Impact of cohesion forces on particle mixing and segregation

The objective of this work is to advance the fundamental understanding of mixing and segregation of cohesive granular materials. Cohesion can arise from a variety of sources: van der Waals forces, electrostatic forces, liquid bridging (capillary) forces. These forces may play a significant role in the processing of fine and/or moist powders in many industries, from pharmaceuticals to materials synthesis; however, despite its prevalence, there is only limited information available in the literature on processing of cohesive materials. Instead, the vast majority of work has been directed at the study of non-cohesive (i.e., free-flowing) particles, and a wealth of information has been learned about the behavior of cohesionless materials. With growing emphasis on controlling the structure of materials at increasingly small length-scales (even tending toward the nano-scale), understanding the effects of particle interactions---which tend to dominate at smaller length-scales---on processing operations has become more important than ever.; This project focuses on the effects of cohesion on mixing and segregation in simple, industrially-relevant, granular flows. In particular, the paradigm cases of a slowly rotated tumbler and the flow in a simple shear cell are examined. We take a novel approach to this problem, placing emphasis on microscopic (particle-level), discrete modeling so as to take as its staring point the well understood interaction laws governing cohesion (capillary, van der Waals, etc.), and build to the view of the macroscopic flow via experiment and Particle Dynamics Simulation. We develop and use discrete characterization tools of cohesive behavior in order to construct a simple theory regarding the mixing and segregation tendency of cohesive granular matter. This theory allows us to analytically determine a phase diagram, showing both mixed and segregated phases, and agrees both quantitatively and qualitatively with experiment. These results have implications for industrial mixing/separation processes as well as novel particle production methods (e.g., engineered agglomerates with precisely prescribed compositions).