Intricate instabilities in drying drops

Complex fluid drops dry with a striking variety of patterns, from the ubiquitous coffee ring to a wide range and scale of cracks. Colloidal suspensions are often used in these experiments, and to date, much of this work has been focused on drops with a relatively low volume fraction of particles. The patterns are thought to be controlled by several factors such as surface wetting properties, evaporation kinetics, and particle interactions. Here, we present a study of the drying of a high-volume fraction suspension drop. We work with highly monodisperse silica and polystyrene colloids made in-house and study both the final pattern morphology, as well as the dynamics of crack growth. In this limit of high concentration, we see intriguing structures emerging: a single dimple appears near the center that then connects to growing radial fractures. The patterns we observe depend on both particle size and concentration. Furthermore, even for similarly sized particles, changing the material of the colloidal particle can qualitatively change the morphology of the crack pattern.