Gaseous diffusion in porous media. Part 3 - Wet granular materials

Abstract

The diffusion of hydrogen through granular materials partly saturated with water was measured by a non-steady state technique previously described. The two types of sample used consisted of solid particles (unimodal poresize distribution) and porous particles (bimodal pore-size distribution), and all measurements were made on samples being dried from saturation. Coefficients of diffusion D were calculated, and for the range over which the larger pores were emptying the empirical equation D = Dv(/v)σ fitted all materials, where epsilon is the fractional air-filled volume, v is the volume occupied by the larger-pore phase, and where Dv is the diffusion coefficient when only this phase is air-filled. For all materials σ approximately equals 4 whether the samples were uniform or of mixed sizes. No such relationship existed over the subsequent range in which the smaller pores were drained. Over this range D must be a function of at least five independent variables - the total porosity T, the crumb porosity c, the shape factor for the crumbs or inter-crumb pores k, the shape factor for the particles forming the crumbs or crumb pores kc, and the moisture content of the sample. The spatial distribution of pores within a porous medium can be as important as the sizes of the pores. The factors k and m, previously introduced as particle-shape factors, now have a greater significance as measures of the geometrical complexity of a porous system. Adding water can either increase or decrease the complexity, depending on the amount added and the nature of the system. The agricultural significance of diffusion between the crumbs Dv and within the crumbs Dc is discussed, and it is suggested that Dc, or its associated complexity factor kc, might be used as an index of soil structure.