The surface suspense

The basic concepts and their explanations

It is a very common concept that the bodies heavier than water do not drown if we put them correctly on the water surface. So the light metal coin, thin aluminum plates and needles float on the water surface, although under Archimedes's law they should sink. The free water surface, as well as the surfaces of some other liquids act as there is a thin membrane on their surface, that prevents the bodies to be drowned. That membrane is only on the surface, and heavier bodies drown according to Archimedes's law.

All that concepts show that free surface of liquid is in the state of tension. The presence of that surface tension can be illustrated by the following experiment. In the circular frame we tie the thread into a loop. The frame and the loop are drowned into lather. The loop has a very irregular form in the lather. But, we pierce the bubbles in the loop, the loop will get the circular form again. So, there is a force working from inside of the bubble that pulls the thread into the radial direction.

Minimal surfaces

The tension of the surface is then a consequence of the added energy in the surface of the liquid due to the forces among molecules. All the natural processes tend to the state of minimal potential energy as a stable state, so the free surface of the liquid tends to take over a minimal surface consistent to the conditions of the work of the outer forces and pot sides' borders. The smallest surface for the given volume has the sphere; so every free liquid that is influenced by the work of the outer forces and that is not limited by the pot would get the form of the sphere. On the Earth water and other liquids disperse into drops, which are a little bit elongated due to work of gravitation and other forces.

The rigid body

Capillarity

If we drown a thin tube into a liquid, the liquid in tube will raise or fall. If we drown a tube into water, the liquid will raise; if drown it into mercury the liquid will fall. That concept is called capillarity. The capillarity is explained by the tension of the surface.