Surface energy measurement

What is surface energy?

Surface energy or surface free energy (SFE) refers to the intermolecular bond disruption when a new surface is created. It is the work required to increase the surface area of a solid material. The typical unit used for surface energy is mN/m (millinewton per meter).

All materials are composed of atoms and molecules that are connected with different kinds of bonds. The strength of these bonds is what determines the surface free energy of the material. There is generally quite a lot of energy tied in the bonds between atoms. The atoms on the surface of the material have excess energy compared to the atoms deep in the material. If the bonds are strong, the material has high surface energy. If the bonds are weaker, the material has lower surface energy.

Metals are examples of materials with high surface energy. High surface energy can make a material relatively unstable, which is why many metals in the presence of oxygen can form a thin oxygen layer on the surface to lower the surface energy. This is because it is natural for systems to pursue a state of low surface energy. Polymers and polymeric materials, on the other hand, have relatively low surface energy levels, and there is no need for surface layers to make the material stable.

Polar and dispersive surface energy

Surface energy is often divided into polar and dispersed fractions. Some materials, such as water and glass, have permanent asymmetry of electron density in molecules. This causes strong polar interactions in the material and is also the reason why water has such a high surface tension.

Disperse interactions in a material are often weaker. They originate from fluctuations in the electron density distribution. These fluctuations can cause charge differences at different locations in the material, resulting in electrostatic attraction between molecules. Examples of dispersed interactions are polyethylene (PE) and polypropylene (PP). They only form dispersive interactions and therefore have low surface energy and low levels of wettability. Many theoretical models for surface energy calculations take both polar and dispersive interactions into account.

Why measure surface energy?

The level of surface energy gives a lot of information about the material. Different levels of surface energy are associated with different characteristics and properties, which makes SFE an important value when considering applications for materials in different fields of use. Surface energy can also be used to understand how materials interact with each other, for example, how a material reacts with water and if the material has adhesive properties.

How is surface energy measured?

Surface energy is measured by measuring the contact angle between the tested surface material and a pure liquid. A liquid with a known surface tension is used to ensure the accuracy and reproducibility of measurements. The data obtained with contact angle measurements can be applied to different theoretical models to calculate the surface energy.

For surface energy measurement purposes, an optical tensiometer with a sessile drop method is most commonly used for obtaining the data. Surface energy can also be calculated from data obtained with a force tensiometer. It is important to note that a force tensiometer measures dynamic contact angle. Surface energy values calculated from receding and advancing contact angles are not comparable with surface energy values calculated from static contact angles.

Depending on the theoretical model chosen for calculations, various types of test liquids are needed. In the typical case of determining two unknowns, the polar and dispersive fractions of interaction, two different liquids with known surface tension levels are used. One liquid needs to be polar and the other very dispersive. Water is often used as the polar liquid, although ethylene glycol and glycerol can also be used. For the other liquid, which needs to be completely dispersive, diiodomethane or alpha-bromonaphthalene are two common options.

What is the connection with surface energy, wettability, and contact angle?

It is natural for a system to pursue a state of low surface energy. This is well explained in the behavior of liquids in a weightless atmosphere, where they tend to form spherical droplets, ending up having the smallest possible surface area. This kind of deformation is not possible In the case of solid materials. This is why solid materials can be wetted with a liquid to reduce free energy. Solid material with high surface energy is very well wettable, meaning that the liquid and the solid have a contact angle well under 90°.

What affects surface energy?

Surface energy measurement is very sensitive to different internal and external factors. Since surface energy is determined by measuring contact angle, the same ground rules apply.

The contact angle is affected by the tested material’s structural and physical properties. For example, the surface roughness, density, and relative moisture content can affect the eventual contact angle measurement results. Both sample contamination and sample cleaning methods can also have a significant impact. Typical contaminants are grease stains from fingerprints. It is never advised to clean the sample with surfactants as a thin barrier will stay on the sample surface after rinsing and may greatly affect the results. Contact angle measurement can also be affected by static electricity. If the sample surface is rinsed and then rubbed dry, static electricity can build up on the surface and alter the results.

The type of liquid chosen for the measurement also significantly affects measured contact angles. Water is the most commonly used liquid. It is advisable to use highly purified water, such as HPLC-grade water or bidistilled water. The same applies to other test liquids. Contact angle test liquids should always be stored in such a way that neither the composition nor properties of the liquid alter over time. For example, liquids sensitive to visible light should be stored in amber containers in the dark. In the case of contact angle test liquids, using plastic containers is not recommended, as plasticizers can migrate to the liquid and affect its properties.

In optical measurements, there is a list of factors that affect the measurement of contact angles, like image size, resolution, contrast, light levels, and depth of field. In force tensiometer measurements, it is important to note that surface energy values calculated from receding and advancing contact angles are not comparable with surface energy values calculated from static contact angles.

Sample requirements and preparation

To get the most accurate and reliable results, preparing surface energy measurement samples with great care is necessary. As suggested in the previous section, the sample cleaning stage greatly affects the results. A good practice is to clean and dry the sample material in such a way that it becomes as clean and neutral as possible.

In repeated measurements, the same spot should never be used. Even with careful handling, cleaning, and drying, the sample will be affected by the placement of the drop, and subsequent measurements can produce inaccurate results. In other words, a new fresh spot on the sample should be used for each repeated measurement.

Need a surface energy analysis?

Measurlabs offers surface energy testing services of high quality with fast results and affordable prices. If you have any questions about your sample or its suitability for the method, our experts are always happy to help. You can contact us through the form below or at info@measurlabs.com.