Rheology and size reduction in roll processing of dense suspensions

In processing of certain materials, including foodstuffs and ceramics, a suspension or paste at particle concentration above the jamming limit for simple-shear flow is used. In this study, the experimental two-roll flow between parallel cylinders with a small ( <100 micron ) gap, smaller than the particle size, and size reduction occurring for such a material is considered, in combination with oscillatory rheology. The solid loading of the paste is well above jamming under simple shear flow, and this is demonstrated by its behavior under large-amplitude oscillatory shear (LAOS), where the development and failure of contact networks can be traced through the stress response. The behavior of the paste in the entry to the gap between the two rollers, which includes strong slip, will be demonstrated based on visualization on multiple axes, and the stresses generated within the material as it jams and passes through the narrow gap with particle fracture takes place. The torques on the two rollers and the normal force generated by the paste are measured as is the gap; the force resisting gap opening due to the counter-pressure from the paste at different solid-liquid ratios is imposed and results will be presented for a range of this force as well as the rotation rates of the two cylinders. The size reduction will also be described as function of the operating parameters. The general fluid mechanical behavior, and the rheological model of the pastes will be discussed in relation to the two-roll flow.