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Non-contact volume determination of free flying nanoliter droplets using an adjustable capacitive measurement bridge

We present a novel electronic circuit for a capacitive non-contact volume sensor to be used for the determination of dispensed liquid droplets in the nanoliter range. Beside the ability of online droplet volume measurement the presented sensor features also sensitivity to certain droplet parameters like droplet velocity, liquid type and even small droplet deformations during the droplet's flight. The sensor principle is based on the capacitive change caused by a droplet while it passes an open plate capacitor. In contrast to the already published results [1, 2], the presented circuit amplifies the change of capacitance by a measurement bridge that feeds a differential amplifier. The required bridge regulation is realised by an adjustable analogue voltage divider combined with an all-pass filter to modulate the reference signal in phase and amplitude. The implementation of the analogue adjustment is crucial to enable the required high sensitivity. The circuit is able to amplify changes in capacitance in the range from ▵C ≈ {0.6 to 2.7 fF} which are caused by single droplets of volumes of V ≈ {20 to 70 nl}). The output voltage signals can reach up to Umax= 2.3V. The sensor sensitivity to the droplet volume reaches Si = 82 mV/nl with an accuracy of ▵V = ± 4 nl. With this sensitivity even droplet deformation occurring within the measurement capacitor can be observed as multiple signal peaks. These are caused from lateral (towards the electrodes) and longitudinal extension of the droplet's shape that influence the effective capacitance by ▵Cmax≈ 0.2 fF.