Flow Behavior of Dense Polydisperse Non-Brownian Suspensions─Shearing under Flow Confinement

Concentrated mixed particulate systems, such as cement, coal slurries, and propellants, exhibit complex behavior due to the presence of non-Brownian particles, high solid fractions, and polydispersity. Typically, the flow behavior of these suspensions is characterized using steady shear. However, given their susceptibility to slip and inertial effects, characterizing their flow behavior is limited to lower shear rates. To minimize the artifacts at intermediate shear rates, one approach is to perform rheological experiments at thin gaps. This study presents the steady shear rheological response of various compositions of model solid rocket propellant suspensions under narrow gaps. Rheo-microscopy experiments were performed to visualize the suspensions in order to analyze the wall slip during shear. The absence of slip in these suspensions was confirmed by comparing the theoretical and measured particle velocities using PIVlab. The primary focus is on how confinement affects the steady shear response of concentrated polydisperse, non-Brownian suspensions. Under confinement, the magnitudes of both shear stress and dynamic yield stress are higher. An increase in the particle pressure of the suspension corresponds to an increase in shear stress. Superimposition of the gap-dependent response onto a master curve using the modified Stokes number indicates a unified description of the confined response. Furthermore, the variation of the effective cooperativity length of the suspension with confinement is studied. Possible confinement-induced microstructural changes are hypothesized and presented.