1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does a greased needle float on water?

  1. Jan 17, 2015 #1
    So I read that if you put grease on a needle it will float on water .

    Why does this happen?
     
  2. jcsd
  3. Jan 18, 2015 #2
    Doesn't this have something to do with densities and immiscibility?
     
  4. Jan 18, 2015 #3

    Drakkith

    User Avatar

    Staff: Mentor

    Not sure. I know you can float a non-greased needle in water too, so I'm not sure the grease is doing much.
     
  5. Jan 18, 2015 #4

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The grease lowers the surface tension of the needle enough that water can't wet it, and the mass of the needle is insufficient to "break" the surface film.
     
  6. Jan 18, 2015 #5
    What is meant by surface tension of needle?
    Even there are some insects which walk on water.
    I think if pressure exerted by needle is insufficient to penetrate water surface(High surface tension of water) then it remains on the surface.
    Greased neede is not able to pierce water surface.Similarly insect footpads cannot pierce water.
    Another fact is observed"If you throw a smooth stone disk parallel to even smoothly flowing river,it bonces to other shore without getting wet as O recall.
     
    Last edited: Jan 18, 2015
  7. Jan 18, 2015 #6
    A lump of grease will sink in water but if flattened out enough it will float same as oil. Covering a needle with it creates enough surface area that the weight of the needle isn't enough to counter act the buoyancy of the geese. Basically the same way ducks float above water due to there oily feathers. Water and greese(oil) don't mix.
     
  8. Jan 18, 2015 #7

    DaveC426913

    User Avatar
    Gold Member

    This is not why ducks float. Ducks float because they are less dense than water, like virtually all birds (around .6 to .8).

    But the oil in their feather does keep water from getting into their feathers, which would waterlog them.
     
  9. Jan 18, 2015 #8
    The oil does more than prevent saturation and ducks are more evolved than simply they float because they're birds. They are able to change the buoyancy using air sacks and there lungs
     
  10. Jan 18, 2015 #9

    DaveC426913

    User Avatar
    Gold Member

    Granted that it is a complex process, but it sounded like you were suggesting that ducks float for the same reason that a needle floats on the surface:
    This is not true; ducks do indeed break the surface tension of the water.

    Oils or no, if they were not less dense than water, they would not be buoyant. (The air sacs and lungs are two of the factors that allow them to be less dense than water - by increasing their volume without increasing their mass.)
     
  11. Jan 18, 2015 #10

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    For the more quantitatively minded, surface tension of water is 70-75 dyne/cm at ordinary temperatures.
     
  12. Jan 18, 2015 #11
    I incorrectly stated above in an attempt to make the reply as short as possible. The point I was trying to make is any object less buoyant than geese will float higher in water when greese is applied. The grease increases the surface area of the ducks feathers while trapping air without decreasing it's buoyancy. The greased needle surface area is increased by the less buoyant grease that both contain air bubbles as well as traps air between the grease and needle. The grease doesn't make the needle float, but the needle is actually suspended inside the floating grease. The grease floats because the needle is increasing the grease's surface area without making it heavy enough to sink.
     
  13. Jan 18, 2015 #12

    DaveC426913

    User Avatar
    Gold Member

    I do not believe this is true.

    A greased needle will float just fine despite having no air bubbles as you suggest.

    The reason it floats is simply because grease is hydrophobic. No part of the needle can penetrate the water and break the surface tension.

    A way to determine which is true is to manually submerge the greased needle. If, as you say, it is trapping air, then it will rise to the surface. But I suspect that it will sink as soon as the surface tension is compromised.
     
  14. Jan 18, 2015 #13
    The needle will eventually sink because it's in a suspension that's floating on the water, sinking it will speed the process by decreasing the surface area of the grease, as well as move it away from the center of the suspension obviously the needle will sink. I can't think of a way to accurately test that.
     
  15. Jan 18, 2015 #14

    DaveC426913

    User Avatar
    Gold Member

    How are you suggesting the surface area of the grease decreases?
     
  16. Jan 18, 2015 #15
    The grease doesn't have a strong enough bound to maintain its shape and volume under water. If the grease isn't evenly distributed it will tip to the heaviest side down. This will put the needle below the surface of the water which will increase grease removal so if the needle returns to water surface it will likely just sink again. Thought maybe cooling the water to just above freezing and the greased needle to the same tempurture to make the grease less malleable but thought that would give an incorrect result.
     
  17. Jan 18, 2015 #16

    DaveC426913

    User Avatar
    Gold Member

    Do you have any references at all to support any of this? It sounds completely made up.
     
    Last edited: Jan 18, 2015
  18. Jan 18, 2015 #17
    Based on personal experience. If you dunk a greased object in water and remove it some of the grease will remain in the water. The amount grease removed from the object will differ depending on the temperature of the water and length of time submerged.
     
  19. Jan 18, 2015 #18
    1.Temperature discussion is deviating the focus.Let us keep the temperature constant s.t. Water remains liquid only.
    2. Personal experience should not be referred to.In science ,facts remain same for everyone.
     
  20. Jan 19, 2015 #19
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why does a greased needle float on water?
Loading...