Effect of Channel Orientation on Thermal Diffusion and Polymer Retention in Thermal Field-Flow Fractionation

The ThFFF retention of polymers is enhanced in certain liquid mixtures. Previous studies of thermal diffusion in such mixtures indicate that the enhancement is due to an additional solubility-based force induced by a solvent gradient in the channel. The partitioning of solvent mixtures in a temperature gradient was shown to be correlated to solvent density, just as the thermal diffusion of polymers is correlated to polymer density.

This work examines the density effect more closely by looking at the effect of channel orientation on both polymer retention and the partitioning of liquids. A difference in the profiles of solvent partitioning with channel orientation indicates the presence of convection currents in an inverted ThFFF channel, yet polymer retention is independent of channel orientation in homogeneous solvents as well as solvent mixtures. The results indicate that the effect of density on polymer retention is rooted in thermal diffusion and not convection, but that either the previously proposed mechanism of enhanced retention is incorrect or convection currents are confined to the channel edges, becoming significant only in the tapered regions of the channel. Finally, the effect of polymer concentration on retention is more pronounced in mixed carrier liquids.