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Science News

Researchers Discover That Sand Behaves Like Water 192

Posted by Soulskill from the not-in-your-stomach dept.
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."
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Researchers Discover That Sand Behaves Like Water More

Researchers Discover That Sand Behaves Like Water

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  • Lol (Score:2, Funny)

    by Anonymous Coward

    Quicksand discovered !!!

  • hmm... (Score:5, Interesting)

    by Anonymous Coward on Saturday June 27, 2009 @08:39AM (#28494283)

    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)

      by Anonymous Coward

      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)

        by JustinOpinion ( 1246824 ) on Saturday June 27, 2009 @10:07AM (#28494835)
        The researchers did consider the effect of air. In fact, the ambient air has the opposite effect: the drag of the air as the droplets fall rips grains out of the droplets, thus working against whatever effect is aggregating them. In particular the authors say in their article [doi.org] (p. 1111):

        For a rough estimate of the cohesive strength we track clusters as they fall and accelerate to a speed at which Stokes drag pulls individual grains off cluster protrusions. Correcting for slight changes in the air viscosity at reduced pressure, this gives values of a few nanoNewtons.

        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):

        It is difficult to distinguish van der Waals from capillary forces because we cannot rule out molecularly thin absorbed films that create tiny bridges between individual asperities24,25. However, we still observe clustering in glass grains stored under vacuum (0.05 kPa) at low humidity (,1%) and also in grains coated with hydrophobic silane.

        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.

        • Re: (Score:3, Informative)

          by raynet ( 51803 )

          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)

            by JustinOpinion ( 1246824 ) on Saturday June 27, 2009 @12:20PM (#28495825)
            Yeah you're right that quote was just about differentiating the contributions from van der Waals and capillary forces. Further in the paper they also explain:

            Whereas the instability of ordinary liquid columns is driven by molecular surface tension, possible mechanisms for droplet formation in granular systems include hydrodynamic interactions with the surrounding gas, inelastic grain-grain collisions, and cohesive forces. Hydrodynamic interactions have indeed recently been associated with fluctuations in the profile of streams falling in air 9; however, from experiments across a wide range of ambient pressures down to 0.03 kPa we find that grain-gas interactions do not drive clustering (Supplementary Fig. S1), in agreement with earlier work 6.

            (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:

            Clustering is observed down to the lowest pressure and the presence of air leads to larger clusters but does not initiate the cluster formation.

        • You read the article!

      • "Go pound air!" doesn't have the same ummph to it.

        "Go pound sand with air!" has promise.

        "Go pound sand with water!" is kinda kinky.

      • Re: (Score:2)

        by Jurily ( 900488 )

        Can't the surface irregularities act like surface tension?

    • Re:hmm... (Score:5, Informative)

      by JustinOpinion ( 1246824 ) on Saturday June 27, 2009 @09:51AM (#28494729)
      For those with access, the actual scientific article is:
      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.

      • Re: (Score:2)

        by radtea ( 464814 )

        However this new piece of work shows rather strikingly that the origin of the force is a very weak form of surface tension

        That's from the Nature summary of the article.

        something that shouldn't happen without surface tension.

        That's from the /. summary of the article. I really don't know why the /. summary mentions that droplets can't form without surface tension when the Nature summary makes it clear that the droplets form due to surface tension. Indeed, the /. summary, read naively, would seem to imply th

    • Gravity?

    • That's peculiar. What's binding the grains together to that extent? Moisture? Electrostatic charge?

      Stan Lee, clearly [wikipedia.org]

    • I say they should ferry some sand up to the ISS and see what happens in null gravity.

    • There was a recent impromptu experiment on the space station where an astronaut shook a bag of power... sugar, flour, I don't remember what and it clumped too. I'm guessing electrostatic... but more to the point... this solves a problem in the early planet formation... namely, gravity is a very weak force. Expecting dust grains to accumulate via gravity alone should a very long time, but having something to jump start the gravitational attraction like the mechanism behind this should speed up that process.

