Physical Components of the Diffusivity Coefficient

Isothermal horizontal infiltration experiments were conducted in 53–63 µm, 74–88 µm, or 105–125 µm glass bead fractions with water, 6% ethyl alcohol water solution, 20% ethyl alcohol water solution, ethyl alcohol, and methyl alcohol. From these experiments, the diffusivity coefficient was determined using the Bruce and Klute (1956) procedure and D (θ) was established for the fluid content, θ, from 0% to saturation. The D (θ) variation in the fluid content range between 3 and 30% was investigated. The functional relationships between D (θ) and the properties of the fluid, the geometry and the solid matrix were investigated. The physical components of D (θ) were determined using: D ( θ ) = (r * 2R * 2. σ   cos   α μ − 4.10 − 4   Δ H v + 5 ) ⋅   10 − 2   θwhere r * and R * are the mean radius of pores and beads, respectively, σ, µ and Δ H v are the surface tension, viscosity and molar heat of vaporization of the fluid, respectively, and α is the fluid‐solid contact angle. The physical significance of the diffusivity coefficient given by Laroussi and De Backer (1975) defining this coefficient as the ease with which the fluid particles spread from a wetting front, is clarified and confirmed.