A particle-scale force approach to granular segregation

Particle segregation in granular flows is challenging to model due to its complex, and sometimes contradictory, phenomenology. State-of-the-art segregation theories rely on configuration-specific closure relations at the flow level, but a general constitutive segregation model is missing. Here, an emerging bottom-up approach for segregation modeling is presented, which relies instead on detailed characterization of the forces relevant to segregation at the particle level. Using discrete element method simulations, we first characterize the driving and resisting forces of segregation on single intruder particles exerted by the surrounding particles in the flow, resulting in scaling laws for a buoyancy-like force, a shear-rate-gradient force, and a Stokes-like drag force. Then, we show how these force models lead to accurate prediction of the propensity for and velocity of gravity-induced size and density segregation, which match extensive experimental and numerical results. Finally, we discuss the extension of the force models from the single intruder limit to the finite-concentration mixture regime, which may lead to upscaled closure relations for continuum modeling of granular segregation.