Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Why Granular Media Are Thermal, and Quite Normal, After All   Two approaches exist to account for granular dynamics: The athermal one takes
grains as elementary, the thermal one considers the total entropy that includes
microscopic degrees of freedom such as phonons and electrons. Discrete element
method (DEM), granular kinetic theory and athermal statistical mechanics (ASM)
belong to the first, granular solid hydrodynamics (GSH) to the second one. A
discussion of the conceptual differences between both is given here, leading,
among others, to the following insights: (1) While DEM and granular kinetic
theory are well justified to take grains as athermal, any entropic
consideration is far less likely to succeed. (2) In addition to modeling grains
as a gas of dissipative, rigid mass points, it is very helpful take grains as a
thermal solid that has been sliced and diced. (3) General principles that
appear invalid in granular media are repaired and restored once the true
entropy is included. These abnormalities [such as invalidity of the
fluctuation-dissipation theorem, granular temperatures failing to equilibrate,
and grains at rest unable to explore the phase space] are consequences of the
athermal approximation, not properties of granular media.

24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph
25
The categories of this paper are: patt-sol
31
The categories of this paper are: ao-sci
22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
21
The categories of this paper are: adap-org
20
The categories of this paper are: acc-phys
34
The categories of this paper are: chem-ph
13
The categories of this paper are: quant-ph
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
8
The categories of this paper are: hep-ex
37
The categories of this paper are: supr-con
11
The categories of this paper are: hep-lat