5008091
story
Comments: 192 +- Science: Researchers Discover That Sand Behaves Like Water on Saturday June 27 2009, @08:34AM
background: url(//a.fsdn.com/sd/topics/topicscience.gif); width:62px; height:75px;
science
background: url(//a.fsdn.com/sd/topics/topicnews.gif); width:60px; height:66px;
news
Xeger writes "University of Chicago researchers have found that streams of sand can behave in a similar manner to liquids, forming water-like droplets when poured from a funnel. To obtain these results, they dropped their expensive high-speed camera from a height of several meters and observed the sand forming into droplets — something that shouldn't happen without surface tension. These findings suggest that conventional engineering wisdom about sand, dirt and other grainy materials needs to be rethought, and that it might be possible to apply fluid dynamics to some solids problems."
Related Stories
I've been in more laps than a napkin.
-- Mae West
All trademarks and copyrights on this page are owned by their respective owners. Comments are owned by the Poster. The Rest © 1997-2009 Geeknet, Inc.
Lol (Score:2, Funny)
Quicksand discovered !!!
Not quicksand (Score:5, Informative)
Quicksand discovered !!!
Quicksand is rather a colloidal suspension requiring an underground water source:
http://en.wikipedia.org/wiki/Quicksand [wikipedia.org]
Parent
Re: (Score:3, Informative)
Quicksand is rather a colloidal suspension requiring an underground water source
Not necessarily [wikipedia.org].
Re: (Score:3, Interesting)
-Oz
hmm... (Score:5, Interesting)
That's peculiar. What's binding the grains together to that extent? Moisture? Electrostatic charge? Just chance mechanical interactions of surface asperities? The first and last are already modelled in some engineering sand models, but I'm not sure they'd be powerful enough to cause droplet formation.
Re: (Score:2, Interesting)
I say it's air. In the places where the stream was thinnest, turbulence began to push the sand toward the thicker sections until it formed blobs. The water will stay in droplet form once it stops moving, but the sand will fall apart without moving air acting against it.
Re:hmm... (Score:5, Informative)
They then go on to measure more careful the strength of the clustering force, and ascribe it to both Van der Waals interaction and capillary forces. They did perform the experiment as a function of humidity to test the effect of water bridging (capillary forces) and found it to be significant. But they provide further data suggesting that Van der Waals forces also play a role. Again from the article (p. 1112):
The fact that clustering still occurs in vacuum suggests air is not crucial to the effect. The precise scaling they observe (e.g. the size and separation of the clusters as a function of time) is not consistent with simple inelastic collisions, and the effect of air would actually be to breakup the droplets, absent any attractive force. What they instead measured was a weak (but sufficient!) interaction between grains, which they ascribe to surface forces and capillary action.
Parent
Re: (Score:3, Informative)
Cannot get to the article you linked to, but the text you quoted doesn't say that they did the test in vacuum, just that they stored the sand in vacuum before testing it to get rid of any moisture.
Re:hmm... (Score:4, Informative)
(Emphasis added.)
For anyone curious, reference 6 is:
Mobius, M. E. Clustering instability in a freely falling granular jet. [aip.org] Phys. Rev. E 74, 051304 (2006). doi: 10.1103/PhysRevE.74.051304 [doi.org]
If you don't have access to Phys. Rev. E., you can read a preprint of the same paper on ArXiv here [arxiv.org].
That paper does measurements down to 0.03 kPa (1/5000 atmospheric pressure), and concludes:
Parent
Re:hmm... (Score:5, Informative)
Had you read their research, you'd know that they tested this, and found it was not the case.
Sadly, it's a lot easier to post snarky comments than it is to do the 3 minutes of research required to determine that the snarky know-it-all was, in fact, wrong.
