Fluid dynamics, Stanford engineers develop a computer that operates on water droplets
Stanford Assistant Professor Manu Prakash, left, and graduate students Jim Cybulski and Georgios Katsikis developed the water drop computer. Photo by Kurt Hickman (courtesy Stanford University).

Researchers have developed a logic machine that uses tiny water droplets as bits of information to be processed. The project is led by Manu Prakesh, an assistant professor of bioengineering at Stanford University and the results were recently published in Nature Physics.

Humans share music by playing an instrument or singing, but also using notations on paper, grooves in vinyl or cascading trains of “0s” and “1s.” The “logic” of the composition, within a particular musical conception, persists through the various mediums.

In the same way, a logic machine, the essential core of a universal computer, can be assembled of various materials. There’s apparently one inside our brains and collective intelligence, at least potentially, and we find them in PCs, laptops, tablets and phones — albeit for the most part tied to specific applications. Many of us are familiar with the idea that a computer can even be assembled using nanomaterials.

What could be the advantage of a “water-based” computer, compared to one assembled from electronic components?

Imagine, if when running computations, Prakesh says, not only information is processed but physical matter is algorithmically manipulated as well. What if, he says, instead of running reactions in bulk test tubes, each droplet carried some chemicals?

Digital into physical

The work being done at Stanford demonstrates the feasibility of processing at the meso scale (10 microns to 1 milliliter). Applying its expertise in manipulating droplet fluid dynamics and knowing what a computer’s operating clock is about, the lab demonstrated “a synchronous, universal droplet logic and control.” 

The team built arrays of tiny iron bars on glass slides, and looking something like a Pac-Man maze. They laid a blank glass slide on top, sandwiched an oil layer between and injected individual water droplets infused with magnetic nanoparticles.

Then they turned on a magnetic field. Every time the field flips, the bars’ polarities reverse, drawing the magnetized droplets in a predetermined direction, like slot cars on a track. Every magnetic field rotation counts as one clock cycle, and every drop marches one step forward with each cycle. Droplet presence or absence indicates the 1s and 0s of binary code.

The lab has demonstrated it can make all the universal logic gates used in electronics by changing the layout of the bars on the chip, including any Boolean logic circuit in the world. It’s demonstrated building blocks for synchronous logic gates, feedback and cascadability. A simple-state machine includes 1-bit memory storage.

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

Future directions

“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 a greater number of operations on a chip,” says graduate student and coauthor Jim Cybulski. “That lends itself very well to a variety of applications.”

Prakash says the most immediate application might involve turning the water-droplet computer into a high-throughput chemistry and biology laboratory. Each droplet becomes its own test tube, with the droplet computer having unprecedented control over the interactions.

The work suggests novel ways of thinking about computation in the physical world, apart from the “physical” limits of computing, and new ways to exploit matter at the mesoscale. Prakash intends to make a droplet-circuit design tool available to interested parties.

“We’re very interested in engaging anybody and everybody who wants to play, to enable everyone to design new circuits based on building blocks we describe in this paper, or discover new blocks. Right now, anyone can put these circuits together to form a complex droplet processor with no external control — something that was a very difficult challenge previously,” Prakash says.

Final summation

The abstract published by Nature Physics provides a few additional details. Water droplets are easy to produce, perform chemical reactions as miniature beakers and carry biological entities, the paper points out. They are manipulated with electric, optical, acoustic and magnetic forces, but these serial-control methods address individual droplets.

Algorithmic manipulation based on logic-operations, on the other hand, automatically computes where droplets are stored or directed, enabling parallel control.

Logic previously implemented in low-Reynolds-number droplet hydrodynamics is asynchronous and thus prone to errors that prevent scaling of operational complexity.

A platform for error-free physical computation via synchronous universal logic uses a rotating magnetic field to enable parallel manipulation of arbitrary numbers of ferrofluid droplets on permalloy tracks. Through the coupling of magnetic and hydrodynamic interaction forces between droplets, researchers developed AND, OR, XOR, NOT and NAND logic gates, fanouts, a full adder, a flip-flop and a finite-state machine.

The platform enables large-scale integration of droplet logic, analogous to the scaling seen in digital electronics, and, conclude the researchers, opens new avenues in mesoscale material processing.