Surface Tension and Soap

What Happens

There was an old commercial for Dawn® dish detergent that said “Dawn takes grease out of your way.” As the voice over said this, a single drop of Dawn was added to a sink filled with greasy water and as the soap hit the surface, the grease “ran away” leaving clean looking water behind. One of the ads can be seen here: The relevant part of the ad is at the very end.

Of course this has nothing to do with Dawn specifically. It just about soap and its effect on surface tension.

You can see the same effect on a pin resting on the surface of water. If a drop of soap is added near (but not on) the pin, the pin will move away from the soap. If you keep adding soap, eventually the pin will sink.

The question then, of course, is why?

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