Effect of viscosity on retention time and hydrodynamic lift forces in sedimentation/steric field-flow fractionation

Particles entrained in fluid flow between parallel bounding walls tend to be driven by hydrodynamic forces, acting perpendicular to the direction of flow, towards certain equilibrium positions between the walls. Sedimentation field-flow fractionation is a technique that is well suited to the measurement of these forces, particularly in the regions close to the walls. Forces much stronger than those due to fluid inertial effects are commonly observed in the near-wall regions. A study is presented here of the influence of carrier fluid viscosity on these hydrodynamic lift forces. The carrier viscosity is varied via two different methods: (1) various ternary mixtures of water, glycerol, and ethanol were used that varied in viscosity while being of constant density and (2) the temperature of the FFF system was raised, changing the carrier viscosity while not significantly altering its density. The near-wall lift forces are shown to be dependent on fluid viscosity. An empirical equation describing the apparent dependence is presented.