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.
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