Studying Soft Interfaces with Shear Waves: Principles and Applications of the Quartz Crystal Microbalance (QCM)

The response of the quartz crystal microbalance (QCM) 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 with the acoustic multilayer formalism. In liquid environments, viscoelastic spectros-copy and high-frequency rheology are possible, even on layers with a thickness in the monolayer range. For particulate samples, rheology is replaced by contact mechanics. The contact stiffness can be derived. Because the stress at the contact is large, nonlinear effects are seen. 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

Studying soft interfaces with shear waves: principles and applications of the quartz crystal microbalance (QCM)

The response of the quartz crystal microbalance (QCM) 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 with the acoustic multilayer formalism. In liquid environments, viscoelastic spectros-copy and high-frequency rheology are possible, even on layers with a thickness in the monolayer range. For particulate samples, rheology is replaced by contact mechanics. The contact stiffness can be derived. Because the stress at the contact is large, nonlinear effects are seen. 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

A modulation QCM applied to copper electrodeposition and stripping

A fast electrochemical quartz crystal microbalance with dissipation monitoring (EQCM−D) was applied to copper electrodeposition and subsequent stripping. Accumulation brings the frequency noise down to the mHz range, corresponding to 0.1 % of a monolayer. With this precision, the apparent mass transfer rate as determined from the time-derivative of the frequency shift can be directly compared to the current. Small but systematic deviations between the two can be attributed to nanoscale roughness. In the voltage range of underpotential deposition (UPD), the apparent mass transfer rate shows peaks and shoulders. The plating additive benzotriazole (BTA) leaves the magnitude of electrogravimetric signals unchanged, but shifts the UPD onset potential. The additive thiourea (TU) promotes UPD and strongly increases the bandwidth

A simple equation predicting the amplitude of motion of quartz crystal resonators

The amplitude of motion of quartz crystal resonators, u 0 , has been calculated on the basis of the transmission line model by Mason ͓Piezoelectric Crystals and Their Applications to Ultrasonics ͑Van Nostrand, Princeton, 1948͔͒. It is predicted to be u 0 =4/͑n͒ 2 Qd 26 U el,0 , where n is the overtone order, Q is the quality factor, d 26 is the piezoelectric strain coefficient, and U el,0 is the amplitude of the driving voltage. This simple result is in good agreement with previous numerical calculations, with an experimental value from the literature, and with our own experimental checks. As a side result, an equation is provided which allows to estimate the active area of the crystal from the product of the motional resistance R 1 and the Q factor

Fouling pathways in emulsion polymerization differentiated with a quartz crystal microbalance (QCM) integrated into the reactor wall

Emulsion polymerization fouling at hot interfaces is studied in situ, making use of a quartz crystal microbalance with dissipation monitoring (QCM-D). The resonator crystal is heated with a ring-shaped thermal pad from the back, turning it into a plate with elevated temperature. Configured to be one of the walls of a small reactor for emulsion polymerization, this resonator is prone to heat-transfer fouling, similar to regular heated parts of process equipment. The fouling kinetics is readily quantified with this QCM. During polymerization at constant temperature (80 °C), some deposition is always observed. However, a film with a thickness of less than 1 μm (determined gravimetrically with the QCM) is sometimes found, which stabilizes the surface against the deposition of much thicker layers. When reaction fouling proceeds directly to thick deposits, a small increase in resonance bandwidth often occurs a few minutes prior to the main transition, presumably caused by coagulum formed in the bulk making first contact with the surface. Furthermore, particle fouling is studied with temperature ramps on nonreactive dispersions. Fouling, if present, is readily observed

Towards vibrational spectroscopy on surface-attached colloids performed with a quartz crystal microbalance

Colloidal spheres attached to a quartz crystal microbalance (QCM) produce the so-called “coupled resonances”. They are resonators of their own, characterized by a particle resonance frequency, a resonance bandwidth, and a modal mass. When the frequency of the main resonator comes close to the frequency of the coupled resonance, the bandwidth goes through a maximum. A coupled resonance can be viewed as an absorption line in acoustic shear-wave spectroscopy. The known concepts from spectroscopy apply. This includes the mode assignment problem, selection rules, and the oscillator strength. In this work, the mode assignment problem was addressed with Finite Element calculations. These reveal that a rigid sphere in contact with a QCM displays two modes of vibration, termed “slipping” and “rocking”. In the slipping mode, the sphere rotates about its center; it exerts a tangential force onto the resonator surface at the point of contact. In the rocking mode, the sphere rotates about the point of contact; it exerts a torque onto the substrate. In liquids, both axes of rotation are slightly displaced from their ideal positions. Characteristic for spectroscopy, the two modes do not couple to the mechanical excitation equally well. The degree of coupling is quantified by an oscillator strength. Because the rocking mode mostly exerts a torque (rather than a tangential force), its coupling to the resonator's tangential motion is weak; the oscillator strength consequently is small. Recent experiments on surface-adsorbed colloidal spheres can be explained by the mode of vibration being of the rocking type. Keywords: Quartz crystal microbalance, Coupled resonance, Biocolloids, Adsorptio

The quartz crystal microbalance in soft matter research: fundamentals and modeling

This book describes the physics of the second-generation quartz crystal microbalance (QCM), a fundamental method of analysis for soft matter at interfaces.From a device for measuring film thickness in vacuum, the quartz crystal microbalance (QCM) has in the past two decades evolved into a versatile instrument for analyzing soft matter at solid/liquid and solid/gas interfaces that found applications in diverse fields including the life sciences, material science, polymer research and electrochemistry. As a consequence of this success, the QCM is now being used by scientists with a wide variet

Thickness of the Soft Layer on Glassy Polystyrene Surfaces

ABSTRACT: The flow behavior of the near-surface region of polystyrene films was studied via the decay kinetics of imprinted corrugation gratings at elevated temperatures. The speed of the decay is the result of a balance between surface tension, on the one hand, and viscous drag, on the other. Depth profiling is possible because the penetration depth of a surface wave is proportional to its wavelength. From the dependence of the decay rate on the penetration depth one concludes that the surface region displays an increased molecular mobility. The soft surface layer has a thickness of about 20 nm