Contact Angle Application- - What is a 'static' contact angle?

A liquid droplet placed on a solid, non-absorbent surface will reach an 'equilibrium' condition where no further spreading of the droplet occurs. The contact angle reading, which is determined at this condition is the 'static' contact angle.

For high viscosity liquids (e.g. glycerin) it is necessary to wait until the spreading stops before a reading is determined. It is important the droplet is applied gently on the specimen surface in order to measure a contact angle, which is as high as possible. If the droplet is pushed into the surface, the droplet will spread out and the contact angle will become lower as the droplet retracts on the surface. In certain applications like roofing materials and wind screen coatings it is of interest to measure the wetting hysteresis , characterized by the advancing/receding contact angles.

Please note the static contact angle can not be used when the test liquid:

1. Penetrates into the substrate because there is no equilibrium condition.
   
   
2. Reacts with solubles at the surface because this will change the surface tension of the liquid.