Multiorifice flow fractionation: Continuous size-based separation of microspheres using a series of contraction/expansion microchannels

We designed a new microfluidic method, multiorifice flow fractionation (MOFF), for continuous size-based separation of spherical microparticles. This method utilizes inertial lift force and momentum-change-induced inertial force generated in a series of contraction/expansion microchannels. The particles were concentrated gradually along the walls of microchannels by those inertial forces as they passed through the channels. The particle trajectory was deflected from the carrier fluid by means of numerous sudden turns formed in a multiorifice channel. The inertial force was induced by this trajectory mismatching between fluid and particle that resulted from the momentum change. The trajectory mismatching induces the lateral drift of the equilibrium position of particle distribution, and its extent is variable according to particle size and flow rate. In the case of polydispersion, the size-based particle separation could be achieved in the specific range of the channel Reynolds number (Re_c). It was found that, at Re_c of 63-91, large polymer particles (∼15 μm) were aligned along the centerline of outlet channel, whereas small particles (∼7 μm) remained near both sidewalls. This method has potential in continuous separation without using a sheath flow.