Liquid Diffusivity

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Liquid Diffusivity

Fluids Engineering and Design Data
Hydraulics and Pneumatics Design and Engineering

Liquid Diffusivity

Transfer of liquid water through porous materials may be characterized as a diffusion-like process:

Equation 1
w^''_l = -ρ · D_l · dγ/dx

Where:

w^''_l = mass of liquid transferred through unit area per unit time, lb/h·ft²
ρ = liquid density, lb/ft³
D_l = liquid diffusivity, ft²/h
dγ/dx = moisture content gradient, ft^-1

D_l typically depends strongly on moisture content.

In measuring materials of high permeability, the finite rate of vapor diffusion through air in the cup may become a factor. Air-film resistance could then be a significant fraction of the sample’s resistance to vapor flow. Accurate measurement of high-permeability materials may require an accounting of diffusive rates across all air gaps (Fanney et al. 1991).

Transient measurement methods deduce the functional form of D_l · γ by observing the evolution of a one-dimensional moisture content profile over time. An initially dry specimen is brought into contact with liquid water. Free water migrates into the specimen, drawn in by surface tension. The resulting moisture content profile, which changes with time, must be differentiated to find the material’s liquid diffusivity (Bruce and Klute 1956).

Determining the transient moisture content profile typically involves a noninvasive and nondestructive method of measuring local moisture content. Methods include gamma ray absorption (Freitas et al. 1991; Kumaran and Bomberg 1985; Quenard and Sallee 1989), x-ray radiography (Ambrose et al. 1990), neutron radiography (Prazak et al. 1990), and nuclear magnetic resonance (NMR) (Gummerson et al. 1979).

Uncertainty in liquid diffusivity measurement is often large because of the need to differentiate noisy experimental data.

References:

ASTM Standard E96
Fanney et al. 1991
Gummerson et al. 1979