Evaporative Drying of Sodium Chloride Solution Droplet on a Thermally Controlled Substrate

We report the evaporative drying-mediated pattern formation of dilute aqueous NaCl solution droplets on a hydrophobic surface made of cross-linked PDMS at elevated temperatures. Experiments were performed with various salt concentrations (0.08 to 2 M). Depending on the substrate temperature (T_S) and initial salt concentration (C_n), a variety of unique deposition morphologies are obtained. Double salt rings are obtained at low C_n (0.08 and 0.1 M) at T_S ≈ 75 °C. With the increase in C_n, the deposition morphology transforms into a single peripheral ring with a central deposition zone that is devoid of any salt deposition. On the other hand, at lower C_n and lower T_S, a single outer ring deposit surrounding a central region covered with dendritic salt deposits is observed. The variation in the morphology as well as the number of rings formed is attributed to the relative strengths of the radially outward capillary-driven flow, inward Marangoni flow, and inward diffusive transport of the salt ions. A morphology phase diagram shows the collective dependence of the final deposition pattern as a function of both T_S and C_n.