Rheometry Using Shear-Wave Resonators

The frequency limitation of conventional rheometers is commonly in the range of 100 Hz. This is not only due to instrument's inertia, also the emergence of wave propagation within the measurement gap requires more involved analysis methods. Mechanical plate resonators exhibiting dominant in-plane motion have been used for viscometry of Newtonian low-viscosity (<0.5 Pa.s) liquids [1]. There, the fluid extends beyond the shear-wave decay length, resulting in robust operation. Outstanding features are low sample volume (20 µl) and excellent sensitivity for low viscosities. We extended the method to viscoelastic fluids [2] and aim at extending the viscosity range to several Pa.s. An optical displacement sensor was implemented which is sensitive enough to measure the oscillation at frequencies below the primary resonance (device specific between 1 and 10 kHz). In addition, the rheometer gap is rebuilt by placing a parallel reflecting surface at controllable distances below the shear-wave penetration depth. Ideally, the deformation profile comes close to a reflected, damped shear-wave [3]. We present analytical tools for calculating viscoelastic moduli as well as experimental results on selected test systems (aqueous polymeric and surfactant solutions, colloidal suspensions), and compare to data obtained using conventional rheometry. Acoustic streaming is observed at high driving amplitudes in low-viscous media, setting a limiting to usable displacement amplitudes.