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Drops falling

  1. Jul 20, 2006 #1
    i noticed something intresting while i was in the shower, my hair was all wet, since i was showering... and i noticed that the "water pillars" falling from my head was fluent, and at a certain distance from my head, it began turning to drops, and the distance between the drops rises as we go farther down from my head.also, the diameter of the "water pillar" is decreasing with the distance

    to simplify, the same phenomenon can be seen on water taps...

    so the first thing that came to my mind was the thought of me standing on a tall building, dropping a ball every second, and simple kinetic laws can lead u easly to the conclusion that as we go closer to the ground the distance between to balls will increase.

    yet two things are unsolved, the devresing diameter, and how can the "fluent to drops" be determined.

    im sure that it has to do something with the bonding power between water molecules, i wonder what system of forces exacly drives the water.
  2. jcsd
  3. Jul 21, 2006 #2
    I remember this same question was raised here - or maybe it was on google groups. But it has to do with surface tension.
  4. Jul 21, 2006 #3


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    I believe that it was Russ who answered this the last time. The gist is that the droplets are constantly accelerating due to gravity, but the droplet release rate doesn't change. The droplets therefore have to lengthen and narrow in order to maintain the same mass flow rate. There might be more to it than that, but that's all that I can remember right now.
  5. Jul 21, 2006 #4

    Doc Al

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    It's simply that the water accelerates as it falls. The rate (mass per second) at which water flows remains the same throughout the stream, but since the speed increases as it falls, the cross-section must decrease. The stream retains a narrow shape (instead of falling straight down like rain) due to surface tension holding things together.

    But once the stream gets too narrow, surface tension breaks it into droplets. And, again, since the droplets go faster as they fall, they get further apart.
  6. Jul 21, 2006 #5
    Another point I see is that, as the stream breaks into droplets, these droplets pull together to form a roughly spherical shape that is wider in it's cross section that the stream. This means that water from the stream contracts in two directions to some extent. These directions would be in the vertical as the drop grows horozontally fatter. As this happens distance is created between the drops that were before spread out in a thin line.
  7. Jul 22, 2006 #6
    god, can u rexplain?

    about my question, i do get the rough picture of how it happens, cuz as the speed grows, the flow must narrow, though if a solid is falling down it wont narrow, it should stay just the same.
    i do understand the contradiction in not narrowing, though i want find the fundemental force which is strenching the water,and at some point seperating it to droplets.
  8. Jul 22, 2006 #7


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    It's a simple, small-scale version of what would happen if you approached a black hole. The bottom of the drop is experiencing more gravity than the top (it's closer to the source), so it gets pulled down faster. Likewise, the whole drop is falling faster than the one above it.
  9. Jul 22, 2006 #8
    Gravity is the fundamental force you seek i think. The formation of discreet dropplets at point x is a property of the viscosity of the fluid, which in turn is a property of the intermolecular forces and internal electrostatics of the fluid. However, it is gravitational acceleration i think that actually seperates the fluid, at point x, the rate at which molecules are accelerating away from their following molcules must exceed the rate at which they attract, therefor steady flow can no longer be acheived?

    Holding a charged rod near water stream such as the one described you can see the effects of charge repulsion/attraction on a steady flow.

    Thats my attempt at rationalising the observation at least :P
  10. Jul 22, 2006 #9
    I have a different view than most here it seems. As the stream falls the leading edge (the bottom) experiences air resistance which causes it to mushroom (for lack of a better term). Like raindrops resemble little parachuttes, not actual drops we like to think of. This actually causes a slowing of the leading edge, which makes the leading edge grow in size as water from behind it catches up. As it grows it creates a pressure difference behind it which allows a very narrow area of the stream,yet again, to catch up to the swolen leading edge. This narrowing of the stream behind the leading edge gets to a point where the attraction of the more massive stream above and the more massive swolen leading edge below cause them to seperate creating some distance between the now drop and the stream.
  11. Jul 22, 2006 #10
    yet, the gravity on the water is the same for every particle, or at least the differece is next to zero. if the particles are attached, and all particles have the same pulling force, there should be no forces between the particles, so i cant seem to find the micro explanation for the phenomenon...
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