(432d) Combined Size and Density Segregation in Granular Materials

In flowing granular mixtures of particles varying in either size or density alone, small particles sink (driven by percolation) and light particles rise (driven by buoyancy). However, when the constituent species vary from each other in both size and density, which particles will rise or sink remains unclear. In particular, modeling the segregation of mixtures of large, heavy particles and small, light particles is a challenge due to the opposing effects of the two segregation mechanisms. Here we use DEM simulations to study combined size and density segregation for a wide range of particle properties and flow conditions. Based on the simulation results, the segregation velocity is accurately modeled by particle size and density ratios through a function that is quadratic in concentration. Concentration profiles predicted by substituting this segregation velocity model into a continuum transport equation match DEM simulation results for quasi-2D bounded heap flows with different combinations of particle size and density ratios. Most surprisingly, the direction of segregation depends on the local concentration for a range of mixtures of large, heavy particles and small, light particles. Funded by NSF Grant CBET-1929265.