Microfluidic systems can enable multiple rapid reactions between chemicals in fluid form. But because forces resistant to fluid flow dominate on a microscopic scale, chemicals may mix together slowly and incompletely. To increase the speed of fluid flow, researchers have used a laser to generate bubbles that burst in microseconds, driving fluid flow.Reprinted with permission of Physical Review Letters.The scientists, from the University of Twente in Enschede, the Netherlands, and from Shimadzu Europa GmbH in Duisburg, Germany, noted that the technique is noninvasive and does not require wires or actuators.In their experiment, they created the bubbles with a pulsed and frequency-doubled Nd:YAG laser from New Wave Research Inc. of Fremont, Calif. The laser produced the bubbles by locally heating the fluid. Because the bubbles disappeared within microseconds, the researchers employed a camera from Shimadzu Corp. of Kyoto, Japan, that operated at 1 million fps. The camera was attached to a Carl Zeiss microscope.The bubbles increased fluid flow to speeds up to 20 m/s, 20 times faster than without the bubbles. The scientists also created fluid vortices that can be used for mixing chemicals. The vortices stirred the liquid at a rate of more than 10,000 rotations per second. The researchers also generated the bubbles in uniquely shaped channels, such as rectangles, squares and triangles (see figure). A square-shaped channel resulted in a bubble that resembled a four-leaf clover.The scientists detailed the results in the June 22 issue of Physical Review Letters.The researchers pointed out that increasing fluid flow with bubbles is applicable not just to microfluidics but also to laser-assisted angioplasty, and to diagnostic and therapeutic ultrasound.