Morphology models for cracking drying droplets*

During the drying of complex fluids, such as polymer solutions or colloidal dispersions, a liquid phase is transformed into a solid. Compared to the case of a pure drop of volatile liquid, the presence of a particulate phase changes the evaporative process and leads to hydrodynamical and mechanical instabilities, sometimes resulting in cracking. The onset of cracking during thin-film deposition, photolithography, and colloidal assembly can ultimately lead to failure of the fabricated product. In contrast, cracking plays an advantageous role in a number of applications ranging from medical diagnostics to high-resolution nano-patterning. Cracking during the evaporative process is an area of ongoing study. We propose a hierarchy of models to capture a range of possible fracture behaviours. In our models, we account for a variety of phenomena, including compaction, evaporation, and substrate interactions. We show that the interplay between these physical mechanisms lead to a variety of crack patterns, which can be seen experimentally.