The phenomenon of granular separation by size is one observed by every child. A package of granola always has the big clumps on top, a bag of tortilla chips has the crumbs on bottom, and a box of mixed nuts always has the Brazil nuts on top. What does each of these situations have in common? Large particles on top; small particles on bottom.
Granular segregation was first studied by Brown in 1939 [1], and was subsequently studied by engineers until 1987 when it was introduced to the physics community as the "Brazil nut problem" [2]. While the problem may seem simple, it is in fact, quite complex and besides being a current active field of research, has recently generated a great deal of controversy with the advent of what is being called the "reverse Brazil nut effect".