CHARACTERIZATION OF HYDRODYNAMIC LIFT FORCES BY FIELD-FLOW FRACTIONATION - INERTIAL AND NEAR-WALL LIFT FORCES

Sedimentation/steric FFF has been used to measure hydrodynamic lift forces exerted on 2-30 mu m latex microspheres driven by flow through a 95 cm long ribbonlike FFF channel of similar to 127 mu m thickness. Following a previous study, lift forces are examined as a function of shear rate, distance from the waif, 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, itis found, as before, that a near-wall lift force proportional to 1/delta (where delta is the particle-wall distance) dominates lift effects at small delta s. As delta 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.