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Water Gastronomy Guild

Playing and Learning

Water's ''Skin'' (surface tension)

(Part of this experiment is adapted from one published in the July-August 2002 issue of the magazine Tiro-Liro, which is published by Ediciones Bayard).

Have you ever wondered why water forms itself into drops rather than spreading around everywhere? The answer lies in the existence of 'Surface Tension' (1)

What you need

A glass, a bowl full of water, a spoon and a paper clip


Down to business
Fill the glass just up to the rim. Then carry on adding water drop by drop. What happens?
	The water rises up over the rim forming a little hill on top, but it doesn't spill. This is because of the water's "surface tension", which keeps the water molecules on the surface together with the rest of the liquid and prevents the water from spilling over. Here's what to do next:

Bend the paper clip as shown in the photograph, so that you can pick it from on top
Lower the clip very gently onto the surface of the water. As you will see, it doesn?t sink!!
What?s going on? The surface of the water has sunk a little on account of the weight of the clip, but the clip just sits there on top of the water. In fact the surface of the water is acting like taut skin, holding up the clip thanks to the existence of Surface Tension.


(1) A more advanced explanation:
The molecules inside a liquid are surrounded on all sides by other molecules, but that isn't the case at the surface of the liquid since there are no more molecules of the liquid above the ones on the surface. If a molecule on the surface is displaced slightly, the molecular bonds with the other molecules get longer, and so a restoring force arises that exerts tension on the molecule again pulling it back to the surface. These forces explain why the clip in our experiment didn't sink in the water. This "restoring force" that acts along a unit of length is called the surface tension coefficient, and its value in the case of water is 0.073 N/m. Up in space (where there's no force of gravity), drops of liquid take on a spherical form, since the sphere is the geometric shape that can hold the greatest volume inside the smallest surface area (and thus the one that requires the minimum expenditure of surface energy possible). Down here on earth, drops are pear-shaped rather than spherical. That is because of the effect of the force of gravity.

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