Drag force and diffusion of small planar structures: A gas kinetic theory analysis and molecular dynamics study.

The drag force on planar structures of arbitrary shape is derived in free molecular flow using gas kinetic theory. The theory is formulated by considering the anisotropic intermolecular potential between the particle and gas molecules, in the limits of specular and diffuse scatterings. The drag force theory formulated by Dahneke [J. Aerosol Sci. 4, 147 (1973)10.1016/0021-8502(73)90066-9] for disklike bodies in free-molecular flow is shown to be a special case of the theory derived herein. The Einstein-Smoluchowski relationship is generalized with anisotropic drag to structures of arbitrary shape. Binary diffusion coefficients of planar molecules (benzene, naphthalene, phenanthrene, pyrene, coronene, and ovalene) in nitrogen are calculated using gas kinetic theory. The diffusion coefficients are in close agreement with available experimental data and with molecular dynamics simulations.