Surface free energy is to solids what
surface tension is to liquids, the work required to increase the area of a substance by one unit area. Actually, the surface free energy of a solid is sometimes also referred as the "surface tension" of the solid substrate. The surface tension of a liquid can be directly measured, but no direct method exists for a solid. Instead the
contact angle between the solid and a liquid is analyzed, which reflect the thermodynamics of the liquid/solid interaction.
The solid is tested against a series of liquids and contact angles are measured. The known properties of the liquids and the measured contact angles are used to calculate surface free energy. Multiple analysis methods exist, making the choice of method and analysis important. Typical methods include:
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Zisman critical surface tension method suitable for low energy surfaces such as fluorinated polymers.
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Fowkes method which splits surface tension into polar and non-polar components.
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Van Oss acid-base method which further splits the polar component into acidic and basic components.
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Schultz method for high energy surfaces such as metals.
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more about surface free energy.