Water Contact Angle as a Quantitative Measurement of Surface Energy
BTG Lab's Surface Analyst™ uses contact angle of highly purified water ballistically deposited on a material to determine surface energy. This surface energy correlates directly to a material's ability to adhere. Contact angle effectively measures surface energy because of its strong correlation to surface energy. And, water, because of its polar nature, is a good indicator indicator of surface energy. Thus, measuring water contact angle quantitatively determines surface energy.
Water, as a highly polar molecule, is sensitive to the polar component of a surface. However, molecules on a surface bond together in two ways: by both dispersive (non-polar) and polar bonds. Ergo, total surface energy entails a calculation of the polar component and the dispersive or non-polar component of a surface. However, water contact angle is only sensitive to the polar component. This raises the question from customers:
If water contact angle only measures the polar component of a surface, can we use water contact angles to determine the total surface energy?
In an article published in the Journal of Adhesion Science and Technology, "Water Contact Angle as a Quantitative Measure of Total Polyethylene Surface Energy" BTG Labs materials scientists explain the validity of water contact angle as a measurement for surface energy.
Polyethylene surfaces were plasma treated to different levels. Contact angle measurements of 5 liquids were then taken on differently treated pieces, and in different areas of those pieces. A trend arose. While the water contact angle (sensitive to the polar component) decreased as surface energy increased, the dispersive (non-polar) component remained the same. Surface treatment did not affect the non-polar component. Thus, because the non-polar component remains constant and unaffected by surface treatments, the polar component–measured by contact angle–may serve as a quantitative measurement of total surface energy.
Read the full article for an in depth description of the oxidized polyethylene experiment and the data on quantitatively measuring surface energy with water contact angle.