Waviness-induced passive particle manipulation of very dilute suspensions in confined microfluidic flows

This study reports a new observation of rapid particle migration in very dilute suspensions flowing through a microchannel. In microfluidics, passive particle manipulation methods based on shear gradients or inertial forces are usually preferred from an energy consumption perspective. Shear-induced migration is known to be particularly slow (requires very long channels) compared to inertial migration. In this case, neutrally buoyant particles usually follow the fluid flow field and could migrate laterally to the flow toward the center, i.e., in the direction of decreasing shear stress. This migration was experimentally observed for dense colloids and suspension flows with high particle concentrations, generally exceeding 25%. However, achieving this migration passively in very dilute suspensions is challenging, typically resulting in the same concentration distribution at the inlet, along the length, and at the microchannel outlet. In this effort, two-dimensional waviness was assigned to the walls of a confined microchannel, and flow visualization was employed to study whether this passive approach has the potential to move the particles toward walls in a very dilute suspension (0.1% concentration) for flow Reynolds numbers < 100 (in the realm of microfluidics). The approach was tested for two different particle sizes ( 2 mu m and 6 mu m) and two flow rates in a pressure-driven flow configuration. The results of particle counting from the captured images showed that wall waviness creates an asymmetric velocity profile for the fluid where the maximum velocity alternately moves closer to the wave peaks or crests on both the channel walls vs. a symmetric parabolic profile in a straight-walled channel. This asymmetry moved the particles toward the wavy walls (closer to the wave crests), where the shear stress was minimum. Even with a highly conservative estimation, it was observed that this migration or settling of the particles into two streams- one near each wall-for the wavy microchannel started to occur at an effective flow length that is less than 10% of the theoretical length required for the particles to migrate to an equilibrium position in a straight microchannel. Remarkably, this waviness-induced particle manipulation was achieved without generating turbulence or secondary flows (no Dean vortices) and, therefore, will markedly assist in controlling the pressure drop increase.