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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."
Quicksand discovered !!!
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.
Haven't they heard of strobe lights?
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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?
The finer the sand the more it acts like this, that's your "water on mars" right there.
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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)
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Is a camel still the ship of the desert?
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.
Huh. /Someone/ has been playing too much of that nifty little toy The Falling Sand Game and calling it research.
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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.
Call me when they can run linux on sand.
(Edit: Please note phone is off, due to slashdotting)
I record my sleeptalking
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.
One of our competitors trademarked the term "hypothesis". From now on, we will call them "boneheaded ideas".
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.
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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.
(Note that I rewrote the equations in plaintext since Slashdot doesn't support all the necessary characters.)
Quicksand discovered !!!
Quicksand is rather a colloidal suspension requiring an underground water source:
http://en.wikipedia.org/wiki/Quicksand
It's Saturday, I was told there would be no maths.
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.
I thought the same thing, however, someone earlier posted a link to the original article, that requires a subscription to actually read, where apparently they say they tried it in a vacuum and achieved the same results
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 different droplet in place. You want to keep individual droplets in frame and follow them as they form and fall. Having the camera fall in unison with the sand seems like a pretty good way to do it.
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.