Walk on Wet Sand: Sun & Heat Causes Drying

  • Thread starter kishtik
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In summary, when walking on a beach that is sunny and hot, stepping on wet sand can cause it to become dry. This is because the pressure from your foot compresses the sand particles, reducing the space between them and squeezing out the water between them. This effect can also be seen in a puddle, where the pressure from your foot pushes the water to the side and creates a temporary "dry" surface. This is also why the sand under your foot may appear drier than the surrounding sand. Additionally, this effect is similar to the line of adhesion that can be seen when pulling up on a piece of Scotch tape that has been swiped with a fingernail.
  • #1
kishtik
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I'm walking on a beach, it's sunny and hot. When I step on the wet sand it becomes dry! Why?
 
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  • #2
Eh?

- Warren
 
  • #3
what? i don't get that
 
  • #4
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.
 
  • #5
Originally posted by kishtik
I'm walking on a beach, it's sunny and hot. When I step on the wet sand it becomes dry!
Where did this happen? I've never made wet sand dry by walking on it.
 
  • #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.
 
  • #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.
 
  • #8
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
 
  • #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.
 
  • #10
Originally posted by UltraPi1
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.
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.
 
  • #11
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.
 
  • #12
No, I didn't mean that. The sand under my foot was drier.
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.
 
  • #13
Originally posted by Chi Meson


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.

Expand on this please... A quick test, and I don't follow what is so amazing about what I just did
 
  • #14
Originally posted by UltraPi1
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.
Please explain wider. Does the sand behave like a spring?
 
  • #15
Originally posted by phatmonky
Expand on this please... A quick test, and I don't follow what is so amazing about what I just did

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?
 
  • #16
Originally posted by Chi Meson
It's not earth-shattering, nor is it "unexplainable," but why does the tape not simply come unstuck and lift straight up?
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.
 
  • #17
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.
 
  • #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.
 
  • #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.
 
  • #20
>Firstly I note this is an old thread, so why resurruct it?
I found it by googling and no one 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.
 
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  • #21
Hello ppzmis.

If you want to use quotes I suggest you click on the 'new reply' button at the top left of the page.

This opens a better reply box, equipped with lots of tools, including quote wrapping code.

I am aware of dilatancy, and yes that is the corect name for the phenomen, but not an explanation.

The sand appears dry becase at least some of the water has flowed elsewhere.
Water flows elsewhere because of a pressure gradient, or not at all.
The pressure gradient is caused by the increased local loading due to the walker's weight.
Once this is removed the pressure gradient will then be in the other direction and the water will flow eventually back.

My comment about walking on water has nothing to do with cornflour fluids, although I am also aware of this phenomenon and the various videos around are quite interesting.
It is simply a comment to observe that in the case of wet sand the pores, and pore fluid is fully connected so pressure can be transmitted sideways and the pressure gradient can lead to flow, which would not be the case with non connected pores.

Incidentally the phenomenon will also not occur with merely damp sand as the fluid is not acting as a connected continuum, so cannot sustain a pressure gradient.

As a final point have you ever heard of sabqua? This sand/salt/water material also has also properties.
 
  • #22
I wonder whether this 'drying' effect would work if the sand grains were all sperical and of equal size. I suspect not - or at least a lot less. For water to be squeezed out of the sand, the space between the grains must decrease due to the foot pressure. The arrangement without your foot must be a minimum energy configuration and this would involve energy due to adhesion and cohesion of the water ('capilliary' effects)- altering the spacing / rotation of the sand particles.
Adding some local pressure could be expected to change the situation and the resulting arrangement could well involved reduced volume between the particles by making them rotate and forcing them closer together.
I don't think this, short term, effect is associated with hydrostatic pressure, which will take longer to affect things; eventually the water would soak back in and the sand settle down into a new equilibrium arrangement.
 
  • #23
I wonder whether this 'drying' effect would work if the sand grains were all sperical and of equal size. I suspect not - or at least a lot less. For water to be squeezed out of the sand, the space between the grains must decrease due to the foot pressure. The arrangement without your foot must be a minimum energy configuration and this would involve energy due to adhesion and cohesion of the water ('capilliary' effects)- altering the spacing / rotation of the sand particles.
Adding some local pressure could be expected to change the situation and the resulting arrangement could well involved reduced volume between the particles by making them rotate and forcing them closer together.
I don't think this, short term, effect is associated with hydrostatic pressure, which will take longer to affect things; eventually the water would soak back in and the sand settle down into a new equilibrium arrangement.

I don't follow your logic.

The usual explanation you will find in geology books is indeed due to dilatation and runs like this:

It is well known in soil mechanics that if you apply uniaxial compression to a cohesionless soil, such as sand, the soil mass expands in a direction orthogonal to the compression. This phenomenon is known as dilatation or dilatancy and is due to rearrangement of the grains from a close packed to a less close packed structure. The volume increase is due to an increase in void volume.

When you walk on the sand your weight applies uniaxial compression and a small raised ridge of sand forms around the footprint perimeter.
The increase in void volume reduces the pore pressure of the water in this sand.
The reduced pressure causes suction which draws in water from the sand underfoot.

I have difficulty with this explanation because there is no such animal as suction.
All such cases are actually a case of pressure difference, the resultant flow being driven from high to low.
Secondly it does not explain why the sand further away is not also dried out.
Finally it does not explain why the footprint depression eventually fills up with water, long before the perimeter ridge collapses back to surface level.

All of these are simply explained by the temporary local increase in hydrostatic pressure directly underfoot.
 
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  • #24
The situation on a beach is not a stable one and changes in detail as you walk towards the sea. We may be discussing different things, depending on where and when we have been 'observing'. The situation if you stand still on / in wet or very wet sand is very different from the dynamic situation of walking over sand that has no obviously visible surface water. I realize that Soil Mechanics experts will have a lot of relevant things to say on the subject but I think that the wet sand on the beach situation is not necessarily the same as for soil. (Except where ploughing is concerned)
This effect (the one I have seen) is more noticeable after the tide has fallen. The sand is almost in a state of semi plastic flow and it is settling down with a lot of water in between the grains. It can be made to flow away from your foot very easily. From memory it forms a slight ridge all the way round and i seem to remember the ridge even has small stretch cracks in it. The top of the ridge will actually admit air into the voids as air flows much more readily than water, initially, and give a lighter colour. Hydrostatic pressure will win in the end and the water will flow in and displace the air. If you dig up wet sand with a spade, you also get distinct colour changes as water squeezes out and air flows into the surface as the pressure is released.
My experience of using builders mortar (pug) tells me that the water in this sort of mix is very mobile. Water can be made to travel very fast as the mix is trowelled and thrown. It also flows very easily from the mortar into brickwork, taking fine cement grains with it.

I have to say that walks on the beach and making sandcastles will never be the same for me again!
 

What causes wet sand to dry quickly in the sun?

The sun's heat causes the water molecules in the wet sand to evaporate, leaving the sand dry.

Why does wet sand dry faster in the sun than in the shade?

The sun's heat is more intense than the ambient temperature in the shade, causing the water to evaporate at a faster rate.

Is it safe to walk on wet sand in the sun?

It is generally safe to walk on wet sand in the sun, but it is important to be cautious as the sand can become very hot and potentially burn your feet.

How long does it take for wet sand to dry in the sun?

The drying time of wet sand in the sun can vary depending on factors such as temperature, humidity, and the amount of water present. Generally, it can take anywhere from a few hours to a full day.

Does the color of the sand affect how quickly it dries in the sun?

Yes, darker colored sand will absorb more heat from the sun and therefore dry faster than lighter colored sand.

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