Flow dynamics of complex fluids: characterization, modeling and flow in milifluidic channel.

Licentiate thesis, 2023

Complex fluids are used in a wide range of applications in biomedical, food, cosmetic, chemical and pharmaceutical industries. Their complex material behaviour arises primarily from their rheological properties and is complimented by factors such as their multi-phase nature, biological component and complex microstructure. A comprehensive understanding of the material rheology is crucial to studying the flow dynamics of these complex fluids in industrial flow configurations. Additionally, to improve and ensure their consistent and efficient production and processing, prediction of their flow properties is of utmost importance. This necessitates the use of non-linear rheological constitutive models that can predict the material response during flow. In this framework, the PhD project aims at improved material characterization of the rheological properties of thixo-elasto-viscoplastic (TEVP) fluids, studying their flow dynamics in various flow configurations and prediction thereafter. In this thesis, three widely used complex fluids, yogurt, Carbopol and Laponite, were studied and their viscoelastic, viscoplastic and thixotropic material responses were extensively characterized. Numerical modeling was performed to predict the thixotropic behaviour of yogurt in Multi interval thixotropic tests (MITT), by curve fitting four phenomenological thixotropic models to hysteresis loop tests. The numerical modeling results show promising predictive capabilities and form the ground work required for flow simulations. This was followed by a microstructural study to investigate the effect of thixotropic material behaviour on the structural evolution of the material. To study the flow dynamics, a non-intrusive optical based imaging solution was developed using the Doppler-optical coherence tomography method to visualize the flow field of complex fluids in milifluidic channels for the first time. Different types/concentrations/structural states of the fluids were considered to investigate the effects of the rheological material functions on the flow field. An ex-situ rheometric method was developed and used to construct the shear stress distribution map inside the channel, which provided useful insights into the structural changes occurring as a result of the competition between the materials' flow and rheological properties. Results obtained from this study will be used to construct and validate simulation models in the future.