Despite its important technological applications, surfactant-enhanced (spontaneous) spreading on a solid surface and how to optimize it on surfaces with different wettabilities are not well understood. Spontaneous spreading involves a surface tension gradient (Marangoni stresses), which enhances spreading over a large area. Experimental observations reveal that the spreading rate and surfactant concentration have an optimum substrate wettability of 60 ± 5° (Hill, R. M. Curr. Opin. Colloid Interface Sci. 1998, 3, 247).This paper discusses why the optimum for surfactant-enhanced spreading requires an initial macroscopic three-phase contact angle of 60 ± 5°. An equation based on experimental evidence allows for the calculation of the surface tension gradient over time using data on the spreading rate, spreading macroscopic contact angle, and droplet spreading radius. This novel approach for estimating the surface tension gradient and explaining the optimum substrate wettability underscores the role of the surface tension gradient, viscosity, and substrate wettability in surfactant-enhanced spreading on solids. The roles of the spreading three-phase contact angle and surface tension gradient in surfactant-enhanced spreading were analyzed, demonstrating that the surface tension gradient contributes more significantly to the spreading rate than the contact angle. Fingering instability formation, an instability at the droplet spreading edge caused by the Marangoni stresses, also serves as evidence of the role that the surface tension gradient plays in surfactant-enhanced spreading. Furthermore, applications of surfactant-enhanced spreading were demonstrated, suggesting potential uses in oil spill removal, leaf pesticide delivery, and oil spill remediation. The goal of the proposed study is to use experimental evidence to develop a model for calculating the optimum spreading rate during the first several seconds of surfactant-enhanced spreading on a solid substrate.

中文翻译：

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表面活性剂增强在固体上的铺展：表面张力梯度、铺展接触角和粘度的作用


尽管表面活性剂在固体表面上的增强（自发）铺展以及如何在具有不同润湿性的表面上进行优化，目前尚不清楚。自发扩散涉及表面张力梯度（马兰戈尼应力），这增强了在大面积上的扩散。实验观察表明，铺展速率和表面活性剂浓度的最佳基材润湿性为 60 ± 5° （Hill， R. M.电流。意见。胶体界面科学1998， 3， 247）。本文讨论了为什么表面活性剂增强铺展的最佳效果需要 60 ± 5° 的初始宏观三相接触角。基于实验证据的方程允许使用有关扩散速率、扩散宏观接触角和液滴扩散半径的数据计算表面张力随时间的变化梯度。这种用于估计表面张力梯度和解释最佳基材润湿性的新方法强调了表面张力梯度、粘度和基材润湿性在表面活性剂增强对固体的铺展中的作用。分析了铺展三相接触角和表面张力梯度在表面活性剂增强铺展中的作用，表明表面张力梯度对铺展速率的贡献比接触角更显着。指法不稳定性的形成，即由马兰戈尼应力引起的液滴扩散边缘的不稳定性，也证明了表面张力梯度在表面活性剂增强扩散中的作用。 此外，还展示了表面活性剂增强撒布的应用，表明在溢油清除、叶子杀虫剂输送和溢油修复方面的潜在用途。拟议研究的目标是使用实验证据开发一个模型，用于计算表面活性剂增强在固体基材上铺展的前几秒钟内的最佳铺展速率。