published an article in Nature that explains the curious attraction found in droplets of everyday food coloring. The paper is the culmination of hundreds of experiments that began in 2009 when Nate Circa was working on an unrelated experiment as an undergraduate at the University of Wisconsin. Circa noticed that when drops of food coloring were placed on a slide they exhibited bizarre behaviors: identical colors would find matches while different colors would seemingly hunt each other.
Circa soon teamed up with Manu Prakash and Adrien Benusiglio who began working on a series of increasingly refined studies to understand why these single droplets appeared to mimic biological processes, resulting in behaviors that looked like chasing, dancing, or avoidance. One of the keys was the interaction of two different compounds found in food coloring: water and propylene glycol. Tom Abate writing for Stanford explains:
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.
This complex behavior is something called artificial chemotaxis which Manu Prakash explains in layman’s terms in the video above:
The physical properties of these fluids give rise to this immense complexity of behavior. For example, chasing and sensing each other, and very much what we call artificial chemotaxis. Chemotaxis is the idea in biology that one single cell can sense where its enemy is, and it brings up all its machinery, and it chases that enemy to try to eat it.
If you really want to get into the nitty gritty of fluid dynamics and molecular physics you can read the full paper in Nature and a bit of a summary on Stanford News. (via, appropriately, F*ck Yeah Fluid Dynamics)