Properties of Fluids

Surface Tension

The surface tension of a liquid is mainly a force that mainly acts to reduce the surface area of a liquid. The directed contracting force which attracts the molecules at the surface of a liquid towards the interior of the liquid is surface tension. The surface tension of liquids depends on the composition of the vapour phase. The surface tension of liquids have many important roles in daily life and also various industrial processes.

Surface tension

Molecular Perspective

There are mainly two types of molecules– interior type and exterior type molecules. Molecules that are on the outside are exterior type molecules and the molecules that are on the inside are interior type molecules. The energy state of the interior molecules is lower than the exterior molecules because the interior molecules are attracted to all the surrounding molecules but the exterior molecules are attracted to only the other surface molecules and to these below the surface. Because of this reason molecules always try to maintain a lower surface area which allows more molecules to have a lower energy state.

In the case of water, the water molecules attract each other because of their opposite charge. Hydrogen is positive and oxygen is negative so they attract each other and stick together. This is the reason there is surface tension and it needs certain energy to break the bonds between the molecules. Water is a liquid which has very high surface tension. Because of this very high surface tension, many things can float on the water whose density is less than water.

Capillary Action

There are two types of forces – adhesive force and cohesive force. The intermolecular force which acts in between the molecules is known as a cohesive force. Now when a liquid is in contact with a surface then there is an attraction between molecules of the liquid and the surface, these forces adhesive forces.

Now consider a very small diameter tube open at both ends is inserted into a liquid. Here we can see the results of adhesive and cohesive force. When the capillary inserted into the water the adhesive forces are stronger than the cohesive forces. So, in this case, water molecules are attracted more strongly to the glass than to each other. So as a result the water surface rises due to the high surface tension of water and the water wets the glass surface. This is capillary action.

Now if we use mercury instead of water and then insert a capillary in it we can see that the mercury surface curves downward against the glass. This is because the cohesive force is stronger than the adhesive force here. The mercury molecules are attracted more to each other than they are attracted to the molecules of a glass surface. So when a capillary is inserted in mercury, mercury falls in glass capillary due to the surface tension and mercury and it does not wet the glass.

Applications of Surface Tension

Surface tension has a huge role in daily life, health and many industrial processes. There are so many techniques that have been developed to modify surface tension.

1. Daily life Example:

a) Small insects such as the water strider can walk on the surface of the water because their weight is very less so they can’t penetrate the water.

b) Disinfectants are mainly the solution of low surface tension so that when we use them in the field they can float on the water and spread out on the cells to destroy them.

c) Soaps and detergents also work on the basis of surface tension. They lower the surface tension of the water so that the soaps and detergents easily soak into the pores and holes.

d) The water bubbles are round because the surface tension of water provides the tension to form the bubble with the water and the surface tension minimizes the bubble into spherical shapes.

e) A small needle can be floated on the surface of the water.

2. Role of surface tension on human health:

Surface tension changes in biological phenomena can determine various diseases in the human body.

3. Industrial applications:

Surface tension is an important factor in industrial processes. In all the industrial plants the R&D departments use the surface tension phenomena to improve the quality of the products. Many operations are used to improve the quality of the product such as detergent formulations. By the use of detergent formulations, we can improve the cleaning properties with more biological surfactants at a lower temperature.

4.  Surface tension is also important for characterization for food, pharmaceutical and packaging products.

FAQs about Surface Tension

Q.1. Is there any difference in surface tension values among tap water, distilled water?

Answer: The surface tension value depends on the number of impurities there are in the water. So if we observe clearly there are more impurities in the tap water than the distilled water.

Q.2. Why rain waters form beads on the surface of the leaf?

Answer: As the surface of the leaf is waxy so the water adheres weakly to wax and strongly to itself hence they forms clusters into drops. The surface tension water gives them spherical shapes.

Q.3. How water striders walk on the water?

Answer: Objects that are less denser than that of water can float on the surface of the water. The water striders use the surface tension of the water to walk on the surface of the water.

Q.4. What are the units of surface tension?

The units of surface tension are Newton per metre (SI unit) and Dyn/cm (CGS unit).

Q.5. What are the methods of measurement of surface tension?

Answer: We can measure the surface tension by using these methods-

  1. Spinning drop method.
  2. Stalagmometric method.
  3. Pendant drop method.
  4. Capillary rise method.
  5. Bubble pressure method.
  6. Sessile drop method.

Q.6. Why raindrops are spherical?

Answer: The raindrops are spherical because of the cohesive forces between the rainwater molecules and the surface tension of the water molecules.

Q.7. Which are the main forces act to create surface tension?

Answer: The main forces are the cohesive force and adhesive force.

Q.8. What is the surface tension of water at boiling point?

Answer: The surface tension of water is zero at boiling temperature.

Q.9. How detergents remove the oil and wax from the clothes?

Answer: We cannot remove the oil and liquid wax spots from the clothes by using normal water. We have to use detergents to remove those spots. So when we add detergents or soaps the surface tension of the water decreases hence the water molecules can reach every corner of the cloths and remove the oil and liquid wax from the clothes.

Q.10. Why oil spreads over the surface of the water?

Answer: Oil spreads over the surface of the water mainly because the surface tension of oil is less than the surface tension of water.

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