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Teapot effect

  1. Oct 21, 2011 #1
    When poured from teapot water has a tendency to run underside the spout. I googled it but could only find that it was not because of adhesion. Even if you coat the spout with paraffin wax or if you use cold water instead of hot water there is no appreciable change.this clearly suggests that no adhesive forces come into play.
    Then what may cause this teapot effect.
     
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  3. Oct 21, 2011 #2
    This isn't physics, but I use a teapot, and find that if you thoroughly clean the spout it tends to happen less often and to a lesser degree. Also in a typical Chinese restaurant the teapots often have a inch or so of plastic tubing on the spout and this seems to help as well (no doubt its also for avoiding chipping).

    Perhaps its a wetting issue? As long as the spout is dry the flow is undisturbed, but any residue wicks moisture into the area and water tries to stick to water?
     
  4. Oct 22, 2011 #3

    A.T.

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    Of course they come into play. Paraffin is just not hydrophobic enough. Small water drops stick to paraffin, even if gravity tries to pull them directly away from it (I just tried with a tealight).

    Here is a study on this that uses superhydrophobic substrates, to make the effect go away:
    http://lpmcn.univ-lyon1.fr/~lbocquet/teapot-prl.pdf

    As expected the effect depends on: wettability (adhesive vs. cohesive forces.), flow velocity, solid surface edge curvature. But with superhydrophobic surfaces the flow separates well, while flow velocity and geometry have little effect.
     
    Last edited by a moderator: Apr 26, 2017
  5. Oct 22, 2011 #4
    A friend of mine was a production potter and he told me that all teapots drip but to minimise this drip he cuts the spout at a slight angle to give the flowing tea a bit of side spin.
     
  6. Oct 22, 2011 #5
    Google
    "Coanda Effect"
     
  7. Oct 22, 2011 #6

    Danger

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    I have several real teapots, and none of them exhibit the problem mentioned in the original post. All, of course, have undercut spouts as a proper pot does.
    The worst offenders are those idiotic stamped-steel teabag-in-warm-water things that are found in restaurants. Some with long spouts also don't dribble upon themselves. Those stubbies are just stupid. To avoid having one of those abominations piss all over your table, hold the lid open while pouring.
     
  8. Oct 22, 2011 #7
    One theory, at one web site says it is important not to clean the teapot, thereby removing the coating of stain that would build up around the lip of the spout.

    Apparently, a patina of tanic acid, as well as a proper spout velocity, together act to break the surface tension at the exit.

    Should I become a tea drinker some time in the future, I hope to be well versed on the perchase of a quality tea pot, as well as having a jump on the professional skills required for correct operation and maintenance.
     
    Last edited: Oct 22, 2011
  9. Oct 22, 2011 #8

    Danger

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    I don't know whether or not that is accurate, but I must admit that I've never cleaned a teapot other than the traditional scalding before making a new brew. I've been using one of my favourites for over 40 years without so much as a rinse.
     
  10. Oct 22, 2011 #9
    Could you wash it and see if it malfuctions? Just for fun. Of course, you should establish some drip baseline tests before washing it...
     
  11. Oct 22, 2011 #10

    Danger

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    I occurs to me that you could use that exact approach in an attempt to open a sex clinic...

    Moving on...
    I could, but I honestly don't think that it would make any difference. All of mine have drip-proof spouts, which is standard as far as I know. After any pour, there will be exactly one drop of tea clinging to the bottom of the spout lip. They're designed that way. Back in the 50's and early 60's, when my mother still made my tea for me, it was the same. One drop—no more and no less.
     
  12. Oct 22, 2011 #11
    I have attached a picture of window frame. If it was all about adhesion then why wouldn't engineers just coat the window frame with oil or anything water repellent instead of making drip groove View attachment untitled.bmp .
     
    Last edited: Oct 22, 2011
  13. Oct 22, 2011 #12

    A.T.

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    Superhydrophobic coating would indeed make sense in a lot of places. I don't know why it is not more widely used. Probably price & durability issues.

    https://www.youtube.com/watch?v=xqvXJRVmh8g

    https://www.youtube.com/watch?v=berp-odsKFo

    Windshields are interesting. But when you drive too slow, the round drops on the superhydrophobic windshield seem more annoying than the blurry water film in the normal windshield. You have to drive faster to blow the drops away, which is not what you might want to do in rain:

    https://www.youtube.com/watch?v=x5bTJcKuTek
     
    Last edited: Oct 22, 2011
  14. Oct 22, 2011 #13

    Andy Resnick

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    It's not a very simple problem to solve. Kistler and Scriven wrote (IIRC) about 40 pages on this:

    "The Teapot Effect: Sheet-Forming Flows with Deflection, Wetting and Hysteresis," S.F. Kistler and L.E. Scriven, Journal of Fluid Mechanics, 263, p. 19 (1994)
     
  15. Oct 22, 2011 #14
    After a month or so the teapot I use will start to dribble. After cleaning the spout the dribble mostly stops for another month or so. I will attempt to explain to the teapot its mistaken ways asap.
     
  16. Oct 23, 2011 #15

    Danger

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    Please do. We've all seen the mayhem that can ensue when teapots run wild.
     
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