Surface tension
Surface tension is the force per unit length at a liquid surface.
Surface tension is the energy per unit area due to having a surface in a liquid. It has the dimension of force per unit length, or energy per unit area. The two are equivalent, but when referring to energy per unit of area, it is common to use the term surface energy, which is a more general term in the sense that it applies also to solids. Surface tension is used for liquids, while surface stress and surface energy are more commonly used for solids. An example of its relevance is the tendency of liquid surfaces at rest to shrink to the minimum surface area possible.
- field
- Physics, chemistry, materials science
- known_for
- Describing the force per unit length at liquid surfaces, causing phenomena such as droplet shape, meniscus curvature, and capillary action
- SI_unit
- Newton per metre (N/m)
- cgs_unit
- Dyne per centimetre (dyn/cm)
- water_value_at_20°C
- 72.8 millinewtons per metre
Lore & Background
Surface tension arises from cohesive forces between liquid molecules. A molecule located away from the surface is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have an equal number of molecules on all sides of them and therefore are pulled inward, creating internal pressure and forcing liquid surfaces to contract to the minimum area. There is also a tension parallel to the surface at the liquid-air interface which will resist an external force, due to the cohesive forces between the molecules.
Reader's Guide
Surface tension is a classic, well-studied property common to all liquids. Its magnitude is connected to the forces between molecules at the surface, and surfactants are often used to reduce it so there is more contact between the liquid and another material, for instance detergents. It can also lead to pressure inside water bubbles, as well as many other phenomena. The balance between cohesion of the liquid and its adhesion to the material of the container determines the degree of wetting, the contact angle, and the shape of the meniscus. When cohesion dominates, wetting is low and the meniscus is convex at a vertical wall; when adhesion dominates, wetting is high and the meniscus is concave. Surface tension is responsible for the shape of liquid droplets, which tend to be pulled into a spherical shape by the imbalance in cohesive forces of the surface layer. In the absence of other forces, drops of virtually all liquids would be approximately spherical. The spherical shape minimizes the necessary 'wall tension' of the surface layer according to Laplace's law. Surface tension can be defined in terms of force per unit length or energy per unit area, and mechanical systems try to find a state of minimum potential energy, so a free droplet of liquid naturally assumes a spherical shape, which has the minimum surface area for a given volume.
Did You Know?
- Water has a higher surface tension (72.8 millinewtons per meter at 20 °C) than most other liquids due to the relatively high attraction of water molecules through hydrogen bonds.
- Surface tension allows objects with a higher density than water, such as razor blades and water striders, to float on a water surface without becoming even partly submerged.
- The forces of attraction acting between molecules of the same type are called cohesive forces, while those acting between molecules of different types are called adhesive forces.
- Surface tension can be measured in SI units as joules per square meter and in cgs units as ergs per cm².
More in Thermodynamics And Statistical Mechanics 1-22
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