Shaping Droplets: Colloids and Instabilities

In this thesis, we investigate several different experimental configurations of sessile droplets; some droplets are evaporating, some contain colloidal particles and some are undergoing instabilities at some point during their lifetime. In the first three chapters of the thesis, we use a water-repellent substrate to force the droplets to retain an almost spherical shape during their evaporation. We investigate the influence of suspended colloidal particles as well as dissolved agents such as salt, on the evaporation process. For colloidal particle suspension droplet we categorise the resulting particle clusters, depending on their initial particle concentration. For salty droplets, we find a strong dependence of the droplet’s fate on the ambient humidity, and some of the droplets will not evaporate fully but reach a stable, liquid volume. In specific droplet evaporation cases, we observe a peculiar oscillation instability of the droplet interface, which we investigate and determine the oscillation frequency of. In the last three chapters of the thesis, we deposit droplets on soft, deformable surfaces, either on a pre-wetted thin film, or a bath of an immiscible liquid. We investigate the ensuing instabilities in both cases (a capillary ripple in the pre-wetted film and an atomisation of the droplet on top of the bath). For the latter case, we also determine the influence of added dye for imaging enhancement. The meaning of the title of the thesis is therefore twofold: either we can shape the droplet, by choosing the substrate accordingly, or the droplets can also shape their surroundings, by deforming either the soft surface underneath, or acting as shaping moulds for suspended colloids and dissolved agents