After more than a decade of research, scientists at Stanford University have created a working computer based on the physical movement of water droplets. It’s a breakthrough in physical computing that gets at the most basic definition of a computer: any programmable device that can carry about logical (mathematical) operations. By combining cutting-edge theory in fluid dynamics with very-much-not-cutting-edge theory in computing, the team was able to create a synchronous computer based entirely on the physics of water.
As you might imagine, a computer based on the physical movement of water is much, much slower than a conventional computer based on the movement of electrons — but that’s beside the point. Nobody expects a new, super-fast liquid CPU, but by applying the principles of computing to the manipulation of matter, lead researcher Manu Prakash and his graduate students hope they can computationally revolutionize other areas of science.

Prakash is actually a bioengineer — his main goal with the project is to create a platform for robust, super-quick chemical testing. Their technique can direct potentially millions of droplets around a chip, simultaneously, and each of these can be loaded with a different chemical for testing. A well-designed chip could make months of complex chemical experimentation into minutes — once the chip has been designed and built, the experiment designed, and the samples made and loaded onto the chip itself.

The system works based on the continual flipping of an applied magnetic field. The chips, which are at present about half the size of a postage stamp, are embedded with tiny iron bars that are easily magnetized; arranging these bars in “Pac-Man-like” mazes provides discreet channels for the droplets to follow. Each droplet is infused with magnetic nanoparticles that make the water responsive to an applied magnetic field, so by flipping the polarity of these bars the team can decide which path each droplet will take through the bar-maze.
The system only works as a general-purpose computer because it is “synchronous,” meaning that it keeps the various operations marching to the same beat — the researchers say they could potentially control millions of droplets at once, with a scaled version of the same technology. In a conventional computer, each of these beats is called a clock cycle — in a water-drop computer, this beat is controlled by the flipping magnetic field. In both cases, the central timing mechanism makes sure that even thousands of different paths and interactions all proceed according to the same schedule, and can thus work together toward computational goals.
Some of the very earliest computers, like the UNIVAC I, had computer memory based on liquid mercury — in essence, the idea of representing computer data with physical matter is not new. What is new is the idea that the physical structure of the chip could be used to direct the movement of matter in a robust, pre-programmed way. In a best-case-scenario, this sort of paradigm shift in the approach to experimental chemistry could cause the sort of exponential efficiency increase electronic computers allowed in regular mathematics.
One big push in the quest truly next-generation medicine is so-called “organ on a chip” technology, which would allow scientists to test the effects of drugs and other substances on certain organs by running those substances through small, high-throughput stand-ins for whole organs of interest. With the ability to quickly and systematically test the interactions of thousands of different substances, that idea might someday plausibly reach the point of “individual on a chip.”
In the more foreseeable future, water-drop computing is a fascinating realization of something that was always theoretically known: computing is a fundamentally physical process (until quantum computing comes of age, I suppose), and as such can be expressed in the medium of physical matter. It’s far less efficient that way — but efficiency isn’t the only goal worth pursuing.