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Thread: Pouring oil on water to calm waves

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    Jon
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    Pouring oil on water to calm waves

    Perhaps you've heard the saying about pouring oil on water, in reference to calming a problematic situation. Turns out that this expression has considerable history, rooted in the physical phenomenon (only a phenomenon to non-physicists, of course) that a small amount of oil, poured onto the surface of a roiling sea, can significantly reduce waves.

    In action:



    This effect appears in ancient writings, as well as, more famously, in a 1773 letter by Benjamin Franklin, in which he recounts his oil-on-water experiment, reproducing an original experiment by Pliny the Elder (AD 23-79).

    "At length being at Clapham, where there is, on the Common, a large Pond, which I observed to be one Day very rough with the Wind, I fetched out a Cruet of Oil, and dropt a little of it on the Water. I saw it spread itself with surprising Swiftness upon the Surface, but the Effect of smoothing the Waves was not produced; for I had applied it first on the Leeward Side of the Pond where the Waves were largest, and the Wind drove my Oil back upon the Shore. I then went to the Windward Side, where they began to form; and there the Oil tho’ not more than a Tea Spoonful produced an instant Calm, over a Space several yards square, which spread amazingly, and extended itself gradually till it reached the Lee Side, making all that Quarter of the Pond, perhaps half an Acre, as smooth as a Looking Glass."
    What's happening here? There is considerable mumbling about "surface tension" and "friction" (concepts no doubt invoked in all online discussions involving physical effects). Both are reasonably applicable; it looks like it might be an example of Kelvin-Helmholtz instability, which occurs at an interface between two fluids of different density. More details on page 115 of this paper by MIT professor Erik L. Mollo-Christensen.

    This effect is especially useful for ships in stormy seas, or lifeboats. Old sailing ships were sometimes equipped with large containers that slowly dispensed oil over the sides to calm the seas. In fact, even today, US law requires that certain lifeboats carry a supply of storm oil, detailed here: 46 CFR 169.529 - Description of lifeboat equipment.

    (x) Oil, storm. One gallon of vegetable, fish, or animal oil must be provided in a suitable metal container so constructed as to permit a controlled distribution of oil on the water, and so arranged that it can be attached to the sea anchor.
    This legally-required storm oil dispenser looks like this:



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