Quartz crystal microbalance.
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Quartz crystal microbalance, or QCM, relies on the ability of quartz crystals to change shape in response to the applied voltage (reverse piezoelectric effect). In particular, quartz crystals cut in a certain way will oscillate in shear-thickness  mode in response to the applied voltage, sending shear waves into the surrounding media. In the image on the left, such a crystal is shown schematically. It has a rectangular cross-section at rest. Top and bottom surfaces will move in opposite directions when voltage is applied.
  In a typical QCM measurement, the resonance frequency and bandwidth (half width of the resonance peak at half height) of a quartz crystal are measured. Measurements can be done in the frequency domain (e.g., with an impedance analyser) or the time domain (exciting the crystals intermittently and measuring the decay of oscillations; this is the approach Q-sense took with their QCM-D system). Both approaches result in two parameters (per overtone): resonance frequency and bandwidth. The two kinds of measurements are related to each other via a Fourier transform (see the diagram on the right).
   These two parameters depend on the environment of the crystal. For example, a crystal in air will oscillate at one frequency, while the same crystal in liquid - at another (red and blue curves in the diagram on the right). Note, that the widths of the resonance peaks also change. The change in the resonance frequency, as well as the change in bandwidth, upon transfer of a crystal from air to liquid is described by the well-known Kanazawa-Gordon relation, which states that both are proportional to the square root of the liquid density times liquid viscosity.
  More information about QCM can be found here. Below, the results of a typical measurement are shown.

  The frequency/bandwidth in air (blue) and the frequency/bandwidth in liquid (red) are related to each other via liquid viscosity and density through the so-called Kanazawa-Gordon relationship (see Borovikov 1976 Instruments and Experimental Techniques 19, 223-224; Kanazawa and Gordon 1985 Anal. Chem. 57, 1771-1772). A frequency-domain measurement (left) is related to the time-domain measurement (right) via a Fourier-transform. Bandwidth is defined as half-width of the resonance peak at half-height.
A typical QCM experiment - adsorption of  liposomes to a surface and bilayer formation:


In this experiment, the resonance frequency and bandwidth of a quartz crystal are monitored as a function of time. The plots show the difference in the resonance frequency (left) and bandwidth (right) relative to that of the bare crystal in liquid. Since initially, there is only a bare crystal in liquid, this difference is zero. At time 0, liposomes were injected. The frequency goes down, while the bandwidth increases, corresponding to adsorption of liposomes to the surface. After reaching the respective extrema, the signals return: bandwidth returns to ~ zero, frequency shift returns to ~ - 25*n Hz, where n is the overtone order. Lines of different color represent measurements done on different overtones, where fn ~ n*f0, with f0 = 5 MHz the fundamental frequency of the quartz crystal used in this experiment. Dissipation is related to bandwidth as D = 2Γ/f.
This type of response (extrema in frequency and bandwidth shifts) has been interpreted by Keller and Kasemo (Biophys. J. 75, 1397, 1998) as adsorption of liposomes and their subsequent decomposition to form a supported lipid bilayer:
    A supported lipid bilayer is formed at the end of the process. Note, that the frequency shifts at this stage are the same on all overtones, so Δf/n ~ constant, and the bandwidth shifts are ~ zero. Therefore, the Sauerbrey relationship can be applied to calculate the mass of the adsorbed layer from the frequency shifts: Δf/n = - (layer density)*(layer thickness) / constant. With Δf/n ~ -25 Hz, the  Sauerbrey constant = 18.8 ng/cm2/Hz, and density ~ 1 g/cm3, the thickness comes out ~ 5 nm, which is in good agreement with what is expected for a lipid bilayer.
     On the other hand, the Sauerbrey relationship can not be used to accurately determine the mass of the adsorbed liposomes, because the bandwidth shift is not zero and Δf/n is not constant. You can learn more about interpretation of QCM data in heterogeneous films here and here.