Surface cracking, during drying

The question of the original location of the cracks must be adressed. Cracks during drying occur from microflaws expected to be present on the surface of the gel. Regarded as a whole, the network is under compression stress. However, Scherer [22] demonstrated that at the extremity of the flaw tips the solid part remains in tension. 

The amount of swelling and contraction in smectites is quite dramatic. Typically, soils containing large amounts of this type of clay will develop cracks that are 30 cm wide at the surface and greater than 100 cm deep, and these cracks will allow surface material to fall into them during dry periods. This characteristic is so unique that these types of soils are given their own name. They are called Vertisols the name is taken from the concept that material from the surface falls to the bottom of the cracks, resulting in inversion of the soil. 

Fig. 6.11 Optical microscopy of the surface lary forces during drying of propanol [Lei 9]. of a 25 pm thick micro PS layer on the Si sub- (c) SEM micrograph of a disintegrated micro strate, showing (a) crack formation in the wet PS film formed at high current density...

Finally, we note the value of rinsing the water-deposited film with methanol. As methanol is more volatile and has a lower surface tension, film cracking due to the capillary stresses exerted by the meniscus of the drying solvent on the walls of nanosized pores is reduced. Similar results have been reported by Goh.22 Films thicker than a few hundred nm exhibit cracks due to stresses that result during drying of the film, even after methanol treatment. Eliminating these cracks will require other modifications of the drying process. 

When the constant rate period is over, vapor phase pore diffusion plays an important role in the cracking of ceramic green bodies. The capillary pressure difference, AP, between the surface and interior of the green body during drying can be estimated from the flux of solvent, j, giving... 

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