Microchannels transport capillary flows

Pervaporation can also be used as a micropump. Isopropanol was placed inside the polyimide membrane microchannels described above, and water was deposited on top of the permeation area. The large selectivity for water transport over alcohol resulted in water permeation at a rate of 70 pm s [263]. In a similar study, a micropump was developed using pervaporation to transport a volume of 300 pL of Ringer s solution out of a membrane over a 6-day period. Capillary forces then induced additional flow into the membrane device to produce a very constant flow of 35 nL min [265]. Although this device did not utilize microchannel architectures, the low fabrication costs and high reliability of such a system make pervaporation an attractive approach to pumping small flow rates for microfluidic devices. 

In addition, microfiuidics makes use of phenomena not available in the macroscopic world. Well-predictable and controllable laminar flow patterns can be achieved by appropriate design of microchannels. Substance transport by diffusion becomes faster as dimensions shrink. Sample volumes are on the order of microliters to picoliters, and flow on the order of pl/min to nl/min can be realized, comparable to the sima-tion in capillary blood vessels. 

In these domains where biological and chemical targets are transported by liquids, capillary actuation of hquids does not require bulky pumps or syringes, or any auxiliary energy sources. The energy source for the flow is the surface energy of the microchannel walls. On the other hand, conventional forced flow laboratory systems require the help of bulky equipment, which is expensive and not portable. 

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