Comment on "Mechanical analog of temperature for the description of force distribution in static granular packings"

TL;DR

This paper critiques a proposed thermodynamic analogy for force distribution in granular packings, arguing that the entropy formulation used is incorrect, leading to inaccurate predictions especially for rigid grains.

Contribution

It identifies flaws in the entropy formulation of Ngan's thermodynamic approach, challenging its validity for modeling force distributions in granular materials.

Findings

The entropy form used in Ngan's model is incorrect.

The method's accuracy diminishes with increasing grain rigidity.

Published results show increasing errors for rigid grains.

Abstract

It has been proposed by Ngan [Phys. Rev. E 68, 011301 (2003)] that the granular contact force distribution may be analytically derived by minimizing the analog of a thermodynamic free energy, in this case consisting of the total potential energy stored in the compressed contacts minus a particular form of entropy weighted by a parameter. The parameter is identified as a mechanical temperature. I argue that the particular form of entropy cannot be correct and as a result the proposed method produces increasingly errant results for increasing grain rigidity. This trend is evidenced in Ngan's published results and in other numerical simulations and experiments.