Thermocapillary pumping of discrete drops in microfabricated analysis devices

A nonmechanical pumping mechanism, thermocapillary pumping (TCP), is descri bed for moving nanoliter- and picoliter-sized drops of liquid within microf abricated flow channels. In TCP, one end of a single drop is heated to crea te a surface tension difference between the ends of the drop. The induced s urface tension difference causes a capillary pressure difference between th e two drop ends and results in drop motion. TCP velocities of up to 20 mm/m in were measured for several liquids at temperature differences between 10 and 70 degrees C. An expression developed for TCP velocity yields predictio ns that agree with experimental velocities within corresponding uncertainty limits. Several techniques for assisting TCP are also suggested when conta ct angle hysteresis, the major factor limiting TCP velocities, is too large . These techniques include using surface treatments to reduce the contact a ngle hysteresis, converging channels to offset hysteresis, or an applied pr essure to assist in movement.