Quartz crystal microbalance damping

Another tool used to study friction on the molecular scale is the quartz crystal microbalance (QCM) introduced in Section 9.4.1. The QCM has been used to monitor the adsorption of thin films on surfaces via the induced frequency shift [385], In the years since 1986, Krim and coworkers could show that the slippage of adsorbed layers on the QCM leads to a damping of the oscillator [472], This damping is reflected as a decrease in the quality factor Q of the oscillator. From the change in Q, a characteristic time constant rs, the so-called slip-time, can be derived. This corresponds to the time for the moving object s speed to fall to 1 /e, i.e. 

Figure 4.17 Shift and change of the resonance frequency of a quartz crystal microbalance, real part of the admittance versus frequency, /q, Wq, resonance frequency and full width at half maximum (FWHM) of the initial gold electrode,/j, w, resonance frequency and FWHM of a gold electrode after formation of a rigid and smooth surface film (no damping), resonance frequency and FWHM of a gold electrode after formation of a viscoelestic and/or rough surface film (strong damping).

The surface acoustic wave device (SAW) is an example of a transducer that is batch fabricated using IC technology and provides improved performance. SAW devices operate at much higher frequencies than the quartz crystal oscillator (or microbalance, QCM), and this results in improved detection limitsl, 29, 30 This can make measurements of absorption into films coating the SAW device possible, under circumstances where the QCM is insufficiently sensitive. On the other hand the ( M can be used in aqueous systems, while the SAW device is essentially restricted to gas phase measurements. Here too, IC techniques have provided means to fabricate thin membranes that can be made to oscillate at frequencies similar to the SAW device, but in a mode that is not over-damped in aqueous solutions. Nevertheless, regardless of the specific oscillator involved, it is the coating films and interfaces that provide the chemical specificity required of the sensor. 

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