A micromachined AC thermal sensor for monitoring the liquid–gas interface in a microchannel

A novel micromachined sensor for monitoring the liquid–gas phase interface in a microchannel is developed based on the three-omega (3ω) thermal-sensing method. As the sensor employs an AC hot-film technique to probe the position and movement of the phase interface, it is more sensitive and noise-resistant than the existing DC technique. Accordingly, it can monitor stagnant or slowly moving interfaces effectively. The sensor is composed of a thin-film heater integrated into a PDMS (polydimethylsiloxane) microchannel fabricated on a glass substrate. The performance of the sensor is analyzed by examining water injection and liquid evaporation processes. The results show good agreement with those by optical inspection. In particular, it is demonstrated that the sensor can effectively monitor a long-term evaporation process taking place over a time scale longer than an hour. These results demonstrate strong potential of the proposed sensor as an integrated real-time probe to monitor a variety of flow features occurring in microfluidic systems.