Morphology modulation in evaporative drying mediated crystallization of sodium chloride solution droplet with surfactant

We report the evaporative drying of an aqueous droplet containing a dilute solution of sodium chloride (NaCl) on a hydrophobic substrate made of cross-linked poly-dimethyl siloxane (PDMS). The salt concentration C-n was varied between 0.08 molar (M) and 2.0 M. The contact line of the evaporating droplets shows significant initial retraction for all C-n, before they get pinned. While the final morphology comprises a few small NaCl crystals deposited around the pinned contact line, in droplets with a low C-n (<0.5 M), it transforms to a single large salt crystal when C-n > 0.7 M with no peripheral deposition. We further show that the deposition morphology drastically changes when an anionic surfactant, sodium dodecyl sulfate (SDS), is added into the salt-solutions. Even in the surfactant-laden droplets, the final deposition morphology changes significantly as a function of C-n. It transforms from a thick SDS ring surrounding a fractal-like deposit of NaCl crystallites at lower C-n to a peripheral deposit of NaCl crystals at higher C-n due to competition between micelle formation and crystallization. However, the crystallographic orientation of the deposited NaCl crystals remains unaltered irrespective of the presence of surfactant.