An experiment with surface tension

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Here's an easy experiment with pepper and soap that allows you to see surface tension - and its destruction.

Photo by John Bush, Tristan Gilet/MIT.

Shake some pepper, carefully, into a cup of water. If you let it fall lightly on top of the water, many of the flakes will stay afloat. They'll wander around, dimpling the surface. Keep going until a good portion of the surface is covered in pepper.


Once you've peppered your water nicely, take a bar of soap and dip it into the center of the water. The moment you do, the pepper all around it should make a run for the hills. Some flakes will drop into the water and to the bottom of the cup. Some will just jet over to the sides. Does pepper hate soap?

No. Surface tension hates soap. Water molecules form bonds – if only briefly and lightly – with other water molecules. These bonds are especially tight on the surface of the water, forming a skin. The pepper rests on that skin.


The entire purpose of soap is to break those bonds. It is a surfactant, something which lowers the strength of the bonds and the tension of water. Soap's long, ropey molecules worm their way through liquids, breaking up whatever is in their path. That's how they clean – picking apart messy molecules and allowing water to sluice them away.

When soap touches one part of the water, it snaps the hold one molecule has on the next molecule. Without this skin to skate on, some pepper drops to the bottom.


The other pepper moves to the sides, but not under its own power. Not all the molecules have been broken up by the soap, only some have. Imagine a large group of people all holding hands and pulling lightly on each other. The soap makes some of those people break their hold. The rest are still holding and pulling. The people whose holds are broken will feel a pull towards one direction, and no pull towards the other. They'll get tugged to the sides. That's what's happening to the water molecules, and they're taking the pepper flakes with them as they're pulled aside.

Via Dr. Holly, and New Zealand Chemistry.