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Load Impedance of Immersed Layers on the Quartz Crystal Microbalance: A Comparison with Colloidal Suspensions of Spheres

2020; American Chemical Society; Volume: 36; Issue: 31 Linguagem: Inglês

10.1021/acs.langmuir.0c01429

1520-5827

Marc Meléndez, Adolfo Vázquez-Quesada, Rafael Delgado‐Buscalioni,

Mechanical and Optical Resonators

The analytical theories derived here for the acoustic load impedance measured by a quartz crystal microbalance (QCM), due to the presence of layers of different types (rigid, elastic and viscous) immersed in a fluid, display generic properties, such as "vanishing mass" and positive frequency shifts, which have been observed in QCM experiments with soft-matter systems. These phenomena seem to contradict the well-known Sauerbrey relation at the heart of many QCM measurements, but here we show that they arise as a natural consequence of hydrodynamics. We compare our one-dimensional immersed plate theory with three-dimensional simulations of rigid and flexible sub-micron-sized suspended spheres, and with experimental results for adsorbed micron-sized colloids which yield a "negative acoustic mass". The parallel behaviour unveiled indicates that the QCM response is highly sensitive to hydrodynamics, even for adsorbed colloids. Our conclusions call for a revision of existing theories based on adhesion forces and elastic stiffness at contact, which should in most cases include hydrodynamics.