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Help on Leidenfrost effect

  1. Mar 31, 2004 #1
    First of all, let me say that i'm a french [in the way that i speak french] student and most probably my sentences will not be as accurate as they should, excuse me in advance.

    To make a long story short, the Leidenfrost effect makes a water drop to stop "boiling" at temperatures higher than 240 Celsius. The principle is as follows: the suddent heating of the water (when it falls on the surface) immediately creates a vapor layer between the liquified water and the surface.

    Note: we have roughly measured the actual temperature of the water at around 80 celsius.

    The same effect happens with a particular plastic surface that as been "polished" (or scratched) with an extremely thin sandpaper OR a surface covered with teflon => there is small "bubbles" of air that gets trapped between the real surface and the water.

    Anyway, what interests me and my group is that when you heat the water ; our optimal temperature (for the oscillating mode we concentrated on) is about 410 celsius, at that point and from a very particular volume of water [the drop constantly evaporate], it starts oscillating at a frequency of about 10Hz and increasing. At that point, the drop has a rounded triangular shape, but it's so fast, you see 6 and arguably 8 tip to the "star" it forms.

    Now, i'd like to know if anybody has ever heard of or work on such thing, we'd especially like to know what exactly happens, what makes the water turn on itself or oscillate [we're not even sure about the movement of the molecules within the drop]. If there is any formula that exists to describe such phenomenon or anything else.

    Also, we are being told it has something to do with surface tension, anyone can help?

    Thank you
     
  2. jcsd
  3. Mar 31, 2004 #2
    Salut,

    I have never heard of such a thing! But I will ask my colleagues about it. I think your english is excellent by the way.
     
  4. Apr 1, 2004 #3
    Suface tension created by the dipole-dipole attraction of water molecules are the reason for this. however, you may want to try a chemistry forum for the answer.
     
  5. Apr 3, 2004 #4
    Is this the definition?

    LEIDENFROST EFFECT - Where a liquid will not wet a surface and becomes insulated from the surface by a layer of vapor if the surface is above a critical temperature of the liquid. The surface is usually well above the liquids boiling point. Water beads seems to last forever in a 400 F frying pan on a stove, where they would boil away in 5 seconds if the pan were only 220 F. McGraw Hill, Dic. of Sci. and Tech. Terms, 5th ed, 1994 (200,000 definitions in one book)

    Thought posting the definition I use in my book "Effects and Their Uses" should be included here.

    Steve Stillman
     
  6. Apr 5, 2004 #5
    Steve: well...i already know that, i've been looking at those water beads for hours, but thanks anyway

    2Pac: hmm...dipole-dipole attraction, that might be the answer...but do you think that would be enough to create a cross wave [now i'm really not sure if it's the right word, in french it's called "onde transversale" where onde = wave] and we observed 6 different modes of oscillation, we are currently studying the 4th mode because it's the one that we have the most facility to recreate
     
  7. Apr 5, 2004 #6

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    You are more likely looking at "vortex shedding" from a "bluff body" --- sorry, no French translation for you.
     
  8. Apr 9, 2004 #7
    hmm, that sounds interesting, i'll look for that :)
     
  9. Apr 18, 2004 #8
    Well, one physics prof. said "I remember hearing of it but forget what it is" ,when I described it per your post he said "oh that sounds right". The other had never heard of the term Leidenfrost effect but when I described it said "oh yeah, I remember learning about that" but neither could point me to a reference for more information. He also said it's the same effect that makes drops of liquid nitrogen dance on the floor or a table top, unless it's a layer of dust that gathers around the surface of the drop instead.
     
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