Thermocapillary Pumping of Discrete Drops in Microfabricated Analysis Devices

( ) A nonmechanical pumping mechanism, thermocapillary pumping TCP , is described for mo®ing nanoliterand picoliter-sized drops of liquid within microfabricated flow channels. In TCP, one end of a single drop is heated to create a surface tension difference between the ends of the drop. The induced surface tension difference causes a capillary pressure difference between the two drop ends and results in drop motion. TCP ®elocities of up to 20 mmrmin were measured for se®eral liquids at temperature differences between 10 and 708C. An expression de®eloped for TCP ®elocity yields predictions that agree with experimental ®elocities within corresponding uncertainty limits. Se®eral techniques for assisting TCP are also suggested when contact angle hysteresis, the major factor limiting TCP ®elocities, is too large. These techniques include using surface treatments to reduce the contact angle hysteresis, con®erging channels to offset hysteresis, or an applied pressure to assist in mo®ement.