Ultrasonic waves in classical gases.

The velocity and absorption coefficient for the plane sound waves in a classical gas are obtained by solving the Boltzmann kinetic equation, which describes the reaction of the single-particle distribution function to a periodic external field. Within the linear response theory, the nonperturbative dispersion equation valid for all sound frequencies is derived and solved numerically. The results are in agreement with the approximate analytical solutions found for both the frequent- and rare-collision regimes.

Viscosity of a classical gas: The rare-collision versus the frequent-collision regime.

The shear viscosity η for a dilute classical gas of hard-sphere particles is calculated by solving the Boltzmann kinetic equation in terms of the weakly absorbed plane waves. For the rare-collision regime, the viscosity η as a function of the equilibrium gas parameters-temperature T, particle number density n, particle mass m, and hard-core particle diameter d-is quite different from that of the frequent-collision regime, e.g.