Modeling yielding behavior of Carbopol solutions using the fractal nature of formed aggregates

Materials like emulsions, gels and foams, present a threshold stress – the yield stress – that delineates the onset of a fluid-like viscous behavior. Below the yield stress they exhibit a viscoelastic behavior with very high viscositities. These materials are present in various industries, including food, pharmaceutical and cosmetics. Due to its non-toxic, clear and odourless properties, fluids like Carbopol solutions are frequently used in experimental studies to understand the mechanical behavior of yield stress fluids, and to impart the appropriate consistency and mechanical characteristics to products. They are characterized as a high molecular weight, hydrophilic, crosslinked polyacrylic acid polymer, featuring a 3D polymer network that is swollen by water and exhibits strong interchain entanglements. To comprehend their mechanical behavior and responses to industrial processes, it is crucial to study their microstructure, and correlate it with the rheological response. This work analyzes and compare the rheology and microstructure of different types of Carbopol solutions and concentrations of neutralizing agents that are used for pH control. The microstructure is visualized by Cryo-SEM technique, and rheology is assessed via rotational rheometry. Furthermore, we employ two models, based on the fractal microstructure aggregates characteristics, to evaluate the impacts of Carbopol and neutralizer concentrations on the rheological properties of the yield stress fluid.