Surface Tension, Capillary Action, and Viscosity

Ever wondered why a yen coin, when carefully placed on water, floats?

Most properties of liquids are dictated by intermolecular forces. The attractive forces between similar molecules in a substance are called cohesive forces.

In water, the cohesive forces pull interior molecules equally in all directions, resulting in  zero net force on average, while surface molecules experience only a downward pull, which causes the molecules to pack closely together.

The interior molecules are more energetically stable than surface molecules because they experience more cohesion, which lowers their potential energy.

Fluids, therefore, try to reduce their potential energy by minimizing the surface area, creating a surface under tension that behaves like an elastic membrane.

The energy needed to increase a liquid’s surface area by a unit amount is called its surface tension, which is often measured in joules per meter-squared.

Adhesive forces, in contrast, exist between dissimilar molecules, such as water and glass.

Upon placing a narrow capillary tube in water, the water molecules spread along the tube's surfaces via adhesion, increasing the liquid's surface area and drawing the rest of the liqu