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Samantha McBride1 Rachael Skye1 Kripa Varanasi1

1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

The coffee-ring effect refers to the phenomena in which a drop of a colloidal suspension evaporates on a substrate and deposits an ordered ring-structure due to the outward radial evaporative flow. This phenomena has been well-investigated due to numerous practical applications including ink-jet printing, microscale separations, and sensing/diagnostics. However, the phenomena has been less quantified for a crystallizing solution. When a drop of salt solution is evaporated, the salt becomes supersaturated and crystals begin to emerge at the substrate. The emerging crystals alter the local wettability of the substrate and fundamentally alter fluid dynamics of evaporation, which in turn alters the form of deposit left behind. Here, we investigate the role of energetic interactions between the substrate, crystals, and solution. We systematically vary the substrate interfacial chemistry by deposition of silane groups of various surface energies, and use four different crystal systems to probe this effect. We find that crystals with favorable interactions with the underlying substrate chemistries alter deposit morphology, while non-interacting crystals behave similarly to colloidal particles, and develop phase diagrams predicting this behavior. These results can be used for fine-tuned control and understanding of crystalline evaporative deposits.

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