Nonintrusion Monitoring of Droplet Motion State <i>via</i> Liquid–Solid Contact Electrification

Droplet motion state monitoring is important in microfluidic applications, such as biomedicine, drug delivery, and metal ion extraction. Here, a nonintrusion monitoring method of droplet motion state via liquid-solid contact electrification is proposed, and a triboelectric droplet motion state sensor (TDMSS) is fabricated. Droplet counting and droplet size monitoring can be realized by signal processing and information extracting of the voltage pulse. The experimental results show that the number of droplets increases linearly with the increase of liquid flow, and the linearity is 0.9854. Moreover, TDMSS can stably monitor the number of droplets in different motion states. In addition, the output pulse width is sensitive to droplet size, and the droplet length ranges from 3 to 13.5 mm. More importantly, TDMSS can realize the function of droplet counting and size monitoring of a conductive liquid and accurate droplet counting under different inclination angles and motion states. This work not only provides a nonintrusion method for droplet motion state monitoring but also makes a solid step for microfluidic sensing technology based on a triboelectric nanogenerator.