Brazil Nut Effect Easily Explained With Fluidization

It is not "first" and it is not "real".

note that the Brazil nut effect occurs even in vacuum, which sinks your explanation.

Maybe they tried to actually produce quantitative results?

Doesn't follow, since the impluse could be greater at the bottom.

Filling gas is not required for either the "Brazil nut effect" or the "reverse Brazil nut effect". No gas, no drag. It does have an effect, but...

2004 suitable for a text horizontal brazil nut effect first reported in the 1930s. References as early as 1998.

What I want to know is could this effect work in a star to preferentially lift larger nucleii to the surface.

David A. Smith

I _said_ it would work in a vacuum by _my_ analysis.

Bret Cahill

I have heard a different explanation:

For a particles to drop down a level, there must be a void underneath the particle, and the void must be large enough for the particle to fit into.

Since, in a randomly shaken container of particles, the odds of finding a small void are greater than the odds of finding a large void, the smaller particles are able to descend more often than the larger particles.

Olin Perry Norton

Why wouldn't that also be true on the way up?

Any explanation must acknowledge that gravity and the force moving the container are not the same on the top as on the bottom and that the bed resides on the bottom longer than on the top, if at all.

If you don't admit that then what happens on the way up will be the same as on the way down and there's no reason for the large particle to gravitate in any direction.

Bret Cahill

A more concise and clear restatement of the above:

Due to gravity the small particles spend more time over the course of a cycle packed at the bottom than the top where they spend more time dispersed.

The larger particle cannot plow down through the packed bed at the bottom but it can make headway up through the small dispersed particles above it whenever it has a higher velocity.

Random verticle velocity is why it works in a vacuum.

Bret Cahill

The particles fall because of gravity.

If there is a void underneath the particle large enough for the particle to fall into, it falls because of gravity.

The particles fall because of gravity.

If there is a void underneath the particle large enough for the particle to fall into, it falls because of gravity.

This is just the void theory with different words. Two masses cannot occupy the same space. Any particle will go down if it can (void beneath it) due to gravity. The larger particle can't plow down through the packed bed at the bottom because there's no void for it. It can go up through the dispersed particles because it can push up and make its own void.

Shaking just generates a random void distribution in the system, which the smaller particles preferentially fill.

Tom.

True but it doesn't explain why the larger particles move to the top.

Why doesn't the large particle beat the smallers ones to the bottom?

Because the particles act like a fluid.

There are voids in both packed beds and dispersed beds but they aren't

100% random when shaking a can of particles.

Over the course of the cycle more particles will tend to be packed at the bottom than the top.

Why don't the particles fill the same at the top and bottom?

No matter the analysis the explanation must mention that gravity packs the smaller particles at the bottom.

Bret Cahill

Dear Bret Cahill:

It doesn't always: "reverse brazil nut effect" "horizontal brazil nut effect"

David A. Smith Bret Cahill

Sure it does. Small voids are more common than large ones, so a small particle is more likely to move than a large one. Gravity biases the motion in one direction, so the small particles can move down more easily than the large ones.

A large particle can't enter a small void. In order for a large particle to go down, you need a large void under it.

Well, no. The two things that really define a fluid are infinite strain under constant shear and the ability to transmit pressure. The small particles can "flow" in a sense, but they don't transmit pressure. The situation is more like rarified gas dynamics.

Since they do depend on local geometry, you're correct that they're not 100% random. But I'm not sure that's important for this effect. Shaking creates voids. Small particles can move into a void more easily than large ones. Gravity makes all the particles want to move down into an available void. Therefore the small particles end up at the bottom.

Gravity. The particles all want to be at the bottom. The smaller ones just have an easier time getting there.

Yep. So far, I think your theory and the void theory are really the same physics, just semantically different.

Tom.

The BN effect doesn't require any theory other than basic Newtonian mechanics.

Fluidization has been studied for over 70 years and even that isn't really necessary. It was just the path that led me to the answer.

. . .

I don't have a theory. All I did was apply Newton's laws in the most conventional mundane manner.

In that case the really interesting question becomes:

How did such a trivial easy-to-explain effect ever get presented as such a baffling curiousity of science in the first place?

Bret Cahill

...

The same effect, on a long time scale is noted in fields subject to freeze-thaw cycles. Large stones and rocks rise to the surface.

As you will immediately notice - momentum effects are not required. Freezing does provide a force which effectively provides some upthrust to the entire substrate - and it is the larger objects which preferentially rise.

How does that sit with your "first real explanation invoving momentum"?

Brian Whatcott Altus OK

But that's not the same effect. That has nothing to do with large objects impacting packed beds of smaller objects or large objects passing through dispersed beds of smaller objects.

. . .

Perfectly.

Bret Cahill

nuclei Brazil nuts are the same density, whereas atomic nuts are not. This is why the stony-ory planets are on the inside and the isy-wispy planets are on the outside.

Dumbass, what will watter or sand do in the same cann?

They need to be kept separate, in different canns.

Bret Cahill