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.

Sedimentation/steric FFF has been used to measure hydrodynamic lift forces exerted on 2-30 μm latex microspheresdnven by flow through a 95 cm long ribbonlike FFF channel of ∼ 127 um thickness. Following a previous study, lift forces are examined as a function of shear rate, distance from the wall, and sphere size. Here, in contrast to the earlier study, measured lift forces are extended downward into a range corresponding to theoretical values of the inertial lift force. After corrections are made for secondary relaxation, it is found, as before, that a near-wall lift force proportional to l/δ (where δ is the particle-wall distance) dominates lift effects at small δ's. As δ increases and this force decays, the measured lift force assumes a value in good agreement with the inertial lift force predicted by the theory of Cox and Brenner as extended by subsequent workers. Over a broad range of conditions explored in almost 300 measurements, the results are consistent with a total lift force that equals the sum of near-wall and inertial contributions. Possible sources of error in the analysis are examined and various explanations for the near-wall lift force are discussed.

During intra‐erythrocytic development, malaria trophozoites digest hemoglobin, which leads to parasite growth and asexual replication while accumulating toxic heme. To avoid death, the parasite synthesizes insoluble hemozoin crystals in the digestive vacuole through polymerization of β‐hematin dimers. In the process, the heme is converted to a high‐spin ferriheme whose magnetic properties were studied as early as 1936 by Pauling et al. Here, by magnetophoretic cell motion analysis, we provide evidence for a graduated increase of live cell magnetic susceptibility with developing blood‐stage parasites, compatible with the increase in hemozoin content and the mechanism used by P . falciparum to avoid heme toxicity. The measured magnetophoretic mobility of the erythrocyte infected with a late‐stage schizont form was m = 2.94 × 10 ⁻⁶ mm ³ s/kg, corresponding to the net volume magnetic susceptibility (relative to water) of Δχ = 1.80 × 10 ⁻⁶ , significantly higher than that of the oxygenated erythrocyte (−0.18×10 ⁻⁶ ) but lower than that of the fully deoxygenated erythrocyte (3.33×10 ⁻⁶ ). The corresponding fraction of hemoglobin converted to hemozoin, calculated based on the known magnetic susceptibilities of hemoglobin heme and hemozoin ferriheme, was 0.50, in agreement with the published biochemical and crystallography data. Magnetophoretic analysis of live erythrocytes could become significant for antimalarial drug susceptibility and resistance determination.