My six-year old daughter observed that oil floating on her soup always forms round blotches. A liquid surface tends to contract to the minimum area due to unbalanced molecular forces at the surface. The molecules at the surface of a liquid are attracted into the body of the liquid because the attraction of the underlying molecules is greater than the attraction by the vapour molecules on the other side of the surface. This inward attraction causes the surface to contract if it can and gives rise to a force in the plane of the surface. The surface tension of a liquid is the force per unit length on the surface that opposes the expansion of the surface area.
Surface tension is responsible for the spherical shape of a bubble.
The surface tension of a bubble acts so that the bubble has a natural tendency to contract and reduces its surface area. Spherical shape has the minimum surface area. Surface tension is also responsible for the formation of spherical droplets, the rise of water in a capillary, and the movement of a liquid through a porous solid. The shape and profile of an oil lens floating on water surface are due to the equilibrium of three surface forces, namely, the surface tension of oil, the surface tension of water and the interfacial tension of oil and water.