What is adhesion in liquids and solids?

• A. The attractive forces between a liquid and a solid surface.
• B. The cohesive forces between two liquids.
• C. The gravitational force on a liquid.
• D. The attractive forces within a liquid.

Answer: (A)

Adhesion is the attraction between unlike substances, specifically between a liquid and a solid surface. This forces the liquid to wet and spread on the solid, which is what allows the liquid to cling to or cling along the surface rather than bead up. In practical terms, adhesion determines how well a liquid wets a material, and is closely related to the concept of wetting and contact angle.

This differs from cohesion, which are the attractive forces within the liquid itself—how strongly the liquid molecules stick to each other. Gravity is a separate influence that affects how a liquid behaves on a surface by pulling it downward, but it isn’t what adhesion describes. The option describing attractive forces within a liquid would be about cohesion, not adhesion.

Understanding adhesion helps explain why, in liquid penetrant testing, a penetrant must wet the surface to flow into flaws and reveal them after development. For example, water wets glass well due to adhesive forces between water and glass, whereas a liquid that does not adhere to a surface will bead up and not penetrate as effectively.