Carbopol gels as model fluids: Influence of the grade

Carbopol dispersions are widely used in the literature as model yield stress fluids, particularly for studying complex macroscopic flows. The prevailing assumption is that they behave as standard elastoviscoplastic materials, facilitating flow modeling and the generalization of findings. However, several studies–though not all–have reported complex flow behaviors under specific conditions, including prolonged transients reminiscent of those observed in thixotropic materials. Notably, these behaviors have been shown to depend on the mixing power applied during the dispersion of microgel particles. In this study, we examine an often-overlooked factor: the specific influence of the Carbopol grade. By analyzing four different grades, we demonstrate that only the simplest grade, Carbopol 980–a densely crosslinked polyacrylic acid without additional functionalization–consistently behaves as a simple yield stress fluid. While the other grades exhibit comparable flow properties under most conditions, significant differences emerge during start-up flows. Specifically, these grades display a notable increase in static yield stress over time at rest–unlike Carbopol 980– likely due to either self-association of functional groups or interpenetration of long dangling chains resulting from the looser crosslinking. Our findings suggest reexamining some previous studies and provide guidance for choosing model materials in future research. • Comprehensive comparison of 4 Carbopol grades across concentrations. • Grade-specific aging behaviors revealed in start-up flow experiments. • Only 980 shows negligible static yield stress aging vs other grades. • Dangling chains/functional groups likely cause grade-dependent rheological. • Carbopol 980 recommended as benchmark for fluid mechanics studies.