Coffision integral between particles of disparate mass

Abstract

Two alternative approaches to expanding the binary collision integral for particles of very different mass in powers of their small mass ratio m/M are examined in detail. One of them in particular is found to be suitable for a subsequent expansion of the above quantity in series of spherical harmonics. This reduces to a simple algebra the labour of determining the angular-moment form of the Boltzmann equation for Lorentzian mixtures.