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 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. 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.
One of the consequences of surface tension is that the pressure on the concave side of a surface is greater than that on the convex side. If there were no excess pressure on the concave side, a bubble could not exist, for it would collapse as a result of the force due to surface tension. Hence, the spherical shape of the bubble is maintained by the excess pressure in the interior of the bubble. The larger the bubble, the smaller the excess bubble pressure. Bubbles burst mainly because of liquid evaporation, which makes the film too thin to hold the bubble together.