Fluid shear thinning effects on particle focusing in a spiral microchannel

Particle focusing behavior in non-Newtonian flows has received increasing attention in the past two decades because of its potential for microfluidic applications. Most previous studies on particle focusing in spiral microchannels are only concerned with the fluid elasticity effect. We present here an experimental study of fluid shear thinning effect on inertial and/or elastic particle focusing in a spiral microchannel. Particles are found to migrate into the radially outer half of the spiral in all our tested polymer solutions regardless of their shear thinning or elasticity effect, contrasting the inner half focusing in a Newtonian fluid. Introducing either of these rheological properties can substantially reduce the Reynolds number, Re, for particle focusing, order(s) of magnitude smaller than the threshold value, Re-th, for the onset of inertial focusing. Therefore, particle focusing in polymer solutions through a spiral microchannel can target low-throughput microfluidic applications with small footprints. The particle focusing position remains nearly unchanged in our tested polymer solutions if Re>Re(th )where the fluid shear thinning effect dominates the elasticity effect. It shifts toward the outer wall of the spiral when Re>Re-th because of the increasing inertia effect. Further increasing Re leads to particle defocusing, and the particle stream can even split into two sub-streams because of probably the impact of fluid shear thinning-enhanced Dean flow.