Researchers Discover That Sand Behaves Like Water 192
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Soulskill
from the not-in-your-stomach dept.
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."
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]
Re:Yawn (Score:1, Informative)
Technically, aren't most processors running linux just baked sand?
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.
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.
Re:Meh... (Score:5, Informative)
(Note that I rewrote the equations in plaintext since Slashdot doesn't support all the necessary characters.)
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]
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.
Re:Not quicksand (Score:3, Informative)
Quicksand is rather a colloidal suspension requiring an underground water source
Not necessarily [wikipedia.org].
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.
Re:Pour sand in a vacuum (Score:3, Informative)
Re:hmm... (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:
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.
Re:hmm... (Score:1, Informative)
Air flow around a cluster of falling particles has a scattering effect, and they *did* redo their experiment in a vacuum to be sure.
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.