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Two Questions: viscosity and sound

  1. Aug 24, 2011 #1
    ive got two basic questions.
    1: Does density effect viscosity, as in is it possible to have a highly dense but low viscosity fluid and vice versa.
    2: I know in space theres no sound but can you hear your own heartbeat and other internal vibrations?
     
  2. jcsd
  3. Aug 24, 2011 #2

    DaveC426913

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    Yes.
     
  4. Aug 24, 2011 #3

    xts

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    1: What about quicksilver and honey?
     
  5. Aug 24, 2011 #4

    DaveC426913

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    Good one. Mercury is very dense yet not viscous.
     
  6. Aug 24, 2011 #5

    xts

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    A bit off-topic, but something I don't understand...
    Do you remember mercury thermometers (banned nowadays)?
    As you finally pull such one out of the obscure place you got it inserted, and the bulb cooled, the mercury in a capilare used to split into several parts, highest of them showing your body temperature. In order to reset the thermometer you had to knock it against the table, causing strong accelleration.

    Question: what is the force keeping the mercury in a fixed place in capilare, strong enough to resist both surface tension and gravity? It couldn't be viscosity, but what else?
     
  7. Aug 24, 2011 #6

    DaveC426913

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    I've never seen one where the mercury stayed in place. The only ones I've seen had a little plug that would ride on top of the mercury and get pushed up by it, then stick in place. You had to shake it to get the little plug to go down again.
     
  8. Aug 24, 2011 #7

    xts

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    I still have one of those (shhhh... don't tell EU authorities!) It is mercury which stays in place. There is no any plug. Yes, you have to shake it or knock it against a table to reset - to drive all mercury to the bulb.

    Usually, as it cools, a bubble of vacuum appears in bottom of the bulb, but sometimes the mercury in a capilare splits to several parts - most often at the place where the capilare is bended - just above the bulb.
     
  9. Aug 24, 2011 #8

    DaveC426913

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    Oh, I think I remember seeing one of these things.

    It had a constriction right above the bottom bulb, where it split into two very narrow capillaries for a moment.
     
  10. Aug 24, 2011 #9

    xts

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    The question stays open: what is the force keeping mercury to stay in place, rather than being retracted to the bulb? The geometry itself cannot cause it - it may help to split, but there must be some resisting force, and viscosity is definitely not sufficient.
     
  11. Aug 24, 2011 #10

    boneh3ad

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    No. The two quantities are independent. You can have a highly viscous, low density fluid (oil) or low viscosity, very dense fluid (mercury).



    While I have never been in space, I would say yes. As long as it has a medium to move through (your body in this case), sound can be heard.
     
  12. Aug 24, 2011 #11

    DaveC426913

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    Ooh! Even better than honey! Oil floats on water.




    It is certain. You an definitely hear your own body's sounds through the conduction of your body.

    It is why
    a] you often hear rushing in your ears (blood flow)
    b] your voice sounds very different to you than to others
     
  13. Aug 24, 2011 #12

    Andy Resnick

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    my suspicion is that it *is* surface tension- mercury does not wet glass. The tensile strength of mercury is about 20 kbar:

    http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5102024

    but I would have to sit down and convert the pressure jump across the interface before knowing if tensile strength is an important factor or not.
     
  14. Aug 24, 2011 #13

    DaveC426913

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    How would surface tension prevent the mercury from descending once it's reached a max and temp drops? Seems to me the fact that it doesn't wet the glass means it would come down easier.
     
  15. Aug 24, 2011 #14

    boneh3ad

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    When the mercury rises, the top isn't a flat line, but a curve called a meniscus. The curvature means that there is a net force generated by surface tension towards the dry side of the tube. When it recedes, it is entirely plausible that the upward force could hold some of it up, and if it pinches off into a bead, it could suspend itself via friction even without wetting the glass.
     
  16. Aug 25, 2011 #15

    Andy Resnick

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    That's why I needed to sit down and calculate:

    The contact angle of mercury on glass is about 140 degrees, and assuming the radius of a thermometer is 0.3 mm, that means the radius of curvature of the meniscus is 0.46mm. Using 485 mN/m as the interfacial energy of mercury in air, the meniscus has a pressure jump of 2*10^-2 bar. This is much smaller than the tensile strength, so nucleation of a surface does not proceed from cavitation.

    The vapor pressure of mercury is 3.6*10^-4 bar, so I suppose severe contamination on the wall (or a surface defect) could be sufficient to nucleate a surface (that is, initiate a dewetting transition). If we assume a thermometer radius of 0.1mm, the pressure jump across the meniscus is now larger (3.2*10^-2 bar), and so the column should be more resistant to 'breaks'.
     
  17. Aug 25, 2011 #16

    xts

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    Thanks for explanation!
    I think you overestimated the diameter of the thermometer capilar.
    Typical thermometer contains no more than 0.1cm^3 of mercury (bulb is 1cm long, about 3-4mm in diameter). As mercury thermal expansion coeff is about 6e-5/K, and the scale is about 1cm per K, the diameter must be in order of 0.03 mm.
    That is why external shape of the capilar is triangle with barely rounded corner, acting as a magnifying glass. If you look at the wrong angle, the mercury is hair-thin barely visible.

    For 0.03mm the effects you described are even stronger.
     
  18. Aug 25, 2011 #17

    Andy Resnick

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    Fair enough- using a diameter of 0.03 mm gives a pressure jump of 4.1 bar across the meniscus. Creating this surface only costs 485 erg/cm^2 *2*pi*0.015 mm *8.3*10^-3 mm = 0.004 erg. To break the column, we make two surfaces for a total of 0.008 erg: not much at all.
     
    Last edited: Aug 25, 2011
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