The quartz crystal microbalance: a tool for probing viscous/viscoelastic properties of thin films

Abstract

Techniques based upon the electrical response of the quartz crystal microbalance QCM have been widely used in laboratories as routine tools. In this article we present and discuss applications of the QCM or its variant, the electrochemical quartz crystal microbalance, EQCM to the viscoelastic characterization of lms. It is pointed out that correlations between the motion of quartz crystal and contacting lms and overlayers as well as the in uence of the electronic circuit on the electric state of the whole system are of fundamental importance in interpreting the results.

Aqueous Sol−Gel Process in the Silica−Metasilicate System. A Microrheological Study

“…As can be seen in Figure the fluid viscosity did not change appreciably during the whole process when compared with results from usual experiments (near the gelation point, sudden viscosity changes from ∼1 mPa s to ∼10 2 mPa s or more have been reported; see ref , pp 304−311). To understand this result it should be noted that in our experiment the amplitude of the shear motion induced by the vibrating crystal was very small (using the physical model of ref we can estimate that the amplitude was on the nm scale, see also ref ). Thus, by preserving the structure of the three-dimensional gel network, the shear motion with small amplitude dissipated energy essentially in the liquid retained by the elastic structure.…”

“…The purpose of this section is to present a brief description of a modified quartz crystal microbalance and show how it can be used to monitor minute mass changes as well as changes in the rheological properties of a contacting fluid medium. For reviews on the quartz crystal microbalance applications we refer to refs , . The microbalance used in our laboratory consists of a thin circular plate cut from an oriented piezoelectric quartz crystal (specifically an AT-cut plate).…”

“…Near resonance the electrical behavior of the compound oscillator (crystal and contacting media) corresponds to that of a series connection of an inductor ( L ‘), a capacitor ( C ‘), and a resistor ( R ‘), known as the motional arm of the equivalent electrical circuit of the oscillator. Due to piezoelectricity, inductance L ‘ is related to oscillating masses, capacitance C ‘ to elastic properties and resistance R ‘ to dissipation in the system. , At the selected resonance frequency, f res = 2π/( L ‘ C ‘) 1/2 the electrical impedance of the oscillator is essentially resistive ( R ‘). The special features of the driver used in our laboratory permit monitoring changes of both the resonant frequency and the resonant resistance R ‘ during a process such as that analyzed in Section 4.…”

“…More specifically, they are helpful in evaluating the mechanical properties of the monitored system in our experiment because the time scale for the kinetics of film formation on the electrode was quite small compared to that for the system's evolution from the sol state (viscous fluid) to the gel state (viscoelastic fluid). Another way to infer the rheological properties of the contacting layer from the EQCM data is to consider the physical model for the compound oscillator. , This approach is based on the physics of wave propagation through the contacting media, namely, air (viscous fluid of low viscosity), quartz crystal (elastic medium), rigid film of negligible thickness, and semi-infinite viscous (sol) or viscoelastic (gel) layer. Thanks to the coupling between the elastic and electrical properties of quartz, it is possible to evaluate the electrical impedance of the compound oscillator, as a function of the parameters characterizing the film (deposited mass, m f ) and the fluid layer (complex shear modulus, G ).…”

“…The rheological properties of the fluid system and the mass of the film deposited initially on the electrode surface were evaluated with the help of the physical model for the EQCM, , as described in Section 2. In the calculation we considered the values for the electromechanical parameters of quartz, those for the crystal geometry, and the experimental value for the solution density, ρ = 1.06 × 10 3 kg/m 3 .…”