How looking at the simplest phenomena can sometimes blow your mind

ElectricAye

Well, after all, this is the sandbox....


blogs.discovermagazine.com/cosmicvariance/2009/06/29/liquid-sand/


It never ceases to amaze me how such simple things can often, upon a second glance, become inexplicably bizarre. Makes me wonder: how much do we really know about Nature? And how much do we think we know?

enjoy,
Mark


PS. No, I am not the Mark who posted that blog.

sylvie369

That reminds me of this:

http://www.nytimes.com/2009/07/07/science/07obsalt.html

"In the process, Dr. Houston and colleagues at Sandia and the University of Pittsburgh discovered that salt can be stretchy. Table salt, which consists of sodium and chloride ions lined up in a rigid crystal, is typically brittle.·But the Sandia researchers found that when they stuck the needle-like tip of an atomic microscope into a salt surface and then pulled, a strand of salt pulled loose and stretched. “We stumbled onto this nanostretching phenomenon quite by accident,” Dr. Houston said, adding, “It’s kind of like Silly Putty.” "

·

Beau Schwabe

ElectricAye,
It's interesting that they say "...this system is still not fully understood - although it is clearly displaying liquid-like characteristics", …. because my first reaction is that since the sand is·in free-fall, and therefore "weightless to earth", the sand itself has microscopic gravitational interactions between the “other" particles of sand that are also in free fall... the same holds true with liquid.· It seems that this interaction is actually understood.· You see the same sort of clumping with the ball landing in the sand with the "after jet" of sand that shoots up from the center of impact (<-- That's actually analogous to a collapsing magnetic field and the back EMF 'spike' generated when a·coil becomes de-energized <-- another topic)· ... further in the article it does make mention that the clumping phenomenon is·possibly the basics for·"... formation of planetesimals/planets".· Although there are several other factors at work here I believe that the general·statement is reasonably accurate.

Very cool anyway, thanks


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

"Liiquid" sand can be a big problem in earthquake zones. The entire downtown area in the city where I live is a "liquefaction zone". This means that, in the case of an earthquake of sufficient magnitude, the soil beneath many of our town's buildings will essentially become liquid, dooming them to certain destruction. The science behind this is the same as in the videos: if you add enough energy to a loose aggregate, it undergoes a phase change, converting from a solid to a liquid.

-Phil

Post Edited (Phil Pilgrim (PhiPi)) : 7/10/2009 5:32:59 PM GMT

ElectricAye

Beau Schwabe (Parallax) said...
....my first reaction is that since the sand is in free-fall, and therefore "weightless to earth", the sand itself has microscopic gravitational interactions between the “other" particles of sand that are also in free fall... the same holds true with liquid. It seems that this interaction is actually understood.....

Beau,

Though i haven't done the calculations, and I really don't know how long this free-fall lasted, I don't think there's enough time for the sand's self gravity to have much effect on the sand particles. In outer space a bunch of sand would have plenty of time to clump up, so long as it wasn't blown around by the solar wind or by interstellar winds or mussed up by some tidal forces from other planets, etc. I thought about electrostatic effects, but it seems to me the most likely effect would be repulsion and not attraction since the sand particles would most likely start out with the same charge. Liquid liquids have surface tension which helps draw liquid drops into balls during weightlessness, and I'm not sure what the sand equivalent of surface tension would be. Are sand particles simply gregarious little creatures? Airflow might also play a part, but the clumps seem to remain plump in a direction I would expect them to start shearing if that were the case and they seem to make no attempt to become aerodynamic, as rain drops do. It just looks weird to me.

As for the ball hitting the sand, it's very similar to what happens when very large meteors hit a hard surface on a planet. There's this weird rebound ejection thing in the center even if the meteor vaporizes on impact, and on the craters of the moon you can sometimes see its result as a "pucker" in the crater's center.

Fun stuff, sandboxes! (Unless, of course, your building happens to be built on one.... sorry, Phil, it was nice knowing you. )

Oldbitcollector (Jeff)

Expanded version with explanation..

www.youtube.com/watch?v=YfYPJZCSI-E

OBC

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ElectricAye

COOL !!!!

Thanks!

Beau Schwabe

"There's this weird rebound ejection thing in the center even if the meteor vaporizes on impact" - This is the result of the initially displaced material suddenly coming back together to reach an equillibrium state. In a 2D version it's like pushing all of the water in a U-tube to one side, and then suddenly letting it go. An impact into Sand, Water, etc. has a similar effect, but in 3D. The reason that the effect is enhanced with the 3D model verses the 2D model is because all of the force vectors that were displaced outward are collaborating back towards the center equillibrium position. The same or similar thing can be described with a collapsing magnetic field, hence the often huge back emf spike.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

ElectricAye

I'm cool with the jet; it's the airborne clumping thing that freaks me out.

