Application of Quartz Crystal Microbalance to Measure the Rheology of Complex Colloidal Suspensions

The United Kingdom nuclear industry is currently experiencing challenges relating to the safe transfer and processing of legacy waste sludge due to its complex physical and chemical properties. The radioactive nature of the sludge makes conventional methods used to characterise its rheology costly. The use of a quartz crystal microbalance (QCM) to measure the rheological properties of concentrated suspensions is considered. The device is small, portable, and can provide rheological data in-situ, allowing for significant cost reductions to be made. The QCM consists of a gold-coated piezoelectric quartz crystal oscillating in the MHz-range when an alternating current is supplied to it. When the QCM is immersed into a medium, a shift in the sensor oscillation frequency (ΔF) is observed, where the magnitude of the shift is proportional to the bulk properties of the medium and the nature of its contact to the resonator surface. In most cases an increased dampening due to a loss in vibrational energy is also observed, i.e. a shift in circuit resistance across the sensor (ΔR), or a broadening in the electrical conductance vs. frequency curve (ΔΓ). When the QCM sensor is submerged into concentrated sludge at fixed pH, ΔΓ or ΔR is proportional to the shear yield stress of the sludge measured using conventional vane viscometry. A stronger suspension network therefore increases both the QCM sensor dampening as well as the suspension bulk yield stress. For sludge composing of large surface area TiO2 (anatase) particles that are sensitive to pH changes, the QCM ΔF and ΔΓ or ΔR response correlated well with changes in the theoretical particle-sensor interaction strength using the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory. No significant resonance shifts were observed at suspension pH values where the particle-sensor interaction is expected to be repulsive. The change in QCM response with particle-sensor interaction allows for the charge characteristics of suspension particles to be inferred. The QCM response when in concentrated suspensions can be explained via the 'point contact load' model based on these findings. Concentration tests using zinc oxide particles was performed at different resonance frequency overtones (5 - 55 MHz initial frequency). An overtone dependency that cannot be explained by the point contact model alone was observed. The QCM data was compared to viscoelastic models where the raw data fitted well with model predictions. The high frequency storage modulus of the sludge correlated well with conventional rheometry data. The QCM response when it is submerged into a concentrated sludge can therefore be described through a combination of the point contact load model and the viscoelastic model, where the relative contribution from each depends on the physical and chemical composition of the suspension particles and the nature of the experiment involved.