Effects of surfactants in the spreading of liquids on solid surfaces

Abstract

Advancing and static contact line experiments using the Wilhemy technique are conducted to understand the interaction of extrinsic conditions (flow, geometry) with intrinsic material parameters (adsorption, diffusion) in dynamic wetting processes, when the liquid contains soluble nonvolatile surfactants. Provided the static contact angle is replaced by the dynamic one, equilibrium models adequately predict both meniscus heights and the force exerted by a moving meniscus on a solid for pure liquid at capillary numbers <10−4. This implies, not surprisingly, that the viscous drag on the wetted area of the solid is negligible, except for a small effect on the contact angle, and that the flow does not have a significant impact on the meniscus shape. However, when the liquid contains a small quantity of surfactant, equilibrium models fail to predict either the correct meniscus heights or the force on the solid, even at capillary numbers of 4 × 10−5. The slow diffusion of surfactant seems to be responsible for transient force responses in both static and dynamic experiments. Peclet numbers for the dynamic experiments with aqueous anionic surfactant solutions vary between 2.1 × 103 and 1.2 × 104, implying that even the soluble surfactants behave as nearly insoluble monolayers within the time scale of the experiments. When adsorption of surfactant on the solid-liquid interface is negligible, as is the case for the anionic surfactant, flow along the liquid surface causes accumulation of surfactant at the contact line causing the force on the solid to decrease continuously over the period of the experiment. If the surfactant has a strong affinity for the solid, as is the case for the cationic surfactant, scavenging on the freshly exposed solid surface can dominate over the flow and decrease the surfactant concentration at the contact line as the meniscus advances. The force on the solid then increases continuously. These concentrations depend strongly upon the solid geometry for low contact angle systems, primarily because of differences in the amounts of new surface created when the liquid first touches the solid. Therefore, extreme care is necessary when extrapolating results from one geometry to another.