CHARACTERIZATION OF A QUARTZ CRYSTAL MICROBALANCE WITH SIMULTANEOUS MASS AND LIQUID LOADING

With a continuum electromechanical model, the electrical admittance has been derived for an AT-cut quartz crystal microbalance (QCM) simultaneously loaded by a surface mass layer and a contacting Newtonian liquid. This admittance expression is used to derive a lumped-element equivalent circuit model that describes the near-resonance electrical characteristics of the QCM under these loading conditions. The resulting model is a modified Butterworth-Van Dyke equivalent circuit, having circuit elements that are explicitly related to physical properties of the quartz, perturbing mass layer, and contacting liquid. The effects of mass and liquid loading on the fundamental and third harmonic resonances are predicted from the model and compared with experimental impedance analyzer measurements. Surface mass accumulation causes a simple translation in frequency of the resonance peak, while increasing the density-viscosity product of the contacting solution causes both a translation and a damping of the resonance peak. With the model, changes in surface mass can be differentiated from changes in solution properties.