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Surface Tension

In this article, we are going to discuss surface tension and its formula, measurements, and examples.

What is the surface tension?

It is defined as the tendency of liquid surfaces to shrink into the smallest possible surface area. Surface tension is a process in which a liquid's surface, when in contact with a gas, behaves like a thin elastic layer. Liquids help to minimize the possible surface area. This term is usually applied when the liquid surface interacts with the gas. It is also referred to as "interface tension" when the surface is between two liquids particles (like water and oil).

Surface tension is determined by the attraction forces between particles inside a liquid and the forces of attraction in contact with solids, liquids, and gases. The work or energy needed to remove the surface layer of molecules in a unit area is essentially equal to the energy responsible for surface tension.

Therefore, surface tension can be expressed in energy units (joules) per unit of area (square meters). At 20 °C (68 °F), water has a surface tension of 0.07275 joules per square meter. In contrast, organic liquids have lower surface pressures, such as benzene and alcohols, whereas mercury has greater surface tension. The net force of attraction between molecules is decreased by increasing temperature and thus reducing surface tension.

Usually, surface tension is calculated in dynes/cm, and the force in dynes is needed to break a 1 cm long film. The surface tension of different fluids is given below in a table:

Liquid Surface Tension (N/m)
Water 0.072
Sodium Chloride 114
Hydrogen 2.4
Helium 0.16
Ethanol 22.0

The formula of Surface Tension

The liquid particles are drawn together by several intermolecular forces, like the Van der Waals forces. The particles are dragged along the surface towards the rest of the liquid, as seen in the image.

It is defined as the ratio of surface force F to the length L along with force. The following formula can calculate surface tension:

T = F/L

Where T is the liquid's surface tension,

F is the force,

and L is the length over which the force works.

SI unit of Surface Tension

The SI unit of surface tension is N/m (Newton per meter), and the CGS (centimeter-gram-seconds) unit is Dyn/cm (dyne per centimeter). It is also helpful to think about it in terms of work per unit area to determine the situation's thermodynamics. In that case, the SI unit is J/m2 (joules per meter square). The unit of cgs is erg/cm2.

The dimension of surface tension

As we above mentioned, the formula of surface tension is:

Surface tension (T) = F/L

We know that F = ma, and after substituting the value of F = ma in the equation, we get:

= ma/L

After the fundamental quantities are added to the equation, we get,

= MLT-2L-1

= MT-2

Examples of Surface tension

Various examples of surface tension are available, some of them as follows:

Floating a needle

A carefully positioned small needle may be made to float on the water's surface even though it is many times as solid as the water. The needle may sink quickly if the surface is moved to break up the surface tension.

Washing and cold water

It is a more powerful wetting agent, and the main reason for washing with hot water is that its surface tension is lower. But if the detergent decreases the surface tension, there might be no need for heating.

Surface tension and droplets

The shape of liquid droplets is responsible for surface tension. Although easily deformed, water droplets appear to be pulled by the surface layer's cohesive forces into a spherical shape.

Walking on water

Tiny insects may walk on water as their weight is not enough to reach the surface, such as the water strider.

Clinical test for jaundice

The surface tension of normal urine is about 66 dynes per centimeter, but it drops to around 55 dynes per centimeter when bile is present. Powdered sulfur is sprayed on the urine surface during the Hay test. It will float in normal urine, but if the bile's surface tension is decreased, it will sink.

Soap and detergents

These help to clean the clothes by lowering the water's surface tension and allowing it to penetrate pores and soiled areas more easily.

Measurements of Surface tension

Various measurements of surface tension are as follows:

  1. Wilhelmy plate method
  2. Stalagmometric method
  3. Bubble pressure method
  4. Du Nouy-Paddy method
  5. Sessile drop method
  6. Spinning method
  7. Capillary rise method
  8. Resonant oscillations of a liquid drop spherical and hemispherical.

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