# Surface Tension: Definition, Formula, Unit, Causes, Examples, Consequences

Surface tension is a liquid property caused by the fact that the molecules of the liquid at the surface are in a different position than those in the liquid’s interior. It is affected not only by the attraction of forces between particles within a liquid, but also by the attraction of a solid, liquid, or gas in contact with it. As a result, it is a phenomenon that occurs when the surface of a liquid comes into contact with another phase (solid, liquid, or gas).

## What is Surface Tension?

Surface tension is a physical property defined as the amount of force required per unit area to expand the surface of a liquid. A fluid surface has a tendency to occupy as little surface area as possible. It is also defined as the phenomenon that occurs when the surface of a liquid comes into contact with another phase (it can be a liquid as well).

“Surface tension is the tension of a liquid’s surface film caused by the bulk of the liquid’s attraction of the particles in the surface layer, which tends to minimize surface area.”

Surface tension forces are caused by intermolecular forces between the molecules of a liquid at its outer boundaries. Liquids tend to have the smallest possible surface area. The liquid’s surface behaves like an elastic sheet.

It is determined not only by the forces of attraction between particles within a given liquid, but also by the forces of attraction of solids, liquids, and gases in contact with it. Surface tension energy can also be thought of as being roughly equivalent to the work or energy required to remove the surface layer of molecules in a unit area.

The addition of substances known as surfactants to a liquid can reduce its surface tension. For example: Adding detergent to water, Pepper sprinkled on water floats, but pepper sprinkled with detergent sinks.

## Formula

Surface Tension is given by the ratio of the surface force (F) to the length of the force acting (L). Thus, surface tension formula can be expressed as:

T = F/L

where,

F – force per unit length
L – length of the acting force
T – surface tension

## Units of Surface Tension

The unit of surface tension (γ)
(CGS system) = dynes per centimeter (dyne cm–1)
(SI system) = Newton per meter (Nm–1)

## Causes of Surface Tension

Surface tension is caused by liquid particle intermolecular forces such as the Van der Waals force. The molecules on the surface are drawn to the liquid’s bottom. Because of cohesive forces, a molecule is dragged in all directions equally by neighboring liquid molecules, resulting in a net force of zero.
As the molecules near the surface are not surrounded by identical molecules, they are pushed inward. Internal pressure is created, therefore causing the liquid surfaces to compress to the smallest possible area. There is also a tension parallel to the surface due to the cohesive structure of water molecules that will resist an external force.

Let us discuss in more detail about the causes.

### Molecular Concept

There are two types of molecules in a sample of water. Those on the outside are called exterior, and those on the inside are called interior. The interior molecules are attracted to all the molecules around them, whereas the exterior molecules are only attracted to those on the surface and below the surface. As a result, the energy state of the molecules on the inside is much lower than that of the molecules on the outside. As a result, the molecules try to maintain a small surface area, allowing more molecules to be in a lower energy state.

### Cohesive and Adhesive Forces

A molecule located away from the surface is pulled equally in all directions by neighboring liquid molecules due to cohesive forces, resulting in a net force of zero. Because the molecules at the surface do not have the same molecules on all sides, they are pulled inward. This generates internal pressure as well as forces liquid surfaces to contract to the smallest possible area. Due to the cohesive nature of water molecules, there is also a tension parallel to the surface at the liquid-air interface that will resist an external force.

Cohesive forces are those that act between molecules of the same type, whereas adhesive forces are those that act between molecules of different types.

The degree of wetting, the contact angle, and the shape of the meniscus are all determined by the balance between the liquid’s cohesion and its adhesion to the material of the container. When cohesion is dominant (adhesion energy is less than half of cohesion energy), wetting is low and the meniscus is convex at a vertical wall (as for mercury in a glass container). When adhesion dominates (adhesion energy is greater than half of cohesion energy), wetting is high and the similar meniscus is concave (as in water in a glass).

