Mechanisms of Mass and Momentum Transport in Cohesive Granular Flows

Cohesive inter-particle forces strongly affect the internal flow structures of granular media and powders. Existing research in non-cohesive flows demonstrates that internal flow structures govern the underlying transport mechanisms of mass and momentum and hence impact the rheology and diffusive mixing properties of granular flows. However, models of cohesive granular flows do not directly recognise the role of these structures. This thesis investigates how cohesive forces influence sheared granular flows. It theoretically derives their diffusive and rheological properties ab initio. These expressions are confirmed using discrete element simulations in a representative volume. This approach provides simultaneous access to continuum-scale properties and internal grain dynamics, facilitating a physically justified model of the structures and mechanisms underlying cohesive granular flows. Analysis of mass diffusion demonstrates that existing scaling laws can fail in cohesive flows. Long-lived mesoscale clusters drive grain kinematics and dramatically increase the diffusivity. A cluster-based theory of diffusion is proposed to account for this prolonged memory effect. This thesis then focuses on the friction law of non-cohesive and subsequently cohesive granular rheology on two fronts. Firstly, it derives an expression for the friction law. This expression differentiates two mechanisms for momentum to diffuse: either through the balanced component of the force-network (as in through force chains) or via fluctuations in grain accelerations. Cohesion enhances friction by strengthening the balanced force-network only. This asymmetric enhancement yields shear-rate weakening effects and concomitant flow instabilities. Secondly, it develops a phase diagram that unifies several scaling laws in cohesive flows. This unification asserts the prominent influence of grain de-bonding modes. These theories advance our understanding of rheology and diffusion in cohesive granular systems. They contribute to finding their long-sought-after physical origin and provide a framework to analyse the dynamics of more complex flows and other types of soft matter