Determination of the Shear Modulus of Thin Polymer Films with a Quartz Crystal Microbalance: Application to UV-Curing.

The photoinduced curing of a light-sensitive varnish was followed, based on a change of the film's shear modulus, G, as determined with a quartz crystal microbalance (QCM). The film thickness was in the range of a few hundred nanometers. Both the storage modulus, G', and the loss modulus, G″, were obtained. The analysis is based on a perturbation calculation. The equations differ from the more commonly used set of equations derived from the small-load approximation and the acoustic multilayer formalism (sometimes termed Voigt-model). The discussion revisits assumptions, accuracy, and limits of the technique. Critical to the analysis is a knowledge of the thickness of the electrode underneath the film.

Partial slip in mesoscale contacts: dependence on contact size.

Using acoustic resonators, we have studied the occurrence and the magnitude of partial slip between glass spheres and polymer surfaces. The measurement relies on the shifts of resonance frequency and bandwidth, Δf and ΔΓ, induced by the contact as well as the dependence of Δf and ΔΓ on the amplitude of oscillation. One often finds a decrease of Δf at elevated amplitudes, which goes back to partial slip (also "microslip"). Building on two different models of partial slip, we derive the frequency-amplitude relation from the force-displacement relation. In accordance with both models, the bandwidth is found to increase with amplitude in the partial slip regime. For the highest amplitudes and largest spheres investigated, one observes a decrease of bandwidth with amplitude, which is interpreted as a transition to gross slip. Deviating from both models of partial slip, Δf is sometimes found to be independent of amplitude in the low-amplitude range. Constant Δf implies linear force-displacement relations. The critical amplitude for the onset of partial slip depends on the contact radius, where partial slip is more pronounced for larger contacts. This finding can be explained by a smooth stress profile at the edge of the contact with no singularity. The stress at the edge might be lowered by nanoscale roughness, by capillary forces, or by the inability of the two surfaces to reestablish a sticking contact at the turning point of the oscillation.

Simple frequency-based sensing of viscosity and dielectric properties of a liquid using acoustic resonators.

The effects of a finite polarizability of a liquid sample onto the series resonance frequency of a piezoelectric resonator are described within the small-load approximation. It is found that the sample's electrical capacitance (the ratio of surface potential and surface polarization at the crystal surface) enters the motional branch of the equivalent circuit, thereby shifting the series resonance frequency. This effect is caused by piezoelectric stiffening. Using a conventional quartz crystal with small hole in the front electrode and exposing this crystal to a variety of different liquids, we demonstrate that derived equation describes the experiment reasonably well. Electric and dielectric effects are particularly strong for torsional resonators, which operate at a frequency of around 56 kHz. These are in commercial use for determination of the viscosity of engine oils. We propose a simple method to in situ switch the strength of piezoelectric stiffening. It allows separation of the viscoelastic parameters from the electric polarizability. It could, for instance, be used to differentiate between gasoline and water being accidentally admitted to the lubricant reservoir. The instrument requires a single resonator and is based on the determination of only 2 frequency shifts.