Contact angle

“The contact angle is defined as the angle the water makes with the surface. In this illustration, a low contact angle (θ<90 ̊) means the surface is hydrophilic. Large contact angles (θ>90 ̊) lead to a surface that is hydrophobic.” (Reference, PEG/PPG PDF) Or, more simply, “The angle where a liquid or vapour interacts with a solid surface”. (Reference) This is important as it can determine if something is hydrophilic and water loving or hydrophobic and water hating.