Viscoelastic particle focusing and separation in a microfluidic channel with a cruciform section.

Considerable attention has been given to elasto-inertial microfluidics, which are widely applied for the focusing, sorting, and separation of particles/cells. In this work, we propose a novel yet simple fabrication process for a microchannel with a cruciform section, where elasto-inertial particle focusing is explored in a viscoelastic fluid. SU-8 master molds for polydimethylsiloxane (PDMS) structures were fabricated via standard photolithography, and then plasma bonding, following self-alignment between two PDMS structures, was performed for the formation of a microchannel with a cruciform section. The particle behaviors inside the fabricated microchannel were experimentally investigated for various flow rates and particle sizes and compared with those inside a microchannel with a square cross section. The experimental results revealed that 3D particle focusing was achieved in the center under viscoelastic fluid flow over a wide range of flow rates without any shear thinning. Even for small particles (∼2 μm), single-line particle focusing was observed in the microchannel with a cruciform section but not in a square microchannel with the same hydraulic diameter (D_h  = 75 μm). The effects of four reflex angles (270°) on particle focusing were quantitatively evaluated through numerical simulation. The simulation revealed that the migration pattern of particles is governed by the combined effect of the reflex angles and fluid inertia, leading to characteristic particle focusing behavior within the cross section of the cruciform microchannel. These findings agree well with the experimental results, which highlight the superior capability of the cruciform microchannel for inertial particle focusing across a wide range of particle sizes.