Why does water bead?
Min Zou, professor of mechanical engineering in the College of Engineering, replies:
A water molecule -- H2O -- consists of two hydrogen atoms and one oxygen atom with the two hydrogen atoms located on one side of the oxygen atom, forming a V-shaped structure. Each hydrogen atom is covalently bonded to the oxygen atom via a shared pair of electrons. Since an oxygen atom is larger than a hydrogen atom and also has more protons in its nucleus, the electrons tend to spend more time orbiting the oxygen atom than the hydrogen atoms. This makes a water molecule polar, with the hydrogen side positively charged and the oxygen side negatively charged. Since opposite charges attract each other, the positively charged hydrogen atom of one water molecule attracts the negatively charged oxygen atom of a neighboring water molecule. That is why water molecules like to stick to one another.
Inside a water droplet, each water molecule experiences pulling forces from other water molecules from all directions, which balances the force acting on the molecule. At the surface of the water droplet, since there are no water molecules outside the surface, the water molecules at the surface are pulled inward by other water molecules inside, trying to form the smallest surface area, or smoothest surface, possible to minimize the energy.
When a water droplet is dropped on a surface, several forces act on the water molecules that are in contact with the surface: the pulling force from the water molecules inside the water droplet, the pulling force from the molecules of the surface material, and the gravity of the water droplet. If the pulling force from the water inside the droplet is larger than the pulling force from the surface and the gravity of the water droplet, the water droplet does not spread; instead, it beads