When water is dripped or spilt o a table what shape is it and why?

Answer 1

Particles inside a liquid like water interact with nearby particles to form a minimum energy state inside the liquid. A physical configuration of particle is in fact stable if it has the minimum possible energy.

The particles on the surface dividing two different materials, for example a liquid and a solid (like water and the table surface) are different from particles in the body of the materials in the sense that they are in contact with particle of different type across the contact surface.

This causes in general the accumulation of energy on the surface, since more energy is needed to maintain the surface with respect to the energy that is needed to maintain the compactness of the bulk of the liquid.

This phenomenon causes the insurgence of the so called surface pressure, that causes the surface area to be minimized so to minimize the surface energy.

This is why droplets, in the absence of gravity (or very small droplet for which gravity is negligible with respect to surface tension) are spheres. As a matter of fact the sphere has the maximum possible ratio between volume and area, that is the smallest area for a given volume.

This happens also for water dropped on a table, it does not form an infinitely thin and wide film, but remains compact to minimize the contact surface with the table and the air. In case, if the water is dropped with a great velocity, the dropping kinetic energy causes the water body to be divided in several small droplets, but each droplet, once the initial kinetic energy is absorbed by droplet formation and attrition, behave so to minimize surface energy.

This discussion naturally assumes that the water is not absorbed by the table surface. If the table surface is porous so to absorb water molecule, gravity and capillary effect causes water to penetrate into the wood structure. Since capillary effect (another consequence of surface tension) is stronger and stronger while the capillary is smaller and smaller, the water will tend to penetrate in very small pores of the wood, so creating a water blot in the wood surface. Naturally this kind of blot is removed with time when water evaporate.

In few cases it is possible that water creates a chemical reaction with the table surface after penetrating into it, for example by polymers hydrogenation in the case of a ruined plastic surface. In this case a blot is formed in the surface that cannot be cancelled by evaporation and remains unless eliminated with chemical methods (for example with suitable detergents).