Parent
Re:hmm... (Score:5, Informative)
John R. Royer, Daniel J. Evans, Loreto Oyarte, Qiti Guo, Eliot Kapit, Matthias E. MÃbius, Scott R. Waitukaitis & Heinrich M. Jaeger "High-speed tracking of rupture and clustering in freely falling granular streams [nature.com]" Nature, 459, 1110-1113 (25 June 2009) | doi:10.1038/nature08115 [doi.org].
The associated "News and Views" (Summary) is:
Detlef Lohse & Devaraj van der Meer "Granular media: Structures in sand streams [nature.com]" Nature, 459, 1064-1065 (25 June 2009) | doi:10.1038/4591064a [doi.org]
The previously-held belief in the field was that this breakup into droplets could be explained by inelastic collisions between the grains. That is, all the sand grains are bouncing off each other, but because these collisions are inelastic (the two particles slow down a bit relative to each other with the collision) the grains will, statistically, aggregate into larger structures.
However this new piece of work shows rather strikingly that the origin of the force is a very weak form of surface tension. In other words, the breakup into droplets occurs for the same reason as it does in water and other liquids... it's just the magnitude of the force that is much smaller. In addition to the high-speed photography the Slashdot summary mentions, they also used atomic force microscopy [wikipedia.org] to directly measure the nanometer-scale cohesive forces between particles. In water, surface tension arises from the (rather strong) cohesive forces between water molecules (each water molecule 'sticks' to its neighbors). In sand, it appears that a very weak nano-scale cohesive force is nevertheless enough to generate macro-scale droplets out of micro-scale particles. The cohesive forces in sand arise from the weak Van der Waals [wikipedia.org] forces (weak, but universal, surface attraction), and due to capillary forces. That is, ambient water bridges the sand particles and causes what is effectively an attractive force, which leads to an effective surface tension.
In the paper, they describe how they vary the particle type and ambient conditions, to demonstrate that these two effects are important. For instance varying humidity alters the cohesion and thus droplet formation. Also, altering the sand particles has an effect: e.g. rougher particles cannot stick to each other as much, thereby reducing this effect.
This is a neat piece of work because it involves just "known" physics. It is demonstrating that well-established physical effects (surface forces and capillary forces) can explain phenomena where their effect was previously assumed to be negligible. The surface tension in these granular media are about 100,000 times smaller than water, yet the exact same effects are observed: the surface tension, weak as it is, tries to minimize surface area. Coupled with well-known instabilities [wikipedia.org], this causes a breakup into droplets.
Parent
Re: (Score:2)
Gravity?
Re: (Score:3, Interesting)
That's not gravity, on these scales it's not quite powerful enough. What you're thinking of is surface tension and the miniscus' formed by the cereal bits. It's actually not that bad of an example of gravity because it is a physical representation of spacetime and something denting it, which is a familiar image if you study physics to any level. I'm not sure what causes this but it obviously is going to have some interesting ramifications.
They dropped their expensive camera? (Score:4, Insightful)
Haven't they heard of strobe lights?
Re:They dropped their expensive camera? (Score:5, Funny)
Besides that, there is also the problem of the greater weight of the camera causing it to fall faster than the lighter grains of sand. Ideally, you'd want to observe the sand in as stationary and synchronized a manner as possible. However, if the camera is moving relative to the sand, it would be difficult to monitor any particular clump of falling sand.
Parent
Re:They dropped their expensive camera? (Score:5, Informative)
Besides that, there is also the problem of the greater weight of the camera causing it to fall faster than the lighter grains of sand. Ideally, you'd want to observe the sand in as stationary and synchronized a manner as possible. However, if the camera is moving relative to the sand, it would be difficult to monitor any particular clump of falling sand.
I have one word to say to you and just one word: Galileo. [jimloy.com]
Parent
Re: (Score:3, Interesting)
Whilst he does explain it ass-backward, you would anticipate a greater resistive effect from thee air on multiple smal grains of sannd, with a proportionally large surface area, wouldn't you?
But yeah, the statement as it stands is bullcrap.
Re: (Score:2)
Whilst he does explain it ass-backward,
Sorry I'd have to agree with the grand-parent as I busted out laughing when I saw weight equals falling faster espically since the first few post where all nerdy....
