Thermophoretic properties of nonspherical particles and large molecules

The Knp ≫ 1 gas-kinetic approach of Waldmann (1959, 1961) is extended to predict the thermophoretic properties of nonspherical aerosol particles, and massive nonspherical gas molecules. Calculations are presented for sphero-cylindrical particles (molecules) of arbitrary aspect ratio, L/R, predicting that in a local temperature gradient grad T they will thermophoretically drift: (i) more rapidly when their major axis is aligned with −grad T; (ii) at a velocity different from that of a sphere of radius R equal to the cylinder radius; (iii) at an angle with respect to −grad T when their major axis is not parallel to, nor perpendicular to, −grad T; and (iv) without a net torque tending to orient the particle with respect to −grad T. Important corollaries are that nonspherical particles in a temperature gradient should also experience new thermophoretically-induced coagulation mechanisms. We predict that the orientation-averaged thermal diffusion factor αT should increase approximately linearly with particle (molecule) length.