  A novel process is described that produces horizontal size segregation in a
vertically vibrated layer of granular material. The behavior is a consequence
of two distinct phenomena that are unique to excited granular media: vibration
which causes the larger particles to rise to the top of the layer, and a
vibrating base with a sawtooth surface profile which can produce stratified
flows in opposite directions at different heights within the layer. The result
of combining these effects is that large and small particles are horizontally
driven in opposite directions. The observations reported here are based on
computer simulations of granular models in two and three dimensions.