    • Re: (Score:2, Interesting)

      by Anonymous Coward

      Well the one thing that comes to mind for me is that sand when heated turns to glass; glass itself is a liquid in the sense that your regular table glass will change shape over time (e.g. start to sag) unlike other objects. Maybe we just miscategorized sand?

  • They dropped their expensive camera? (Score:4, Insightful)

    by hey! ( 33014 ) on Saturday June 27, 2009 @08:43AM (#28494297) Homepage Journal

    Haven't they heard of strobe lights?

    • 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.

      • Re:They dropped their expensive camera? (Score:5, Informative)

        by samriel ( 1456543 ) on Saturday June 27, 2009 @09:08AM (#28494467)

        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]

        • Re: (Score:3, Interesting)

          by Nursie ( 632944 )

          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)

            by ae1294 ( 1547521 )

            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)

              by tsstahl ( 812393 )

              When does something changing from a liquid to a solid change models?

              When light wavicles hit it?

              *rimshot*

              • Re: (Score:2)

                by ae1294 ( 1547521 )

                When light wavicles hit it?

                Umm ok... please recheck the thread and get back to me on your wavicles idea... thanks...

            • Re: (Score:2)

              by radtea ( 464814 )

              When does something changing from a liquid to a solid change models?

              When it's convenient for the knowing subject. Stuff is what it is. We categorize it in various ways for our own convenience. Our categories are constrained by the way stuff is, but not determined by it. An inability to grasp this fact explains most of epistemology for the past several thousand years. Philosophers think the world must either fully determine (realists) or leave completely undetermined (subjectivists) our concepts.

              Depend

              • Re: (Score:2)

                by ae1294 ( 1547521 )

                coal--engineering a soft-rock mine can sometimes look more like fluid mechanics

                So basically what you are saying is the mater is unimportant and you can use which ever system of equations you like that is most convenient for you? This seems a little at odds with the "everything in its place" type of physics I'm use to hearing about but this isn't my field so I'll take your word on it.

          • 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)

          by hey! ( 33014 )

          I have seven words to say to you: no we will not let you go!

        • Re: (Score:2)

          by Quothz ( 683368 )

          I have one word to say to you and just one word: Galileo. [jimloy.com]

          You might consider reading the article you linked. Galileo was closer to the mark than Aristotle but not fully correct. As it points out, a more massive object does accelerate more quickly in a vacuum. Outside of a vacuum, a larger object encounters more friction and accelerates more slowly. Mind you, both effects are infinitesimally small in terms of this experiment; as usual, BaG's point is worth a moment's thought but fails to apply to the issue at hand.

          • Re: (Score:2)

            by ae1294 ( 1547521 )

            m. Outside of a vacuum, a larger object encounters more friction and accelerates more slowly.

            Damnit.. Didn't I watch a video of a guy on the moon test this with a golf ball and a feather and they both hit the (moon) ground at the same time?

            Fuck this... I'm going back to bed...

            • Re: (Score:2)

              by Quothz ( 683368 )

              Damnit.. Didn't I watch a video of a guy on the moon test this with a golf ball and a feather and they both hit the (moon) ground at the same time?

              Probably you did. As I noted, the effect of the smaller mass is infinitesimally small - it can be discounted entirely unless you start dealing with very massive objects or very high initial speeds.

              But you should be aware that the effect is there: For example, say someone drops a white dwarf star over you near a black hole, and you decide that spaghettification is preferable to a fiery death, so you let yourself fall, secure in the knowledge that you'll fall at the same speed. Well, then, you'll be in for

              • Re: (Score:2)

                by ae1294 ( 1547521 )

                But you should be aware that the effect is there: For example, say someone drops a white dwarf star over you near a black hole, and you decide that spaghettification is preferable to a fiery death, so you let yourself fall, secure in the knowledge that you'll fall at the same speed. Well, then, you'll be in for a nasty surprise (hint: It's warm!).

