Modeling and Experimental Study of Steric Inversion in Field-Flow Fractionation (FFF)
Frederick R. Phelan Jr., Barry J. Bauer
Modeling of the separation of spherical particles in field-flow fractionation under conditions spanning the normal to steric transition is studied and is compared to experimental results. The separation process is simulated using a Brownian dynamics method in which the particle motions are governed by a Langevin equation which takes into account the drag force due to fluid flow and the Brownian force. The steric effect is accounted for by a boundary condition governing the point of closest approach. Elution fractograms through the device as a function of particle size, and throughput and cross flow flowrates are shown and compared with experimental data and theory. Both simulation results and experimental data for mean elution time are in good agreement with the steric inversion theory of Giddings and indicate that the steric transition occurs when the particle diameter is approximately 600 nm (for the given experimental conditions).The steric transition observed in FFF for spheres shows that interactions with the boundary are important, and the particle shape is bound to sharply influence the nature of this interaction. This finding has important implications in regard to the separation of more complex structures such as nanotubes in FFF.