Angle of Contact

When a liquid surface meets a solid surface -- for instance, where the curved surface of water inside a glass tube meets the tube's inner glass wall -- the liquid surface meets the solid at a definite, characteristic angle, called the angle of contact, denoted $\theta$. It is defined as the angle, measured inside the liquid, between the tangent drawn to the liquid's free surface at the point of contact and the tangent drawn to the solid surface at that same point, both tangents drawn at the line where the liquid, the solid, and the surrounding air (or vapour) all meet.

What determines the angle of contact. The value of $\theta$ for a given liquid-solid pair is determined by the relative strength of two different kinds of intermolecular force:

• the liquid's cohesive force -- the force of attraction between the liquid's own molecules, and
• the adhesive force -- the force of attraction between the liquid's molecules and the molecules of the solid surface.

If the adhesive force (liquid-to-solid) is comparatively stronger than the liquid's own cohesive force (liquid-to-liquid), the liquid "prefers" to spread out over, and cling to, the solid surface -- the liquid is said to wet the solid, and the observed angle of contact is small (acute), often close to $0°$ for a liquid that wets a surface very strongly. Water on a clean glass surface is the standard example: water's adhesion to glass is stronger than its own cohesion, so water wets glass, spreading into a thin film with a small, acute angle of contact.

If, instead, the liquid's own cohesive force is comparatively stronger than its adhesive force to the solid, the liquid "prefers" to pull itself together rather than spread out over the solid -- the liquid does not wet the solid, and the observed angle of contact is large (obtuse), sometimes well over $90°$. Mercury on glass is the standard example: mercury's own (metallic) cohesion is far stronger than its adhesion to glass, so mercury does not wet glass at all, instead beading up into rounded droplets and showing a large, obtuse angle of contact where it does touch a glass surface. …