What are surface energy and tension and how are they measured?

Surface energy and tension are fundamental concepts in the fields of physics and chemistry, with practical applications across various industries such as the manufacture of adhesives, paints, coatings, and cleaning processes. In this article, we explain what they are, how they are measured, and why they are important.

What Is Surface Free Energy (SFE)?

Surface Free Energy refers to the amount of energy present at the surface of a material due to the unbalanced forces experienced by molecules in that region. Inside a material, molecules are surrounded by other similar molecules that exert equal forces in all directions. However, at the surface, molecules do not have neighbors on all sides, which creates a higher-energy state.

This energy is crucial because it determines how materials interact with their environment, including how they adhere to, repel, or are wetted by other substances. For example, materials with high surface energy tend to be more adhesive and interact more easily with liquids.

What Is Surface Tension?

Surface tension is a property of liquids that describes the tendency of their surfaces to contract in order to occupy the smallest possible area. This phenomenon occurs due to the cohesive forces between liquid molecules, which are stronger at the surface where molecules are not completely surrounded by similar ones.

For example, surface tension is the reason why small insects can walk on water or why raindrops have a spherical shape. It also plays an essential role in industrial processes, such as detergent formulation or coating manufacturing.

How Is Surface Free Energy Measured?

Measuring the free surface energy of a solid involves evaluating its interaction with different liquids. The most common methods include:

1. Liquid drop method: This technique consists of placing small drops of liquids with known properties on the surface of the material and analyzing how they spread. Depending on the shape of the drop, surface energy can be calculated using mathematical models such as Owens–Wendt or Fowkes.
2. Contact angle or tensiometer: This is a specialized device that allows direct measurement of the drop shape and enables highly accurate calculation of surface energy.
3. Dynamic methods: Some advanced approaches use techniques such as advancing and receding contact angles to obtain more detailed measurements.

How Is Surface Tension Measured?

The surface tension of a liquid is measured using various instruments and methods, including:

1. Du Noüy ring method: This method uses a wire ring immersed in the liquid. As the ring is pulled upward, the force required to break the liquid’s surface film is measured, allowing surface tension to be calculated.
2. Pendant drop method: This method analyzes the shape of a drop suspended from a capillary. It is especially useful for viscous liquids or liquids with low surface tension.
3. Pressure bubble method: A gas bubble is generated in the liquid, and the pressure required to form it is measured. This method is ideal for measuring surface tension under dynamic conditions.
4. Advanced optical methods: By using high-resolution cameras and specialized software, deformation patterns on the liquid surface can be analyzed.

Why Is It Useful to Know Surface Energy and Surface Tension?

Understanding and measuring surface energy and surface tension has practical applications in many sectors:

1. Adhesives and coatings: It allows the selection of materials that bond better or provide improved protective properties.
2. Pharmaceutical and cosmetic industries: It helps formulate products that spread evenly over skin or other surfaces.
3. Cleaning and surface treatment: It is key to developing detergents and processes that effectively remove dirt or residues.
4. Nanotechnology: At the nanoscale, these properties influence the design of devices and innovative materials.
5. Quality control: In industrial production, knowing these parameters ensures products meet the desired performance standards.

Understanding how surface energy and surface tension work not only makes it possible to optimize industrial processes, but also to improve the design and functionality of products we use in our daily lives.

Example of a Project Made in ATRIA

At ATRIA Innovation, we have applied the concepts of surface energy and surface tension in various projects. Below, we present one example:

One of our clients encountered a paint adhesion problem in samples supplied by a new vendor. They found that when the parts were reintroduced into their polishing process, paint adhesion improved. This indicated that the samples were not arriving in suitable condition from the supplier, forcing the client to add an extra processing step, consuming additional time and resources.

They contacted us to identify the root cause of the problem, and we proposed measuring the surface energy of samples from both the previous supplier and the new one. It was observed that samples from the new supplier had lower surface energy. In addition, topographic measurements were carried out using a confocal microscope to analyze surface roughness. This analysis revealed that the roughness of the supplier’s parts and that of the parts processed by the client were completely different.

Projects like this demonstrate how understanding and manipulating surface energy and surface tension are essential for developing innovative solutions and improving the properties of materials in industrial applications.

Would you like to know the surface energy or surface tension of your materials? Contact us!