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Walking on wet sand

  1. Jan 28, 2004 #1
    I'm walking on a beach, it's sunny and hot. When I step on the wet sand it becomes dry! Why?
     
  2. jcsd
  3. Jan 28, 2004 #2

    chroot

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    Eh?

    - Warren
     
  4. Jan 28, 2004 #3
    what? i don't get that
     
  5. Jan 28, 2004 #4

    FZ+

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    I'll harzad a guess.

    Sand is composed of essentially impermeable silicates, in the form of small particles. Wet sand has water between these particles.

    When you step on the sand, you compress them, reducing the space between the sand particles. Thus, in a way, you are literally squeezing the water out. Your body, being hot, may also help.
     
  6. Jan 28, 2004 #5

    turin

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    Where did this happen? I've never made wet sand dry by walking on it.
     
  7. Jan 28, 2004 #6
    I agree with FZ+
    Having been born and raised on the Florida coast for 20 years(now in Montana), I have seen this alot. Often one also notices that the beach sand immediately surrounding the foot impression is noticably wetter than before, as if the water was squeezed-out laterally to a large extent.
     
  8. Jan 28, 2004 #7
    The remaining sand from the foot impression is not "completely" dry, just much drier than before. Also, in some cases where the sand is super-saturated the effect does not occur or barely so.
     
  9. Jan 28, 2004 #8

    chroot

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    Well, okay. Of course, FZ+ is correct. When you step on the wet sand, you are increasing the pressure in the water between the grains. The pressure causes the water to flow out from under your foot, "drying" the sand.

    In the degenerate case of just stepping in a puddle without any sand, you're already aware that the pressure created by your foot pushes the water to the side. In other words, your foot is "drying" the surface under it when you step in a puddle -- if you accept using the word "drying" so loosely. When you release the pressure by lifting your foot, the water, of course, flows right back in.

    The same thing happens with the wet sand, except that the sand creates a substantial resistance to the flow of water. The water takes a few seconds (or minutes) to flow back in.

    - Warren
     
  10. Jan 28, 2004 #9
    I think I get what kishtik is talking about.

    My reply would be that the sand around your foot is raised higher than what it was before you stepped on it, and when this happens the water drains out. Essentially you raised the sand above the water table.
     
  11. Jan 29, 2004 #10
    No, I didn't mean that. The sand under my foot was drier.
    The reason I used "drying" is that noticeable color difference. The wet sand was sort of brown, but after I stepped on it, it became yellower.

    Thanks for intersest.
     
  12. Jan 29, 2004 #11

    Chi Meson

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    I read about this effect in a book (Maybe it was a magazine article, this was I'd say ten years ago) that listed a number of still unresolved puzzles. THis was one of them. DOes anyone know of this book? THe only other "puzzle" I recall was the thing about scotch tape:

    If you lay a piece of Scotch tape on a flat surface (like glass), so that it is NOT stuck; then you use a fingernail to swipe across the tape, making a thin line that IS stuck to the surface; then you pull the end of the tape straight up. YOu will see that the line of adhesion propagates along the tape-surface. IT's quite nifty.
     
  13. Jan 29, 2004 #12
    Well then it would be a little of both explanations. The sand under your foot while it's still in the sand would still be wet, although less water per volume. Lifting the foot would release the pressure thereby allowing the sand to raise slightly in which the water would flow out before it flowed back in.
     
  14. Jan 29, 2004 #13
    Expand on this please... A quick test, and I don't follow what is so amazing about what I just did
     
  15. Jan 29, 2004 #14
    Please explain wider. Does the sand behave like a spring?
     
  16. Jan 29, 2004 #15

    Chi Meson

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    OK: so you had a piece of tape that was not stuck tot he surface except for one thin line across the width of the tape; you then pulled upward on one end of the tape; there is a subsequent force that pulls the leading edge of the tape downward in order to keep a continuous line adhered to the surface.

    It's not earth-shattering, nor is it "unexplainable," but why does the tape not simply come unstuck and lift straight up?
     
  17. Jan 29, 2004 #16

    turin

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    Lever action? When you pull up on one end, the pivot point is the line that is stuck, so the other end goes down. But, by going down, it will come into contact with the table.

