Determination of the critical surface tension using an automatic wetting balance

Abstract

The purpose of this research was to classify fluoropolymer surfaces with regard to their wettability by various liquids. Thin solid blades, coated with various fluoropolymers, were suspended from a force transducer balance. The blades penetrated various hydrocarbon/air and hydrocarbon/water interfaces, and the curves of force acting on the blade versus displacement were recorded. The contact angles hydrocarbon/solid/air and hydrocarbon/solid/water were calculated using: COS θ = t γL and COS i θ = T i γi where θ, θ i : contact and interfacial contact angle; γ L, γ i: surface and interfacial tension; and τ, τ i : adhesion and interfacial adhesion tension. Cos θ versus γ L for various γ L were plotted and γ c (for cos θ = 1) was determined. It was found that there are two critical surface ( γ c ) and critical interfacial ( γ c i ) tensions due to contact angle hysteresis. Three different fluoropolymer surfaces were investigated. When the surface can be used in the form of a well-defined blade and when enough liquid to be tested is available, the method was found to be useful, rapid, and reproducible. Under these circumstances it can be used in place of Zisman's well-known sessile drop method.