The quartz crystal microbalance (QCM) is an exceptionally simple device.  A small disk of a piezoelectric material is subjected to an AC-voltage, which gives rise to a thickness shear vibration.  If the frequency of the electrical excitation matches the acoustic resonance frequency of the plate, the amplitude of oscillation (and the current through the electrodes) becomes large.  In this way, an acoustic resonance is probed by electrical means.  The sharpness of the acoustic resonance of quartz plates much exceeds the sharpness of most electrical resonances.  Quartz crystals therefore are used to control the frequency of most modern clocks.

When the crystal comes into physical contact with some kind of sample, the resonance frequency shifts.  Of course, such disturbances are avoided for clocks, but quartz resonators are also powerful sensing devices.  For instance, the deposition of a film onto the crystal surface is easily detected by the induced frequency, even if the film is only is a molecular monolayer.  Using a quartz resonator in this way, it turns turn into a "quartz crystal microbalance (QCM)".

The group is concerned with complex samples such as a biofilm, a surface-attached hydrogel, a sand pile, or a froth.  With complex samples, it is essential to build the analysis on all available parameters.  Apart from the frequency shift, these are

 

• the bandwidth of the resonance
• the shifts of frequency and bandwidth on multiple overtones
• the dependence of these shifts on the amplitude of oscillation>/i>
• dependence of these shifts on whether or not the front electrode is grounded/

 

Data analysis always relies on the small-load-approximation.  As long as the frequency shift is much smaller than the frequency itself, Δf is proportional to the in-phase component of the average lateral stress at the crystal surface.  The shift in bandwidth, &#ΔΓ, is proportional to the out-of-phase component of the stress.  These stresses may be (numerically) computed by the Finite Element Method even for complex samples, thereby making them accessible to experiments using the QCM.