... 4 Introduction largely conceptual, this fills an important void in a theory of seminal importance in the physics of granular media. The statistical mechanics framework of Edwards has ... the problem of segregation when a mixture of grains of two different sizes was shaken [16, 17]. An equivalent granular ‘Hamiltonian’ was written down and solved to increasing levels of sophistication. ... [20], who equated it to the tangent of the angle of repose, by defining it to be the ratio of shear and normal stresses on an inclined pile of sand. While the work of Bagnold [5, 6] made it clear...