Composition Dependence of the Thermal-Diffusion Factor of a Dusty Gas

The composition dependence of the thermal-diffusion factor αT of a dust–gas mixture is derived. In the case of monatomic gas, (αT)−1 is shown to vary linearly with mole fraction, but nonlinear contributions arise in the case of a polyatomic gas as a result of inelastic gas–gas collisions. Application of the result to a description of thermal transpiration in terms of the “dusty-gas” model yields a correction to the previously derived equation for thermal transpiration and may have a significant effect on the determination of rotational collision numbers from thermal transpiration measurements.