Cracking of Surface Films

Cracking in Drying Colloidal Dispersion

Shrinkage crack patterns have been studied over many years by reseachers in disciplines as diverse as Earth Sciences and Art History. Similar patterns appear time and again over several orders of magnitude in scale and in a wide variety of materials. Our present research concentrates on silica films around 100 microns in thickness.
Cracking in a Thin Film A liquid drop placed on a flat substrate is clearly thinnest at its edge, with a spherical cap shape and a given contact angle with the substrate. A sufficiently large drop is flattened in the middle by the effects of gravity such that it can be considered as a film, but the very edges must still meet the substrate at the same contact angle. As the film dries, liquid evaporates from the whole film in a uniform way, however since there is less liquid in the thin section at the edge, this area reaches close packing first. Capillary stresses cause a pressure gradient which pulls fluid from the bulk into the packed region. This fluid brings more particles with it, expanding the packed region so that a "compaction front" is seen to move across the sample. If the pressure in the sample reaches the maximum pressure sustainable by the packed bed, then a drying front follows the compaction front as the fluid recedes. Otherwise, the film remains wet until compaction is complete before drying out as one item.
A number of different crack patterns may be observed forming as the film dries. These include front following cracks, which proceed a certain distance behind the compaction front, and the island type cracks similar to those seen in dry mud. Understanding the conditions which control the type of cracking which occurs is a main focus of our current research.