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Published May 1, 2004 | public
Journal Article Open

Capacitive sensing of droplets for microfluidic devices based on thermocapillary actuation


The design and performance of a miniaturized coplanar capacitive sensor is presented whose electrode arrays can also function as resistive microheaters for thermocapillary actuation of liquid films and droplets. Optimal compromise between large capacitive signal and high spatial resolution is obtained for electrode widths comparable to the liquid film thickness measured, in agreement with supporting numerical simulations which include mutual capacitance effects. An interdigitated, variable width design, allowing for wider central electrodes, increases the capacitive signal for liquid structures with non-uniform height profiles. The capacitive resolution and time response of the current design is approximately 0.03 pF and 10 ms, respectively, which makes possible a number of sensing functions for nanoliter droplets. These include detection of droplet position, size, composition or percentage water uptake for hygroscopic liquids. Its rapid response time allows measurements of the rate of mass loss in evaporating droplets.

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© The Royal Society of Chemistry 2004 Received 5th December 2003, Accepted 4th May 2004. First published on the web 25th June 2004 The authors gratefully acknowledge financial support for this work from the National Science Foundation (CTS and DMR divisions), US Army TACOM-ARDEC and NASA's Microgravity Fluid Physics Program.


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