Let us take a reference particle which is assumed to be stationary and all other particles are approaching towards it. Consider a spherical shell surrounding the reference particle as shown in the figure below.
Fig. 10.9: Theoretical spherical surface surrounding a reference particle
Number of particle crossing the spherical shell per unit area per unit time can be written by taking into account the diffusion of particles due to concentration gradient given by Fick’s law and energy of attraction between the particles. Thus flux ‘j’ is given by
where r is the center to center distance between the two particles, N is number of particles per unit volume and ƒ is friction factor. Second term in right hand side of eq. (10.17) would be zero for r greater than 2R because we have assumed that energy of interaction between two particles is zero.
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