A coat, for example of adhesive, paint or ink, will only stick to a surface if the liquid coat can wet the surface. This wetting is primarily prevented by contamination of the surface (above all by oils, grease and wax) or by insufficient surface energy of the material.
Wettability is tested with test inks of a known surface tension. To get accurate values for the surface energy, it is necessary to determine the contact angle generated by a drop of a test liquid on the surface. Another method is using a series of test inks of various surface tensions and determining from which surface tension a drop of the liquid on the surface will no longer keep the shape of a drop but will dissolve. To ensure that the coat or the adhesive sticks to the surface, de-greasing is required before coating or bonding. The degreasing effect is measured on the cleaned components by means of a wetting test. The wettability is measured by means of various test liquids applied on the surface to be tested. If the test liquid wets the surface at a contact angle of 90°, degreasing of the surface is considered successful. To determine the exact degree of wetting, the contact angle must be accurately determined.