But here's a question. When does something changing from a liquid to a solid change models? Donno might be just as stupid as the above but I think it's interesting that sand can exist in several different states including a liquid.
Re: (Score:2)
What? NO..
I think you missed what I was saying. You know, when you heat sand, it melts into a liquid. Reading this thread made me wonder about all the little rules that govern how liquids behave vs how solids behave and what happens in the in-between states and how that relates to this.
But never mind I wasn't expecting the Spanish Inquisition this morning.
Re: (Score:2)
anticipate a greater resistive effect from thee air on multiple smal grains of sannd
For someone making so much sense, you sure type like a retard :)
Re: (Score:3, Interesting)
I have seven words to say to you: no we will not let you go!
I knew a prof that did that! (dropped a camera) (Score:2)
He was a film prof, not physics, however. He rigged up a pulley system, so you could film a Point Of View sequence for someone thrown down a stairwell. The friction from the rope and pulley would slow down the acceleration and fall, but the camera could be run at a slower speed to compensate. At the last moment, you could grab onto the rope (with thick gloves) and save the camera. A bit of spin and/or off-center mounting of the camera would give you a more chaotic feel.
Effective and cheap.
Re: (Score:2)
A strobe light wouldn't work for this. (Score:3, Insightful)
The strobe light effect you mention appears to slow down, stop, or reverse falling droplets, but is merely an illusion. The individual droplets in each frame are actually replaced by successive droplets that are sufficiently similar-looking to give the illusion that you're seeing one individual droplet frozen in space.
With the sand example, the droplets are visibly different in size and shape. You don't want some sleight-of-hand trick with a strobe light, where you turn out the lights and quickly put a diff
Mars (Score:5, Interesting)
Re: (Score:2)
Knudsden number (Score:4, Interesting)
We see a stream of sand dividing up into 'drops'. It has been suggested that these 'drops' of sand are not being held together by internal forces, but by the air currents. The sand is arranging itself into shapes that can fall through the air, and horizontal oscillations of the air may be causing the column to break up into these 'drops'. I am not sure that is wholly the case - the video shows an intriguing 'satellite' droplet after a main one, a lot like you get with liquids.
So, could you get the same effect on Mars? You have less than 1/100th of the pressure, so we might expect the forces from the air to be proportionately weaker. There is also a characteristic length - the mean free path - which is the distance an atmospheric particle will travel before it hits another. If the geometry of what we are looking at - in this case, the sand - goes beneath the mean free path, then the flow changes. There is a dimensionless number called the Knudsden number which describes the point in which this change occurs. The man free path in the earth's atmosphere is about 0.1 micron, so on Mars it will be about 10 microns, which is probably still smaller than sand, so the Knudsden number is still below 1.0. My guess is you may get these 'droplets' on mars, but the effect is a lot weaker ad you would need a much longer drop for the effect to show itself. I hope the people repeat the experiment under vacuum. If you still get the effect in vacuum, then it must be something else.
Powders can behave a lot like liquids provided they keep moving. They can leave tracks that look a lot like liquids. I suspect some of the things we see on Mars may have been formed by powders. However, most of these mechanisms are particles moving over each other under the influence of gravity, and don't really use the atmosphere as the sand may be doing here. However, I started off as a major sceptic on water on Mars, but the evidence of shorelines (which you wouldn't get with powders unless there was something to keep them moving) is beginning to win me over. We shall see.
Here's my usual pet peeve with journalism like this. The motion of powders is a fascinating topic, and it doesn't really need dressing up as the 5th state of matter that baffles scientists. It is not a forgotten topic in science. Fluidized beds are used in industrial chemistry. They tend to be a bit unpredictable, because when they slump, it can be very hard to get them going again, which is what makes them unpredictable.
Parent
Water on Mars? (Score:3, Funny)
The finer the sand the more it acts like this, that's your "water on mars" right there.
