Shaking cereal

Why is it that when you shake a box of cereal all the big bits magically float to the top? The magic also works on boxes of popcorn.

For the same reason that air bubbles will rise to the top of water, they’re less dense.

I am certain there is a proper term and equation to describe this behaviour, but consider the following:

It is easier for the smaller elements of the group to fall through the spaces between the larger elements than vice versa. The larger elements do not float upwards so much as the smaller elements all go downwards.

And I nominate this as the most appropriate poster to answer a General Question that I have ever seen.

Preview is your friend!:smack:

I was referring to cornflakes replying to a GQ about cereal…

Oh, never mind.

I’ll give it a shot…

When you are shaking a box, all the small bits start filling gaps in the cearal due to the larger bits underneath them. It’s hard to explain it in words: imagine that bowling balls are packed together, but the gaps are large enough to allow pingpong balls to drop through.

Since the amount of cereal particles are the same, this has the effect of pushing the larger bits to the top as the volume of the particles below it increases. There’s quite a bit on segregated particle settling, but I’m not too sure how detailed you want it.

It also explains why the powdery stuff is always at the bottom of cereal boxes. Since they are so small, they can fall through the gaps formed by almost all the particles. They’re also the tastiest bit of the cereal :slight_smile:

It’s not about particle size. It’s about density, as cornflakes mentioned. Essentially, the agitated particles behave like a liquid. A cork, or anything less dense than the substrate, will float on top (nothing has to fall anywhere) and a marble, or anything denser than the substrate, will sink to the bottom, regardless of its size.

So, you are saying that a large bran flake is denser than a smaller bran flake? 'Splain, please.


Strike that and reverse it.

The particle size (if the particles are the same material) is related to the [bulk] density of the media.

But, you right Gary T. I forgot my fluidised particle bed lectures :smack:.

PS DrFidelius: Bulk density is considered to the total density of the media including all the gaps between the particles. That’s why a packet of large bran flakes will a lower bulk density than a smaller bran flakes. The density of the actual particles is the same.

Should read “…than a packet of smaller…”

So the smaller pieces fall into the spaces between the larger pieces, making the larger pieces rise in the column.

I am glad you explained that so very well.

I’m going to clarify a bit, expound a bit, and retract a bit.

I misinterpreted the OP question. Had it read “Why do the bigger pieces float to the top,” I would have understood it to mean larger pieces of the cereal (e.g. flakes) as opposed to smaller pieces of the cereal. The word “bits” had me thinking of Lucky Charms, which is described as having marshmallow bits mixed in with the oat cereal. The marshmallow bits are larger than the oat pieces, hence “big bits.”

Years ago in school I saw an interesting film that showed how powders act like liquids when agitated so as to mix some air with the powder. One example of this was jostling a bag of dry cement to enable it to pour out. Another fascinating demonstration was blowing compressed air into a tub of the dry cement powder. A cork floated, a nail sank – it acted as though it were water rather than powder (the nail would just sit on top without the airstream mixed in). In that case, it was clearly the density of the cork and of the nail, compared to the density of the powder/air mix, that made the difference between floating and sinking.

I have used ashtrays made of a can partly filled with kitty litter. The cigarette butts tend to be pushed down into it, but when it is shaken, they float right to the top. Though I haven’t tried it, I daresay a marble – easily 20 times the size of a kitty litter granule – would not rise like that. Again, it seems clear that the densities of the butts and of the marble are the key factors in floating vs. sinking. Getting back to the cereal, I envisioned the marshmallow bits floating right up when the box was shaken, hence my answer above.

Now it appears that the OP question had to do with larger vs. smaller bits or pieces of the cereal itself. I gather that particle size is a factor, so apparently I was wrong to dismiss it.

Here is an interesting article that addresses some of these issues. It mentions particle size and fluid dynamics (which I believes relates to density as I and Rabid_Squirrel have mentioned) as both being factors:

It’s called the Brazil Nut Effect, and it is, in a large part, about particle size. When you agitate, the smaller particles can fall through the gaps while the larger ones can’t. It works for rocks in the soil working thir way to the surface, mixed nuts and other situations where there the large particles are not necessarily less dense than the smaller ones.

That it also works with e.g. popcorn, where the smaller particles are more dense, only serves to confuse the issue. Density plays a role, but it’s not the only or overriding factor.

Here is one article that explains that it’s a complex issue- note the mention of “it’s not just the grains’ different sizes”

Here is an abstract that mentions that it’s a size effect, but density matters as well.

Has it occurred to you that maybe shaking the packet causes the bits at the bottom to break? The ones at the top have room to move about when you shake, so they stay intact, the ones at the bottom dont have room to move, so they break.

No kidding? If I didn’t know better, I’d think that’s what the link in my second post said, too. :rolleyes:

Awhile back, I started a thread asking why a farm field can cough up big, fat, tractor-rattling rocks every year. Many of these same answers came up. Though the rocks are much denser than the dirt, they rise because they are bigger.

I wonder if this would work with every box of fluid-behaving objects, like tax receipts or body parts.