Moisture movement, driving potential

The driving potential assumed for moisture movement based on Equation 39 is the moisture concentration Other driving potentials may also be assumed. Table I lists the potentials that have been proposed, the resulting transport coefficients, and their relationships to D in each case (59). Although one or more of these other potentials may be more descriptive of the driving force for moisture movement, the discussion that follows will be restricted to the diffusion coefficient because it is so well established in the literature, and can be related to any of the others. Furthermore, it appears unchanged in the unsteady-state diffusion equation (Pick s second law), unlike any of the other coefficients. Thus Pick s second law may be written, for one dimension, as... 

Waananen et al. (1993), in their review of drying models, note that most models in their final form express the driving force for moisture movement in terms of a moisture concentration gradient. However, the true potential for transfer may be different, namely, differences in chemical potential, as explored in greater detail by Keey et al. (2000). In theory, the diffusion coefficient will be independent of moisture concentration only if the moisture is unbound) but concentration-independent diffusion coefficients have been successfully used in some cases over a wide range of moisture contents. 

To understand the mechanism further, a microscopic investigation on the moisture content in clay is required. The osmotic suction potential was introduced as the driving force of moisture movement, described in Section 31.3, and was successively applied to the prediction of moisture movement in wet clay [15,16]. The theoretical analysis on the two-dimensional moisture transfer of cylindrical clay was performed taking into account the effects of both osmotic suction and strain-stress caused by the shrinkage [17,18]. However, only the transient mass-transfer equation was analyzed, assuming a constant drying rate on the external surface of the... 

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