Angle of contact

Angle of contact:

When a liquid surface comes in contact with a solid surface, it forms a meniscus, which can be either convex or concave. The angle of contact, between a liquid and a solid surface is defined as the angle between the tangents drawn to the free surface of the liquid and surface of the solid at the point of contact, measured within the liquid. 

When the angle of contact is acute, the liquid forms a concave meniscus Fig. 2.18 (a) at the point of contact. When the angle of contact is obtuse, it forms a convex meniscus Fig. 2.18 (b). For example, water-glass interface forms a concave meniscus and mercury-glass interface forms a convex meniscus. This difference between the shapes of menisci is due to the net effect of the cohesive forces between liquid molecules and adhesive forces between liquid and solid molecules.

a) Shape of meniscus:

i) Concave meniscus - acute angle of contact: 

Figure below shows the acute angle of contact between a liquid surface (e.g., kerosene in a glass bottle). Consider a molecule such as A on the surface of the liquid near the wall of the container. The molecule experiences both cohesive as well as adhesive forces. In this case, since the wall is vertical, the net adhesive force (AP) acting on the molecule A is horizontal, Net cohesive force (AC) acting on molecule is directed at nearly 45° to either of the surfaces. Magnitude of adhesive force is so large that the net force (AR) is directed inside the solid. For equilibrium or stability of a liquid surface, the net force (AR) acting on the molecule A must be normal to the liquid surface at all points. For the resultant force AR to be normal to the tangent, the liquid near the wall should pile up against the solid boundary so that the tangent AT to the liquid surface is perpendicular to AR. Thus, this makes the meniscus concave. Such a liquid wets that solid surface.

ii) Convex meniscus - obtuse angle of contact:

Figure below shows the obtuse angle of contact between a liquid and a solid (e.g., mercury in a glass bottle). Consider a molecule such as A on the surface of the liquid near the wall of the container. The molecule experiences both cohesive as well as adhesive forces. In this case also, the net adhesive force (AP) acting on the molecule A is horizontal since the wall is vertical. Magnitude of cohesive force is so large that the net force (AR) is directed inside the liquid. For equilibrium or stability of a liquid surface, the net force (A) acting on all molecules similar to molecule A must be normal to the liquid surface at all points. The liquid near the wall should, therefore, creep inside against the solid boundary. This makes the meniscus convex so that its tangent AT is normal to AR. Such a liquid does not wet that solid surface. 

iii) Zero angle of contact: 

Figure shows the angle of contact between a liquid (e.g. highly pure water) which completely wets a solid (e.g. clean glass) surface. The angle of contact in this case is almost zero. In this case, the liquid molecules near the contact region, are so less in number that the cohesive force is negligible, i.e., AC = 0 and the net adhesive force itself is the resultant force, i.e., AP = AR . Therefore, the tangent AT is along the wall within the liquid and the angle of contact is zero. 

iv) Angle of contact 90° and conditions for convexity and concavity: 

Consider a hypothetical liquid having angle of contact 90° with a given solid container, as shown in the figure. In this case, the net cohesive force AC is exactly at 45° with either of the surfaces and the resultant force AR is exactly vertical (along the solid surface). 

b) Factors affecting the angle of contact:

The value of the angle of contact depends on the following factors:

i) The nature of the liquid and the solid in contact.

ii) Impurity: Impurities present in the liquid change the angle of contact.

iii) Temperature of the liquid: Any increase in the temperature of a liquid decreases its angle of contact. For a given solid-liquid surface, the angle of contact is constant at a given temperature.