This paper presents an automated droplet metering and ejection system based on capacitive sensing for precise handling of minute liquid volumes in microfluidic applications. The system incorporates a miniaturized parallel-plate capacitive sensor fabricated using a four-layer PCB with an integrated shielding layer to minimize external interference. By detecting changes in the dielectric constant of the liquid, the sensor enables real-time volume monitoring within the microchannel. A NE555-based oscillation circuit converts capacitance variations into frequency signals, achieving high-response and delay-free detection. System control is implemented using an STM32 microcontroller, which coordinates real-time frequency tracking with pneumatic actuation and solenoid valves to realize pulsed liquid injection and quantitative dispensing. A sensor prototype with a full-scale range of 1 μL was developed and evaluated using liquids covering a wide range of dielectric constants, including deionized water (ε = 78.4), 4 mol/L NaCl solution (ε = 58), anhydrous ethanol (ε = 28), and fluorinated oil (ε = 2). Experimental results demonstrate high sensitivity across all tested media, reaching 5137 Hz/μL at ε = 2, with sensitivity increasing at higher dielectric constants. The system achieves a minimum sampling volume of 50 nL, with maximum sampling errors of ±10 nL for water and ±20 nL for fluorinated oil, confirming excellent adaptability and precision. An integrated pulsed gas-jet module enables controlled droplet ejection. This work provides a flexible and accurate liquid handling platform suitable for microfluidic analysis, bio-detection, and microreaction applications.

Microfluidics, Capacitive Sensor, Droplet Metering, Quantitative Sampling