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Wetting refers to the contact between a Fluid and a surface, when the two are brought into contact. When a liquid has a high Surface Tension (strong internal Bonds ), it will form a droplet, whereas a liquid with low surface tension will spread out over a greater area (bonding to the surface). On the other hand, if a surface has a high surface energy (or Surface Tension ), a drop will spread, or wet, the surface. If the surface has a low surface energy, a droplet will form. This phenomena is a result of the minimization of interfacial energy. If the surface is high energy, it will want to be covered with a liquid because this interface will lower its energy, and so on.

The primary measurement to determine wettability is a Contact Angle measurement. This measures the angle between the surface and the surface of a liquid droplet on the surface. For example, a droplet would have a high contact angle, but a liquid spread on the surface would have a small one. The contact angle and the surface energies of the materials involved are related by the equation:
:: \gamma\ _{sv} = \gamma\ _{sl} + \gamma\ _{lv} \cos heta\ \

where \gamma\ is the surface tension between two substances and s, v, and l correspond to the solid, vapor, and liquid substances in a contact angle experiment respectively.

A contact angle of 90° or greater generally characterizes a surface as not-wettable, and one less than 90° means that the surface is wettable. A wettable surface may also be termed Hydrophilic and a non-wettable surface Hydrophobic . Superhydrophobic 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-leaf effect". This characteristic of spreading out over a greater area is sometimes called 'wetting action' when discussing Solder s and Soldering .

Wetting is often an important factor in the bonding ( Adherence ) of two materials. It is also the basis for Capillary Action , the ability of a narrow Tube to draw a liquid, even against the force of gravity.