  A binary mixture of particles interacting via long-ranged repulsive forces is
studied in gravity by computer simulation and theory. The more repulsive
A-particles create a depletion zone of less repulsive B-particles around them
reminiscent to a bubble. Applying Archimedes' principle effectively to this
bubble, an A-particle can be lifted in a fluid background of B-particles. This
"depletion bubble" mechanism explains and predicts a brazil-nut effect where
the heavier A-particles float on top of the lighter B-particles. It also
implies an effective attraction of an A-particle towards a hard container
bottom wall which leads to boundary layering of A-particles. Additionally, we
have studied a periodic inversion of gravity causing perpetual mutual
penetration of the mixture in a slit geometry. In this nonequilibrium case of
time-dependent gravity, the boundary layering persists. Our results are based
on computer simulations and density functional theory of a two-dimensional
binary mixture of colloidal repulsive dipoles. The predicted effects also occur
for other long-ranged repulsive interactions and in three spatial dimensions.
They are therefore verifiable in settling experiments on dipolar or charged
colloidal mixtures as well as in charged granulates and dusty plasmas.
