The flexibility of FFF stems from the large set of transverse external fields that can be used. Gravitational fields, centrifugation, electrical fields, magnetic fields, dielectrophoretic fields, acoustic fields, photophoretic fields, cross-flows (both symmetrical and asymmetrical) and thermal gradients have all been used to generate non-uniform solute concentration distributions.

But FFF also depends on a non-uniform flow profile. It is generally taken for granted that the profile in a channel obeys Poiseuille flow. By changing the flow profile we may change the retention ratio (the mean velocity of the solutes normalized by the mean fluid flow).

One way to alter the flow profile is to have a slip wall. Usually, fluids flowing over a solid surface obey a no-slip boundary condition, meaning that the speed goes to zero. However, modern nanoengineering can construct ultrahydrophobic surfaces by nanopatterning structures on the surface that reduce the drag on the fluid.

Another idea is to superimpose Poiseuille flow and electroosmotic flow with a thin Debye layer. This would create a non-zero velocity at the wall (called the Smoluchowski slip velocity, if you’d like to look it up).

With these motivations in mind, we investigated how the retention ratio would change for various slips. We found an number of interesting conclusions:

1. Hydrodynamic chromatography never benefits from slip.
2. In a moderate external field Normal-Mode FFF can be somewhat improved by slip at the depletion wall (the top wall).
3. Slip at the depletion wall typically increases the range of Steric-Mode FFF with little impact on resolution.
4. Normal- and Steric-Mode FFF persist for weaker fields when there is slip at the depletion wall.

But the most interesting result by far is that we once again found a novel operational-mode. If the accumulation wall (the bottom wall) has an extremely large slip and the external field is strong then the retention ratio rises surprisingly rapidly over a small range for the tiniest particles. Mathematically, this looks like the hydrodynamic chromatography regime of FFF but the particle size range and the resolution are more comparable to Normal-Mode FFF so we call this new mode slip-FFF.

Find more details or the citation for this in our article in the Journal of Chromatography A.