  We have calculated microscopically bulk viscosity of hadronic matter, where
equilibrium thermodynamics for all hadrons in medium are described by Hadron
Resonance Gas (HRG) model. Considering pions and nucleons as abundant medium
constituents, we have calculated their thermal widths, which inversely control
the strength of bulk viscosities for respective components and represent their
in-medium scattering probabilities with other mesonic and baryonic resonances,
present in the medium. Our calculations show that bulk viscosity increases with
both temperature and baryon chemical potential, whereas viscosity to entropy
density ratio decreases with temperature and with baryon chemical potential,
the ratio increases first and then decreases. The decreasing nature of the
ratio with temperature is observed in most of the earlier investigations with
few exceptions. We find that the temperature dependence of bulk viscosity
crucially depends on the structure of the relaxation time. Along the chemical
freeze-out line in nucleus-nucleus collisions with increasing collision energy,
bulk viscosity as well as the bulk viscosity to entropy density ratio
decreases, which also agrees with earlier references. Our results indicate the
picture of a strongly coupled hadronic medium.