    That's my guess.
     
  18. Jan 29, 2004 #17

    Chi Meson

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    I recall that this is part of it, but since the tape is very flexible, it does not produce a sufficient lever arm to fully explain it. There was also something to do with the bottom side of the tape (the sticky side) is pulled with slightly more force causing a sheering at the point of contact. Since the sticky substance was more elastic than the cellophane,and since it was on the outside of the curve, it gets slightly thinned and then... I can't remember.

    BUt it is similar to the drying sand question; the full answer is a combination of several reasons, its just no one has spent the time to research it completely.

    It's also similar to that guy who got the IGnobel prize for studying why shower curtains billow in. Turns out it was not quite so simple as air convection.
     
  19. Nov 11, 2010 #18
    Hi all,

    ignore all the answers given above they are all wrong! The phenomenon being described is called "Dilatancy" and was discovered by Reynolds about 100 years ago. It works only when you have well compacted sand that contains just enough water to cover all the individual grains. When you stand on the sand you create a stress / force which causes the sand to move. In order for the sand to move / flow individual grains have to be able to move past one another. Imagine a bunch of oranges stacked as you might see them at a grocers. The first layer has them all tightly arranged and then the second layer sits down into the gaps between the oranges on the first and third layer. Now imagine trying to move one of the oranges in the second layer. In order to move it the oranges on the first and third layer mut move down and up respectively to enable the orange to move. Effectively the volume of the pile of oranges or grains of sand increases with bigger gaps in between. So when you put your foot down on the sand it shoves sand out the way but in doing so the volume in between grains has to increase temporarily to allow the grains to move relative to one another. Consequently all the fluid at the surface is sucked by surface tension into the extra gaps made by the rearrangement of the sand. Since there is no longer any fluid at the surface the grains of sand are now dry.

    Some things you might like to try are try putting your foot down slowly and very quickly. Doing it quickly has a much bigger effect since the grains have less time to rearrange resulting in bigger gaps. You might also like to get some cornstarch and dissolve it in just enough water to make it a viscous fluid. Then try hitting it hard with your fist. The surface will go from shiny to matt indicating that the fluid is retreating from the surface.
     
  20. Nov 11, 2010 #19
    Firstly I note this is an old thread, so why resurruct it?

    Secondly the answer is given in any (good) book of soil mechanics.

    Any load taken by wet soil (including sand) is distributed between the pore fluid pressure and the granular material.

    Can you walk on water?

    Well no, so as your foot applies pressure to the pore fluid (water) it displaces it away from the bearing area because the pore fluid is greater immediately under your foot than nearby.

    The balance of your weight then falls upon the sand granular structure.
     
  21. Nov 11, 2010 #20
    >Firstly I note this is an old thread, so why resurruct it?
    I found it by googling and noone had actually answered the question so why not. Why do people get so uptight about this just ignore it if it bothers you.

    >Secondly the answer is given in any (good) book of soil mechanics.
    Yes most things are answered in books but the point of a forum is some people might not have a book on soil mechanics;-) . Some people state the answer is given in any (good) books on soil mechanics and then go on to correct an answer incorrectly.

    >Any load taken by wet soil (including sand) is distributed between the pore fluid pressure >and the granular material.
    The point of the question wasn't how the load was distributed but why the sand appears dry. This is to do with Dilatancy.

    >Can you walk on water?
    >Well no, so as your foot applies pressure to the pore fluid (water) it displaces it away from >the bearing area because the pore fluid is greater immediately under your foot than nearby.
    Water no but a shear thickening fluid as in this case yes:

    Hence the comment about stepping on the sand slowly or quickly. You might want to learn something about shear thickening colloidal fluids before you make comments like that.

    >The balance of your weight then falls upon the sand granular structure.
    When you stand on the sand you sink slightly it therefore flows. You apply a stress and it flows. This flow causes granulation, shear thickening and then the fluid jams supporting a load. At that point you are right but the reason the surface appears dry is caused by the flow as you step on the sand.
     
    Last edited by a moderator: Sep 25, 2014
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