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Water Surface Tension

  1. Sep 27, 2011 #1
    Hey guys,

    I'm working on a project which demonstrates the venturi effect, by flowing gas through a nozzle (which is housed in a 2 piece housing) and it flows out the exit, the air is picking up water along the way. We have noticed that water droplets are forming in the corners of the housing and not flowing back to the bottom to be picked back up by the venturi nozzle, we believe this is condensation forming there.

    A question came up on how the surface energy of the housing material (plastic) plays a role in this along with surface roughness. At first I thought a material with a higher surface energy would allow the water to wet the surface better thus allowing it to drain back down. But after thinking about it for awhile I'm wondering if the opposite is true, a lower surface energy material would make for a more sphere like droplet which would make it harder for the droplet to hang on into the corner. I started to change my thinking after taking a couple of different materials and putting a droplet of water on them, it seemed that the materials with the lower surface energy thus a more sphere like shape "slid" along the surface easier.

    Bottom line, I have two questions:

    1. Do you think a lower surface energy material will "hang" onto a droplet of water more so than a higher surface energy material?

    2. Why do corners seem to "hang" onto droplets in the first place?

    Thanks,
    Pete
     
  2. jcsd
  3. Sep 27, 2011 #2

    Andy Resnick

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    If I understand you, making the surface hydrophobic (high interfacial energy) will result in smaller droplets that could pushed out of the housing by the airflow. As the interfacial energy decreases, water more easily wets the surface, which could result in larger volumes of water in the housing.

    Droplets probably rest in the interior corners because the airflow is more stagnant, allowing droplets to grow over time.

    Or am I not understanding you?
     
  4. Sep 28, 2011 #3
    Andy,

    My understand is, and perhaps i'm wrong, a higher energy surface will allow water to wet more easily and vice versa. I suppose the question is, if water is building up in an interior corner would it be better to have a higher or lower surface energy material? My thought is a lower surface energy material will create a more sphere like droplet which I think will allow the droplet to fall back into the base more easily.
     
  5. Sep 28, 2011 #4

    Andy Resnick

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    That's exactly backwards- the lower the interfacial energy, the more easily water wets the surface. For example, Rain-X is a hydrophobic (high interfacial energy) coating for windshields.

    Fluid management is outside my expertise- for example, I could picture use of hydrophilic surfaces just in the interior corners to trap a limited amount of water, and high energy surfaces elsewhere to move the excess water out. OTOH, use of high interfacial energy surfaces everywhere could *prevent* droplets of water from forming in the first place.
     
  6. Sep 28, 2011 #5
    Hmm, here is where I got my data from: http://web.mit.edu/nnf/education/wettability/wetting.html

    Which states: When the solid has a high affinity for water - in which case it is called hydrophilic (high energy e.g. glass)- water spreads. In the opposite case of hydrophobic (low energy e.g. teflon) surfaces, water does not spread but, instead, forms at equilibrium a spherical cap resting on the substrate with a 'contact angle'.

    I also "backed" that up by remembering very low surface energy materials (some plastics) are very hard to bond to because they don't let the adhesive wet the surface, which makes sense.

    I appreciate the help, hopefully we can figure out the right answers.
     
    Last edited by a moderator: Apr 26, 2017
  7. Sep 28, 2011 #6

    Andy Resnick

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    There could be some confusion because of the need to account for the air-solid interface. If the water-solid interface has a lower energy than the air-solid interface, water will displace air and wet the surface. If the water-solid interface has a higher energy than the air-solid interface, air will displace the water.
     
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