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 liquid with it by cohesion. The liquid in the capillary rises until gravity overcomes the adhesive and cohesive forces. This phenomenon is called capillary action.

For water, the adhesive forces between water and glass are stronger than the cohesive forces, resulting in a concave, or inward-curving, meniscus. For mercury, the cohesive forces outweigh the adhesive forces, resulting in a convex meniscus.

Viscosity expresses a liquid's resistance to flow and is often measured in poise, or grams per centimeter-second,

Consider methanol and glycerol. Although both form hydrogen bonds, glycerol has three –OH groups instead of one and forms more hydrogen bonds per molecule. This results in a stronger attraction between glycerol molecules, making glycerol more viscous than methanol.

Viscosity is influenced by molecular shapes. Hydrocarbons of increasing molar masses and lengths interact over greater areas and entangle more frequently, resulting in stronger dispersion forces and higher viscosities.

Lastly, viscosity depends on temperature. Increased thermal energy weakens intermolecular forces, corresponding to molecules moving more freely and liquids thus flowing faster at higher temperatures.