The wettability of a material is its tendency to make liquids spread out over its surface. It is very important for procedures such as coating and bonding, in which an adhesive has to form the largest possible area of contact to maximize adhesion. When the liquid is water, hydrophilicity is the more commonly used term.
Wettability is a direct function of surface energy. As a general rule, the higher the surface energy of a material, the more the liquid will spread out over the surface. Plasma activation increases wettability by raising the surface energy of almost any material.
Wettability can be quantified by measuring the contact angle of a droplet. When contact angles are below 90°, that is generally good enough for most technical applications.