Surface tension is a force per unit length (or surface energy per unit area) acting in the plane of the interface between the liquid and the bounding surface. It is the extra energy that the molecules at the interface have as compared to the interior.
Units and dimension of surface tension:
SI unit of surface tension = Nm-1
CGS unit of surface tension = dyne cm-1
Dimensions of surface tension = [MT-2].
The extra energy possessed by the molecules of the surface film of unit area compared to the molecules in the interior is called surface energy.
SI unit of surface energy is Jm-2.
Angle of contact
The surface of liquid near the plane of contact, with another medium, is in general curved. The angle between the tangent to the liquid surface at the point of contact and solid surface inside the liquid is termed as the angle of contact. It is denoted by θ.
- Adhesive force > cohesive force: Liquid wets the solid surface and has concave meniscus.
- Adhesive force < cohesive force: Liquid does not wet the solid surface and has convex meniscus.
- Adhesive force = cohesive force: Liquid surface is plane.
The values of angle of contact depending on the following factors:
- Nature of the solid and the liquid in contact.
- Cleanliness of the surface in contact.
- Medium above the free surface of the liquid.
- The temperature of the liquid.