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Following a book of science experiments for children with my daughter, we did the following:

  • put some water in a shallow dish;
  • put some ground pepper floating on the water surface;
  • touch the water surface with the tip of a small stick previously dipped in liquid soap;
  • then the floating pepper escapes radially from the soap.

But I was at a loss to explain why. I mumbled something about “surface tension”, but didn't convince even myself.

What was going on?

I am pretty sure that the pepper in itself plays no role but showing the surface motion of water. Am I correct?

DaG
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1 Answers1

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The "soap molecules" are so called amphiphiles: They've got one end that likes water ("polar" or "lipophobic") and the other ("lipophilic" or "fat-loving") doesn't.

These amphiphile molecules like to form a single-molecule thick film on water, all standing up in an orderly fashion (foot in the water), so if you put a drop of it on a water surface, it spreads very wide, until the film is monomolecular.

Or, until something pushes back. In your case the pepper, when its particles are pushed together so they also form a tightly packed surface. Waiting longer, the soap will likely act as a detergent, wrap the pepper particles in a bilayer, and drag them under the surface.

Search for, e.g., surfactants, detergents for some pictures.

As to your last question: Some water gets dragged along with the surfactant, but no, there is no large lateral movement of water on the surface. The spreading front you see is just the soap.

Karl
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