The contact angle is the angle in which the liquid interface meets the solid surface. The contact angle should be as small as possible to have an effective penetrant material.

Contact angle is the angle that a drop of liquid makes as it meets the surface or interface of another phase, usually a solid

Angle of contact is the angle between a tangent to the liquid surface at the interface and the solid surface inside the liquid. For concave surfaces, it is acute . For convex surface , it is obtuse.

There are many ways to measure a contact angle, but the simplest way is to use a goniometer. The image of the droplet is taken with a high resolution camera; the angle can then be measured using a protractor.

It is a right angle