Piezoelectric resonators have been in use as mass-sensing devices for almost half a century. More recently it was recognized that shifts in frequency and bandwidth can come about by a diverse variety of interactions with the sample. The classical “load” consists of a thin film, which shifts the resonance frequency due to its inertia. Other types of loading include semi-infinite viscoelastic media, rough objects contacting the crystal via isolated asperities, mechanically nonlinear contacts, and dielectric films. All these interactions can be analyzed within the small-load approximation. The small-load approximation provides a unified frame for interpretation and thereby opens the way to new applications of the QCM.

We have studied the temperature dependence of swelling of a polystyrene brush in cyclohexane with quartz resonators and ellipsometry. The swelling of the layer, which has been covalently bound to a silicon oxide surface, continuously increases as the temperature is raised. The data from the quartz resonator display a “film resonance,” which indicates that the thickness of the brush is on the order of the wavelength of transverse sound. The quantitative analysis shows that the acoustic thickness as determined with the quartz resonator is higher than the optical thickness from ellipsometry. We argue that this difference is partly due to the high contrast achieved in acoustical measurements, which makes the quartz resonator most sensitive to dilute regions in the tail of the segment density profile. The relation between the acoustic thickness and the hydrodynamic thickness is discussed.

The response of the quartz crystal microbalance (QCM, also: QCM-D for “QCM with Dissipation monitoring”) to loading with a diverse set of samples is reviewed in a consistent frame. After a brief introduction to the advanced QCMs, the governing equation (the small-load approximation) is derived. Planar films and adsorbates are modeled based on the acoustic multilayer formalism. In liquid environments, viscoelastic spectroscopy and high-frequency rheology are possible, even on layers with a thickness in the monolayer range. For particulate samples, the contact stiffness can be derived. Because the stress at the contact is large, the force is not always proportional to the displacement. Nonlinear effects are observed, leading to a dependence of the resonance frequency and the resonance bandwidth on the amplitude of oscillation. Partial slip, in particular, can be studied in detail. Advanced topics include structured samples and the extension of the small-load approximation to its tensorial version.