# Migration reversal of soft particles in vertical flows

## TL;DR

This study investigates how non-neutrally buoyant soft particles migrate in vertical flows, revealing flow-dependent migration reversal based on particle buoyancy and flow direction, with potential applications in particle separation.

## Contribution

The paper uncovers flow-dependent migration reversal of soft particles based on buoyancy, combining simulations and analytical calculations for small deformations.

## Key findings

- Light particles migrate to off-center streamlines in downward flow.
- Heavy particles migrate to the center of downward flow.
- Flow reversal reverses particle migration direction.

## Abstract

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.