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Sifting Cereal - why does our cereal seem to sort itself?

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Ilmarinen
1195044.  Thu Jun 16, 2016 5:40 am Reply with quote

So, I know this was mentioned in a previous series, but things may have updated since, so I'll ask anyways; has there been a definitive explanation given for why in a packet of cereal the contents have a habit of sorting and separating?

Should there in fact be no definitive answer, I have my own theory I'd be glad to discuss with anyone.

 
14-11-2014
1195057.  Thu Jun 16, 2016 9:35 am Reply with quote

Quote:
this was mentioned in a previous series

It was, FTR: https://www.youtube.com/watch?v=gCPfpzJpi-Q

 
Zziggy
1195069.  Thu Jun 16, 2016 11:22 am Reply with quote

It seems kind of obvious to me that the smaller bits would sink to the bottom over time.

Which probably means I'm due a klaxon.

 
PDR
1195082.  Thu Jun 16, 2016 11:56 am Reply with quote

I had assumed it was a "density" thing rather than a "size" thing - I assumed what was going on was an example of the buoyancy concept, with vibration/shock/bump making the mixed particles exhibit a fluid behaviour.

But I could be wrong. It's not unheard of*.

PDR

* But it is very, very rare!

 
Zziggy
1195083.  Thu Jun 16, 2016 12:00 pm Reply with quote

PDR wrote:
I had assumed it was a "density" thing rather than a "size" thing

Given that we're essentially talking about particles of breakfast - what's the difference?

(That's an honest question - I'm crap at applied maths).

 
suze
1195086.  Thu Jun 16, 2016 12:04 pm Reply with quote

That YouTube link is blocked in the UK and therefore not much use to most of us, but Zziggy and PDR are both in essence right.

The layperson - and I certainly include myself in this - perhaps expects that the bigger bits go to the bottom because they're the heaviest, but that's not what happens.

Anyone who wants the physics should look up the term granular convection, but it explains why the Brazil nuts come to the top of the muesli box, and why the stones come to the top in your flower bed.

 
PDR
1195090.  Thu Jun 16, 2016 12:14 pm Reply with quote

Zziggy wrote:

Given that we're essentially talking about particles of breakfast - what's the difference?

(That's an honest question - I'm crap at applied maths).


Lots of ways to describe it - pseudo-hydrostatic pressure, entropy, displacement etc. The bottom line is that the denser material puts more mass into a given space than the less dense material, so a configuration with the particles stacked in order of decreasing density is the lowest energy state, so when subject to random agitation (you know - like when you hear Michael Gove talking) sufficient to fluidise the powder the mixture will tend towards that lowest energy state.

PDR

 
Ilmarinen
1195112.  Thu Jun 16, 2016 3:04 pm Reply with quote

Thanks for your replies, I get the feeling I'm going to have quite some reading material for a while as I look up granular convection. I do have one extra question though. Could the surface of the granule have an impact on its movement through the material? I thought perhaps density on its own might not be sufficient explanation as to why granules move as they do, also being affected by the surface area they present to other particles. By that I mean perhaps a more rough and irregular shape might be more easily pushed by the granules around it, and the smoother granules would transfer downward more easily as impacts from other granules would have a greater likelihood of glancing off.

 

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