Simultaneous and continuous measurement of shear elasticity and viscosity of liquids at multiple discrete frequencies

Abstract

This paper presents the extension of the classical torsional resonance viscometer towards the characterization of linear viscoelastic fluids. By simultaneously tracking multiple resonance frequencies, shear viscosity and elasticity can be monitored at multiple discrete frequencies at the same time. The proposed method is applied to the well-established sensor design of a simple rod-like structure, hence enabling a robust and versatile measurement system that can be used in different applications, such as in-line measurement or hand-held devices. In order to simultaneously control different resonance frequencies, multiple independent phase-locked loops are used, of which each subsystem is responsible for one natural vibration. An analytical description of the relationship between measurement parameters and physical fluid parameters is presented that is valid both for Newtonian and linear viscoelastic fluids. Consequently, calibration of the sensor is possible using Newtonian fluids only. To demonstrate the sensor, viscoelastic polymer and surfactant solutions are investigated at five frequencies between 2 kHz and 20 kHz. Additionally, the test fluids are characterized by means of a classical rotational rheometer (low frequency range) and diffusing-wave spectroscopy (high frequency range). The comparison of all methods shows very good agreement and validates the application of the sensor as resonant rheometer for viscoelastic fluids.