Passive particle manipulation using inertial and elasto-inertial microfluidics have received substantial interest in recent years and have found various applications in high throughput particle sorting and separation. For separation applications, elasto-inertial microfluidics has thus far been applied at substantial lower flow rates as compared to inertial microfluidics. In this work, we explore viscoelastic particle focusing and separation in spiral channels at two orders of magnitude higher Reynolds numbers than previously reported. We show that the balance between dominant inertial lift force, dean drag force and elastic force enables stable 3D particle focusing at dynamically high Reynolds numbers. Using a two-turn spiral, we show that particles, initially pinched towards the inner wall using an elasticity enhancer, PEO (polyethylene oxide), as sheath migrate towards the outer wall strictly based on size and can be effectively separated with high precision. As a proof of principle for high resolution particle separation, 15 µm particles were effectively separated from 10 µm particles. A separation efficiency of 98% for the 10 µm and 97% for the 15 µm particles was achieved.
Fig: Overview of particle focusing principle in spiral microchannels. (A) Schematic illustration of particle focusing in elasto-inertial microfluidics. Under the influence of Dean drag forces (FD), particles migrate along the Dean vortices, and depending on the position the particles experience additional strong inertial lift forces (FLS and FW) and elastic forces (FE). How these forces acting on a particle focused in a distinct point (positions 1–4) are highlighted. Note, there are vertical lift and viscoelastic forces acting on the particles but are negligible at the center line. Inset, COMSOL simulation showing a of a skewed mean flow (contours) and cross-sectional flow (arrows). (B) Inertial and elasto-inertial particle focusing. Fluorescence image of 15 µm particles flowing through the spiral in Newtonian (left) and non-Newtonian (right) fluid using PEO as elasticity enhancer. In a Newtonian fluid, the particles are focused at the inner wall and for Non-Newtonian fluid at the outer wall. Scale bar: 500 µm.
Furthermore, we demonstrate sheath-less, high throughput, separation using a novel integrated two-spiral device and achieved a separation efficiency of 89% for the 10 µm and 99% for the 15 µm particles at a sample flow rate of 1 mL/min—a throughput previously only reported for inertial microfluidics. We anticipate the ability to precisely control particles in 3D at extremely high flow rates will open up several applications, including the development of ultra-high throughput microflow cytometers and high-resolution separation of rare cells for point of care diagnostics.