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Rheo-PTV analysis of complex fluids Pourzahedi, Ali

Abstract

Complex fluids are ubiquitous in the everyday life. Their study enables a better perception of the surrounding environment as it can be used to model a variety of industrial, biological and geophysical applications. This study presents a novel optical capillary rheometer that benefits from the advantages of both capillary rheometry and flow visualization. The experimental setup developed in this research was applied to three different case studies, each highlighting an aspect of the rheological behavior of complex fluids. In the first case study, the slip mechanism of various concentrations of carbopol gel, as a simple yield stress fluid, was systematically investigated with respect to its material properties. Presence of a fully plugged flow and a thin layer of a Newtonian solvent, lubricating the unyielded gel at wall stresses below the yield stress was confirmed. The wall slip behavior of the carbopol gels at wall stresses above the yield stress was studied as well. It was shown that the slip velocities have a power-law correlation with the wall shear stress below the yielding point, and a linear correlation after the yielding point. Furthermore, the sliding threshold below which the gel sticks to the wall, was found to have a linear relationship with its yield stress and the fluidity of its solvent. The start-up flow of laponite as a thixotropic yield stress fluid was examined in the second case study. The time dependent behavior of the fluid was studied in terms of the variations of shear stress at a constant flow rate. Finally, the behavior of synovial fluid at high shear rates was investigated as the third case study. Synovial fluid of the knee joint undergoes extensive range of shear rates during high physical activities. Rheological behavior of synovial fluid was studied at shear rates as high as 7000 s-1, which is seven times greater than the maximum shear rate typically measurable by a rotational rheometer. The method developed in this research enables to further study the complex behavior of fluids that are challenging using the conventional rheometry methods.

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