In the case of complete wetting, the liquid molecules are strongly attracted to the solid molecules then the liquid drop will completely spread out on the solid surface, corresponding to a contact angle of 0°. If the water contact angle is smaller than 90°, the solid surface is considered hydrophilic and wettable. If the water contact angle is larger than 90°, the solid surface is considered hydrophobic. In the case of ultra-hydrophobic materials with the so-called lotus effect, the contact angle approaches the theoretical limit of 180°.

Contact angle is important wherever the intensity of the phase contact between liquid and solid substances needs to be checked or evaluated: coating, painting, cleaning, printing, hydrophobic or hydrophilic coating, bonding, dispersing etc.

Surface tension is the cohesive force of molecules at the surface of a substance attracting toward one another to take up the least possible surface area. In a simple word, it measures how much force it takes to keep a liquid together. Contact angle and surface tension are two terms never far apart. Contact angle analysis is used to measure the quality of a solid surface. Surface tension analysis is used to measure the quality of a liquid.