  We theoretically study sedimentation-diffusion equilibrium of dilute binary,
ternary, and polydisperse mixtures of colloidal particles with different
buoyant masses and/or charges. We focus on the low-salt regime, where the
entropy of the screening ions drives spontaneous charge separation and the
formation of an inhomogeneous macroscopic electric field. The resulting
electric force lifts the colloids against gravity, yielding highly
nonbarometric and even nonmonotonic colloidal density profiles. The most
profound effect is the phenomenon of segregation into layers of colloids with
equal mass-per-charge, including the possibility that heavy colloidal species
float onto lighter ones.
