FFF is similar to chromatography methods in that materials are separated
by transport velocity, but in place of a retention media the separation is carried
out in a thin, open channel with bulk flow in the longitudinal direction and a
separation field (centrifugal force, electric field, thermal gradients, or cross-flow)
in the perpendicular direction. Particles are driven to the wall by the separation
field and average particle distance from the wall is determined by the competition between the separation field and the size-dependent diffusion of particles against the concentration gradient. Since the narrow channel has a parabolic flow profile (laminar flow), the particles farthest from the wall are in the highest velocity streamlines and therefore travel the fastest. Sedimentation FFF (SdFFF) uses centrifugal force to generate the separation field.