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.