Real-time mass sensing and dynamic impact monitoring of printed pico-liter droplets realized by a thermal-piezoresistive self-sustained oscillator

In this work, we successfully demonstrated a real-time mass sensor utilizing a MEMS-based self-sustained thermal-piezoresistive oscillator (TPO) fabricated by a standard silicon-on-insulator (SOI) MEMS process. From the measured data recorded by the frequency counter, not only a real-time frequency change with respect to the added mass can be detected but the dynamic impact and solvent evaporation process of the droplets was also successfully observed. The proposed MEMS TPO mass sensor has several key features, including real time monitoring, fast response time, high mass sensitivity of 3.4 Hz/pg, and most importantly very high mass resolution of only 100 fg estimated from the TPO's Allan deviation. Therefore, the proposed technology is expected to benefit both the fundamental science (high-resolution mass sensors for physical, chemical, and bio fields) and the industrial applications (such as PM2.5 sensors).