Gel during drying

Liquid in gel during drying water alcohol (% %) Activity (gg 0.5 hr1) Volume (cc g ) inside pores of diameter ... 

Dieudonne P, Hafidi Alaoui A, Delord P, and Phalippou J. Transformation of nanostructure of silica gels during drying. J. Non-Cryst. Solids 2000 262 155-161. 

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.

Fig. 8.8. Rate of water loss from alumina gel versus water content of gel during drying for various initial thicknesses (0.75 0 3.0 1.8 A 0.8 mm). From Dwivedi [22].

The volume shrinkage of the gel during drying induces an increase of its stiffness. At a given time, the solid network is no more compliant and the meniscus recedes in the pores. At this moment, the stress is maximum since the curvature radius corresponds to that of the shrunk pore (assumed cylindrical). Associated to evaporation, the liquid flows from the core of e gel to the surface. This flow is hindered by the solid arms of the gel. A gel is badly permeable because the size of the pores lies mainly within the range 0.2-10 nm indicating that a gel is a mesoporous material. According to the Darcy s law, the liquid flow, /, is related to permeability, D, by the relation ... 

This is analogous to the swelling of polyacrylamide gels when immersed in water (21], where the lower chemical potential of the pore liquid drives flow into Che gel. During drying, the source of liquid is the interior of the body, so flow is toward the surface. 

Figure 4.4. Growth ofUsteria innocua on the surface of a gelatin gel during drying at 18°C. The points correspond to the experimental measurements and the lines correspond to the predictions of the model...

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