  Non-neutrally buoyant soft particles in vertical microflows are investigated.
We find, soft particles lighter than the liquid migrate to off-center
streamlines in a downward Poiseuille flow (buoyancy-force antiparallel to
flow). In contrast, heavy soft particles migrate to the center of the downward
(and vanishing) Poiseuille flow. A reversal of the flow direction causes in
both cases a reversal of the migration direction, i. e. heavier (lighter)
particles migrate away from (to) the center of a parabolic flow profile.
Non-neutrally buoyant particles migrate also in a linear shear flow across the
parallel streamlines: heavy (light) particles migrate along (antiparallel to)
the local shear gradient. This surprising, flow-dependent migration is
characterized by simulations and analytical calculations for small particle
deformations, confirming our plausible explanation of the effect. This density
dependent migration reversal may be useful for separating particles.
