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

Abstract

A simple theoretical model for the size selectivity, S(d), in the lift mode of retention in field-flow fractionation (FFF) is developed on the basis of the near-wall lift force expression. S(d) is made up of two contributions: the flow contribution, S(d,f), arising from the variation of the flow velocity at center of particle due to a change in particle position with particle size, and a slip contribution, S(d,s), arising from the concomitant change in the extent of retardation due to the presence of a nearby channel wall. The slip contribution is minor, but not negligible, and amounts to 10-20% of the overall size selectivity. It contributes to reduce S(d) in sedimentation FFF but to enhance it in flow FFF. S(d) would steadily increase with particle size if the flow profile was linear (Couette flow). Because of the curvature of the flow profile encountered in the classical Poiseuille flow, S(d) exhibits a maximum at some specific particle size. The model predicts a significant difference in S(d) between sedimentation FFF and flow FFF, arising from the different functional dependences of the field force with particle size between these two methods. The predictions are in good agreement with the various S(d) values reported in the literature in both sedimentation and flow FFF. On the basis of the model, guidelines are given for adjusting the operating parameters (carrier flow rate and field strength) to optimize the size selectivity. Finally, it is found that S(d) generally decreases with decreasing channel thickness.

Principles of transverse flow fractionation of microparticles in superhydrophobic channels

“…(2)(3)(4)(5)(6)(7)(8)), the lateral displacement can be further enhanced by using textures with larger L. Therefore, our results can guide the design of superhydrophobic microfluidic devices for efficient sorting of microparticles.…”

“…Many of these technologies belong to a class of separations known as a field-flow fractionation (FFF), where suspended particles (in a broad sense, including solid particles, fluid droplets, microcapsules or cells, typically of radius a = 1 − 5 µm) migrating in a microchannel are displaced perpendicular to the direction of flow under the action of external (gravity, electric, magnetic, etc) cross fields. 6,7 In the conventional FFF the external vertical force is balanced by the so-called "hydrodynamic wall-induced lift force". As a result, particles of different size, shape, density and/or surface properties migrate forward by retaining at equilibrium distances, δ , from the wall, which leads to their spreading and batch sorting in the flow direction.…”

Conductivity-Dependent Flow Field-Flow Fractionation of Fulvic and Humic Acid Aggregates

“…This behavior is postulated to result from increased aggregation, i.e., increased size, of the humic acid aggregate as concentration increased, resulting in a change in the separation mechanism from predominantly normal (Brownian) mode flow FFF to predominantly steric (hyperlayer or lift) mode flow FFF ( Figure 3). The steric inversion point in flow FFF has been reported to occur at approximately 1 µm [8]; it is also influenced by shape. In normal (Brownian) mode flow FFF, submicrometer particles elute before larger particles, whereas in steric (hyperlayer, lift) mode, micrometer-sized particles elute before smaller particles [8].…”

“…The steric inversion point in flow FFF has been reported to occur at approximately 1 µm [8]; it is also influenced by shape. In normal (Brownian) mode flow FFF, submicrometer particles elute before larger particles, whereas in steric (hyperlayer, lift) mode, micrometer-sized particles elute before smaller particles [8]. Because reversal of the elution order for Aldrich HA and PSS (Mw = 5400) was observed (Figures 2 and 3), the conclusion is that the predominant particle size became greater than 1 μm between a concentration of 2.6 and 5.2 mg·mL −1 .…”

Adverse-Mode FFF: Multi-Force Ideal Retention Theory

“…Most complications negatively impact separation [48]. Such complications may include flow profile distortion by the external field [43,49,50], hydrodynamic interactions leading to wall-induced lag [51,52], finite-aspect ratio channels [53], slip [54] and concentration effects [55].…”