Re: A Coffee Break
It is probably for the same reason that the big rocks end up at the toe of the pile & not in the centre. Perhaps you will have to find a coffee stacker and study its operation. ■
Segregation
The effect noted is common to vibrated beds, known as 'The Brazil Nut effect' due to the selective stratification that occurs with such nuts. An impressive demonstration that John Williams of Bradford University used to carry out was to put a large steel ball bearing at the bottom of a small container and cover it with fine sand. After tapping the underside for a short while, the steel ball popped out of the surface and stood very proud, showing that the density difference was much overcome by the mechanics of the system. The basic principle is that if any displacement occurs it is easy for a fine particle to slip into a void past or under a large particle, but the opposite can not take place, so the accumulated build up progressively lifts the ball.
Percolation of this form takes place in the draining repose surface of a non-mass flow hopper, leaving a layer of fines to progressively pass down the bed as the hopper empties. Although it is common for the main residue discharging to end with a predominance of coarse particles previously deposited around the outside edges of the hopper, these is a final 'flash' of fines as the final layer empties.
Segregation is very pernicious and has some odd effects, non queerer than the vertical stratification that takes place in a long rotating cylinder. It is amazing to behold, but similarly occurs because a fine particle can penetrate a bed of coarse, but not the reverse, so any initial congregation of a few fines forms a self-reinforcing bed to hold out the coarse. Some migration occurs and the bands become progressively fewer and thicker. ■
A coffee break
At every place I've worked, so far, the instant coffee seems to knowingly contradict the principles of segregation which we all, on these forums, espouse.
When I dig in with the spoon the big grains always have finer grains underneath. I've studied the storage strata at length, through the glass or perspex, and found that there is a layer of fines residing just above the bed layer, with more, smaller, pockets of fines dispersed above. After exherted vibration compaction attempts the big grains are back at the top next morning. The ascendance of the coarser grains is also observed to be independent of the repose of the jar or its contents.
Can anybody enlighten me? Is there a Maxwellian Demon amusing itself at my ignorance? Does the phenomenon scale up to bulk storage of instant coffee, if indeed instant coffee is ever really stored in bulk? ■
John Gateley johngateley@hotmail.com www.the-credible-bulk.com