Onset of Dynamic Wetting Failure in the Presence of Surfactants

Abstract

Wetting phenomena are omnipresent in nature and play an important role in various industries. During a dynamic wetting process, an ambient fluid is displaced by another one (the wetting fluid) on a solid surface. Studies have shown that dynamic wetting fails when the process speed exceeds a certain critical value. In the coating industry, wetting failure leads to air entrainment which degrades product quality and thus limits the production rate. Therefore, dynamic wetting failure has attracted the attention of many researchers due to its industrial significance. Surfactant molecules are common additives in coating liquids due to their special nature. They are often used as wetting agents to reduce surface tension and enhance wetting of the coating liquids. Despite the wide usage of surfactants in coating liquids, relatively little is known about the influence of surfactants on dynamic wetting failure in coating processes. The objective of this thesis is to advance the fundamental understanding of the influence of surfactants on dynamic wetting failure. A hydrodynamic model is developed to describe the displacement of a receding fluid by an advancing fluid in a rectangular channel and a curtain coating geometry. Numerical results show that Marangoni stresses resulting from surfactant concentration gradients along the interface promote the onset of dynamic wetting failure due to the thinning of the receding phase between the fluid interface and the solid substrate. In a study on curtain coating, numerical results show that hydrodynamic assist mainly arises from the hydrodynamic pressure generated by the inertia of the impinging curtain. This pressure leads to a strong capillary-stress gradient that pumps air away from the DCL and thus increases the critical substrate speed for wetting failure. A complementary experimental study is also conducted where a solid substrate is driven through a tank filled with surfactant solutions. It is found that the critical substrate speeds associated with the onset of wetting failure in surfactant solutions are increased when surfactant concentration increases. The comparisons between numerical and experimental results shed light into the mechanisms through which surfactants influence dynamic wetting failure. For the more viscous solutions used in this work, the change in the critical speed as surfactant concentration increases can be explained by accounting for the decrease in equilibrium surface tension and static contact angle when surfactants are present. However, for the lowest viscosity liquid, the comparison suggests other surfactant-induced mechanisms (e.g., Marangoni stresses) may play a more important role.