                Well I didn't read that in your post and I didn't check the link but I'm aware of the effects of gravity on space/time so I can understand how a massive objects attraction to another massive object will lead to them meeting sooner than a less massive object but as far as I was aware on earth it's rather a constant, not withstanding the effects of air resistance. The original poster was under the impression that if you drop two balls of the same size, one being twice as heavy it would hit the ground first, w

              • Re: (Score:2)

                by mark-t ( 151149 )
                During the Apollo 15 mission, one of the astronauts performed a demonstration dropping both a feather and a hammer. It may be that video you are referring to.

        • Re: (Score:2)

          by 4D6963 ( 933028 )
          I have another word for you, well two, terminal velocity [wikipedia.org].

    • 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.

    • Just send a small transparant container of sand on the vomit comit or a space shuttle...

    • 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

      • Re: (Score:2)

        by hey! ( 33014 )

        That's true for water droplets as well. You don't get a perfect picture, but you can see that the phenomenon is periodic.

  • Mars (Score:5, Interesting)

    by __aaydvd4604 ( 915212 ) on Saturday June 27, 2009 @08:44AM (#28494305)
    Interesting.... I've always wondered how those Martian erosion patterns could definitively be ascribed to surface water, perhaps they will have to rethink that now?
    • Surely this research would be most applicable in very small scale scenarios?
    • The effect as described in the scientific paper relies on van der Waals interaction and also capillary forces. In other words, a significant portion of the force/effect comes from ambient water that coats the particle surfaces, and creates adhesion by bridging between particles when they touch.

      So this suggests that sand will act most "liquid-like" (breaking into droplets, flowing, etc.) when there is atmospheric water.

      I agree that this kind of data on granular media will have an effect on the interpre

    • Knudsden number (Score:4, Interesting)

      by Richard Kirk ( 535523 ) on Saturday June 27, 2009 @10:48AM (#28495187)

      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.

      • Re: (Score:2, Funny)

        by Anonymous Coward

        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.

        This is my favorite sentence ever. The infinite loop of causation without any real explanation of cause is just totally brilliant.

  • Water on Mars? (Score:3, Funny)

    by koan ( 80826 ) on Saturday June 27, 2009 @08:46AM (#28494321)

    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)

      by mrsquid0 ( 1335303 ) on Saturday June 27, 2009 @09:12AM (#28494507) Homepage

      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.

    • 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.

  • It's probably just due to static electricity.

    • Re:Meh... (Score:5, Informative)

      by JustinOpinion ( 1246824 ) on Saturday June 27, 2009 @10:21AM (#28494943)
      Nope. The researchers thought of that, too. But they ruled-out electrostatic charging. From the article [doi.org] (p. 1111):

      In principle, cohesion might arise from a variety of sources, including electrostatic charging, capillary or van der Waals forces. ... a rough estimate of the cohesive strength ... gives values of a few nanoNewtons. To compare this to any electrostatic forces present, we obtain the distribution of charges on the grains by applying a uniform electric field perpendicular to the falling stream and tracking individual grain trajectories (see Supplementary Information). For both glass and copper, we find the streams are neutral overall but contain a small fraction of positively and negatively charged grains, up to a roughly q_max = +/- 100,000 electron charges per grain (Supplementary Fig. S2). Still, this gives attractive electrostatic forces a maximum F_max = (1/4*pi*e_0)q_max^2/d^2 ~= 0.1 nN for grains with diameter d = 100 micrometer, too weak to be the dominant cohesive force. (Here e_0 = 8.85 * 10^-12 C^2 N^-1 m^-2 is the permittivity of free space.) Furthermore, experiments with conductive, silver-coated 100-micrometer-diameter glass spheres produce clusters identical to experiments using uncoated spheres, emphasizing that electrostatic forces do not drive the observed clustering.

      (Note that I rewrote the equations in plaintext since Slashdot doesn't support all the necessary characters.)

  • It's the air. (Score:3, Insightful)

    by vettemph ( 540399 ) on Saturday June 27, 2009 @08:48AM (#28494333)

    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)

  • News at 10 (Score:3, Funny)

    by auric_dude ( 610172 ) on Saturday June 27, 2009 @08:54AM (#28494367)
    Is a camel still the ship of the desert?

