Asymmetrical Flow Field-Flow Fractionation, AFFFF, is a simple and robust approach for separation of nano- and micron- size particles in solutions and dispersions. The technology is well developed and widely used by large and by small pharmaceutical companies. A leading product in this area is ECLIPSE from Wyatt Technology Europe GmbH. The design of ECLIPSE is based on careful analytical study of fluid flow and of separation in a microchannel.

Further improvement in the performance of the devices can be achieved via mathematical modeling and computer simulation. Three dimensional flow simulations allow for obtaining a detailed view on the flow within the spacer, as well as on the particles transport there. Compared to the analytical considerations, the CFD (Computational Fluid Dynamics) simulations provide more detailed information in the case of complicated geometry. This information supplements the analytical considerations in optimizing the flow regimes and in further improving the design and the performance of the device. In particular, simulations with different size and/or location of the injection pipe allow to study the influence of these parameters on the focusing area. Another important parameter influencing the focusing zone is the flow rate distribution between the two inlets in the case of injection through a separate pipe. In this case the CFD simulations allow for observing the location of the focusing zone for each reasonable flow rate distribution for any selected shape and size of the spacer. The size and the shape of the focusing area influences significantly the particles separation during the elution stage, and therefore its study is useful for further improvement of the design of the fractionation device. CFD simulations allow for evaluation and pre-selection of designs, without building expensive prototypes.

CFD simulations allow to study not only the focusing zone, but also the time history of the fractionation during the elution stage. In this way, an optimized flow control at the inflow during the elution stage can be selected.

A sketch of the device and the separation process

A computational domain together with patchlines obtained in one of the simulations.

The focusing of a tracer (coloured in blue) and its transport during the elution stage.