An aqueous computer based on water droplets rolling around a superhydrophobic surface has been developed by researchers in Finland. See flip-flop memory logic in action:
From the abstract to the paper: “When water droplets impact each other while traveling on a superhydrophobic surface, we demonstrate that they are able to rebound like billiard balls.” We present elementary Boolean logic operations and a flip-flop memory based on these rebounding water droplet collisions. Furthermore, bouncing or coalescence can be easily controlled by process parameters. Thus by the controlled coalescence of reactive droplets, here using the quenching of fluorescent metal nanoclusters as a model reaction, we also demonstrate an elementary operation for programmable chemistry.”
So, what is making the droplet flip and flop so that it goes left-right alternately. I asked team leader Robin Ras to explain:
“You can see that the droplet that is standing still is actually located in a bistable depression (in the shape of an 8). Each depression is slightly off-axis, so that when the moving droplet hits it, it kicks the droplet at an angle. As in classical Newtonian mechanics (for action there is an equal and opposite reaction), the first moving droplet is going into the other direction (and slowing down as it transfers its momentum) and fits into the other depression.” Hence the alternating. “Simple but beautiful,” Ras told me.
More droplet logic videos.
Mertaniemi, H., Forchheimer, R., Ikkala, O. & Ras, R.H.A. (2012). Rebounding Droplet-Droplet Collisions on Superhydrophobic Surfaces: from the Phenomenon to Droplet Logic, Advanced Materials, n/a. DOI: 10.1002/adma.201202980