This sets up capillary forces by the interfacial tension between t These capillary forces provide the driving force for moving water through the pore surface. Small pores develop greater forces than do large pore At the beginning of the falling-rate period at point C in Fig. 9.5-lb. the water is being brought to the surface by capillary action, but the surface layer of water starts to recede belov the surface. Air rushes in to fill the voids. As the water is continuously removed. a point continuous films across the pores reached where there is insufficient water left to maintain and the rate of drying suddenly decreases at the start of the second falling-rate period at point D. Then the rate of diffusion of water vapor in the pores and rate of conduction of heat in the solid may become the main factors in drying In fine pores in solids, the rate-of-drying curve in the second falling-rate period may con form to the diffusion law; the curve is concave upward, as shown in F 9.5-2b. For very porous solids, such as a bed of sand, where the pores are large, the rate-of-drying curve in the second falling-rate period is often straight, and hence the diffusion equations do not apply. 3. Effect of shrinkage. A factor often greatly affecting the drying rate is the shrinkage of the solid as moisture is removed. Rigid solids do not shrink appreciably, but colloidal and fibrous materials such as vegetables and other foodstuffs do undergo shrinkage. The most serious ef fect is that there may be developed a hard layer on the surface which is impervious to the flow of liquid or vapor moisture and slows the drying rate; examples are clay and soap. In many foodstuffs, if drying occurs at t high a temperature, a layer of closely packed, shrunken cells, which are sealed together, forms at the surface. This presents a barrier to moisture m gration and is known as case hardening. Another effect of shrinkage is to cause the material to and change its structure. This can happen in drying wood Sometimes, to decrease these effects of shrinkage, it is desirable to dry with moist air This decreases the rate of drying so that th effects of shrinkage on warping or hardening at the surface are greatly reduced