A smooth surface is desired, while spraying colours (on a car) or  while
printing a photograph. Surfaces made by deposition/evaporation of
particles can vary in their roughness. Depending on the rules of
deposition/evaporation one may get a phase transition (non-equilibrium)
from a rough (disordered) surface to a smooth (ordered) one. 
The criticality, thus obtained, falls generically in  KPZ universality
class [ PRL 56, 889 (1996)].
   KPZ transition in 2-dimension is less understood.  The following
interesting paper describes the surface growth in  a experiment on 
a pile of rice.
Ref : Avalanche dynamics, surface roughening, and self-organized criticality:
           Experiments on a three-dimensional pile of rice  ,  (pdf file is here)     

         C. M. Aegerter, R. Günther, and R. J. Wijngaarden

 , Phys. Rev. E 67, 051306 (2003)

I would like you explain their results:
1) Describe their Experiment
2) Explain their  result
3) Compare  KPZ exponents in 1, 2 and 3D
 4) Can you think of a simple model ?