(95h) Quartz Crystal Microbalance As a Technique to Probe the Rheological Properties of Particulate Suspensions
World Congress on Particle Technology
Monday, April 23, 2018 - 3:30pm to 3:45pm
QCM is a simple mechanical device where an AT-cut quartz sensor is driven into oscillation in the MHz range (5 MHz in the current study). The oscillatory motion of the sensor and the dampening of the sensor following excitation can provide great insight to the physical properties of deposited and/or adsorbed layers, with the technique providing nano-gram resolution . Our previous work (to be discussed briefly in the current presentation) showed that the oscillatory resistance correlated to the shear yield stress of colloidal sediments. This was the first study to demonstrate that QCM can provide meaningful data when interacting with concentrated particle suspensions. The interaction potential between the sensor and suspension remained constant, and the yield stress of the suspension varied by increasing the particle concentration. The simple approach of submerging the resonating sensor into the sediment and measuring the air-to-sample resistance shift, showed that the magnitude of the resistance shift increased exponentially with increasing sediment yield strength â i.e. the motion of the sensor becomes increasingly constrained by the overlying sediment.
The current research explores the frequency and resistance responses of the QCM when the strength of interaction between the sensor and particle suspension is varied. The typical sensor used in the study is gold-coated and the particle type was TiO2 (titania). Titania was chosen because it is well defined in literature, and its anatase phase exhibits an isoelectric point at pH ~6.5, hence the particles were positively charged in acidic conditions and negatively charged in basic conditions. Across the pH range the gold sensor remained negatively charged with slight dependence on the pH . At a fixed solids concentration (16.2 vol%) the pH of the titania suspension was varied between pH 3 and pH 10 and the suspension shear yield stress varied between 0 Pa and ~64 Pa, with the maximum yield stress measured at the suspension isoelectric point. Submerging the QCM into the titania samples and measuring the frequency and resistance responses across the pH range, the trending responses were complex with the magnitude of the resistance peaking (+ve) at pH 4, decreasing to pH 6.0, and remaining independent of pH in basic conditions. While the suspension yield stress is changing, the response of the QCM appears to be more sensitive to the interaction potential between the sensor and the overlaying suspension. Atomic force microscopy showed that the attractive force between a titania sphere and QCM gold sensor decreased as the pH was increased from pH 4 to 6.5. Beyond this pH both surfaces were negatively charged and the overall interaction potential was purely repulsive, with the colloidal system stabilized by electrical double layer forces. As such, in basic conditions both the frequency and resistance of the QCM sensor were independent of the suspension pH, thus the response did not track the changes in suspension yield stress. This series of experiments highlights the importance of a strong (positive) interaction between the sensor and particle suspension to measure suspension rheological properties. However, it does also highlight the ability of QCM to measure interaction forces between colloidal particles and the sensor.
Finally, the gold-coated sensors were coated with a 200 nm thick layer of titania. The sensor resistance showed a peak value at the suspension isoelectric point (pH ~6.5), indicating strong interaction between the suspension and sensor, with the resistance reducing as the pH became slightly more acidic and basic. Such response remains qualitatively consistent with the change in sediment yield stress, although the work remains ongoing to fully understand the QCM response.
The current research will for the first time present the application of QCM to probe the colloidal forces and rheology of concentrated particulate suspensions. Through further development and fundamental understanding of the QCM signal, the technique can be potentially applied to act as a simple QC rheometer.
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