A wonderful article in Science (press release and article): Manu Prakash and Neil Gershenfeld describe how to use bubbles in a microfluidic device to carry on-chip process control information, while also performing chemical reactions.
Microfluidics is a fascinating technology that deals with the control and manipulation of microliter and nanoliter volumes of fluids. Fluids flow through microfabricated channels (see figure), allowing the delivery of precise quantities of reactants and the precise control of chemical reactions.
Previously, control has been achieved via external valves and control systems. The authors present channel geometrics that exploit nonlinear behavior of bubbles in microfluidic flows to perform logic operations (e.g., "a bubble has arrived on channel A AND B") and to store bubbles. (E.g., the figure shows three AND-gates connected in a ring oscillator. A bubble flows clockwise around the ring until it joins a stream.)
Quoting the press release: "Controlling chemical reactions will likely be a primary application for
the chips. It will be possible to create
large-scale microfluidic systems such as chemical memories, which store
thousands of reagents on a chip (similar to data storage), using
counters to dispense exact amounts and logic circuits to deliver them
to specific destinations."