Surface Tension | Its All Important Concept

Cohesion and Adhesion

First of all we try to understand some terms useful in surface tension study. Liquid has properties like Cohesion. Cohesion is a property in which one molecule of liquid attracting another nearer molecule. Adhesion is a property in which the fluid molecules are attracted by solid surface contact with it. In short, we can say that the force between similar molecules is Cohesion and the force between dissimilar molecules is adhesion.

Let’s take an example.

If we drop mercury droplet on any surface, it tries to form in droplet because Cohesion is higher than the adhesion force. You will get a notice that Mercury droplet does not stick on the solid surface. The Mercury will try to stay away from the solid surface; it will not wet solid surface.

Now let’s take another example if we consider water particles fall on the surface. It will spread all over the concrete surface. It happens because of the adhesive force is more significant than Cohesive force in that case. The angle of contact between the liquid and the solid surface can describe wetting and non-wetting of the surface.

Surface Tension
Wetting and non-wetting of liquid credit Hisoki

Refer the above figure the liquid gas and solid surface interface the liquid will where the solid surface when the angle is less than 90 degree (π/2). The wetting of the surface is increasing with decreasing the angle. If the angle is more than 90 degree, the liquid will not wet solid surface. The angle depends on the nature of the surface, types of liquid, solid surface, and cleanliness.

If we consider pure water comes in contact with the clean glass surface. The angle is 0 (zero) degree in that case. If we add impurities in the water: The angle will increase with the addition of impurities. As we have discussed the Mercury is non-wetting liquid, so the angle is lies from 130 to 150 degree.

Surface Tension

In liquid, the molecules are lying below the free surface. Every molecule of liquid is attracting to the molecule nearby. The molecular Cohesion force is the same in all direction. All the forces are the same in magnitude and opposite in direction. So, it will get cancel in liquid. It can be the reason for equilibrium in liquid. There is no resultant force present in the liquid.

Suppose we considered topmost molecules of liquid lying at a free surface as we know that there are no liquid molecules over them. So here, they are getting attracted by liquid molecules lying below them. This free surface liquid molecules will feel pull force interior of the liquid. This force acts like elastic force. Expended per unit area of the surface is called surface tension.

The Surface tension is denoted by Sigma (σ). Surface tension occurs at liquid-gas interface, liquid-liquid interface. The reason behind surface tension is an intermolecular attraction because of Cohesion.

Let’s understand it in depth by considering some practical examples,

  • You have seen liquid droplet of a spherical shape. The reason behind its spherical shape is surface tension.
  • You might be noticed that if we thoroughly pour water inside the glass. Even if the glass is filled, still we can add some water above the glass limit.
  • Suppose we will experiment with a thin glass tube on the water surface. We can quickly notice a capillary rise and depression inside a thin glass tube.
  • Birds can drink water from the water body due to surface tension.

Though pressure and the gravity force are higher than surface tension force, the surface tension force plays an important role when there are free surface and small dimensions. The unit of surface tension is N/m. The magnitude of surface tension depends on the following factors:

  • Type of liquid
  • Type of surrounding state gas, liquid or solid
  • The kinetic energy of molecules
  • Temperature of molecules

If we increase the temperature of substance like liquid, the intermolecular attraction is decreasing because the distance between molecules increases. The surface tension depends on intermolecular attraction (Cohesion). The value of surface tension for liquid is taken for air as a surrounding medium,

The surface tension for the air-water interface is 0.073 N/m.

The value of surface tension decreases with increasing temperature.

Capillary

If a narrow tube is dipped into the water, the water will rise inside the tube at a certain level. This type of tube is called a capillary tube, and this phenomenon is called the capillary effect. Another name of the capillary effect is the meniscus effect.

The capillary effect is due to surface tension force. The capillary rise and depression are happening because of cohesion and adhesion intermolecular attraction. The adhesion force between tube surface and a water molecule is higher than the Cohesion force between water molecules. Because of this, the water molecules can be observed in concave shape on the tube surface.

Weight of liquid rise or depress in small diameter tube

= ( Area of tube * Rise or fall ) * ( specific weight )

= (π/4 *d2*h) w

Verticle component of surface tension force

= σ cosθ * circumference

= σ cosθ * πd

If we consider equilibrium then upward force balances downward force, so the component of force is given as,

( π/4 * d2 *h * w ) = σ cosθ * πd

H = ( 4 σ cosθ/ wd )

Capillary tube

It can be observed from an angle that if the angle is between 0 to 90 degree, the value of h is positive, concave shape formation and capillary rise. If the angle is between 90 and 180 degrees, the h is negative, convex shape formation and capillary depression.

If the liquid is Mercury, then the effect is wholly turned opposite. In the case of Mercury, the Cohesion force is more significant than the adhesion force. Because of these, the Mercury molecules form convex shape on the tube surface.

The capillary effect is inversely proportional to the tube diameter. If you want to avoid the capillary effect, then you should not choose a small diameter tube. The minimum tube diameter is recommended for water, and Mercury is 6 mm. The surface inside the tube should be clean.

Evaporation

Evaporation is defined as a change of state from liquid to gaseous. Operation rate is dependent on the pressure and temperature condition of liquid.

Consider one example,

Suppose, the liquid is inside the closed vessel. In this vessel, the vapour molecules possess some pressure. It is called vapour pressure. If the vapour pressure starts decreasing then the molecule starts leaving from liquid surface very fast, this phenomenon is known as boiling.

In boiling, the bubbles are formed inside the liquid. This bubble travels near to higher pressure zone and collapses due to higher pressure. These collapsing bubbles are exerting significantly higher pressure around 100 atmospheric pressure. This pressure causes mechanical erosion on metal. Commonly, this effect is called Cavitation. It is required to study and design hydrodynamic machinery considering Cavitation.

