  We report on the scaling between the lift force and the velocity lag
experienced by a single particle of different size in a monodisperse dense
granular chute flow. The similarity of this scaling to the Saffman lift force
in (micro) fluids, suggests an inertial origin for the lift force responsible
for segregation of (isolated, large) intruders in dense granular flows. We also
observe an anisotropic pressure/stress field surrounding the particle, which
potentially lies at the origin of the velocity lag. These findings are relevant
for modelling and theoretical predictions of particle-size segregation. At the
same time, the suggested interplay between polydispersity and inertial effects
in dense granular flows with stress- and strain-gradients, implies striking new
parallels between fluids, suspensions and granular flows with wide application
perspectives.
