A history and state-of-the-art of accommodation coefficients

Abstract

This paper reviews theory and measurements of transport processes between small particles and the surrounding gas. Evaporation and condensation coefficients and gas uptake coefficients are of particular interest. There has long been a great difference in coefficients reported by different experimentalists, and much of this disagreement is considered in this overview. A brief review of the kinetic theory of gases is provided to describe molecular transport to or from a surface when the mean free path of molecules, ℓ, is large compared with the particle dimensions, that is, when the Knudsen number is large. For a sphere of radius a the Knudsen number, Kn = ℓ / a. The condition Kn >> 1 is called the free molecule regime, and for Kn << 1 continuum theory applies. It is shown that accommodation coefficients cannot be determined by experiments operating in the continuum regime. At intermediate Knudsen numbers (the Knudsen regime) transport theory is more difficult, but results based on solution of the Boltzmann equation describing the evolution of the molecular velocity distribution are reviewed. With the advent of high-speed computers transport theory has been supplemented by molecular dynamics calculations, and these calculations are often at odds with experimental measurements of accommodation coefficients. Examples are provided. Experimental methods surveyed include Knudsen cell methods, jet tensimetry, electrodynamic levitation experiments, expansion cloud chamber measurements, and vibrating orifice aerosol generator (VOAG) techniques. VOAG measurements are particularly useful for studying processes over small times, and theory and results related to VOAG experiments are presented. More recent experimental measurements of condensation coefficients involving the use of molecular beams are reported. There is a growing body of evidence that accommodation coefficients are of order unity in many cases, but coefficients smaller than 0.01 are still reported, particularly for uptake coefficients.