Growing non-equilibrium length in granular fluids: from experiment to   fluctuating hydrodynamics
  Velocity correlations in a 2D granular fluid are studied in experiments and numerical simulations. The transverse component of the velocity structure factor reveals two well defined energy scales, associated with the external "bath temperature" $T_b$ and with the internal granular one, $T_g<T_b$, relevant at large and small wavelengths respectively. Experimental and numerical data are discussed within a fluctuating hydrodynamics model, which allows one to define and measure a non-equilibrium coherence length $\xi$, growing with density, that characterizes order in the velocity field. 
Giacomo Gradenigo, Alessandro Sarracino, Dario Villamaina, and Andrea   Puglisi
cond-mat.stat-mech