Dimples and Voids in Dense Drying Drops

Despite their simple appearance, sessile droplets of colloidal suspensions display a diverse range of phenomena when they dry, from ring depositions to a variety of mechanical instabilities. A multiplicity of factors can influence the final dried pattern such as evaporation kinetics, surface chemistry, colloidal interactions, particle anisotropy, and the volume fraction of the suspension. We observe a novel "Dimple-Void" instability in dense drying suspension drops; this instability appears when the colloidal volume is at and above 30 percent of the drop volume and forms on the order of tens of milliseconds. Here we present an experimental study of the dynamics and morphology of this instability using high-resolution, high-speed imaging and profilometry, and show how the colloidal particle size as well as material affects the dynamics of this system.