This application note provides a brief introduction why it is important to measure contact angles and surface tensions of different surfaces and liquids. Additionally, a brief comparison to other surface characterization techniques is given. Finally, a list of application areas and industries where these properties are of importance with short example will be presented. More about what contact angle and surface tension are and how they can be measured can be found in the other application notes AN #101- AN #106.
Why measure contact angle and surface tension?
The large influence of surface chemistry has only recently been acknowledged in many industrial fields. In order to meet the challenges of the increased competition and the challenges of the feature in improving our quality of life and protecting our environment, the development of new surface-engineered materials and products are the focus of many fields of today´s scientific research. The precise characterization and knowledge of the properties of surface engineered materials and liquids are of utmost importance in the development of new, better performing products with improved qualities. The knowledge of the surface properties of raw materials and how to manipulate these properties of the material to fit a certain application has become a key role this process.
Measurement of contact angles and surface tensions provides a better understanding of the interactions between solids and liquids or liquids/liquids. These interactions play a key role in understanding adhesion, material wettability, biocompatibility, lubricity of solid surfaces as well as the wetting, washability, spreading and adsorption of liquids. Contact angle and surface tension measurements provide the information needed for development and modification of liquids and solid surfaces using today´s sophisticated surface engineering techniques. Hence, almost any solid or liquid surface can be modified to fit an application.
How come contact angle and surface tension are so important ?
Molecules inside (bulk) a liquid/solid are in every direction affected by equal attraction forces, whereas the molecules at the surface lack a neighbor towards the air phase and therefore they have larger attraction forces towards the liquid/solid than air (see figure below). This leads to a situation where the interface has excess free energy. This excess free energy is characteristic for any liquid or solid. For liquids a spontaneous contraction of the surface will take place due to this free energy and the Surface Tension of a liquid is a direct measure of it. In the case of solids a contraction is hardly ever seen, but still this free energy is present at the interface of a solid. However, now it is called Surface Free Energy (instead of Surface Tension as for liquids) and can be accessed by measuring the contact angle of a series known liquids placed on the solid surface. The dimension of Surface Tension and Surface Free Energy is mN/m.
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