Effect of polymer concentration in co-flowing viscoelastic separation of submicron particles

Manipulation and separation of submicron particles such as extracellular vesicles (EVs), viruses and bacteria have broad applications in biotechnology and diagnostics. Viscoelastic microfluidic technology has emerged as a powerful technique for high-resolution particle sorting in non-Newtonian fluids. Viscoelastic co-flowing system is the most popular design for particle separation in viscoelastic microfluidics, and has been successfully employed for the separation of EVs, bacteria and cancer cells. However, current studies mainly focus on particle differential migration in viscoelastic fluids of low polymer concentration, and the effects of high polymer concentration on particle migration and separation are still largely unexplored. In this work, we investigate the migration behaviour of 100 nm and 500 nm particles in a viscoelastic co-flowing microfluidic system of high polyethylene oxide (PEO) concentrations. The effects of PEO concentration of sample and sheath flows, the flow rate ratio (FRR) of the sheath to sample flows and the total flow rate on the particle migration and final equilibrium positions were studied. At low PEO concentrations, large (500-nm) particles migrate fast toward the channel centre and small (100-nm) particles exhibit slow migration. In contrast, at specific high PEO concentrations, an intriguing reversed phenomenon appears where small (100-nm) particles migrate fast and focus at the channel centre, while 500-nm particles remain near sidewalls. Finally, we successfully applied this phenomenon for the separation of binary submicron particle mixture and achieved separation purities of 88% and 87% for 100-nm and 500-nm particles, respectively. This work demonstrates the potential for optimising size-based submicron particle separation in the co-flowing system by tuning high polymer concentrations.