A Wireless Passive Capacitively Coupled Contactless Conductivity Detection (WPC4D) for Microfluidic Flow Monitoring

Using the capacitively coupled contactless conductivity detection (C4D) principle in microfluidic systems has attracted researchers from various fields. Different measurement methods using this structure in microchips have been developed to overcome the difficulties and limitations of the conventional C4D structures. In this study, we proposed and implemented a passive wireless method utilizing C4D structure to monitor the fluidic flow based on the principle of a passive wireless LC sensor combined with the microfluidic C4D structure. NaCl with different concentrations has been used in the experiments for evaluating the performance of the proposed structure. The dependence of the resonant frequency shifts on the conductivity of the fully filled channel and in-flow occurred micro-droplets were examined. The experimental results reveal that the detector works well to detect the NaCl concentration ranging from 0.01 M to 1 M. In particular, the resonant frequency of the sensing resonator decreases as the concentration of NaCl, i.e., the conductivity of the solution, increases. Based on the shift in resonant frequency and the acquired reflection coefficient S
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value, the generated in-flow micro-droplets can also be detected. This work demonstrates that the implementation of LC wireless sensing technique in fluidic flow detection is reasonable, and it will lay the foundation for future research on the miniaturization and simplification of sensing-microfluidic systems.