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Surface tension-Introduction

This article covers an Introduction to Surface Tension through concepts of surface tension of water, its meaning and goes into detailed concepts surrounding surface tension with surface tension examples.

The brilliance of any Physics concept is how obvious it is in our day to day lives, but we often simply ignore the cause. Take for example dipping a cotton bud in oil, the oil rises beyond the level to which the bud is dipped. Why does this happen? Or when you mix water and oil together, why do they always separate, no matter how hard you stir? They might sound like two very different things but surprisingly, the reason behind both phenomena is the same – the concept of physics that governs the actions of all fluids – surface tension.

Surface Tension Meaning

To put it simply, surface tension can be defined as the tendency of liquids to shrink to avail the minimum surface area possible. This is caused by the attraction between each molecule of the liquid, which wants to stay together and reduce the space between each other.

This force of attraction is also true for liquids interacting with gases or solids. In fact, this is the reason that water tends to stick to the sides of a beaker even when poured out. Or oil forming a film on the surface of the water. 

To understand the meaning of surface tension, we must first understand surface energy. Surface energy is the work done per unit area to create and maintain a surface. This is a result of the mutual attraction between the molecules of the liquid.

Let us understand this using a surface tension example. If you ever see small insects in water, they do not drown. This is because their weight is not enough to break the mutual force between the surface layer of water molecules. Thus, the insect moving is unable to generate enough surface energy to break the surface tension of water.

Surface Tension Of Water

The cohesive force between water molecules is what presents as surface tension in water. Water molecules cling to each other. But because there are fewer water molecules to cling to on the surface, as one surface comes in contact with air, an even stronger bond is formed between the molecules present on this surface layer. Thus, a strong surface tension now exists which creates a film between the air and the water.

The same thing happens when oil and water come into contact. Because oil does not ionize and dissolve, it separates out, which causes water molecules to cling together and form a clean barrier between them and the oil molecules.

Surface Tension Examples

  1. The shape of droplets – You must have noticed how no matter how and where and when you see drops of any liquid, they tend to form the same droplet shape. This is because of surface tension. The cohesive force between the molecules on the surface layer draws the droplet into its spherical shape.
  2. The shape of bubbles – It is for the same reason that bubbles are round. The surface tension of water causes them to collapse into each other and form a spherical shape. As a result, the air is trapped on the inside of a sphere, thus forming what we popularly call bubbles.
  3. Tents leak on touch – Tents are generally waterproof, but if you touch the surface, you will see water beginning to seep through. This is because the surface tension of water keeps the molecules from passing into and thus through the pores in the cloth material. But when you touch the surface, that tension breaks and the seepage begins, effectively destroying the waterproof property of the tent.
  4. How soaps clean – Soaps and detergents readily decrease the surface tension of water. This allows them to easily penetrate the surface and get into the pores and nooks of any material. This allows for the dirt and other microorganisms to be flushed out, giving us a clean surface.
  5. Hot water cleans better than cold water – Soaps are not the only way to decrease the surface tension of water. Heating water weakens the force of attraction between the molecules. As a result, the force now needed to break the tension is lowered considerably. This is why it is recommended to wash items in hot water over cold water for a deeper cleanse. 
  6. How disinfectants work – Similar to soaps, disinfectants are solutions with very low surface tension. This allows them to penetrate the cell wall of bacteria and fungi and kill them to sterilize a surface.
  7. Walking on water – Ever seen an ant or a small insect drown? No. They seem to have the magical property of walking on water. This is simply because their weight is not enough to break the surface tension of water. This means they can actually walk all over the thin film water created on its surface where it meets the atmosphere.
  8. Capillary action – Notice how the water forms a lifted edge on the sides of the glass it has been poured in, with a dip in the middle? This is because of the surface tension of water in contact with the glass surface. This phenomenon is observed in water bodies across the globe.

This article mainly discusses the concept of surface tension regarding the surface tension of water. All fluids exhibit surface tension. It is one of those fundamental properties of matter that ensures an array of day-to-day activities and natural phenomena go on undisturbed. Understanding this concept is of paramount importance for future clarity on more advanced concepts of physics and mechanics.

Some mathematical observations and theories will shine further clarity on the topic. There are many more concepts related to this topic we have not discussed yet, however, the bit that we have should offer enough clarity of thought for you to learn more on the topic.