Layering in bidisperse granular flows at high size ratios

Segregation in flowing granular mixtures having a large difference in particle diameters differs qualitatively from mixtures having a small difference. In particular, gravity driven flows of size-bidisperse mixtures with diameter ratios ($R$) greater than about three can exhibit such phenomena as reverse segregation and layering at high concentrations of the fine (smaller) particles. We investigate layering of size-bidisperse granular mixtures in a simple uniform shear flow in the absence of gravity using Discrete Element Method (DEM) simulations with $2<R<6$ and large particle concentrations by volume in the range $0.05<c_l<0.6$ . Surprisingly, even in the absence of shear rate gradients and pressure gradients, layers of large particles form parallel to the shear planes for $R$ greater than about 2.7 and $c_l$ greater than about 15%. The distance between the centers of adjacent large particle layers is approximately the sum of the diameters of one fine particle and one large particle and is largely independent of $R$ and $c_l$ . The mechanism behind the formation of these layers is likely shear induced collisions between neighboring layers that stabilize layers when the mean intra-layer concentration is sufficiently large.