Putting oil on troubled waters
Introduction
Wind blowing over water builds waves. The surface tension of the water plays an important role in the formation of the smallest waves. Wind pushes on these small waves adding energy to the waves and making them grow. Change the surface tension of water by adding a little oil and you can change how water waves grow.
Material
To Do and Notice
Notice how there are few if any waves at the upwind edge of the pond, and that much larger waves hit the downwind shore.
Go to the upwind shore and drop one drop of olive oil or dishwashing liquid into the pond.
Notice that the oil spreads into a large patch and that water waves do not grow in this patch.
Notice that as the patch of oil blows out into the pond waves enter the patch of oil and die. Waves start to grow again beyond the downwind side of the oil slick.
What's Going On?
The smallest waves in water, those waves the size of your finger or smaller, are called surface tension waves. The "skin" of water behaves like an elastic membrane if you blow wind over it, it ripples like a flag.
The wind then blows into the upwind slope of these surface tension waves, pushing them, and adding energy to them, so that they grow in height and wavelength. When they grow bigger than a finger their motion is no longer controlled by surface tension, but it is determined by gravity instead, and the waves become "gravity waves." Even these gravity waves have surface tension waves on their surface.
Almost anything you add to the surface of water will decrease its surface tension: oil and soap for example.
When you decrease the surface tension you decrease the surface tension waves, and the wind no longer adds energy to the waves, so the waves die out where the water is covered by a thin layer of oil or soap.
So What?
In storms small boats used to dangle oily rags over the side to surround themselves with a circle of oily water and thus reduce the size of the waves colliding with the boat.
Etc.
Big waves have little waves that sit on them and
ride them
And little waves have lesser waves, so on ad infinitem.
Paraphrase of a classic poem about bugs.
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Scientific Explorations with Paul Doherty |
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17 Oct 2002 |