## Dimensional Formula

The formula of (γ) = F/L

Also, we know that Force = ma

Putting this value in the equation, we get (γ) = ma/L

Therefore, putting fundamental quantities; MLT-2L-1

On solving it further, we get MT-2

So, the dimensional formula for surface tension is MT-2.

## Examples

• Insects and other small animals that are denser than water can walk across its surface without sinking.
• It is also responsible for the rounded shape of water droplets on a surface.
• Because of the interaction between the different surface tension values of ethanol and water, as well as the faster evaporation of alcohol compared to water, tears of wine forms rivulets on the glass of any alcoholic beverage (not just wine).
• The tension between two dissimilar liquids causes oil and water to separate. The term “interface tension” is used in this case.
• Soaps and synthetic detergents thus have a cleaning action.
• A needle floats on the surface of water. Despite the fact that the density of a needle or a paper clip is greater than that of water, the surface tension along the depression is sufficient to counteract the gravitational force that pulls down on the metal object.
• Filling a glass of water to the brim as well as filling a pen’s nib with ink

## Consequences of Surface Tension

Spherical shape of liquid drops: The lowest energy state of a liquid is when the surface area is the smallest. Surface tension attempts to minimize the liquid’s surface area. Drops of liquid are spherical because a sphere has the least surface area for a given volume.

Rise of a liquid in capillary tube: When one end of a capillary tube is immersed in a liquid that wets glass, the liquid rises to a certain height inside the capillary tube. This rise is caused by the inward pull of surface tension, which therefore pushes the liquid into the capillary tube. It is the same reason that oil rises into the wick of an oil lamp, water beneath the earth’s surface rises in plants, and ink rises in blotting paper. In the case of liquids that do not wet glass, such as mercury, the level inside the capillary falls below the level outside, whereas the upper surface of a liquid that wets glass is concave, whereas the upper surface of mercury is convex.

Soap and detergent cleaning action: Soaps and detergents lower the interfacial tension between water and grease, which results in the emulsification of grease in water and thus washing away of dirt that adheres to the greases.

## Methods of Measurement

Spinning drop method: This method is ideal for measuring low interfacial tensions. The diameter of a drop within a heavy phase is measured while both are rotated.

Pendant drop method: Even at high temperatures and pressures, the surface as well as interfacial tension can be measured.

Drop volume method: It is a technique for calculating interfacial tension as a function of interface age. A liquid of one density is pumped into a liquid of another density, and the time between drops produced is measured.
Capillary rise method: It involves immersing the end of a capillary in a solution. The surface tension is related to the height at which the solution reaches inside the capillary by the equation discussed below.

## Effect of Nature of the liquid and Temperature

Nature of the Liquid: It is a property caused by the intermolecular forces of attraction between liquid molecules. The higher the intermolecular forces of attraction, the higher the liquid’s surface tension. Hence, liquids, where molecules have large attractive intermolecular forces will have large surface tension.

Temperature: With an increase in temperature, a liquid’s surface tension typically decreases until it reaches zero at the critical temperature. The decrease in surface tension that occurs as temperature rises is brought on by the molecules’ increased kinetic energy, which thus leads to a reduction in the intermolecular attraction.

## References

• Arun Bahl, B. S. Bahl & G. D. Tuli, Essentials of Physical Chemistry, S. Chand and Company Ltd., New Delhi, 2012
• Lange’s Handbook of Chemistry (1967) 10th ed. pp 1661–1665 ISBN 0-07-016190-9 (11th ed.)
• https://byjus.com/physics/surface-tension/
• https://en.wikipedia.org/wiki/Surface_tension#Effects_of_surface_tension
• https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Properties_of_Liquids/Surface_Tension#:~:text=Surface%20tension%20is%20the%20energy,the%20liquid%20(e.g.%20water%20vs.
• Petrucci, Ralph H., et al. General Chemistry: Principles and Modern Applications. Upper Saddle River, NJ: Prentice Hall, 2007
• https://chemistry-desk.blogspot.com/2011/06/important-consequences-of-surface.html