Chipping in to microfluidics

physics.ubs.ca Imagine stepping off the edge of a swimming pool, only to find that your foot deflects the surface of the water without breaking it, as if held by some impenetrable skin. As you walk forward, the water continues to support you; but if you take a running leap and bring your full weight down on the surface, then it snaps open to envelop you without a splash. Rather than plunging to the bottom of the pool, however, you stop abruptly as your kinetic energy is instantaneously dissipated in the fluid. You flail your arms in an attempt to return to the pool's edge but make no progress, merely bouncing back and forth with each stroke. While such an experience would come as some surprise to a human used to experiencing life on the macroscopic scale, this is precisely how fluids behave when confined to micrometre-wide channels. On the micro-scale, surface tension and viscosity dominate fluid dynamics, as our imaginary swimmer would discover (see box on page 27). These phenomena cause the chaotic turbulence that characterizes macroscopic flow to disappear and be replaced by "laminar flow" in which fluid flows in parallel layers with little or no mixing between them. Physicists, chemists and biologists are seeking to exploit the novel physical properties of this microfluidic regime for applications ranging from materials synthesis to drug discovery. Handling liquids is a big part of nearly all experimental chemistry and biology, and it is usually carried out with traditional hand-held micropipettes or robotic liquid-handling systems. The advantage of microfluidic technology is that it allows far smaller, sub-nanolitre, volumes of precious reagents to be manipulated precisely. This economy of scale means researchers can carry out exhaustive experiments that would otherwise be prohibitively expensive. Microfluidic technology also allows experiments to be miniaturized and automated on compact "lab-on-a-chip" systems, thereby freeing scientists from repetitive work while making experiments more accurate and reproducible. Less than 20 years old, this idea is transforming biological and medical research in the same way that the miniaturization of electronics has benefited computation, and it is set for wider applications in clinical and environmental sensing. But microfluidic research means much more than just miniaturizing chemical and biological experiments. The unique physical properties of the microenvironment, including laminar flow, allow experiments that are difficult or impossible in macroscopic systems, ranging from the analysis of single cells to highly efficient mixing for protein crystallization. And recent work that incorporates droplets and bubbles into microfluidic systems is even opening up a new world of chemical computation in which digital bits of information and chemical payloads are carried in the same package.