Researchers Discover That Sand Behaves Like Water

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."

Comments

Re:They dropped their expensive camera?

[samriel]

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.

Re:hmm...

[JustinOpinion]

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.

Re:It's the air.

[vux984]

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.