What is the temperature of a granular medium?

TL;DR

This paper explores the applicability of thermodynamic temperature concepts to granular media across different regimes, analyzing how a temperature-like quantity can be defined and measured, and examining the implications of the Fluctuation-Dissipation Theorem.

Contribution

It provides a comprehensive review of defining and measuring temperature in granular systems using the Fluctuation-Dissipation Theorem across various regimes.

Findings

Temperature-like quantities can be defined in all regimes.

The Fluctuation-Dissipation Theorem's validity varies across regimes.

Interpreting granular temperature remains complex and context-dependent.

Abstract

In this paper we discuss whether thermodynamical concepts and in particular the notion of temperature could be relevant for the dynamics of granular systems. We briefly review how a temperature-like quantity can be defined and measured in granular media in very different regimes, namely the glassy-like, the liquid-like and the granular gas. The common denominator will be given by the Fluctuation-Dissipation Theorem, whose validity is explored by means of both numerical and experimental techniques. It turns out that, although a definition of a temperature is possible in all cases, its interpretation is far from being obvious. We discuss the possible perspectives both from the theoretical and, more importantly, from the experimental point of view.