(a) Define angle of contact (b) Draw sketches to show angles of contact for a capillary tube dipped vertically in (i) water; (ii) mercury.

Answer Details

(a) Angle of contact

The angle of contact for a liquid in contact with a solid is the angle, measured through the liquid, between the solid surface (the wall of the container) and the tangent to the liquid surface (the meniscus) at the point where the liquid, the solid and the air meet.

Its value shows whether the liquid wets the solid or not:

• If the angle of contact is acute (\(\theta < 90^\circ\)) the liquid wets the surface, the adhesive forces between liquid and solid being stronger than the cohesive forces within the liquid.
• If the angle of contact is obtuse (\(\theta > 90^\circ\)) the liquid does not wet the surface, the cohesive forces being stronger than the adhesive forces.

(b) Sketches of the angles of contact

(i) Water in a clean glass capillary tube. Water wets glass because adhesion (glass–water) exceeds cohesion (water–water). The liquid therefore climbs the walls, giving a concave meniscus that curves upward at the edges, and the water rises in the tube. The tangent to the meniscus at the wall lies almost along the glass, so the angle of contact is acute, close to \(0^\circ\) for pure water on clean glass.

(ii) Mercury in a glass capillary tube. Mercury does not wet glass because cohesion (mercury–mercury) exceeds adhesion (glass–mercury). The liquid is pulled away from the walls, giving a convex (dome-shaped) meniscus, and the mercury is depressed below the outside level. The tangent to the meniscus at the wall turns steeply away from the glass, so the angle of contact is obtuse, about \(140^\circ\).