Drop position sensing in digital microfluidics based on capacitance measurement

In this work, we present an electronic circuit able to sense the droplet position in Electro-Wetting On Dielectric (EWOD) systems. The drop position is determined measuring the equivalent capacitance of the EWOD electrode, whose value varies according to the presence of the fluid over the pad. In the presented system, the capacitance measurement is achieved through the 'Frequency Shift Oscillator' method: the EWOD electrode is inserted in an oscillator circuit, whose operating frequency is inversely proportional to the electrode capacitance value. A microcontroller, included in the system, counts the number of rising edges at the output of the circuit determining the oscillation frequency. The oscillator has been simulated and subsequently fabricated on a double layer printed circuit board. A very good agreement between simulations and experiments has been achieved. The value of obtained sensitivity is not lower than 1.2 kHz/pF that corresponds to a minimum detectable capacitance variation of 0.167 pF. This value is well below the variation of capacitance due to the presence of the droplet above the EWOD electrode and demonstrates the suitability of our circuit as a successful drop position sensor.