On the Frequency and Q Factor Response of the Quartz Crystal Microbalance to Liquid Overlayers
Doctoral thesis, 1995
The quartz crystal microbalance (QCM) is an ultra-sensitive weighing device. It consists of a piezoelectric quartz crystal, often in the form of a disk, which is sandwiched between a pair of evaporated electrodes. When these are connected to an electronic oscillator, the crystal can be made to oscillate in a very stable manner at its resonant frequency, f, due to the piezoelectric effect. If a thin, rigid film is deposited evenly over one or both of the electrode surfaces in such a way that it does not slip on the surface, the resonant frequency decreases proportionally to the mass of the film. By measuring the resonant frequency, masses well below 1 ng/cm2 can be gauged. Traditionally, the QCM has been used in gaseous or vacuum environments but has lately been employed as a mass sensor in liquids. However, in a liquid, several other factors besides the deposited mass may influence the resonant frequency, and the linear mass to frequency relationship might no longer hold.
A large fraction of this thesis work has been devoted to developing techniques to simultaneously and reliably measure the resonant frequency and the Q factor (inversely proportional to the power dissipation) of a QCM operating in either air or liquid. The other part of the work has concerned applications of the QCM technique where the change in resonant frequency may not be linearly related to the change in deposited mass. These include the deposition of a mass that (i) is viscous (i.e., not rigid), (ii) is slipping on the surface, (iii) is not deposited evenly over the electrode(s), (iv) causes a change in the dielectric constant or conductivity of the medium contacting the QCM, and/or (v) changes the coupling between the crystal motion and the surrounding liquid. It is shown that in some of these cases, it is still possible to obtain a reliable measure of the deposited mass if the Q factor is measured simultaneously with the resonant frequency.
The thesis is based on six papers. Papers I and II describe methods to simultaneously measure the Q factor and the resonant frequency of a QCM. Paper III treats how films that are not rigid and may slip on the QCM electrode(s) influence f and Q. Paper IV shows how liquid deposits that are not deposited evenly over the electrode surface affect f and Q. Paper V discusses how the electrical properties (dielectric constant and conductivity) of a liquid in contact with a QCM influence f and Q. Finally, Paper VI is an in situ study of protein adsorption onto a thiol modified gold surface using the QCM. The latter paper shows, among other things, how the knowledge from Papers I-V is used in a 'real' measurement situation. Generally, the results of this thesis significantly advance the understanding of how a QCM operates and can be utilized in liquid phase measurements.
quartz crystal microbalance