Contactless sensing of the conductivity of aqueous droplets in segmented flow

We present an electrode arrangement for the inline measurement of the conductivity of droplets in segmented flow by impedance spectroscopy. We use a thin-walled glass capillary with electrodes contacting the outer surface, so that the contactless measurement of conductivity of the liquid within the capillary is possible. The surface of the glass capillary is silanized resulting in a single hydrophobic surface across which droplets can freely move. We model the impedance of such insulated electrodes and use the model to optimize the electrode system. Measurement of solutions with various salt concentrations allows the performance of the electrode structure to be characterized. Subsequently, the measurement of the impedance response of the aqueous segments in two-phase flow was demonstrated. Measurements were firstly performed with an impedance analyzer and subsequently with a multi-sine measurement setup that is better suited to high-speed measurement of droplets. Previous electrical measurements of segmented flow sensed the difference in dielectric constant between the aqueous phase and the carrier fluid through measurement of capacitance. This work describes an electrical measurement of the conductivity of droplets in segmented flow, that is, the sensor senses a variable property of the droplet itself.