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Researchers Discover That Sand Behaves Like Water

Posted by Soulskill on 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."

15 of 192 comments (clear)

  1. hmm... by Anonymous Coward · · 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.

    1. Re:hmm... by JustinOpinion · · Score: 4, Informative

      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, 459, 1110-1113 (25 June 2009) | doi:10.1038/nature08115.

      The associated "News and Views" (Summary) is:
      Detlef Lohse & Devaraj van der Meer "Granular media: Structures in sand streams" Nature, 459, 1064-1065 (25 June 2009) | doi:10.1038/4591064a

      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 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 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, this causes a breakup into droplets.

    2. Re:hmm... by JustinOpinion · · Score: 5, Informative
      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 (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.

    3. Re:hmm... by mosch · · Score: 4, 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.

  2. Mars by __aaydvd4604 · · Score: 5, Interesting

    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?

  3. This is called granular flows by Saba · · Score: 5, Interesting

    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.

  4. The Falling Sand Game by SomeGuyFromCA · · Score: 4, Funny

    Huh. /Someone/ has been playing too much of that nifty little toy The Falling Sand Game and calling it research.

    --
    if the answer isn't violence, neither is your silence / freedom of expression doesn't make it alright
  5. Re:They dropped their expensive camera? by BadAnalogyGuy · · 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.

  6. Re:They dropped their expensive camera? by samriel · · 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.

  7. who ya gonna call? by JackSpratts · · Score: 5, Interesting

    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.

    1. Re:who ya gonna call? by Anonymous Coward · · 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.

  8. Re:Meh... by JustinOpinion · · Score: 4, Informative
    Nope. The researchers thought of that, too. But they ruled-out electrostatic charging. From the article (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.)

  9. Not quicksand by rlseaman · · Score: 4, Informative

    Quicksand discovered !!!

    Quicksand is rather a colloidal suspension requiring an underground water source:

    http://en.wikipedia.org/wiki/Quicksand

  10. Re:It's the air. by vux984 · · 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.

  11. Re:Meh... by Bin_jammin · · Score: 4, Funny

    It's Saturday, I was told there would be no maths.