Migration reversal of soft particles in vertical flows

Non-neutrally buoyant soft particles in vertical microflows are investigated. We find for light soft particles in downward Poiseuille flow cross-streamline migration (CSM) to off-center streamlines and for heavy particles CSM to the center. In both cases a reversal of the vertical flow direction and the related shear gradient causes a reversal of the migration direction. This gravitational driven CSM of soft particles occurs also in linear shear flows: heavy (light) particles migrate antiparallel (parallel) to the shear gradient. The surprising, flow-induced migration (reversal) is characterized by simulations and analytical approximations, confirming our plausible explanation of the effect. This might be applied for separating particles.