Step into the Ring: the role of particle shape on deposition patterns in dense drying droplets

Sessile droplets of colloidal suspensions display a diverse range of phenomena when they dry, and one of the most common of these are the ring stains from spilled drops of coffee on your countertop. This occurs because a radial fluid flow is induced by strong evaporation at the pinned edge of the droplet. This flow transports suspended particles to the edge of the droplet yielding the ring pattern seen upon drying. Many factors can influence the final dried pattern such as evaporation kinetics, surface wetting, colloidal interactions, particle anisotropy, and the volume fraction of the suspension. Due to the challenge of synthesizing rod shaped particles, much less attention has been given to investigating how rod particulate suspensions compare with their spherical counterparts, and initial studies suggested that suspensions with rod shaped particles would not form ring patterns upon drying at all. Here we present a broad experimental study of the dynamics and morphology of sessile dried deposits for silica particles suspended in water with aspect ratios varying from 1 to 11 and volume fractions ranging from 0.01 to 0.4. We observe that a ring deposition forms for all particle aspect ratios at low concentrations. In general, we find that the final dried pattern, quantified using both microscopy and optical profilometry, is controlled both by the concentration of the suspension and the aspect ratio of the colloids present in the suspension.