We demonstrate the active manipulation of nanoliter liquid samples on the surface of a glass or silicon substrate by combining chemical surface patterning with electronically addressable microheater arrays. Hydrophilic lanes designate the possible routes for liquid migration while activation of specific heater elements determines the trajectories. The induced temperature fields spatially modulate the liquid surface tension thereby providing electronic control over the direction, timing, and flow rate of continuous streams or discrete drops. Temperature maps can be programed to move, split, trap, and mix ultrasmall volumes without mechanically moving parts and with low operating voltages of 2-3 V. This method of fluidic actuation allows direct accessibility to liquid samples for handling and diagnostic purposes and provides an attractive platform for palm-sized and battery-powered analysis and synthesis. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1537512͔Miniaturized automated systems for liquid routing, mixing, analysis, and synthesis are rapidly expanding diagnostic capabilities in medicine, genomic research, and material science.1 Liquid flow in microchannels can be regulated by pressure gradients, 2 thermocapillary pumping, 3 electrokinetic forces, 4,5 or magnetohydrodynamic pumping.6,7Electrowetting 8,9 and dielectrophoresis 10 have also been used to move droplets on an open surface. These techniques typically require high operating voltage and high electrolyte concentrations. We demonstrate a different method for liquid handling and transport that uses programmable surface temperature distributions, in conjunction with chemical substrate patterning, to provide electronic control over the direction, timing, and flow rate. This method capitalizes on the large surface-to-volume ratio inherent in microscale systems since the gas-liquid and liquid-solid surface energies are modulated to induce and confine flow. It works equally well with polar or nonpolar liquids, requires no mechanically moving parts and operates at very low voltages. The open architecture is best suited to liquids of low volatility. Encapsulation schemes that retain the free liquid-air interfaces can minimize evaporative loss.Local heating of a liquid film at a position x reduces the surface tension ␥(x) to produce a thermocapillary shear stress •n ϭ"␥ϭ(ץ/␥ץT)"T that pulls liquid toward regions of cooler surface temperature T. [11][12][13] Since Tץ/␥ץ is essentially temperature independent for all liquids, the driving force and flow direction are proportional to "T. For a thin flat liquid layer, the average flow speed and flow rate ͑per unit width͒ are given by (x)ϭh(x,t) •n /2(x) and Q(x,t)ϭh(x,t)(x,t), where h(x,t) is the film thickness and (x) the local viscosity. This phenomenon forms the basis of our microfluidic device for actuating continuous streams and discrete droplets.The sample layout 14 for driving continuous streams is shown in Fig. 1͑a͒. Hydrophilic stripes connect pairs of 4.5-mm-wide square reservoir pads; the...