A Microfluidic Viscometer With Capacitive Readout Using Screen-Printed Electrodes

Microfluidic co-flow viscometers require the measurement of the interface position between two fluids (one sample and one reference fluid) flowing in parallel inside a microfluidic channel. From the interface position, the flow rates of both fluids and the known viscosity of the reference fluid, the viscosity of the sample fluid can be calculated. The detection of the interface position is usually done by optical means, therefore requiring optical access, and often coloring of the liquids or suspension of tracer particles to achieve optical contrast. In this contribution we present a device that is capable of determining the interface position in an alternative manner by capacitive measurements. The fully polymeric microfluidic chip is fabricated by simple processes requiring no specialized equipment. The capacitive sensor is fabricated by screen-printing. In order to enhance sensitivity, an insulation coating made from PMMA and barium titanate is applied. To assess the interface detection by capacitive means, measurements using different water/glycerol mixtures are performed and compared to optical images of the interface position, establishing a relationship between capacitance and interface position. The capacitive measurement does not require coloring of the liquids or suspension of tracer particles, if there is a dielectric contrast at the measurement frequency.