Viscoelastic second normal stress difference dominated multiple-stream particle focusing in microfluidic channels

Particle focusing in viscoelastic fluid flow is a promising approach for inducing particle separations in microfluidic devices. The results from theoretical studies indicated that multiple stream particle focusing can be realized with a large magnitude of the elastic second normal stress difference (N₂). For dilute polymer solutions, theoretical and experimental studies show that the magnitude of N₂ is never large, no matter how large the polymer molecular weight nor how high the shear rate. However, for concentrated entangled polymer solutions, the magnitude of N₂ becomes large at high shear rates. Therefore, in order to test the hypothesis that N₂ can be used to induce multiple particle stream focusing behavior, we perform the systematic study of the effects of increasing carrier fluid polymer concentrations in a microchannel containing fluorescent particles. In a dilute polymer solution, multiple particle stream focusing is not observed, even at high shear rates and large dimensionless Weissenberg number values (Wi ≈ 30) at which the elastic first normal stress difference (N₁) and the viscosity shear-thinning should be very large, while in a concentrated entangled polymer solution, we observe that particle streams focused upon the channel centerline bifurcate to form two symmetric off-channel particle streams at higher shear rates. This particle focusing behavior is different from previous multiple-stream focusing phenomena, and that we attribute to the influence of the second normal stress difference N₂. This N₂ induced multiple stream focusing phenomenon provides a different approach for manipulating the particle trajectory and separation in a microchannel.