Generalized hydrodynamics of thermal transpiration, thermal force and friction force

Transport processes in a rarefied gas are treated phenomenologically by means of generalized hydrodynamics with appropriate boundary conditions. The integral mass and heat flows through a capillary of arbitrary width are calculated from the differential constitutive laws and boundary conditions. The coefficients connecting these flows with the driving forces - pressure and temperature differences - automatically fulfil the well-known integral second law requirements and Onsager symmetry which de facto had been incorporated in the basic generalized hydrodynamics and boundary conditions. The formula obtained for thermal transpiration can account for the experimental data over a wide pressure range. Furthermore, expressions for the frictional and thermal forces on a spherical particle suspended in a gas are derived. These expressions have the correct behavior all the way from the ordinary hydrodynamical to the Knudsen regime. In particular, thermal force data are well represented, with suitably chosen surface coefficients, for small and large ratios of the particle/gas heat conductives.