Field Flow Fractionation
Field-Flow Fractionation (FFF) is a well-established separation technique based on a simple concept: separation of a mixture of different species can occur when solute particles are carried through a channel by a parabolic flow profile and also acted on by an external force applied perpendicular to the flow. Shrinking FFF systems to microscopic sizes is a tantalizing idea for size separation in lab-on-a-chip devices. However, miniturization must be approached with caution since effects due to the small scales can be subtle and easily overlooked.
Traditional FFF retention theory has two forms: one for normal-mode FFF, in which small particles elute before larger particles, and a second for steric-mode FFF in which larger particles elute before smaller particles. These two different theories implicitly assume some unspecified transition from normal-mode to steric-mode operation (this is sometimes called steric-inversion since the elution order flips).
Since we are thinking about microfluidic devices, we extend the retention theory to explicitly account for particle size and to better estimate the solute velocity. By doing this, we arrive at a single retention theory that not only encompasses both normal- and steric-mode FFF but also predicts two additional operational modes. At the tiniest particle sizes, the hydrodynamic chromatography limit of FFF is found and at the larges sizes (when the diameter of the particles approaches the size of the separation channel) the solutes slow and we predict a transition to a previously unexpected operation mode (with the same elution order as normal-mode FFF) called Faxen-mode FFF.
Non-ideal effects can complicate matters and foremost amongst these in microfluidic channels are hydrodynamic interactions with the walls, which effectively increase the drag coefficient of large particles, causing them to slow. We have recently collaborated with Michel Godin's research group to theoretically, computationally and experimentally study this phenomenon.