Suppose you have a can of mixed nuts and dry fruits. You have a liking, particularly for the Brazil nuts, and you see no more of them left in the can. What would you do to really make sure none are left? You would shake the can; the nuts being bigger than the other nuts would surely rise to the top. And there you are! Finding and relishing even the last two or three nuts would make you happy.
Although the study of the Brazil nut effect or its mechanism might be a little abstruse, the applications of the effect are quite famous. It is an old technique used in household chores like sifting, or as seen in the above example. On shaking a mixture containing elements of varying sizes, the ones that are large move to the top. There have been many efforts to find out the 'why' and 'how' behind this commonly observed process.
It is a phenomenon where large grains rise to the surface in a granular mixture (containing grains of different sizes) on shaking.
Theoretical studies or experiments trying to understand this effect use inedible objects like beads of variable sizes. They are referred to as grains, as in a granular mixture. The Brazil nut effect, seemingly quite simple, has become a controversial topic due to differences of opinion about its mechanism. There are many theories explaining the prevalent force behind this effect. Factors like interstitial (space between the grains) air pressure, friction, and the vibration amplitude and frequency, etc., and their significance in the process are the topics for dissimilar opinions.
This is the exact opposite of the Brazil nut effect, where large granular particles go down or sink in a mixture. However, this is not universally acknowledged.
Argumentation favoring this contrary effect is understood to take place under certain given conditions. However, it is argued against due to the lack of experimental evidence.
There are many experiments conducted by researchers to prove various hypotheses explaining the reasoning behind the Brazil nut effect, which have either concluded in confirming the effect or the reverse of it. Listed below are some such mechanisms or hypotheses:
- Free Fall: The tendency of smaller grains to fill in the space left underneath the large grains is the general conceptual understanding here. This is assumed to happen during the 'free fall' portion of the shaking (when the container is accelerating downwards at a rate higher than g).
- Convection: It was put forth as another causal process by way of a 1993 experiment. It had one large grain in a cylindrical tube with other smaller grains. Intermittent shaking was said to have formed convection currents, bringing the large grain to the top. The effect of the geometry of the container on granular convection can be seen in this case. Convection did not prove as an evidence for RBNE with a cylindrical container. However, it was observed that the convection currents in a martini glass-shaped object conveyed large grains to the bottom.
- Condensation: This is considered to predict RBNE. It was experimented using molecular dynamics simulation. The basic proposition was that, grains below a certain temperature condense into a solid bundled state, keeping the individual grains in confined positions. In cases of binary grains, based on their ratio of mass and diameter, either of them was confined or moved up, as the smaller grains occupied the voids in between the larger grains. So, this mechanism, through simulation, could exhibit either a BNE or an RBNE.
- Inertia: While shaking, when the container is being accelerated downwards at a higher rate than g, it is still moving up. Here, the larger grains break through a thin layer of smaller grains due to their inertia. This is possible, as the smaller grains are moving quite slowly. On coming back down, smaller grains will be in their previous positions. Thus, in each shaking cycle, the larger grains will have moved up some distance.
Out of the many mechanisms put forth and the experiments conducted to examine their validity, each one is found to have some strengths and weaknesses. With various competing theories, including convection, the reorganization of grains, condensation, or percolation, and the entropy or gravity, there is no one particular proven mechanism justifying the process of the Brazil nut effect.