A liquid does not wet a solid surface if the angle of contact is:

A liquid does not wet a solid surface when the angle of contact is greater than 90 degrees. This concept is tied to the interactions between the molecules of the liquid and solid surface. When the contact angle is greater than 90 degrees, it indicates that the cohesive forces within the liquid are stronger than the adhesive forces between the liquid and the solid. Consequently, instead of spreading out over the surface, the liquid beads up, showing that it does not have a tendency to wet the solid.

For instance, think of mercury on glass. Mercury has a contact angle far greater than 90 degrees, meaning it does not wet the surface of glass and instead forms droplets. This behavior is critical in applications where wettability is essential, such as in inkjet printing or painting, where good wetting ensures even coverage.

Other angles, such as less than 30 degrees, equal to 90 degrees, or even equal to 180 degrees, indicate different wetting behaviors. Angles less than 90 degrees suggest the liquid spreads over the surface (wetting), while a 90-degree angle indicates a neutral wetting situation. An angle of 180 degrees implies the liquid is not in contact with the solid at all, which is a special case of