Phil Pilgrim (PhiPi)

The airborne clumping may have nothing to do with attractive forces. Some sand particles, due to variable air resistance, will fall faster than others. They will tend to run into the ones ahead of them, forming clumps. The same thing happens with cars on a highway when they can't pass each other. Traffic backs up behind slower vehicles, leading to clusters of cars rather than a steady stream.

-Phil

CounterRotatingProps

" Here we demonstrate that the cluster formation is driven by minute, nanoNewton cohesive forces that arise from a combination of van der Waals interactions and capillary bridges between nanometre-scale surface asperities. " 1)

(I understood that --- until the last word [noparse]:)[/noparse]

Asperity: "When the surfaces are subjected to a compressive load, the asperities plastically deform, increasing the contact area between the two surfaces ..." 2)

Ah!

They clump like raindrops, with the leading edge rounding as it drops. (They're unlike raindrops, however, in that raindrops all have a micrograin of dust in the center, around which the water vapor condenses.)

What I'd be interested in seeing is what the sand does when the drop column is a full vacuum.

- H

__________________________________________________________________
1) http://www.nature.com/nature/journal/v459/n7250/abs/nature08115.html
2) http://en.wikipedia.org/wiki/Asperity

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ElectricAye

Phil Pilgrim (PhiPi) said...
The airborne clumping may have nothing to do with attractive forces....The same thing happens with cars on a highway when they can't pass each other. Traffic backs up behind slower vehicles, leading to clusters of cars rather than a steady stream.

-Phil

Phil,

I'm not buying this explanation. Sand particles don't use cell phones, apply cosmetics using a rear view mirror, Twitter, or get in their circa 1984 Cadillac and make their way to the Senior Center.

You'll have to do better than that.

Phil Pilgrim (PhiPi)

Okay, I did a simulation using the following parameters:

1. 200 grains of sand are falling in single file, each with a randomly assigned terminal velocity (±5%).

2. Simulation starts with all grains having reached terminal velocity and evenly spaced, five vertical units apart.

3. Simulation progresses for 5000 steps. At each step, for each grain:

····a) The grain's position is decremented by its velocity.
····b) If it's within one vertical unit of the one below it, it's set to one vertical unit above that grain. (i.e. Grains are assumed to be one unit tall, and not able to pass or squish each other.)

4. At each 50th step, the positions of all the grains relative to the top grain are printed, along with the step number.

5. The printout is copied and pasted to Excel to create a scatter chart of the 201 time steps.

Here's the chart:



As you can see, slightly random terminal velocities are sufficient to cause the clumps to form and for small clumps to join to form bigger clumps. Van der Waals forces may well be necessary to keep the clumps from falling apart again due to the faster grains tumbling around the slower ones, but the grains start out too far apart for those forces to have an effect in forming the clumps.

-Phil

ElectricAye

Phil,

Honestly, I'm very, very impressed by your efforts on this. Your results are clearly on the road toward a rational explanation of this phenomenon. And yet... I notice your resulting spacings seem to verge on random when compared to the quasi-periodicity I detect in the falling sand. Might other, more mysterious forces, be at work here? And why would falling sand grains experience different terminal velocities if we were to assume falling sand grains would experience very little air drag thanks to a kind of drafting effect of being inside a falling column? In other words, just for example, how different would be the terminal velocities of PSWPs (people skydiving without parachutes) if they were falling stacked atop one another a mile high through the air?

You get an A for effort. Your labor toward proving the ascendency of rational thought is duly noted. However.... the sands are still awaiting the grand hour of their secret's revelation.

Phil Pilgrim (PhiPi)

The falling sand grains initially experience acceleration, which naturally causes them to separate at first. (I did not simulate this part.) So they're not "stacked atop one another" once they finish the acceleration phase and reach terminal velocity. This allows the non-uniform effects of air resistance to come into play.

Reviewing the movie again, I would have to say that neither the size nor the spacing of the various clumps appears anywhere near uniform.

Anyway, my simulation is very rudimentary and was meant only to provide a plausible explanation for clump morphogenesis, not so much for clump behavior after formation.

-Phil

ElectricAye

Phil Pilgrim (PhiPi) said...
...
Reviewing the movie again, I would have to say that neither the size nor the spacing of the various clumps appears anywhere near uniform....

Of course you're right, Phil. I was just hoping to milk another cool simulation out of you.

But I'd also be willing to bet that, in addition to the effect you have so skillfully illustrated, there is some kind of resonance/oscillation phenomenon taking place at the pouring orifice, which gets enhanced as the particles move down and experience precisely what you've shown in your simulation.

my sincere thanks for working this out. I'm sure a lot of people will think twice now before applying cosmetics in their rear view mirror.

cheers,
Mark