  A granular mixture of particles of two sizes that is shaken vertically will
in most cases segregate. If the larger particles accumulate at the top of the
sample, this is called the Brazil-nut effect (BNE); if they accumulate at the
bottom, the reverse Brazil-nut effect (RBNE). While this process is of great
industrial importance in the handling of bulk solids, it is not well
understood. In recent years ten different mechanisms have been suggested to
explain when each type of segregation is observed. However, the dependence of
the mechanisms on driving conditions and material parameters and hence their
relative importance is largely unknown. In this paper we present experiments
and simulations where both types of particles are made from the same material
and shaken under low air pressure, which reduces the number of mechanisms to be
considered to seven. We observe both BNE and RBNE by varying systematically the
driving frequency and amplitude, diameter ratio, ratio of total volume of small
to large particles, and overall sample volume. All our results can be explained
by a combination of three mechanisms: a geometrical mechanism called void
filling, transport of particles in sidewall-driven convection rolls, and
thermal diffusion, a mechanism predicted by kinetic theory.
