The Angle of Contact (Surface Tension) – concepts

In this post, we will discuss the concepts of the Angle of Contact which is related to surface tension. Let’s define and understand it first.

Define the Angle of Contact

How much curved a meniscus will be, is expressed in terms of angle of contact. The angle of contact, θ; between a liquid and a solid is the angle enclosed between the tangents to the liquid surface and the solid surface measured inside the liquid. (see the figure below)

Angle of Contact is an acute angle for a concave meniscus and it is an obtuse angle for a convex meniscus – explain

Figure 1 (a) shows a concave meniscus and Figure 1 (b) convex meniscus.

P is a point of contact of liquid with the glass wall. PQ is tangent at P to meniscus.

Angle RPQ is the angle θ of contact.

Obviously, θ is the acute angle for a concave meniscus and is an obtuse angle for a convex meniscus.

Angle of Contact depends on these factors

The value of Angle of Contact θ depends on

(i) nature of liquid and solid in contact

(ii) the nature of the medium above the free surface of the liquid.

And, θ does not depend on the inclination of the solid to the liquid surface.

Angle of Contact examples

The angle of contact of mercury with glass is about 1400.

The angle of contact for water and glass is about 8⁰–10⁰.