Re:Water on Mars? (Score:4, Interesting)
The evidence for water on Mars is stronger than just erosion features. There is chemical evidence as well. Still, this does call into question how wide-spread the water was in the highland areas.
Parent
Re: (Score:2)
The finer the sand the more it acts like this, that's your "water on mars" right there.
A good example of that would have been observed by anyone changing the toner in one of the old high-end HP colour laser printers. You could see the highly liquid nature of the fine grain toner through the translucent plastic cartridges. That stuff sloshes.
Meh... (Score:2)
Re:Meh... (Score:5, Informative)
(Note that I rewrote the equations in plaintext since Slashdot doesn't support all the necessary characters.)
Parent
Re:Meh... (Score:5, Funny)
Parent
It's the air. (Score:3, Insightful)
Maybe this tells us more about what the air is doing than what the sand is doing. Chaotic particles spiraling down end up it in each others draft and stay there. (think nascar drafting)
Re: (Score:2)
that's what i thought too...
repeating the test in a vacuum would test this hypothesis pretty easily.
Re:It's the air. (Score:5, Informative)
repeating the test in a vacuum would test this hypothesis pretty easily.
And if you'd read the full article you'd know that they did test in a vacuum. And they still formed droplets.
Parent
Re: (Score:3, Funny)
That may have been the only time I've seen Nascar related to anything remotely intelligent. I applaud you sir.
Re: (Score:2)
News at 10 (Score:3, Funny)
This is called granular flows (Score:5, Interesting)
We typically regard the size of the particles to be larger than 1Âm. Any smaller and you have to start to take into account interparticle forces such as electrostatics and Van der Waals.
Trying to work out exactly how granular media behaves is tricky. Sometimes it behaves like a solid (sand on a beach, say -- you don't sink into it) and sometimes it behaves like a fluid (you can pour the grains of sand from a beach through your fingers). The example given here shows how it can behave inbetween solid objects (mechanics) and liquids (fluid dynamics). There's a large body of statistical and simulation results that try to understand what's going on, but nothing exists like Navier-Stokes does for liquids.
There's a lot of strange and unintuitive behaviour that arises out from studying these sorts of materials, and it's *extremely* important to industry. For example how granular media has a self-sorting behaviour when you subtly vary the size or mass of each particle.
The article shows another example of it.
The Falling Sand Game (Score:4, Funny)
Huh. /Someone/ has been playing too much of that nifty little toy The Falling Sand Game [fallingsandgame.com] and calling it research.
So about those "rivers" and "lakes" on mars (Score:3, Insightful)
If sand can flow like water then perhaps the lakes and rivers shown by "water" like flow on mars were just created by sand flow.
Yawn (Score:4, Funny)
(Edit: Please note phone is off, due to slashdotting)
Pour sand in a vacuum (Score:2, Redundant)
Evacuate and try it again...
Re: (Score:3, Informative)
!News (Score:4, Insightful)
For example, most houses are built to "float" in the soil like a boat. For structures that won't "float", like skyscrapers, they have to drive piles down to bedrock.
who ya gonna call? (Score:5, Interesting)
Re:who ya gonna call? (Score:4, Informative)
physicists may have just figured this out but special effects guys have known about it for decades.
With all due respect to special effects guys, they were aware of the phenomenon, but had no explanation. Physicists have also been aware of the phenomenon for decades. What this new work does is provide an explanation. From an explanation we can then move to understanding nature and rationally building technologies based on the knowledge.
Again, props to the FX people for coming up with such cool solutions. But your comment makes it seem like all that is necessary is observation. Science is about much, much more. It is about reproducible observation, experimentation, modeling, explanation, theory, and understanding.
Parent
This is interesting (Score:2)
So the "surface tension" in the sand is probably due to either friction of grains of sand rubbing together, or gravity. I doubt that it's due to charge (as in water), and I'd put my money on friction.
Re: (Score:2)
No, the Slashdot editors just got around to noticing. Make more sense now?