Surface tension – Why are water drops spherical?

Surface Tension

You may have noticed how insects are able to walk on water surface or wondering why a drop of water is spherical or perhaps how a gently placed needle is able to float on water despite being a metal. Well, wonder no more for this is due to a force known as surface tension.

Surface Tension is a force acting perpendicularly to the surface of a liquid making it to appear as a thin solid layer thereby allowing the occurrence of the aforementioned phenomena.

Surface tension in a liquid

Molecular explanation

Let us take a look at the molecular explanation of surface tension from  the picture above.

In water, each molecule is held to the surrounding molecules by strong hydrogen bonds. Molecules in the center of the liquid are completely surrounded by other molecules, so these forces are exerted in all directions. However, molecules at the surface do not have any water molecules above them to pull them upwards. Because they are only pulled sideways and downwards, the water molecules at the surface are held more closely together. This is called surface tension.

Surface tension is the energy, or work, required to increase the surface area of a liquid due to intermolecular forces. Since these intermolecular forces vary depending on the nature of the liquid (e.g. water vs. gasoline) or solutes in the liquid (e.g. surfactants like detergent), each solution exhibits differing surface tension properties. Whether you know it or not, you already have seen surface tension at work. Whenever you fill a glass of water too far, you may notice afterward that the level of the water in the glass is actually higher than the height of the glass. You may have also noticed that the water that you spilled has formed into pools that rise up off the counter. Both of these phenomena are due to surface tension.

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