MICROFLUIDICS CAN BE DRIVEN BY HEAT rather than by electric fields. Microfluidics is to the mixing of fluids (including studies of blood, DNA, etc.) what integrated circuits are to the processing of electrical signals: transactions occur quickly, controllably, in a very small space. But instead of excavating small channels in a substrate and propelling tiny fluid volumes around the nano-sized system of aqueducts customary in microfluidics (see Update 367), Princeton professor Sandra M. Troian and Dawn Kataoka, now at Sandia Laboratories(CA), have moved tiny liquid rivulets around a silicon wafer using temperature gradients. The capillary movement of the micro-fluids can be programmed because (1) the liquid surface tension varies with temperature and even a gradient of 3 or 4 K will cause a fluid to seek out a cold region, and (2) a lithographically applied pattern of chemical modifications on the substrate (the equivalent of an invisible scent marker or a chemical levee) further constrains the droplet rivercourses. Thus streams of hydrophilic and hydophobic molecules, zooming across the substrate along neighboring lanes, can be shunted together at some desired meeting point. The advantages of thermo-capillary action over electronic-driven fluidics are that the use of high electric fields and the precision carving of channels are not necessary; everything happens on a plane, making easier the task of building micro-electromechanical (MEMS) "labs-on-a-chip." Troian (609-258-4574, stroian@princeton.edu) will report on her research at the APS division of fluid dynamics meeting in New Orleans, November 21-23: www.nd.edu/~apsnd/)