Surfactant-Enhanced Spreading on Solids: Roles of the Surface Tension Gradient, Spreading Contact Angle, and Viscosity

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.