A self-powered triboelectric microfluidic system for liquid sensing

Triboelectric nanogenerator (TENG) technology is expected to be applied in various areas, such as mechanical and chemical sensors, self-powered electrochemical systems, and electricity generation for personal electric devices. Ever since the triboelectric behavior between liquids and dielectrics has been demonstrated, liquid-based TENGs have been invented for energy harvesting and chemical sensors. However, previous researches utilized droplet-based TENGs, thus requiring surface treatments, such as superhydrophobicity, to effectively generate triboelectric signals. In this study, we utilized the electrical reaction of liquids with an external electric field generated by the triboelectrification for liquid sensing without additional surface engineering. To evaluate the sensing performance, we fabricated a polydimethylsiloxane (PDMS)-based microfluidic channel and injected different liquids, such as water, NaCl solutions, and CrO3 solutions. The triboelectric outputs were noticeably decreased maximally injecting the liquids 6 times and drastically changed depending on the types of liquids. To analyse this behavior, we theoretically investigated the working mechanism of our liquid sensor based on the electrical behavior of ions and polar molecules. Furthermore, we measured the capacitance after injecting the liquids into the PDMS microfluidic channel and analysed the zeta potential of each liquid. Finally, we demonstrated a single electrode mode-based liquid sensor that could verify liquids with multiple methods, such as triboelectric signal analysis and resistance measurement. Based on the results, we demonstrated the possibility for the detection of liquid types and impurities in liquids. We expect that our PDMS-based microfluidic channel, as a multiplex liquid sensor, could provide a simple way to detect liquid types and impurities in liquids for biomedical and chemical industries.