A contact angle is the angle that a solid surface makes with a liquid when they come in contact with each other. The angle is determined by the properties of the solid material and the liquid it carries. The inter-molecular forces of cohesion and adhesion determine the contact angle created.
When the contact angle is less, the adhesive forces are stronger and the liquid molecules interact more with the solid molecules than the liquid ones. When the contact angle is large, the cohesive forces are stronger and the liquid molecules interact more among themselves than with the solid molecules.
A contact angle is formed at the boundary where the three phases of liquid, solid and gas interact. A contact angle less than 90 degrees indicates that the liquid wets the surface; contact angle greater than 90 degrees, the surface is said to be non-wetting. A contact angle can be either static or dynamic.
A static contact angle is measured when the three phase boundary is not moving and the droplet is standing on the surface. A dynamic contact angle is measured when the three phase boundary is moving. A contact angle can be measured using optical and force tensiometry, Sessile drop method, Wilhelmy plate method and capillary rise method. A contact angle also determines the wettability of a solid and is very important in sewer and water pipe manufacturing.
Pipes that don't have the appropriate contact angle with the liquid they carry can be rehabilitated using trenchless methods such as cured-in-place pipe (CIPP), sliplining and pipe bursting.