Resuspension of heavy micro-particles in elongational flow.

In this study, we explore re-suspension of non-Brownian spherical particles from under a flow directed upwards from the surface. The flow has been arranged in a microfluidic channel enabling detailed measurements of the particle lift. Simple dimensional arguments predict that for a flow with normal component of the velocity near the bottom surface, the particle lift against the gravity does not depend on the particle size within a range of the particle radii. The upper and lower limits of this range could be determined from Reynolds number on a particle to be below unity and the particle diffusion length against the gravity to be less than the particle size. This provides a range of sizes where the phenomenon can take place. For the polystyrene particles or micro-organisms, the range can be estimated from a few to a thousand microns. Our experiments with polystyrene beads reveal a very weak dependence of the lift event versus the radius of the beads, while the small discrepancy between the prediction and the experiment is attributed to the details of the flow arrangement. The results suggest that the re-suspension process indeed depends on the strength of the flow and does not depend on the size of the re-suspended particles within the considered limits.