Scientific Method —

Computing with bubbles

Researchers develop microfluidic logic gates.

Microfludic devices were a subject of extreme hype back in the 80s and 90s. Although devices with a single purpose have been commercialized, the promised general analytical "lab-on-a-chip" never emerged. One of the key reasons for this is control. A general device requires that one can change where the fluid goes and what processes happen to it. That requires programming, which is generally done with electronic devices, while control is obtained through the thermal and mechanical properties of the device and fluid. Unfortunately, the coupling between the electronic control signal and the fluidic control mechanisms is not that great, meaning your fluid control is less than perfect.

To make an end-run around this problem, a group of researchers have developed logic gates that use bubbles as their ones and zeros. Simple logic gates are made by dividing the tube in two and directing the bubbles into either tube. The flow regime is such that the surface tension holding the bubble together prevents it from breaking up and the decision (e.g., which tube to take) is determined by speed of the flow in each tube as the bubble hits the entrance. Since the presence of bubbles in the tube changes the pressure, they also change the flow rate. That means that one bubble can control another bubble, simply by being there. There are some complications: unlike electrical circuits which can add or remove current to generate ones and zeros, the microfluidic version conserves the number of bubbles. Thus a logical AND gate presented with a bubble and no bubble will still emit a bubble (oops). To generate logical functions and preserve all the bubbles, the researchers had to combine logical gates, such as AND and OR. They demonstrated a whole lot of different logical functions such as AND-OR, NOT-AND, a flip-flop, and a ring oscillator.

The ultimate goal is to use the bubble logic to control the where the bubbles go, which then reduces the problem of controlling a general purpose lab-on-a-chip device to a single electrical-fluid coupling, in the form of a programmable bubble generator.

A fascinating bit of technology, but I must warn everyone: those movies are hypnotic.

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