Evaporative pattern formation by Aqueous Sodium Chloride solution at ambient and elevated temperature conditions

Droplet evaporation on solid substrate is of interest since last few decades due to its growing importance in several applicative fields. Presence of crystalline materials within a liquid drop makes the evaporation process critical as along with complex flow patterns inside an evaporative droplet an additional phenomena of crystallization takes place. By employing simple drop-deposition technique on solid substrate at ambient conditions conversion from ring-like to uniform deposit was observed with increasing NaCl salt concentration. NaCl-rings were obtained at higher salt concentration instead of a single deposit on addition of surfactant in the salt solution i.e., the deposition patterns were altered in presence of surfactant. At elevated temperatures, double ring to single ring conversion was noticed with stepwise increase in salt concentration. At higher salt concentration the inner ring disappeared and merged with the outer one to form a single thicker ring with the evolution of a no-deposition zone at the center of the droplet. The flow dynamics within the droplets and the mechanism of the drying pattern formation are qualitatively discussed.