Pneumatic pumping of liquids using thermal transpiration for lab-on-a-chip applications

This paper reports, for the first time, the pumping of water in microfluidic glass channels due to a pressure difference generated by an integrated Knudsen pump. The Knudsen pump operates by the principle of thermal transpiration which generates a pressure difference along a channel with a thermal gradient, and whose channel height is on the order of the mean free path of air (60 nm at 1 atm). A two mask process using microscopic glass slides is used to form hydrophobic, sealed channels with two heights. Shallow channels (80 nm) are used for the Knudsen pump, and water flows in the deep channels (12 µm). The hydrophobic coating repels water from the channels until a thermal gradient is applied across the shallow channel, reducing the pressure in the shallow channel by thermal transpiration, and drawing water into the deep channel. This pump can be readily integrated into many other microfluidic platforms because it has no moving parts, uses low voltages, and requires no additional materials.