Size Selectivity in Field-Flow Fractionation: Lift Mode of Retention with Near-Wall Lift Force

Flow Field-Flow Fractionation with a Thickness-Tapered Channel

This study introduces the thickness-tapered channel design for flow field-flow fractionation (FlFFF) for the first time. In this design, the channel thickness linearly decreases along the channel axis such that the flow velocity increases down the channel. Channel thickness is an important variable for controlling retention time and resolution in field-flow fractionation. Especially, in the steric/hyperlayer mode of FlFFF, in which particles (>1 μm) migrate at elevated heights above the channel wall owing to hydrodynamic lift forces, the migration of long-retaining smaller-sized particles can be enhanced in a relatively thin channel or by increasing the migration flow rate; however, an upper size limit that can be resolved is simultaneously sacrificed. A thickness-tapered channel was constructed without a channel spacer by carving the surface of a channel block such that the channel inlet was deeper than the outlet (w = 400 → 200 μm). The performance of a thickness-tapered channel was evaluated using polystyrene standards and compared to that of a channel of uniform thickness (w = 300 μm) with a similar effective channel volume in terms of sample recovery, dynamic size range of separation, and steric transition under different flow rate conditions. The thickness-tapered channel can be an alternative to maintain the resolving power for particles with an upper large-diameter limit, faster separation of particles with a lower limit, and higher elution recovery without implementing the additional field-programming option.

Principles of transverse flow fractionation of microparticles in superhydrophobic channels

We propose a concept of fractionation of micron-sized particles in a microfluidic device with a bottom wall decorated by superhydrophobic stripes. The stripes are oriented at an angle α to the direction of a driving force, G, which generally includes an applied pressure gradient and gravity. Separation relies on the initial sedimentation of particles under gravity in the main forward flow, and their subsequent lateral deflection near a superhydrophobic wall due to generation of a secondary flow transverse to G. We provide some theoretical arguments allowing us to quantify the transverse displacement of particles in the microfluidic channel, and confirm the validity of theoretical predictions in test experiments with monodisperse fractions of microparticles. Our results can guide the design of superhydrophobic microfluidic devices for efficient sorting of microparticles with a relatively small difference in size and density.