Researchers Discover That Sand Behaves Like Water 192
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."
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.
Mars (Score:5, Interesting)
Re:hmm... (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.
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.
Re:hmm... (Score:1, Interesting)
TFA says "The droplets formed because of instabilities in the subtle atomic forces that attract sand grains to each other."
Personally I'm thinking that the air flow around a falling object might have a concentrating effect. I hope they redo their experiments in vacuum to be sure.
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.
Re:They dropped their expensive camera? (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:hmm... Gravity!! (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.
Re:They dropped their expensive camera? (Score:3, Interesting)
I have seven words to say to you: no we will not let you go!
who ya gonna call? (Score:5, Interesting)
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.
Re:Not quicksand (Score:3, Interesting)
-Oz
Re:hmm... (Score:2, Interesting)
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?