Thermocapillary pumping of discrete drops in microfabricated analysis devices

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Abstract
A nonmechanical pumping mechanism, thermocapillary pumping (TCP), is described for moving nanoliter‐ and 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 velocities of up to 20 mm/min were measured for several liquids at temperature differences between 10 and 70°C. An expression developed for TCP velocity yields predictions that agree with experimental velocities within corresponding uncertainty limits. Several techniques for assisting TCP are also suggested when contact angle hysteresis, the major factor limiting TCP velocities, is too large. These techniques include using surface treatments to reduce the contact angle hysteresis, converging channels to offset hysteresis, or an applied pressure to assist in movement.

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