(475i) Fractal like Patterns in Drying Droplets of Colloidal Dispersions

Droplets of colloidal solutions are known to form a rich tapestry of patterns upon drying on flat substrates. Their drying patterns range from the well-known coffee ring deposit, tunable ring structures and periodic crack formations. These patterns form at different drying conditions specified by temperature, humidity, substrate features and properties of the dispersion itself. In this study, we report the observation of a highly tenuous, fractal like connected structures in drying droplets of polystyrene colloids that are modified with a surface adsorbed layer of poly (vinyl alcohol) PVA. These fractal networks are highly persistent across a range of particle volume fractions, PVA concentrations, substrate features and to some extent, polymer type. We argue that the formation of these networks is due to the combination of three phenomena – an immersion capillary force that drives particles at the depinning stage in the drying process, a repulsive interaction between the drying substrate and the particles and finally a polymer mediated particle-particle bridge. We support our argument through a series of experiments where we purposefully nullify each of the phenomena, or a combination of the above-mentioned phenomena and demonstrate that the fractal patterns are no longer observed. The results of this study demonstrate the complex physics that underscores the drying of colloidal particles and suggests the need for theoretical developments to describe the drying physics vis-à-vis the above-mentioned interaction forces.