Durgesh Prasad Kavishvar
Doctor of Philosophy
Department of Chemical Engineering University of Toronto
Abstract
Yield stress is a characteristic stress depicting the flow behavior of many complex materials. When the applied shear stress is lower than yield stress, materials exhibit solid-like behavior, transitioning to liquid-like behavior for shear stress>yield stress. Conventional rheometers often struggle with sensitivity in measuring low yield stress, lack real-time measurement capabilities, and are prohibitively expensive. These limitations are particularly pronounced in biological materials such as blood and mucus, where variations in yield stress can signify underlying health conditions. For instance, yield stress of blood from patients with cerebrovascular and cardiovascular diseases, hypertension, sickle cell disease, among others, exceeds that of healthy blood. Similarly, yield stress of mucus from individuals battling conditions such as cystic fibrosis or asthma can be several orders of magnitude higher than that of healthy lung mucus. Also, yield stress measurements prove valuable for quality assessment in various industries such as oil & gas as well as food industry.
In this work, we investigate, through experiments, scaling analysis, and simulation, the yielding behavior of various complex materials in a Hele-Shaw microfluidic extensional flow device (MEFD). We propose the MEFD as a new microfluidic rheometer capable of measuring a low yield stress ranging from 5 mPa−5 Pa. The design of the MEFD is such that it enables a gradient in shear stress, shear stress, such that shear stress is lower near the center or stagnation point, and higher away from the stagnation point. For a yield stress fluid, we observe that, below a certain flow rate, shear stress exceeds yield stress only in the outer region, leading to stagnation or unyielding of the fluid in the inner region. Our simulation study also corroborates the experimental findings, demonstrating the existence of an unyielded region near the stagnation point of the extensional flow. We apply scaling analysis to deduce yield stress by measuring this size of the unyielded region at center. We validate the scaling relationship using Carbopol solutions of various concentrations (0.015 to 0.3%), measuring yield stress as low as ~10 mPa to ~1 Pa, and comparing these measurements with a standard rheometer.
Furthermore, we showcase the applicability of our rheometer by measuring yield stress of human blood samples ranging between 30−80 mPa for a range of hematocrits as well as yield stress of porcine gastric mucins (20%) of 0.7 Pa. We also demonstrate measurements of yield stress of clay suspensions (2 to 6%), useful in oil & gas applications. We also show a proof of concept for food industry by measuring yield stress of a lactic drink of roughly 7 mPa. Our microfluidic rheometer offers several other advantages, including real-time measurement capability (with measurement time as short as 4 s), low volume requirement (<1 ml), ease of cleaning and reuse, and cost-effectiveness. Therefore, it attracts diverse applications in clinical research, particularly in disease detection, as well as in industries such as food and oil & gas for quality assessment.
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