Computer that operates on water droplets

Engineers have developed a synchronous computer that operates using the unique physics of moving water droplets.

Computers and water typically don't mix, but in a Stanford bioengineer's lab, the two are one and the same. The computer is nearly a decade in the making, incubated from an idea that struck Manu Prakash, an assistant professor of bioengineering at Stanford, when he was a graduate student. The work combines his expertise in manipulating droplet fluid dynamics with a fundamental element of computer science — an operating clock.

Because of its universal nature, the droplet computer can theoretically perform any operation that a conventional electronic computer can crunch, although at significantly slower rates. Prakash and his colleagues, however, have a more ambitious application in mind. "We already have digital computers to process information. Our goal is not to compete with electronic computers or to operate word processors on this. Our goal is to build a completely new class of computers that can precisely control and manipulate physical matter. Imagine if when you run a set of computations that not only information is processed but physical matter is algorithmically manipulated as well. We have just made this possible at the mesoscale," said Prakash. 

The ability to precisely control droplets using fluidic computation could have a number of applications in high-throughput biology and chemistry, and possibly new applications in scalable digital manufacturing.

Computer clocks are responsible for nearly every modern convenience. Smartphones, DVRs, airplanes, the internet — without a clock, none of these could operate without frequent and serious complications. Nearly every computer program requires several simultaneous operations, each conducted in a perfect step-by-step manner. A clock makes sure that these operations start and stop at the same times, thus ensuring that the information synchronises.

The results are dire if a clock isn't present. It's like soldiers marching in formation: if one person falls dramatically out of time, it won't be long before the whole group falls apart. The same is true if multiple simultaneous computer operations run without a clock to synchronise them, Prakash explained.

A magnetic clock

Developing a clock for a fluid-based computer required some creative thinking. It needed to be easy to manipulate and also able to influence multiple droplets at a time. The system needed to be scalable so that in the future, a large number of droplets could communicate amongst each other without skipping a beat. Prakash realised that a rotating magnetic field might do the trick.

Prakash and graduate student Georgios 'Yorgos' Katsikis built arrays of tiny iron bars on glass slides that look something like a Pac-Man maze. They laid a blank glass slide on top and sandwiched a layer of oil in between. Then they carefully injected into the mix individual water droplets that had been infused with tiny magnetic nanoparticles.

Next, they turned on the magnetic field. Every time the field flips, the polarity of the bars reverses, drawing the magnetised droplets in a new, predetermined direction, like slot cars on a track. Every rotation of the field counts as one clock cycle, like a second hand making a full circle on a clock face, and every drop marches exactly one step forward with each cycle.

A camera records the interactions between individual droplets, allowing observation of computation as it occurs in real time. The presence or absence of a droplet represents the 1s and 0s of binary code, and the clock ensures that all the droplets move in perfect synchrony, thus the system can run virtually forever without any errors.

"Following these rules, we've demonstrated that we can make all the universal logic gates used in electronics, simply by changing the layout of the bars on the chip," said Katsikis. "The actual design space in our platform is incredibly rich. Give us any Boolean logic circuit in the world and we can build it with these little magnetic droplets moving around."

The results have been published in Nature Physics. The current paper describes the fundamental operating regime of the system and demonstrates building blocks for synchronous logic gates, feedback and cascadability — hallmarks of scalable computation. A simple-state machine including 1-bit memory storage (known as 'flip-flop') is also demonstrated using the above basic building blocks.

Manipulating matter

The current chips are about half the size of a postage stamp, and the droplets are smaller than poppy seeds, but Katsikis said that the physics of the system suggests it can be made even smaller. Combined with the fact that the magnetic field can control millions of droplets simultaneously, this makes the system exceptionally scalable.

"We can keep making it smaller and smaller so that it can do more operations per time, so that it can work with smaller droplet sizes and do more ... operations on a chip," said graduate student and co-author Jim Cybulski. "That lends itself very well to a variety of applications."

Prakash said the most immediate application might involve turning the computer into a high-throughput chemistry and biology laboratory. Instead of running reactions in bulk test tubes, each droplet can carry some chemicals and become its own test tube, and the droplet computer offers unprecedented control over these interactions.

From the perspective of basic science, part of why the work is so exciting, Prakash said, is that it opens up a new way of thinking of computation in the physical world. Although the physics of computation has been previously applied to understand the limits of computation, the physical aspects of bits of information have never been exploited as a new way to manipulate matter at the mesoscale (10 microns to 1 millimetre).

Because the system is extremely robust and the team has uncovered universal design rules, Prakash plans to make a design tool for these droplet circuits available to the public. 

Image caption: Stanford Assistant Professor Manu Prakash, left, and graduate students Jim Cybulski and Georgios Katsikis developed the water drop computer.