Midair collisions enhance saltation
  Here we address the old question in Aeolian particle transport about the role of midair collisions. We find that, surprisingly, these collisions do enhance the overall flux substantially. The effect depends strongly on restitution coefficient and wind speed. We can explain this observation as a consequence of a soft-bed of grains which floats above the ground and reflects the highest flying particles. We make the unexpected observation that the flux is maximized at an intermediate restitution coefficient of about 0.7, which is comparable to values experimentally measured for collisions between sand grains. 
Marcus V. Carneiro, Nuno A. M. Ara\'ujo, Thomas P\"ahtz, Hans J.   Herrmann
physics.ao-ph cond-mat.soft physics.comp-ph physics.flu-dyn