Steric mode separation occurs in Field-Flow Fractionation (FFF) when particles experience size exclusion effects with the boundaries. Under such conditions, larger particles are more highly entrained by the throughput flow and elute faster than smaller ones [1,2]. The steric inversion of spheres suggests that an even stronger steric effect might be obtained for rodlike particles due to their great length, if they could be oriented in the gradient direction, normal to the flow direction. One way to induce such alignment is by the use of an electric field. Anisotropic particles like nanotubes experience a torque which acts to align them in an electric field, whether the field is uniform or non-uniform. This could be useful in the context of nanotube separation as a means for separating tubes by type, if either metallic or semi-conducting types could be preferentially oriented relative to the other under different field conditions.

In this work, modeling of particle separation in Electric Field, Field-Flow Fractionation (EF-FFF) is examined using a Brownian dynamics method [3], in which the forces and torques arising from an AC electric field acting in the gradient direction have also been incorporated [4]. In order to align while in flow, the tubes must overcome both the shear field and Brownian motion. This is governed by a dimensionless group which is the ratio of the dielectrophoretic potential energy to kT. The torque and material property requirements needed to trap the particles in “wobbly alignment” against the force of the imposed flow and Brownian motion are estimated. The simulation results show that E-field aligned rods can be induced to exhibit steric inversion, and the results for perfectly oriented ellipsoids are in good agreement with a modified steric inversion theory of Giddings [2]. The application of the method to the fractionation of tubes of different type will be discussed.

References

1. J. Janca, in Field-Flow Fractionation, (Marcel Dekker, New York, 1987).

2. J. C. Giddings, Separation Science and Technology 13, 241 (1978).

3. F. R. Phelan Jr. and B. J. Bauer, Chemical Engineering Science 62, 4620 (2007).

4. T. B. Jones, in Electromechanics of Particles, (Cambridge University Press, New York, 1995).