Heterogeneity, suspension, and yielding in sparse microfibrous cellulose gels 2: strain rate-dependent two-fluid behavior

Complex fluids are widely used in formulated products to impart rheological properties like stability, performance, and aesthetics. Yield stress fluids are a particularly important example, enabling applications like particle suspension, surface coating, and therapeutic delivery. Recent work has shown that particle suspension in yield stress fluids can be a strong function of yielding and flow heterogeneities, especially in anisotropic fiber dispersions. This work uses a microbubble technique to study the deformation before and during yielding of microfibrous cellulose suspensions. We note significant variations in suspension performance as a result of fiber rearrangement and heterogeneities. Strong strain rate dependencies are found to vary local network yield strength, and confocal microscopy quantifies structural reinforcement and deformation rate effects. The observed behavior indicates a two-fluid interpretation may help interpret sparse network flow and suspension properties.