Liquid Viscosity and Density Measurement with Flexural-Plate-Wave Sensors

摘要

Micromachined flexural plate wave (FPW) sensors can detect subtle variations inthe density and viscosity of liquids. Liquid density causes a mass-loading effect that lowers the sensor operating frequency. In viscous liquids, the attenuation coefficient of the acoustic wave is proportional to the square-root of the viscosity. We discuss recent results that explore the density-sensing precision and the viscosity-sensing range of FPW sensors. A micromachined FPW sensor (wavelength 100 micrometers, plate thickness 6.0 micrometers, operating frequency in water 5.7 MHz) detected the density of a variety of solvents and aqueous salt solutions with an error less than 0.003 g/(cu.cm) Most of this error was likely due to the uncertainty with which we know the true density and sound velocity of each sample. The same sensor was used to measure the viscosity of aqueous polymer solutions (poly(ethylene glycol) and hydroxyethyl cellulose). The sensor accurately measured the viscosity of solutions with molecular weights up to around 10,000. Polymer solutions with MW >20,000 appeared water like to the sensor (i.e., there was little or no attenuation of the acoustic waves).The degree in apparent viscosity measured with the FPW sensor occurs for polymers with relaxation frequencies greater than or equal to the FPW frequency.