Hexagons and Interfaces in a Vibrated Granular Layer
  The order parameter model based on parametric Ginzburg-Landau equation is used to describe high acceleration patterns in vibrated layer of granular material. At large amplitude of driving both hexagons and interfaces emerge. Transverse instability leading to formation of ``decorated'' interfaces and labyrinthine patterns, is found. Additional sub-harmonic forcing leads to controlled interface motion. 
I.S. Aranson, L.S. Tsimring, and V.M. Vinokur