The underlying physical mechanism behind the BNE is still not well understood. There are a number of competing theories as to what the dominant mechanism is, including reorganization of grains, convection, condensation/percolation, and entropy/gravity. The last of these is intriguing because it comes from a statistical mechanics argument and can be used to determine the nature of the crossover from the BNE to the RBNE without having to worry about the microscopic details of the interaction. It can be shown that a statistical mechanical model of a binary granular mixture undergoes a phase transition from a “paramagnetic” phase in which varying the mass ratio of the grains smoothly alters the average difference in height of the large and small grains to a “ferromagnetic” phase in which this difference in height jumps sharply as a function of mass ratio. The critical exponents of this transition in mean field theory are identical to those found for the Ising model. Experimental studies of the BNE are still somewhat inconclusive
regarding the existence of the RBNE in the parameter regimes predicted by theory, although the effect has been observed [8]. Since one of the main features that separates some theoretical mechanisms from others is their prediction regarding the RBNE, testing this effect in a laboratory setting is of the utmost importance to unravelling the Brazil nut problem.