Drying Colloidal Suspensions: Simple Patterns and Complex Flows

Sessile drops of colloidal suspensions display a diverse range of phenomena when they dry, from the ubiquitous coffee ring to a wide variety and scale of cracks. There are numerous factors that influence the final drop pattern such as surface chemistry, evaporation kinetics, colloidal interactions, as well as particle anisotropy and size. Much of the previous work on this problem has been focused on suspensions where the volume fraction of particles is at most 1%. Here, we present a broad experimental study of drying suspension droplets with volume fractions ranging from 0.1% to 45%. We work with highly monodisperse silica colloids made in-house via the Stöber process and examine both the final pattern morphology and the dynamics of crack formation in the drying droplet. In the limit of high concentration, we see intriguing structures emerging: a single dimple appears near the center of the droplet that then connects to growing radial fractures. Analysis of these features seems to suggest that they result from a transition from a Stokes-dominant flow at lower volume fractions to a "Darcy-like" flow at higher volume fractions.