Fig. 2. Particle focusing in Newtonian and non-Newtonian fluid. Samples (50 µL/min) introduced from inner and outer wall of the spiral inlet were studied for (A) Newtonian fluid and (B) Non-Newtonian fluid at a total flow rate of 1 ml/min. Images were taken at the outlets of the spiral (highlighted in blue on the spiral design). The best separation of 1 and 7 µm particles could be obtained using a non-Newtonian fluid when the sample was introduced at the outer wall of the inlet. Scale bar:100 µm.
Following, to better understand particle migration and focusing in a non-Newtonian fluid, a detailed analysis was performed (Fig.3). The sample was introduced at the outer wall, and images were taken at four regions of the spiral channel (region 1-4) in addition to the outlet (region 5) (Fig.3A). Initially, the sheath flow pushes all the particles toward the outer wall (region 1). At region 2, the smaller particles start to migrate towards the inner wall. This is seen in the image as they spread everywhere in the channel. At region 3, most of the smaller particles have reached the inner wall. At region 4, all the smaller particles have now reached the inner wall and stay focused. The smaller particles can then be effectively collected through the inner outlet (region 5). To investigate the influence of Re, the 1 µm and 7 µm particles were pushed at different flow rates and the particle distribution was imaged at the outlet (Fig.3B). The sample flow rate was kept constant at 50 µL/min, while the sheath flow rate was increased systematically from 100 µL/min to 1 mL/min (Re = 5 to 35, Wi =0.023 to 0.165). As expected, at a low flow rate (Re = 5), both the 1 and 7 µm particles are spread out partly due to insufficient inertial and Dean forces. At relatively low total flow rate there is also insufficient pinching effect of the PEO buffer (1:2 ratio), which will result in the spreading of the particles. As the flow rate increases, a gradual spreading of 1µm was observed and at a flow rate of 500 µL/min (Re = 18), the particles reach to the inner wall. As the flow rate was increased further to 700 µL/min (Re = 25), it was observed that 1 µm particles start to focus toward the inner wall. The particles gradually move close to the inner wall as the flow rate reaches up to 900 µL/min (Re = 32). In contrast, 7 µm particles stay focused toward the outer wall and the higher the flow rate more tightly focused. The particles, initially having a broader distribution of positioning at the outer wall, at flow rates of 100 µL/min (Re = 5), are well focused as the Re was increased (Re = 12 – 32). Note that while the Dean forces quickly move the smaller particles laterally toward the inner wall, the migration toward the outer wall in the second round is significantly slowed due to dominant shear-induced lift force counteracting the influence of Dean and elastic forces pushing the particles toward the outer wall again. This phenomenon eventually enables high resolution particle separation. Calculated Wi for different volumetric flow rates is shown in supplementary table S1.