Contact Angle – Surface Energy

Contact angle is a measure of the surface energy of a surface relative to the surface tension of a fluid in contact with it.  Water is used to measure contact angle in normal paper and paperboard tests.  If the surface energy is high relative to the fluid, the contact angle will be low. If the surface energy is low relative to the fluid, the contact angle will be high.  A rapid decrease in contact angle with time likely means the test fluid is being absorbed by the substrate.  Contact angle cannot be measured on highly-absorptive surfaces.