  Discovering fundamental principles governing the dynamics of granular media
has been a long-standing challenge. Recent predictions of detailed balance
steady states (DBSS), supported by experimental observations in cyclic shear
experiments of planar granular systems, called into question the common belief
that the detailed balance principle is only a feature of equilibrium. Here, we
first show analytically that DBSS in planar granular dynamics arise when a
certain conditional cell order distribution is independent of the condition. We
then demonstrate that this condition is met in rotational shear experiments,
which indeed also give rise to robust DBSS. This suggests that DBSS not only
exist but are also quite common. We also show that, when the unconditional cell
order distribution maximises the entropy, as has been found recently, then this
distribution is determined by a single parameter - the ratio of splitting and
merging rates of cells of any arbitrary order. These results simplify the
modelling of the complex dynamics of planar granular systems to the solution of
recently proposed evolution equations, demonstrating their predictive power.
