  This study numerically and analytically investigates the dynamics of a rotor
under viscous or dry friction as a non-equilibrium probe of a granular gas. In
order to demonstrate the role of the rotor as a probe for a non-equilibrium
bath, the molecular dynamics (MD) simulation of the rotor is performed under
viscous or dry friction surrounded by a steady granular gas under gravity. A
one- to-one map between the velocity distribution function (VDF) of the
granular gas and the angular distribution function for the rotor is
theoretically derived. The MD simulation demonstrates that the one-to-one map
accurately infers the local VDF of the granular gas from the angular VDF of the
rotor, and vice versa.
