Zhong Xin, Ren Junheng, Chong Karen Siew-Ling, Ong Kian-Soo, Duan Fei
School of Mechanical and Aerospace Engineering , Nanyang Technological University , 50 Nanyang Avenue , Singapore 639798 , Singapore.
State Key Laboratory of Multiphase Flow in Power Engineering , Xi'an Jiaotong University , Xi'an 710049 , P. R. China.
Langmuir. 2019 Apr 2;35(13):4509-4517. doi: 10.1021/acs.langmuir.9b00170. Epub 2019 Mar 22.
Wetting transitions induced by varying the components in a solution of a drying droplet can lead to its evolving shape on a textured surface. It can provide new insights on liquid pattern control through manipulating droplet solutions. We show the pronounced transitions of wetting for surfactant solution droplets drying on a micropyramid-patterned surface. At low initial surfactant concentrations, the droplet maintains an octagonal shape until the end of drying. At intermediate initial surfactant concentrations, the early octagon spreads to a square, which later evolves to a stretched rectangle. At high initial surfactant concentrations, the droplet mainly exhibits the "octagon-to-square" transition, and the square shape is maintained until the end. The octagon-to-square transition occurs at similar temporal volume-averaged surfactant concentrations for the various initial surfactant concentrations. It results from the dependence of the surface energy change of spread over the micropyramid structure on the temporal volume-averaged surfactant concentration. At high initial surfactant concentrations, the accumulation of the surfactant near the contact line driven by outward flows could raise the local viscosity and enhance the pinning effect, leading to the great suppression of the "square-to-rectangle" transition.
通过改变正在干燥的液滴溶液中的成分所引发的润湿性转变，能够导致其在有纹理的表面上呈现出不断变化的形状。这可以为通过操控液滴溶液来控制液体图案提供新的见解。我们展示了在微金字塔图案表面上干燥的表面活性剂溶液液滴的显著润湿性转变。在低初始表面活性剂浓度下，液滴在干燥结束前保持八边形形状。在中等初始表面活性剂浓度下，早期的八边形会扩展为正方形，随后演变成拉伸的矩形。在高初始表面活性剂浓度下，液滴主要呈现“八边形到正方形”的转变，并且正方形形状会一直保持到干燥结束。对于各种初始表面活性剂浓度，八边形到正方形的转变发生在相似的时间体积平均表面活性剂浓度下。它是由铺展在微金字塔结构上的表面能变化对时间体积平均表面活性剂浓度的依赖性导致的。在高初始表面活性剂浓度下，由向外流动驱动的表面活性剂在接触线附近的积累会提高局部粘度并增强钉扎效应，从而极大地抑制“正方形到矩形”的转变。
