Chemotaxis

In the scientific world, solving mazes used to be the dominion of experimental psychologists. Now chemists are getting in on the action, but they've swapped the traditional lab rat for a drop of liquid. A team led by Northwestern University chemistry professor Bartosz A. Grzybowski has shown that an acidic droplet can successfully navigate a complex maze (J. Am. Chem. Soc., DOI: 10.1021/ja9076793).

"I personally find most exciting that such a simple system can exhibit apparently 'intelligent' behavior," Louisiana State University chemistry professor John A. Pojman comments. "This approach may be useful as a pumping method for microfluidics or a way to convert chemical energy to mechanical motion in small devices. I am eager to see if they can generalize it to other types of gradients," he says.

The droplet, composed of 2-hexyldecanoic acid in either dichloromethane or mineral oil, travels several centimeters through a maze with a pH gradient. The pH is high at the maze entrance and low at its exit. Once in the maze, the droplet travels toward the lower pH, and in doing so, Grzybowski notes, it always finds the shortest path through the maze.

Acid-base chemistry and surface tension are what drive the drop. "The interfacial reaction between the acid in the drop and the base in the channel creates convection rolls or 'swirls,' " Grzybowski explains. "The fluid mechanics plays out such that the swirl facing lower pH is more pronounced, and so the drop is effectively pushed in this direction."

A droplet travels through a maze with a pH gradient.