Humidity-induced diffusion

Both thermal transpiration and humidity-induced diffusion require a porous partition within the plant tissue, with pore diameters of <0.1-3 pm (Armstrong et al., 1994), which is the molecular mean free path length for Knudsen diffusion. Armstrong et al. (1991a) defined Knudsen diffusion... 

Humidity-induced pressurization is the result of vapor pressure differential between the leaf and atmosphere separated by a porous partition (plant cell membrane) (Figure 7.16). The total pressure will be greater on the more humid side. Humidity-induced diffusion is more important than thermal transpiration because it can be increased with temperature and can function at a constant temperature, as well as across temperature gradient (Armstrong et al., 1991a, 1991b). 

The relative importance of each of the processes described above is difficult to ascertain because all the processes function simultaneously and independently, and various interactive factors regulate each process. Humidity-induced pressurization is usually the most dominant process, regulating convective flows in many wetland plants. Species with cylindrical culms and linear leaves usually have internal pressurization potential (Brix et al., 1992) and may be able to grow in deeper water than species dependent on root oxygenation due to diffusion only (Brix et al., 1992). 

The horizontal wind components, potential temperature and specific humidity are used in METRAS as forcing fields. Changes in the scalar quantities temperature and humidity are mainly induced by advection and diffusion, i.e. processes that depend on the wind. Therefore, we define the conditions for writing the model results only in dependence of changes in the horizontal wind components. The model results were alternatively written at regular intervals (3 h, 6 h), if the... 

Figure 7 shows a compilation of diffraction patterns recorded during B - D transitions which were induced by a reduction of the relative humidity. The behavior of reflections III and IV is similar to that described above for the D -> B transitions. However the changes in the diffuse scattering are more complex. Initially (Fig. 7 a) the molecule is in a well-defined semicrystalline B-form and, in the centre of the pattern, the diffuse scatter is concentrated on layer-lines 2 and 3. As the relative humidity falls, these layer-lines become less sharp and diffuse scatter appears in the region between them. When the layer-line spacing of reflection IV is between 31 A and 34 A (Fig. 7 b, c, and d) the diffuse scatter retains the crosslike shape of the B-DNA transform, but when the spacing falls to between 24 A and 27 A this scatter is similar in overall intensity distribution to the D-DNA diffraction pattern (Fig. 7e). Sharp reflections begin to appear (Fig. 7e) with spacings characteristic of the D-form and grow in intensity and sharpness until a fully crystalline D form is attained (Fig. 7f).

In this present case, we will consider the nature of modes of sorption and permeation of water in polymers in terms of degradation induced changes in polymer composition. These changes can accentuate the effects of temperature-humidity cycling on water sorption and diffusion to cause major changes in properties with progressive degradation. 

In the case of composite laminates, new problems linked to the anisotropy of diffusion paths, the eventual role of interfacial diffusion and the role of pre-existing or swelling-induced damage appeared in the mid-1970s. The interest was mainly focused on the effect of humidity on carbon fibre/amine crosslinked epoxy composites of aeronautical interest. For the pioneers of this research (Shen and Springer, 1976), the determination of diffusion kinetic laws appeared as the key objective. Various studies revealed that, in certain cases, diffusion in composites caimot be modelled by a simple Pick s law and that Langmuir s equation is more appropriate. Carter and Kibler (1978) proposed a method for the parameter identification. At the end of the 1970s, the kinetic analysis of water diffusion into composites became a worldwide research objective. Related experimental results can be summarized as follows. 

Upon subsequent exposure to ambient environments in several humidity and temperature-controlled chambers these curvatures varied with time due to the two aforementioned contradictory time-dependent mechanisms. While the time-dependent moisture diffusion process serves as a stress-inducing mechanism, the time-dependent relaxation acts to reduce the level of those stresses. Note that relaxation depends on moisture content through the shift factor an m). 

Permeability is defined as the product of the diffusion constant and the solubility coefficient. Water vapor permeability (WVP] is defined as the time rate of water vapor transmission through unit area of flat material with a definite unit thickness induced by unit vapor pressure difference between two specific surfaces, and under indicated temperature and humidity conditions. (ASTM E 96, Standard Test Method for Water Vapor Transmission of Materials]. 

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