Continuous sheath-free focusing of microparticles in viscoelastic and Newtonian fluids

A simple microfluidic device is reported for continuous, sheath-free focusing of microparticles in both viscoelastic and Newtonian fluids. Gradually contracted and suddenly expanded structures in the flow direction were designed on one side of the microchannel to induce asymmetric distribution of the elastic force on the cross section in the viscoelastic fluid. Driven by the elastic force and the viscous drag force, microparticles were focused in a single position located near the straight wall of the channel. In the Newtonian fluid, these contracted and expanded structures were utilized to generate highly curved streamlines which induced momentum-change-induced inertial force on particles. Under the combined effect of this force and the inertial lift force, the particles were also focused in a single stream in the Newtonian fluid. The influences of flow rate, particle size, polymer concentration, fluid viscosity and channel geometry on the particle focusing were investigated in the paper. In comparison with other particle focusing devices, the present microchannel can focus microparticles in a single stream in both the viscoelastic and the Newtonian fluids in a short channel length without the aid of sheath flow. With simple structure and easy operation, this device could be potentially utilized in a variety of particle focusing processes in biomedical applications, such as the enrichment of cells.