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Surface Tension

posted Jul 2, 2015, 3:54 PM by Upali Salpadoru   [ updated Jul 2, 2015, 6:22 PM ]

What is Tension?  Nelly and Sally are pulling a rope. Nelly exerts a force to left while the force of Sally is to the right. Force has a direction. What is the direction of the force on the rope? There is no direction. Therefore it is not a force; we call it tension.

Fig.1 Nelly and Sally exert forces in opposite directions.

Anything that has a mass has an attractive force. Closer the distance between them, greater the attraction. Greater the mass greater the attraction.

Newton stated, “The attraction is directly proportional to mass while inversely proportional to the square of the distance”.

The diagram shows how the molecules are arranged in a minute section of a glass containing some water. In the glass heavy molecules are packed close to each other as their inter molecular force is high. In water they are apart but still exert a force on each other. In air molecules are far apart and it is easy for us to walk and run.

Fig.2 A molecular diagram.

If you look at the water molecules carefully, you may notice that the topmost layer is not getting a pull upwards. They are getting pulled down and on the sides. This has a skin effect on the top surface of liquids.

Then again look at the right edge of the liquid layer. It has got elevated. Do you know why?

In a thin tube the curvature of the liquid level can be very well seen. It is called the meniscus.

This is due to the greater attraction caused by the molecules of the solid container.

Here are some examples demonstrating surface tension.

Fig. 3. Water boatman, an insect that can walk on water.

Fig. 4. A coin can float in water.
Place the coin very carefully in water. When its floating add a drop of a detergent and watch.

This has to be an Aluminium coin.

It is much easier to float a steel needle or a paper clip.

Fig. 5 How the water rises in thin tubes.  Thinner the tube higher the level.

Some practical features of this phenomenon:-

  1. Water gets aborbed into paper and cloth.
  2. Ground water rises up better in clay soil than in sandy soil.
  3. Some fountain pens draw ink by this method.

Molecules in a mad rush

Ask a friend,   “Do steel needle sink or float?”  They are sure to say that they will sink as iron is denser than water. Now get some sewing needles and place them gently on water. If you fail to put it the first time, it will be difficult to float. Then you must dry it and stick in a lump of butter before you try again. Butter can keep the water molecules out.

That’s not all...

Fig.1. Say, "Abra Cadabra"

Now tell your friends to insert their fingers into the water without touching them. Observe what happens. (Nothing much will happen other than getting their fingers wet) Now you must have dipped your index finger into a dish washing solution. It should be done so that your friends cannot see. First dip your fingers, except the prepared one and show them nothing happens.

Now comes the important stage. Wave your magic wand over the water saying “ Abra… Cadabra” several times. While doing this lightly touch the water with your index finger. The molecules will go mad. Only this finger should have the detergent.

The top layer of molecules in a liquid are packed tightly. Every layer under that has molecules pulling up and down. The top layer does not have similar molecules above it. Due to this fact the liquids show a skin effect at the surface. This is the reason for a needle to float in water. This effect is called " surface tension". Soap or a detergent can break this surface.