Classical flow-FFF is a separation technique in which a perpendicular cross flow is imposed upon a channel flow of dilute particulates [1]. Competition between various flow mechanisms drives particles of different sizes to discrete average positions in the cross flow direction. Separation is achieved due to the different residence times of the particles based upon their position in the parabolic velocity profile in the throughput direction. A number of different mechanisms can be exploited to achieve separation in FFF. Normal mode separation in FFF applies to particles which are small enough to undergo significant Brownian motion, and whose size is small compared to the cross flow gap size. In this case, smaller particles, which are more diffusive, have an average position closer to the centerline and elute faster than larger particles. Steric mode separation occurs when the particle layer in FFF is strongly compressed, and thus, larger particles are more highly entrained by the throughput flow than smaller ones. The steric transition region is the range in which normal mode FFF undergoes a transition to steric FFF due to increasing particle diameter. In this work, modeling and experimentation of particle separation under conditions spanning the normal to steric transition is examined. 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 [2,3]. 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 [4]. Both simulation and experiment show that the steric transition occurs when the particle diameter is in the range of 300 to 600 nm (for the given experimental conditions). The implications of the present results for the separation of nanotubes in FFF will be discussed.

References: [1] Josef Janca, Field-Flow Fractionation (Chromatographic Science), Marcel Dekker, New York, (1987). [2] A. Satoh, Introduction to Molecular-Microsimulation of Colloidal Dispersions, Elsevier Science B.V., Amsterdam, The Netherlands, (2003). [3] S. Kim, and S.J. Karrila, Microhydrodynamics: Principles and selected applications, Butterworth-Heinemann, Stoneham, (1991). [4] J.C. Giddings, Separation Science and Technology, 13, 241-254, (1978).