Crucial role of sidewalls in velocity distributions in quasi-2D granular   gases
  Our experiments and three-dimensional molecular dynamics simulations of particles confined to a vertical monolayer by closely spaced frictional walls (sidewalls) yield velocity distributions with non-Gaussian tails and a peak near zero velocity. Simulations with frictionless sidewalls are not peaked. Thus interactions between particles and their container are an important determinant of the shape of the distribution and should be considered when evaluating experiments on a tightly constrained monolayer of particles. 
J.S. van Zon, J. Kreft, Daniel I. Goldman, D. Miracle, J.B. Swift,   Harry L. Swinney