Influence of humidity on the cracking patterns formed during the drying of sol-gel drops

Optical microscopy has been used to study the sequence of events associated with drying of a silica-based sol-gel. Drops of sol containing 50 wt% silica were deposited on glass slides and dried in cells containing sulphuric acid as a drying agent. The drops, on the slide, were about 3.5 mm in diameter and before the onset of drying had a spherical cap shape. At the start of drying the rim of the drops became pinned to the glass surface. The humidity of the drying environment had a strong effect on shrinkage within the drop and the processes that occurred during drying. In very dry environments evaporation occurred primarily from the edges of the drop and resulted in a radial concentration gradient in the gel. Shrinkage resulted in a pattern of radial cracks that spread from the outside in a direction orthogonal to the iso-concentration profiles. Cracking was preceded by shear deformation. Drying was more uniform in humid environments and occurred from both the rim and the top surface of the drop. Wrinkling of the skin of gel that formed on the surface of the drop was followed by radial and circumferential cracking, except in very humid environments. The results are attributed to the influence of the radius of curvature of the drop surface on the rate of evaporation and the relative rates of flow and diffusion in the drying sol-gel. (C) 2002 Kluwer Academic Publishers.