Double-line particle focusing induced by negative normal stress difference in a microfluidic channel

Abstract

Particles suspended in diluted viscoelastic fluids migrate in the transverse direction of the fluid flow towards equilibrium locations determined by spatial normal stress distributions across the cross-section of microfluidic channels. Polymer solutions with a negative first normal stress difference exhibit unexpected fluid behaviors such as material contraction after die extrusion and filament compression of semiflexible biopolymer gels in abrupt shear flow. The lateral particle migration was investigated in a hydroxypropyl cellulose (HPC) viscoelastic fluid with a negative first normal-stress difference. Unlike common viscoelastic fluids with positive normal stress differences, double-line particle focusing was identified in a microfluidic channel, which was caused by the negative first normal stress difference. More importantly, unique particle migration with different-sized particles in a microchannel was observed in which bigger particles were double-line focused along the channel walls while smaller particles were single-line focused at the center. A new particle focusing mechanism was suggested to demonstrate this unique double line focusing behavior of particles in the viscoelastic fluids.