The contact angle or wetting angle is the angle formed by the tangent to the drop contour with the surface of the solid at the three-phase point when observing the projection of the horizontal drop on the solid body. According to the physical definition, a surface with a contact angle of smaller 90° is hydrophilic (wettable) and greater 90° hydrophobic (non-wettable). By means of plasma treatment the contact angle can be modified (increased or reduced). Via a suitable plasma process or application of a suitable coat in a plasma process, hydrophilic surfaces can be made hydrophobic (and vice versa by hydrophilic coats). The determination of the surface energy pursuant to DIN 55660-2 is based on measuring the contact angle.