Structural Biochemistry/Unique Properties/Surface Tension

Surface Tension of Water


Surface tension is caused by the cohesive intermolecular forces of water with other water molecules at the surface of liquid water. This property can be observed when water forms droplets on a surface, when a curvature forms on the top of a filled cup of water, and when reptiles walk on the surface of water. Surface tension is usually measured in units of force per length (dynes/cm) or units of energy per area (ergs/cm^2).

The Physics of Surface Tension
In the molecular view, water can be separated into two groups - free water molecules in the bulk and water molecules that are interacting with each other on the surface. Because water molecules at the surface can only interact with water molecules underneath and to the side of them, a resulting force exists and points towards the interior of the bulk, indicating that the energy of water molecule at the surface of water is higher than that of water molecules in the bulk. Therefore, water must receive energy to move from the interior to the surface. In another point of view, surface tension of water can be seen as the energy distribution on the surface of water. Since energy is required to create a large surface area, liquids such as water tend to reduce their surface by forming spheres, which has the lowest surface/volume ratio.

Examples


The Water Strider is a typical example that reveals the life sustaining ability of surface tension. The Strider has three pairs of long hydrophobic legs that are covered with wax. Since hydrophobic substances do not form interaction with water, the hydrophobic legs of the Water Strider prevent it from sinking. This hydrophobic property allows water to exert an opposite force on the Strider and to support its weight. This opposite force comes from surface tension. Another example in which plants transport water from roots to stems is also strong evidence that surface tension and cohesion help sustain life on earth.

Reference
Timothy A. Isgro, Marcos Sotomayor, and Eduardo Cruz-Chu. "Case Study: Water and Ice"