Front, drying

On the other hand, when the membrane is saturated, transport still occurs. This transport must be due to a hydraulic-pressure gradient because oversaturated activities are nonphysical. In addition, Buechi and Scherer found that only a hydraulic model can explain the experimentally observed sharp drying front in the membrane. Overall, both types of macroscopic models describe part of the transport that is occurring, but the correct model is some kind of superposition between them. - The two types of models are seen as operating fully at the limits of water concentration and must somehow be averaged between those limits. As mentioned, the hydraulic-diffusive models try to do this, but from a nonphysical and inconsistent standpoint that ignores Schroeder s paradox and its effects on the transport properties. 

Once the drying front enters the green body, the drying rate decreases abruptly. This decreasing rate continues until the large pores are essentially free of liquid, except for that which is trapped at the point of contact between the particles as shown in the inset of Figure... 

Instead of an abrupt drsdng front, there is a diffuse drying front (see Figure 14.2) [2] in which the capillaries (or pores) smaller than those... 

FIGURE 14.2 The drying front in a green body composed of monoeized 0.5 fxm SiOa particles. In this photo, saturated pores are white and empty pores are black. The drying front is fractally rough on the size scale of the partides (a) but smooth on a larger scale (b). Taken from Shaw [2]. Reprinted by permission of the Materials Research Society. 

The stress in ceramic film during drying have been measured by Chiu and Chima [19]. They have shown that the tensile stress is at its maximum value when the drying front penetrates the surface of the film, and this maximum stress is dependent on the size of the ceramic particles in the film. 

This result is also suggested by a steeper decrease of aqueous vapor pressure below 0°C (Table 2). Throughout the sublimation stage, a nearly constant subzero temperature is held in the portion of the specimen in which most of the ice crystals remain, but in the portion where the drying front has passed the specimen temperature rises quickly in accordance with ambient temperatures (Figure 4). 

Fig. 2 Velocity of the drying front in an infinite plate of wood for different temperatures and watar contents.

Fig. 8.7. Schematic illustration of the formation of a liquid-vapour meniscus in the pores of a gel during drying (a) and (b) are in the CRP with (b) the critical point situation (c) is in the FRPl with/ being adsorbed films at the unsaturated pore walls, w is the width of the drying front.

The liquid-vapour interface in the pore system is called the drying front. As shown by Shaw [7] it is fractally rough on the scale of pores, but stable on larger scale. The smaller the pore (and particle) size, the smaller the width w of the drying front (see Fig. 8.7c) with w defined as the distance from the most advanced to the least advanced part of the front. Shaw found that the value of w decreased as the velocity of the front increased ... 

Note that Eq. (8.6b) implies that the drying front roughens w increases) as it advances into the gel body because with a given external pressure, the pressure gradient in the drained region decreases and so does v. The existence of a certain width of the drying front will have important consequences for the drying characteristics of films as will be discussed later. 

Beyond the critical point (FRPl) the gel expands slightly as drying continues. This implies that around the liquid-vapour interface (the drying front) differential strains (and stress) gradients are relatively large. 

Fig. 8.11, Schematic illustration of stress formation and stress relaxation by cracking at the tip of a crack with length C. a and Oc are the externally applied stress and the stress at the crack tip respectively. Zone I is the stress relief zone, w represents the irregular drying front zone w, in zone II...

Another reason might be the more pronounced effect of the (irregular) drying front with width w in thinner specimen. If w becomes comparable with the membrane thickness, CRP and FRPl overlap and cannot be distinguished clearly. 

The gel network in the unsaturated dry part expands slightly when the drying front proceeds because capillary forces no longer act here and the stress... 

Equation (8.14) shows that the risk of cracking increases with increasing width of the drying front w (larger cracks) and with increasing drying rate. 

The first uncertainty concerns the fact that the theory is derived for bulk materials, thin-walled bodies or thick films. It is not certain whether extrapolation to thin films is allowed. This is particularly the case when the width of the drying front approaches the film thickness. 

T.M. Shaw, Movement of a drying front in a porous material, in C.J. Brinker, D.E. Clark and D.R. Uhlrich (Eds.), Better Ceramics Through Chemistry II. Materials Research Society Symposia, Vol. 73. Materials Research Society, Pittsburgh, PA, 1986, pp. 215-223. 

Routh, A.F. Russel, W.B. Horizontal drying fronts during solvent evaporation from latex films. AIChE J. 1998, 44 (9), 2088-2098. 

The visible solvent front is the dry front. The wet front has an R( value of about 0.75, and a pale yellow band may be seen at this point (before application of the locating reagent) due to traces of iron (Figure 9.1). It should... 

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