Angle of contact

(a)
(b)

1. Angle of contact, for a solid and a liquid is defined as the angle between tangent to the liquid surface drawn at the point of contact and the solid surface inside the liquid.

2. The angle of contact of a liquid surface on a solid surface depends on the nature of the liquid and the solid.

Case I: When θ < 90°:

The liquid surface curves up towards the solid. This happens when the force of cohesion between two liquid molecules is less than force of adhesion between the liquid and the solid. If such a liquid is poured into a solid tube, it will have a concave meniscus.
For example, a glass rod dipped in water, or water inside a glass tube.

Case II: When θ > 90°:

The liquid surfaces get curved downward in contact with a solid. In this case the force of cohesion is greater than the force of adhesion.

In such cases, solids do not get ” wet “. When such liquids are put into a solid tube, a convex meniscus is obtained.
For example, a glass rod dipped in mercury or mercury within a solid glass tube.

Also Read:

Surface Tension
Surface Energy
Excess Pressure inside a soap bubble
Capillarity & Ascent Formula
Viscosity , Stoke’s Law & Terminal Velocity
Poiseuille’s formula

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