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Surface tension and surface free energy

Surface tension and surface free energy play a key role in solving adhesion problems.

Surface tension

Measuring surface tension is key in numerous sectors: electronics, printing, coating, pharmaceuticals, chemicals and petrochemicals, textiles, cosmetics, food & beverage, optics, automotive and packaging.

The example of a glass half-full of water might be helpful to understand the concept of surface tension.

  • Within the liquid, every single water molecule is surrounded on all sides by other water molecules; the surrounding molecules attract the central molecule equally in all directions, leading to a zero net force
  • Above the surface level, there is air. Water molecules below the liquid’s surface pull molecules at the surface level inward.

When we pour a small amount of water on a surface, it tends to shrink down in size and it forms perfectly spherical droplets: this spherical shape is the result of cohesive forces among molecules inside the droplet. As previously established, whereas inside the liquid molecules pull each other in every direction, the molecules at the surface level are only pulled inward, causing the liquid to contract and therefore forming a spherical shape.

This example helps to understand that in order to pull a molecule towards the surface we need to counter the force pulling it inward by providing energy.

Surface free energy

Differently from liquids, solids cannot alter their shape, but they can determine the behaviour of a liquid in contact with their surface. Surface energy is measured as the energy required to increase the surface area of a liquid by a unit of area.

Different materials can have a high, low or extremely low surface energy value. Solid materials such as metals, metal oxides or glass have high surface energy, whereas the majority of polymers - and plastic materials in general - have a lower surface energy value.

Disperse and polar molecules

When we spill a liquid substance on the surface of a solid material, the two materials’ molecules interact with one another. A different ratio between polar molecules and disperse molecules completely changes the way in which a solid reacts to different liquid substances.

For instance, if we pour oil on a Teflon (also known as polytetrafluoroethylene) non-adhesive pan, it spreads into the pan quickly and evenly, as the two elements aid the formation of a chemical bond. If we pour water instead of oil on the same exact pan, the liquid contracts and forms droplets. This is a result of the interaction between water and Teflon, marked by vastly different chemical compositions (one has a higher number of disperse molecules, while the other has a higher number of polar molecules).

The correlation between surface tension and surface free energy is explained by the Dupré Equation, which determines that:

  • A liquid adheres to a solid If the liquid’s surface tension is lower than the solid’s surface energy
  • A liquid cannot adhere to a solid if the surface energy value is lower than the surface tension value.

Corona and Plasma treatments allow the material’s surface free energy to increase, thus helping a liquid to spread out evenly on a horizontal flat surface, to improve its adhesion value.