Characterization of particle behavior in non-Newtonian fluid
For particle focusing, while scaling differently, all the three forces
(FL, FE and FD) affect
the particles and as the forces are fully developed the particles will
be fully focused at the equilibrium position close to the outer wall. By
carefully optimizing the geometry, flow rate and by prepositioning
particles closer to the outer wall at the inlet, we show here that it is
possible to achieve focusing and high resolution particle separation.
Particles above a size cut-off are fully focused while smaller particles
will follow the Dean vortices and differentially migrate towards the
inner wall in spiral channels. We initially characterized the spiral
chip to understand the focusing and separation behavior using 1 and 7 µm
particles, keeping in mind the average size of RBCs and WBCs to be\(\geq\)7 µm while bacteria to be \(\sim\)1 µm. Fig.2 shows how
pre-positioning the particles at the outer wall of the spiral in a
viscoelastic flow enables size based particle separation at extremely
high total volumetric flow rates (Sample + sheath = 1 mL/min). Different
sized, 1 µm (green) and 7 µm (red), particles were suspended in
Newtonian (1X PBS) and non-Newtonian (PEO) fluid and prepositioning them
at inner and outer wall of the inlet was compared by taking images near
the outlet. For Newtonian fluid, it was not possible to separate the
particles at a flow rate of 1 mL/min independent of the sample
introduction position. In contrast, in a non-Newtonian fluid, starting
the sample from the inner wall results in the spreading of 1µm particles
toward the center of the channel from the inner wall, while 7 µm migrate
to the center of the channel and are unfocused. However, when the sample
is introduced from the outer wall , 1 µm particles migrated towards the
inner wall while the 7 µm stayed well-focused at the equilibrium
position at the outer wall. Clearly, introducing the sample closer to
the particle focusing position allows particles above a certain cut-off
to be fully focused throughout the entire channel length.