The importance of direct measurement of dynamic contact angles during the wetting of solids by surfactant solutions

Abstract

The dynamic contact angle serves as an appropriate measure of wettability in industrial and laboratory processes that involve moving contact lines. This angle can be obtained from a capillary force measurement, provided the local surface tension at the contact line is known a priori. Even when the liquid contains surface active contaminants, this value is commonly assumed equal to the equilibrium surface tension. Wilhemy plate experiments conducted here show that such an assumption may be invalid under certain operating conditions. The primary reason for this is that surfactant transport by flow and diffusional interchange with the bulk phase can cause a transient surfactant concentration at the contact line or a steady dynamic value. In either case this concentration as well as the corresponding surface tension need not be the equilibrium value. The key dimensionless group impacting this behavior is the Peclet number, arising from the dimensionless surfactant flux balance at the liquid-vapor interface. If it is small, it is safe to assume that the dynamic surface tension at the contact line is equal to the equilibrium value. Liquids involved in practically relevant wetting processes quite commonly contain surfactants, either because they are deliberately added or because of inadvertent contamination. In order to make conclusions about wettability in such systems, it may be necessary to measure dynamic contact angles directly.