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.