The angle of intersection of the interface between two fluids at a solid surface. It is measured from the solid surface through the water phase, or in an oil and gas test through the oil phase.

A contact-angle test uses carefully captured and preserved samples of reservoir oil to determine the wetting preference. A droplet of the crude oil is suspended between two parallel plates of quartz or calcite inside a simulated formation water bath at reservoir temperature and sometimes at reservoir pressure. By periodically displacing one of the plates sideways, a contact angle is determined at the side of the droplet where water is forcing the oil from the solid. A small angle indicates water-wetting preference while a large angle indicates oil-wetting. Angles near 90 are intermediate-wetting. Different minerals display different wetting preferences, although most are more likely to be water-wet.