Analysis and Metric Development for the Study of Viscoelastic Thin Films Utilising a Quartz Crystal Microbalance

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Abstract

The aim of this thesis is the creation of a set of tools for the quartz crystal microbalance (QCM-D) that aid in the measurement and quantification of soft viscoelastic thin films and experimental work demonstrating their use. The QCM-D is an acoustic technique that monitors structural changes occurring at the sensor’s surface via changes in the sensor’s resonance frequency and the rate of mechanical energy loss (dissipation). As a first approximation, the frequency shifts are used to measure mass changes on the sensor’s surface, and dissipation shifts used to quantify changes in the rigidity of the film. Use of the QCM-D responses in this manner requires that the film is acoustically thin and rigid, limiting its application to soft films.To quantify mass and viscoelastic changes using the QCM-D, soft films either need to be approximated to a thin, rigid layer, or the frequency and dissipation responses modelled using a viscoelastic model. Such an approximation leads to the encompassment of all the viscoelastic properties into the single dissipation measurement in addition to potentially introducing errors in mass calculations. Existing commercial software allows for the deconvolution of film parameters such as the shear modulus and viscosity by fitting experimental data to a viscoelastic model. This analysis can only be done after the experimental data is collected however, and provides no guidance on future experiments, also commonly requiring an initial estimate of the parameter values under investigation.I have developed an experimental optimisation tool, termed the total parameter matrix sensitivity (TPM-sensitivity). It is defined as the Jacobian determinant of the QCM-D responses with respect to the parameters under investigation, e.g. the film’s height, density, viscosity and shear modulus and the bulk fluid’s density and viscosity. TPM-sensitivity is a measure of how readily resolvable and separable the film and bulk are when analysing the QCM-D responses. This enables the user to select the most mathematically important harmonics, and using this I was able to experimentally resolve the viscoelastic information of a soft film using frequency responses alone.I have also defined a classification system which categorises the QCM-D responses relative to a perfectly rigid and thin film. This provides guidance on the level of analysis required to gain information about the film parameters, with the limitations of commonly applied rules of thumb also demonstrated. Examples using these computational tools and metrics are also presented with data I obtained experimentally and from the literature. Of the experimental investigations, the curing process of a bulk elastomer is of particular importance due to the film being both soft and acoustically thick, demonstrating QCM-D use for a film not complying to either of thecommonly used film approximations.

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Software generated to aid QCM-D users can be found at https://github.com/88tpm/QCMD and downloaded.

Keyword(s)

QCM-D; Sauerbrey; TPM-sensitivity

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