Introduction to Surface Energy

The Basics of Surface Energy

All materials have properties such as density or melting point that help us characterize and differentiate between them. Surface energy is another property of materials: it’s a measure of how attracted a material’s molecules are to each other and to other materials’ molecules. Surface energy is a good measure of how easy or hard the surface may be to adhere.

See how different levels of surface energy can make a material hard or easy to bond.

What is Wet Out?

“Wet out” or “wetting” occurs when a liquid flows or spreads out when applied to the surface of a solid. Wetting allows the adhesive to achieve intimate contact with the surface of a material to be bonded. Depending on the surface energy of the material, an adhesive will naturally wet and flow over the surface or have a tendency to bead up. Wet out is a necessary condition to form an adhesive bond.

Good wet out - water spreads out
Poor wet out - water beads up

Wetting out on Higher- and Lower-Surface-Energy Materials

Materials with higher surface energy are easier to wet out than materials with lower surface energy. Consider the sauce pans in the images above. If the pan has a higher surface energy, the water droplets will flow across the surface into a thin film. If the pan has a lower surface energy (perhaps by adding a non-stick coating), the water droplets will bead up. This will be explored more in the next sections.

Measuring Surface Energy

How easy or hard it is for an adhesive to wet out a material’s surface depends on the surface energy of the material. Surface energy is, as you might expect, expressed in units of energy per area. Typically, it is measured in a unit called “dynes” where 1 dyne/cm is equal to 1 mJ/m.

Most commonly, surface energy is measured with water. When a droplet of water is placed on a surface, it will bead up to some extent. Theoretically, if the surface energy is zero, the droplet will be a perfect sphere. Conversely, if the surface energy is infinitely high, the droplet will form a perfectly uniform film. For anything in between, we can measure the angle made at the edge of the droplet. (For a sphere, that angle would be 0°; for a film, it would be 180°.)

In case you were wondering:

Why use water for measuring surface energies?

Water is a common choice for measuring surface energy for a few reasons. Most importantly, it is readily available. It is also easy to purify, relatively inexpensive, low viscosity, and achieves equilibrium very quickly on the surface – typically within 60 seconds. Imagine using something like pitch to measure surface energy. It may take a few hundred years!

Learn about the Pitch Drop experiment, the world’s longest running lab experiment

The Science of Adhesion Educational Series

The Science of Adhesion Educational Series is designed to be a comprehensive introduction to Adhesion Science and the use of tapes and adhesives in design applications. Visit the Science of Adhesion page to view all articles or choose from the selected topics below.