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Atmospheric and Oceanic Optics

2014 year, number 1

Photophoresis and accommodation

Institut für Experimentalphysik, Johannes-Kepler-Universität, A-4040 Linz (retired), Lüfteneggerstraße 15, A-4020 Linz, Austria
Keywords: aerosols, vertical transport, photophoresis, radiometer forces, accommodation


There are two types of photophoretic forces, the ΔTS - (Crookes) and the Δα-force (Knudsen). This paper deals with the fundamental problem of distinguishing by experiment between both forces by using their different dependencies on pressure. We explored gravito-photophoresis of individual particles from three materials differing in their physical properties (carbon amorphous, crystalline, aluminium). On the grounds of available aerosol theory representation of the majority of cases is not feasible. Resolvable special cases and the data in their entirety, however, secure unambiguously earlier hypotheses that the force of gravito-photophoresis is normally caused by differences in the accommodation coefficient (Δα) over the surface. That conclusion is confirmed by a method for determining the size of particles from the force-pressure diagram which yields reasonable results. Hypothetical application of gravito-photophoresis to atmospheric aerosols now obtains empirical corroboration. Some aluminium particles surprisingly show changes in behaviour depending on pressure and irradiance. Here, the levitating force is determined not only (as usual) by the momentary irradiation, but also by the previous history of this factor. We assume that with some metals the irradiation can induce variations in the accommodation coefficient. Results of the investigations are applied to clarifying lasting problems of electro-photophoresis and apparent longitudinal photophoresis.