High throughput particle focusing
In viscoelastic flows through curving microchannels, the following dominant forces affect particle focusing: lift force (FL), elastic force (FE), and a curvature-induced Dean’s drag force (FD). The combined interaction results in particles migration and focusing. While previous work from our group and others have mainly focused on particle migration towards an equilibrium position using either inertial and elasto-inertial microfluidics, in this work we show that it is possible to pre-position particles at the inlet and keep the particles fully focused throughout the channel length strictly based on particle size. To this end, for the first time, we show that particles above a certain size cut-off, prepositioned at the lateral equilibrium position will remain fully focused throughout the channel, independent of length, in flow through spiral microchannels. Smaller particles are affected differently by the dominant forces and entrapped into the Dean vortices and continuously migrate away from the equilibrium position. By optimizing the spiral geometry and length, we show it is possible to fully migrate the smaller particles close to the inner wall for efficient separation By. Experimentally, by simply introducing a viscoelastic sheath buffer (PEO) into a two-inlet spiral microchannel, the blood will be pinched toward the outer wall. The pre-positioned blood cells will remain fully focused while smaller bacteria readily migrate towards the inner wall and can be extracted. Fig.1 shows a schematic illustration of how the method works. A sheath fluid pinches the sample containing large and small particles to occupy a narrow stream at the outer wall of the inlet. As the hydrodynamic forces are developed, the larger particles remain focused due to the balance between the three main forces while the smaller particles are trapped in the Dean vortices and differentially migrate away from the outer wall. Finally, the smaller particles reach the inner wall and can then be separated. Note, the total flow rate (sample + sheath) is extremely high, in the range of mL/min. Furthermore, the focusing phenomena differ from inertial microfluidics, where the focusing point is toward the inner wall laterally and at two vertically focusing points as a result of balance between FL and FD. We evaluated several parameters that influence the particle focusing behavior, preposition of the particle at the inlet, flow rate and particle size. To this end, we first used microparticles to examine the focusing phenomena before evaluating the spiral for bacteria separation.