Superposition 'Chirp' Rheometry: a new technique for the rapid rheological characterisation of complex fluids with transient microstructures.

The flow and deformation properties (i.e. the Rheology) of complex fluids, which arise due to the fluids microstructure, are often a critical factor in achieving a product functionality. In this context, "products" may range from printed electronic components manufactured using functional inks, to the 'biological products' of physiological processes such as blood coagulation - i.e. the blood clot. However, the effect of complex flow conditions (which are inherent to the 'process') on the fluid microstructure, and hence flow properties, is difficult to study - especially if the process is dynamic (e.g. where the sample is undergoing curing, gelation, clotting, drying etc). This research proposal aims to develop, test and demonstrate a novel technique for characterising changes in the rheology (and microstructure) of flow-sensitive complex fluids occurring in response to a flow condition. It is proposed to develop a technique which uses 'chirp' waveforms (i.e. frequency modulated waveforms) to probe the time-dependent flow and deformation properties of fluids as they are experiencing flow. Successful delivery of the project will provide industrial and academic rheologists, product formulation specialists, manufacturers and process designers with (i) an extremely powerful tool for rapidly characterising rheological/microstructural changes occurring in response to a sustained shear flow, (ii) new insights into the microstructural consequences of gelation under unidirectional stress, and (iii) a framework for interpreting parallel superposition rheometric data.

This proposal will develop and test a novel approach to superposition rheometry experimentation and interpretation that will facilitate rapid characterisation of the relationship between complex flow conditions and the rheological/microstructural properties of process materials. The research is of particular relevance to applications involving processing materials with complex, shear sensitive microstructures which impact on product performance. There are numerous examples of such applications but particular focus is given in the present proposal to functional inks used for printed electronics (representing a future high-value manufacturing technology) and collagen (which represents a rapidly gelling material with shear sensitive microstructures and has vast applications in bioscience and tissue engineering). It is anticipated that the techniques developed as part of the proposed research will allow both academia and industry to develop an improved understanding of the flow-microstructure-rheology relationship and allow better process design, optimisation and control, to deliver improved products and processes. Consequently, the research has the potential to deliver both economic impacts through the invention of new and more reliable products/processes and environmental impact through an associated reduction in waste and energy requirements.

There is also significant potential for future healthcare applications. Within the last decade it has been established that the incipient blood clot microstructure (as probed by through the rheological properties of coagulating blood at the sol-gel transition) provides an excellent biomarker for abnormal coagulation and is sensitive to a number of pathophysiological conditions (e.g. stroke, cancer) and the bulk flow stresses imposed on the forming clot. It is anticipated that the outcomes of the present proposal will lead to a better understanding of the effects of non-linear flow induced shear stresses on the evolution of blood clot microstructure, the new superposition technique better representing physiological conditions of clot formation than small amplitude oscillatory shear alone. Moreover, the new technique will provide a means of better understanding clot evolutionary responses in situations where an underlying prothrombotic pathophysiological condition, or exposure of flowing blood to foreign surfaces (such as a ventricular assist device) markedly accelerates the rate of coagulation. Existing rheometric technologies are often hard pressed to fully characterise the rheological responses of clotting blood under such circumstances. It is anticipated that the outcomes of the present work will undergo a series of preclinical assessments under the auspices of the NHS-based Welsh Centre for Emergency Medicine Research in due course. The proposed work hence has the potential for both societal and economic impacts on medical device design and optimisation, coagulation diagnostics and theranostic approaches to disease management, thereby impacting wider society in terms of both the economic burden of disease management and associated clinical outcomes for individual patients.