Viscosity and Density Measurements of Glycerol-Water Mixtures Utilizing a Novel Resonant MEMS Sensor

We present a resonant MEMS sensor for viscosity and mass density measurements of liquids. The device is based on Lorentz-force excitation and has an integrated piezoresistive readout. The sensing element is a rectangular vibrating plate suspended by four beam springs. Due to in-plane vibrations, the plate contribution to the overall damping is low. Additionally, the plate increases the moving mass of the sensor and therefore the quality factor of the resonant system. Two of the plate-carrying springs comprise piezoresistors. With two additional resistors on the silicon rim, they form a half Wheatstone-bridge. Through the conductive layer of the beam springs a sinusoidal excitation current is driven. In the field of a permanent magnet, the Lorentz force excites plate vibrations resulting in a bridge unbalance. The sensor was experimentally tested with glycerol-water mixtures at constant temperature. We recorded both the frequency and the phase of the bridge output. By evaluating the properties of the resonant system, it is possible to extract the viscosity and the mass density of the mixtures simultaneously.