Contact angle, the angle at which a liquid droplet meets at a solid surface, is another way to characterize polymer surfaces. Contact angle (θ) is a measure of the wetting ability of the liquid on a solid surface. Generally, due to low surface energy, liquids will not wet polymer surfaces and the contact angle will be greater than 90°. The liquid molecules are more attracted to other liquid molecules as compared to the polymer surface. Because the polymer surfaces are solid surfaces, surface tension cannot be measured in a traditional way such as using a Wilhelmy plate. Instead, contact angles can be used to indirectly estimate the surface tension of polymer surfaces. This is accomplished by measuring the contact angles of a series of liquids on a polymer surface. A Fox and Zisman plot of cos θ versus surface tensions of the liquids(γ) gives a straight line which can be extrapolated back to determine the critical surface tension of the solid (γ).
Where:
: is the contact angle
: is the surface tension of the liquid
: is the critical surface tension of the solid
The variable β was previously determined to be approximately 0.03 to 0.04. While the actual surface tension of the solid polymer surface cannot be determined, the Fox and Zisman plot serves as an estimate. However, this estimate may be skewed if there are significant intermolecular forces between the surface and the liquid. Also, this plot is not applicable for binary mixtures of liquids dropped onto a polymer surface. Some estimated surface tensions of different polymers and the contact angles of different liquids on polymer surfaces is shown below.