Operational-modes of field-flow fractionation in microfluidic channels

Through a careful consideration of the retention ratio for field-flow fractionation (FFF), we show that a single unified ideal retention theory can predict a wide range of separation behaviours including hydrodynamic chromatography, normal-mode FFF and steric-mode FFF by introducing the concept of a device retention parameter. We determine the critical device retention parameter above which normal-mode does not exist and there is no clear distinction between hydrodynamic chromatography and steric-mode FFF. Numerical analysis of the elution order as a function of particle size quantitatively predicts the transitions between these regimes. The resulting map of the operational-modes shows each of the regions and their connectivity, and so may guide future device design. By extending this analysis to account for the variation of stress over particle surfaces, a hitherto unreported regime called Faxén-mode FFF is predicted, which has the same elution order as normal-mode FFF. This mode arises when particle sizes approach the channel height, as can occur when microfluidic devices are utilized for FFF. The transition from steric-mode to Faxén-mode FFF is numerically mapped and approximations for each transition are presented.