Cavitation has both sides beneficial and non-beneficial. As we know that Cavitation cause erosion in metal, so it is non-beneficial

Some new research areas recently suggest that hydrodynamic Cavitation is useful for some chemical and wastewater treatment. So here, hydrodynamic Cavitation is a beneficial concept.

The vapour pressure of the liquid firmly depends on temperature: It increases with increase in temperature. At the temperature of 20°C, the vapour pressure of water is 0.235 N/cm2. The vapour pressure of Mercury is 1.72* 10-5 N/cm2.

If we want to avoid Cavitation in hydraulic machinery: We should not allow liquid pressure to fall below vapour pressure at the local temperature.

You might have thought many times that why the Mercury is used inside the thermometer and manometer. Why not other liquid?

Your answer is here; the Mercury has the lowest value of vapour pressure with high density. Its make Mercury suitable for use in thermometer and manometer. 

Find the capillary effect in a tube of diameter 4mm. When the liquid is water

Questions & Answers

1) What is the difference between Cohesion and adhesion?

Cohesion is an attraction force of molecules between the same matter whereas adhesion is an attraction between molecules of different matter.

2) The Mercury is tried to stay away from the surface, why?

In Mercury, cohesion force is greater than adhesion force. Because of this, Mercury is called non-wetting liquid.

3) What is condition for wetting and non-wetting of liquid with the surface?

The liquid will wet the solid surface is less than 90 degree. If the angle is greater than 90 degree, then the liquid will not wet the solid surface.

4) Explain about surface tension

The liquid molecules on the free surface are getting attracted by liquid molecules lying below them. This free surface liquid molecules will feel pull force interior of the liquid. This force acts like elastic force. Expended per unit area of the surface is called surface tension. The Surface tension is denoted by Sigma (σ). Surface tension occurs at liquid-gas interface, liquid-liquid interface. The reason behind surface tension is an intermolecular attraction because of Cohesion.

5) Give some practical examples of surface tension.

  • You might be noticed that if we thoroughly pour water inside the glass. Even if the glass is filled, still we can add some water above the glass limit.
  • Suppose we will experiment with a thin glass tube on the water surface. We can easily notice a capillary rise and depression inside a thin glass tube.
  • Birds can drink water from the water body due to surface tension.

6) What is the unit of surface tension?

The unit of surface tension is N/m.

7) Give the value of surface tension for air-water and air-Mercury interface at standard pressure and temperature.

The surface tension for the air-water interface is 0.073 N/m.

The surface tension for the air-Mercury interface is 0.480 N/m.

8) What is the capillary effect?

If the narrow tube is dipped into the water, the water will rise inside the tube at a certain level. This type of tube is called a capillary tube, and this phenomenon is called the capillary effect.

9) Is there any relationship between the capillary effect and the surface tension? If yes, what?

Yes. The capillary effect is due to surface tension force. The capillary rise and depression are happening because of cohesion and adhesion intermolecular attraction.

10) Define: Boiling, Cavitation

Boiling: The vapour bubbles form inside the liquid due to temperature and pressure change. The boiling is a change of state) from liquid to vapour.

Cavitation: The formation of a vapour bubble inside machinery due to pressure of the liquid falls below the saturated vapour pressure.

Multiple Choice Questions

1) For wetting liquid, the angle of contact θ should be ________

(a) 0                       (b) θ < π/2                           (c) θ >π/2                            (d) None

2) For non-wetting liquid, the angle of contact θ should be ________

(a) 0                       (b) θ < π/2                           (c) θ >π/2                            (d) None

3) Surface tension value decrease with __________

(a) Constant pressure

(b) Increase in temperature

(c) Increase in pressure

(d) Decrease in temperature

4) If the value angle lies between 0 and 90, then what happens in capillary effect?

 (a) h is positive with concave shape formation

(b) h is negative with concave shape formation

(c) h is negative with convex shape formation

(d) h is positive with convex shape formation

5) Why Mercury is used in thermometer and manometer?

(a) High vapour pressure and low density

(b) High vapour pressure and high density

(c) Low vapour pressure and low density

(d) Low vapour pressure and high density

6) What is approx. collapsing pressure of bubbles in cavitation phenomena?

(a) Around 20 atmospheric pressure

(b) Around 50 atmospheric pressure

(c) Around 75 atmospheric pressure

(d) Around 100 atmospheric pressure

7) What is the value of vapour pressure of water at 20° C temperature?

(a) 0.126 N/cm2

(b) 0.513 N/cm2

(c) 0.235 N/cm2

(d) 0.995 N/cm2

8) What is the value of vapour pressure of Mercury at 20° C temperature?

(a) 1.25* 10-5 N/cm2

(b) 1.72* 10-5 N/cm2

(c) 1.5* 10-5 N/cm2

(d) 1.25 N/cm2

Conclusion

This article is presented you to understand the concept of surface tension, capillary effect, Cavitation, evaporation and its effects. Some of the practical examples is included in this article to represent it practically. The effort was made to make you correlate fluid mechanics concept with your day to day life.

To learn more on fluid mechanics, please click here.

About Deepakkumar Jani

I am Deepak Kumar Jani, Pursuing PhD in Mechanical- Renewable energy. I have five years of teaching and two-year research experience. My subject area of interest are thermal engineering, automobile engineering, Mechanical measurement, Engineering Drawing, Fluid mechanics etc. I have filed a patent on "Hybridization of green energy for power production". I have published 17 research papers and two books.
I am glad to be part of Lambdageeks and would like to present some of my expertise in a simplistic way with the readers.
Apart from academics and research, I like wandering in nature, capturing nature and creating awareness about nature among people.
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Also refer my You-tube Channel regarding “Invitation from Nature”