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Breaking surface tension in a water droplet (within a 0.88mm tube)

  1. Jan 18, 2013 #1
    Hi,

    I wasn't actually sure where I should post this.

    I am trying am experiment where I need to briefly break the surface tension of a droplet to then allow another droplet to form in it's place.

    I don't want to use any chemicals as that would effect the test further down the line.

    Is there an element that will naturally break the surface tension of water when it is briefly in contact with it?

    Basically I can get the water to form a droplet just inside the tube (which is 0.8mm ID) with a concave surface at the end of the tube, but due to its size it will then just sit there (so to speak), and I need it to drain from the tube without using any increased pressure or chemicals....

    I have thought of wicking, but that is way too slow (tested it and it does work), although I have been informed that silica strands are far better.

    Any help is appreciated.

    Cheers

    Farnet
     
  2. jcsd
  3. Jan 18, 2013 #2
    Just to make it a bit clearer.

    I am using capillary action to draw the water horizontally to the end of the tube and even with the tube pointing slightly down there isn't enough force for the water to drip out.

    I had a light bulb moment and thought that if I can somehow break the surface tension at the end of the tube, then it might act like a dripping tap.

    I don't want to contaminate the water, so I am looking at any way that I can achieve this with no chemical, electrical or mechanical intervention, I have been searching the net for various ideas but I am getting nowhere.

    Apologies to those of you who are thoroughbred scientists, this is my pet project and as such not a professional, just budding enthusiast :-)
     
  4. Jan 18, 2013 #3
    *****NOTE*****

    This was originally posted in the Chemistry forum, as I though they might have a solution, but it seems they didn't.

    I work with a team of physicists, and they weren't sure of the answer, but saying that, they are VERY specialised.
     
  5. Jan 18, 2013 #4

    Andy Resnick

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    Not sure what your overall apparatus/application is- why are you doing this?

    My first thought was to try thermocapillary flow, but water may not exhibit this effect:

    http://www.mie.utoronto.ca/labs/tkl/publications/SefianeWardRecent.pdf

    Can you push the water out by applying positive pressure to the other end? Can you treat the ends of the tube to be hydrophobic ('painting' it with Rain-X or another fluoropolymer)? Can you use another immiscible fluid that better wets the tube (say, oil) to displace the water?
     
  6. Jan 20, 2013 #5

    chemisttree

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    Go back to the chemistry forum...
     
  7. Jan 21, 2013 #6
    lol.... thanks
     
  8. Jan 21, 2013 #7
    If you break the surface tension at the end of the tube then the water will not be pulled towards the end. The simplest solution would be perhaps to use a tube that gradually increases in diameter, with the last bit bent downwards. That should be possible if you know a glass blower. Then I expect that the water will go towards the end and drip out, if the hydrostatic pressure at that end is lower than at the inlet.
    You should make sure that the net pressure is positive over the full length of the tube, else the water will not on its own reach the outlet.
     
  9. Jan 21, 2013 #8
    Hi Harrylin,

    That was my thoughts, so I have bent the tube so that the last 8mm is almost pointing vertically down.

    I thought about expanding the end and / or cutting a 'V' into the into the lower part of the end, but when the tube itself is only 0.8mm ID the I'll need someone with a steady hand like a brain surgeon to do that sort of work.

    I did manage to flare the end of one tube with a pin, but it looked more like some abstract glasswork than a scientific experiment :-)
     
  10. Jan 21, 2013 #9
    Probably the best way to do that is to start with a big tube that is drawn to a smaller diameter - as is done for making glass micro pipettes, like the ones that are used in life sciences: a bit like the following, but more gradual and less extreme
    - https://en.wikipedia.org/wiki/Micropipette
    Someone who is into glass blowing will surely be able to that with the right equipment, but I don't. It's art work. :tongue:
     
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