CHARACTERIZATION OF NEAR-WALL HYDRODYNAMIC LIFT FORCES USING SEDIMENTATION FIELD-FLOW FRACTIONATION

Abstract

In field-flow fractionation (FFF), a family of high resolution techniques for the separation of particles and polymers, the measured retention time of entrained particles eluting through a thin (50-500 μm) parallel plate channel is determined by the transverse forces acting on the particles during their migration. For particles in the size range ∼l-100μm an applied transverse driving force (in the present case sedimentation) rapidly brings the particles into balance with hydrodynamic lift forces, so that the two force vectors are equal in magnitude but opposite in direction. By subjecting latex microspheres of known size and density to a specified rotation rate in a centrifuge, the applied sedimentation force is known and thus the magnitude of the lift forces is immediately obtained. The transverse particle position can be determined from the measured particle retention time. Thus lift forces can be determined as a function of particle size, transverse position, and flowrate. This strategy has been used to characterize the lift forces acting on spherical particles driven close to one channel wall. This characterization promises to be useful for the improved understanding and optimization of FFF, crossflow membrane separation, and other processes involving particle motion close to walls In this study, 519 retention measurements were made on eight latex microsphere standards of 2.0-44.6 μm diameter using flowrates of 1.88-38.0 mL/min in a 254 μm thick FFF channel subjected to 7-343 gravities. Multiple linear regression analysis yielded an expression for the lift force F_L of the form where a is the particle radius, η is the fluid viscosity, s₀ is the undisturbed shear rate at the wall, δ is the distance of closest approach of the sphere to the wall, and C is a coefficient that may itself depend on other system parameters. Possible sources of experimental error were examined but none changed the general form of the above equation. Since the expression is inconsistent with an inertial lift mechanism it is postulated that the lift forces observed are related to lubrication phenomena.