Investigating the rheological behavior of a simple yield stress fluid under shear flows

Many soft materials display unique and complex rheological behavior characterized by a transition from a solid-like to a fluid-like state upon the application of a force that exceeds the threshold to flow, known as the yield stress. Yield stress fluids are found in a wide range of commonly encountered materials including microgels, emulsions, and foams, and have been widely studied by rheologists over the last several decades. Carbopol is a popular polymeric microgel system as it displays simple, non-thixotropic rheological behavior and is typically seen as an ideal yield stress fluid. Previous research has demonstrated the reproducible behavior of shear stress in Carbopol systems, but measurements of the normal stress differences are either limited in scope or demonstrate chaotic behavior. Here a Carbopol yield stress fluid is evaluated to examine how experimental test history effects subsequent constant shear strain rate experiments, and how the shear stress and normal stress differences evolve during and after being subject to a period of zero stress known as recovery