Wetting indicates the tendency of a liquid to cover a solid surface. The degree of wetting (wettability) is determined by a balance between adhesive and cohesive forces. Adhesive forces between a liquid and a solid cause a liquid drop to spread across the surface. Cohesive forces within the liquid cause the drop to ball up and avoid contact with the surface.
The wettability of a material can be measured using the sessile contact angle technique, in which a drop of a liquid is deposited on the sample surface and the angle between the solid surface and the tangent to the drop profile at the drop edge is measured.
The contact angle (θ) is the angle at which the liquid-vapor interface meets the solid-liquid interface. The contact angle is determined by the resultant between adhesive and cohesive forces. The tendency of a drop to spread out over a flat, solid surface increases as the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability.
A contact angle less than 90° (low contact angle) usually indicates that wetting of the surface is very favorable, and the fluid will spread over a large area of the surface. Contact angles greater than 90° (high contact angle) generally means that wetting of the surface is unfavorable so the fluid will minimize contact with the surface and form a compact liquid droplet. For water, a wettable surface may also be termed hydrophilic and a non-wettable surface hydrophobic. Super hydrophobic surfaces have contact angles greater than 150°, showing almost no contact between the liquid drop and the surface. This is sometimes referred to as the "Lotus effect".