This paper proposes a dimensionless analysis for precise estimation of microsphere behaviors in elasto-inertial microfluidic separation, focusing on the interface between Newtonian and viscoelastic fluids. Using Reynolds number, modified Weissenberg number, and modified elastic number, the balance between inertial and elastic lift forces is investigated. A new dimensionless number, representing the Newtonian fluid stream width divided by microsphere diameter, is introduced to predict microsphere migration across the co-flow interface. Experimental results using polystyrene microspheres (2.1 and 3.2 µm) in a co-flow of water and polyethylene oxide solution validate the theoretical estimations. The method achieves submicron separation of microspheres (2.1 and 2.5 µm) at high throughput, purity (>95%), and recovery rate (>97%), with application demonstrated in separating platelets from *Escherichia coli*.

Hyunwoo Jeon, Song Ha Lee, Jongho Shin, Kicheol Song, Nari Ahn, Jinsoo Park