Bubble power holds key to micro-pumps

Making tiny gears and valves to miniaturise that most basic of machines, the pump, is a major headache for engineers developing micro-devices such as fuel cells for cellphones. Now scientists at Johns Hopkins University in Baltimore, Maryland, have side-stepped the problem by using collapsing bubbles to drive liquid around a device small enough to be integrated into a chip.

Zhizhong Yin created the "blinking-bubble" pump in a 120-micrometre-wide liquid channel by warming a section towards one end of the channel to form a vapour bubble. As the bubble grows, it splits the liquid into two unequal parts. Then the heat is turned off, the bubble collapses and the two columns of fluid rush towards each other to fill the gap. Because the shorter column is lighter, it picks up more speed than the longer end. "It's counter-intuitive because the shorter liquid column pushes the longer one each time, like a hammer hitting a nail," says team member Andrea Prosperetti. The "blinking" as new bubbles grow and collapse creates the pumping action.

"Any miniaturised hydraulic system needs a micro-pump," says Prosperetti. And while "lab-on-a-chip" devices have to be chained to macro-sized syringe pumps at the moment, micro-pumps could make them portable.

The blinking-bubble pump could be used in a variety of applications, says Surya Raghu of Advanced Fluidics in Maryland, a company which makes micro-fluidic devices. It could be used to drive methanol and water around micro fuel cells that might one day replace batteries in portable electronic devices, he says. "It gets past the hurdle of having to assemble many pump components on such a small scale - a real challenge in the development of mmicro fuel cells."

Raghu also suggests that the micro-pump would be ideal for pumping coolant fluids over electronic chips, which would be more efficient than using air fans for cooling. The lack of moving parts means that the micro-pump would also be reliable and cheap to make, he adds.