  • This is called granular flows (Score:5, Interesting)

    by Saba ( 308071 ) on Saturday June 27, 2009 @08:54AM (#28494375)
    Sand belongs to a group of things called granular media. This includes things like pellets, ores, polymers, etc.

    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)

    by SomeGuyFromCA ( 197979 ) on Saturday June 27, 2009 @08:57AM (#28494403) Journal

    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)

    by RichMan ( 8097 ) on Saturday June 27, 2009 @09:05AM (#28494453)

    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)

    by sleeponthemic ( 1253494 ) on Saturday June 27, 2009 @09:07AM (#28494461) Homepage
    Call me when they can run linux on sand.

    (Edit: Please note phone is off, due to slashdotting)
  • The video shows sand droplets forming but some of the smaller droplets are falling more slowly than the larger droplets. This indicates the drop column has air in it.

    Evacuate and try it again...

    • Re: (Score:3, Informative)

      by nloop ( 665733 )
      I thought the same thing, however, someone earlier posted a link to the original [doi.org] article, that requires a subscription to actually read, where apparently they say they tried it in a vacuum and achieved the same results

  • !News (Score:4, Insightful)

    by Thelasko ( 1196535 ) on Saturday June 27, 2009 @09:45AM (#28494685) Journal
    Okay, maybe the sand forming droplets is news. However, my old college roommate is a structural engineer. On more than one occasion he told me that structural engineers consider soil to be a highly viscous fluid.

    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)

    by JackSpratts ( 660957 ) on Saturday June 27, 2009 @10:04AM (#28494811) Homepage
    physicists may have just figured this out but special effects guys have known about it for decades. 25 years ago in ghostbusters when the stay puft marshmallow man panic causes a fire hydrant to fail (in miniature), the fountain of "water" shooting out of it is actually diatomaceous earth. shot from above in high speed it looks amazingly real.

    • Re:who ya gonna call? (Score:4, Informative)

      by Anonymous Coward on Saturday June 27, 2009 @10:41AM (#28495133)

      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.

  • 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.

    • friction creates heat, which would tend to drive them apart. Unless you're talking about air; and, they found the same effect occurs in a vacuum. As to gravity... Force due to electric charge is like 36 orders of magnitude stronger than gravity. So, if there's any chance it's electric charge, then that's the most likely reason.

    • Re: (Score:2)

      by PPH ( 736903 )
      Electrostatic attraction between sand grains. Generated by the friction of the sand flowing through the funnel.

  • This is well known. Earth quakes can cause sandy soil to flow and cause buildings to sink.

    The fluidic properties of particulates are used to process ores, grains, tobacco dust and flour for example.

  • In other news, today scientists announced that sand is wet. Well duh! What are we paying these guys for? I could have told you that... wait, sand? _Sand_ is wet? WTH?
  • showed this when they used sand to stop a bank robber and his get away car by blowing air under the sand and it fluctuated just like water would have and caused the guy and car to sink, and once the air was stopped the sand solidified again.

  • Well, you know what they say, camels are the ships of the desert.

  • seem to act like water: my kids seem to want to spend my money that way :-(

  • I for one welcome our new found sand droplet forming overlords and I am more then willing to narc on all those sand castle building sons of bitches!

  • Flowing sand has been considered as a model of hydrodynamics for quite some time. They were using it in studies of turbulence when I was at the Santa Fe Institute 10 years ago. One of the effects of turbulence is the formation of vorticies that persist for much longer than one would expect from the physical characteristics of the sand. The effect was noted and named "self-organizing criticality" 20 years ago http://www.iop.org/EJ/abstract/1402-4896/1990/T33/001 [iop.org] . The result appears as though surface tension

  • everything else is kinda moot.

    1) How uniform are the sizes of each grain?
    2) Static charges?
    3) Aerodynamics? (see 1)

    It may appear to behave like water, no chance I am going to wash my knob with it though.

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