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Introduction to Surface Energy

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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.

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

  • Example of good wet out – water spreads out

    Good wet out - water spreads out

  • Example of poor wet out – water beads up

    Poor wet out - water beads up

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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.

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Measuring Surface Energy

  • Surface energy is expressed in units of energy per area or “dynes”. 1 dyne/cm is equal to 1 mJ/m.

    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°.)


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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


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