Instability and Soft Patterning Laboratory, Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Pin 721 302, West Bengal, India.
Soft Matter. 2018 Oct 3;14(38):7883-7893. doi: 10.1039/c8sm01370e.
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 Cn was varied between 0.08 molar (M) and 2.0 M. The contact line of the evaporating droplets shows significant initial retraction for all Cn, 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 Cn (<0.5 M), it transforms to a single large salt crystal when Cn > 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 Cn. It transforms from a thick SDS ring surrounding a fractal-like deposit of NaCl crystallites at lower Cn to a peripheral deposit of NaCl crystals at higher Cn 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.
我们报告了在交联聚二甲基硅氧烷（PDMS）疏水基底上含有氯化钠（NaCl）稀溶液的水基液滴的蒸发干燥。盐浓度 Cn 在 0.08 摩尔（M）至 2.0 M 之间变化。所有 Cn 的蒸发液滴的接触线都显示出明显的初始回缩，然后才被固定。虽然最终形态包括围绕固定接触线沉积的几个小的 NaCl 晶体，但在 Cn <0.5 M 的低 Cn 液滴中，当 Cn > 0.7 M 时，它会转变为没有外围沉积的单个大盐晶体。我们进一步表明，当向盐溶液中添加阴离子表面活性剂十二烷基硫酸钠（SDS）时，沉积形态会发生剧烈变化。即使在含有表面活性剂的液滴中，最终沉积形态也会随着 Cn 的变化而显著变化。由于胶束形成和结晶之间的竞争，它从较低 Cn 时围绕 NaCl 微晶分形状沉积物的厚 SDS 环转变为较高 Cn 时的 NaCl 晶体外围沉积物。然而，沉积的 NaCl 晶体的晶体取向仍然不变，无论是否存在表面活性剂。
