Dynamic Viscosity Measurements of Colloids Using Quartz Crystal Resonators

The objective of this work is to understand the relationship between the rheological properties of complex fluids like colloidal dispersions and the response of a Thickness Shear Mode (TSM) Quartz-Crystal Resonator (QCR). Such fluids are often non-Newtonian because their steady shear viscosities are shear-rate dependent and because they exhibit viscoelastic behavior. Due to practical interests in obtaining viscosity information at high shear rates, in the order of 10 s\u27 , the focus of this work is on corresponding high frequencies, in the order of 10 MHz, oscillatory measurements. The use of piezoelectric devices, like the TSM-QCR, is expected to provide relevant viscoelastic information directly with additional possibilities of fast-response for in-line or on-line process sensing. The TSM-QCR frequency response was analyzed to obtain complex viscosities of several colloidal dispersions over the frequency range of 3 - 50 MHz. The test dispersions included latex suspensions, coating formulations, and polymer solutions. The effects of flow rate, concentration, and particle size are reported. The measurement technique was validated using water and three alcohols as reference liquids. The average deviation between experimental and predicted values for the magnitude of the complex viscosity |η*| as a function of frequency for the reference fluids was ~10%. The TSM- QCR is shown to be sensitive to the properties of the test suspension as a whole, and not the suspending medium alone. The |η*| vs. frequency response from TSM-QCRs operating at high frequencies (\u3e 1 MHz) showed the same continuing decreasing trend as the response from rotational rheometers operating at low frequencies (\u3c 100 Hz). Comparison of shear-rate dependent steady-shear viscosity from high pressure capillary rheometer with the |η*| as a function of angular frequency from TSM-QCR shows consistent trends as expected from the Cox- Merz rule. The concentration effect on |η*| of a coating formulation followed a Krieger- Daugherty type relationship. Flow-through operation showed that the |η*| drops by 25% when NRe changes from 0 to 500 and drops to 40-60% when NRe reaches 2100. This study shows promise of TSM-QCR for characterizing high-frequency complex viscosity of practical colloidal dispersions in static as well as flow-through conditions