Rheology and thermodynamics of a granular pile

From the onset of the subject, granular media have been defying the toolkit of statistical mechanics thus hindering our understanding of their thermodynamical and rheological properties. In the present work, we offer a resolution to this problem in the case of static granular media by considering a collective behavior of 2D identical balls forming a granular pile in the gravity field. Besides the uncertainty due to rough substrate on which the pile is built, we uncover another one due to ambiguities occurring in the positions of some interior balls. Both are responsible for the thermodynamic description of the granular pile. In particular, a pile is characterized by three temperatures: one is infinite, the other is negative, and the third is of higher-order. Also, considering the fields of ball displacements and the normal force deviations from the ideal isosceles triangular structure of a regular pile reveals the hyperbolic nature of the displacement field and ability of the force field to change the characteristic type from hyperbolic to elliptic. The latter property not only explains the origin of force chains and provides an adequate description of the rheology of static granular media, but turned out to be instrumental for understanding the granular media thermodynamics.