Surface Tension

What happens if you place a sewing-needle in a glass of water?

It sinks immediately.

But if you place the needle onto the water surface very carefully with tweezers, it will remain on the surface and float! In that way, you can also float paper clips, lightweight coins, etc. It works even better if you place the objects on a piece of blotting paper first, and then carefully remove the paper from under the objects as soon as the it has become saturated.

How Does This Work?

If you look at the water surface closely, it almost looks like a skin, on which the objects rest. You can observe something similar if you fill a glass of water to the brim, and then carefully add a few additional drops: The water does not flow over, but forms a small heap. Its surface looks like a taut skin, through which nothing can run out.

Actually, certain forces act in the water surface, which keep the surface together tightly. They are caused by the mutual attraction between the water molecules, and this is called surface tension. In other liquids, for example petrol, the cohesion between the particles is not so strong, therefore, they have a much lower surface tension in comparison to water.

Where else can you observe surface tension?

Surface tension is the reason why drops of water always try to take on the smallest possible surface area and therefore form a spherical shape.

Water striders can move on the water’s surface without sinking in thanks to the surface tension. They are supported by the skin just like the needle in the experiment.

Can this surface tension also be disturbing?

Sometimes, the high surface tension of the water is unwanted: It is the surface tension’s fault that water does not penetrate certain fabrics, but rolls off instead. For that reason, when doing the laundry, certain substances are added to the water, which reduce the surface tension: Soaps, detergents and washing-up liquids contain such substances, called tensides. They disturb the cohesion between the water molecules, and thus considerably reduce the surface tension. Now the water is able to wet the fabric.

Repeat the experiment at the beginning with water, to which you have added a drop of dishwashing liquid.

Now the surface tension is no longer sufficient to let the objects float, and you can’t heap up a hill of water any more.
A water strider would also be brought to a fall by dishwashing liquid in the water!