Theoretical and experimental analysis of the sedimentation kinetics of concentrated red cell suspensions in a centrifugal field: Determination of theaggregation and deformation of RBC by flux density and viscosity functions

The flow properties of blood are mostly determined using various viscometri c approaches, and described in terms of a shear rate or shear stress depend ent apparent viscosity. The interpretation of results are rather difficult, especially at low shear rates when particle sedimentation and migration wi thin the viscometer gap are significant. By contrast, analysing the separat ion process in concentrated RBC suspensions in a centrifugal field also yie lds information about the viscosity function, including particle-particle i nteraction and deformation parameters. In this paper, the sedimentation pro cess is approached by means of the theory of kinematic waves and theoretica lly described by solving the corresponding one-dimensional quasi-linear par tial differential equation based on viscosity/flow function as a function o f volume concentration. The sedimentation kinetics of rigid spherical RBC s uspended in saline and normal RBC suspended in Dr-saline solutions were inv estigated by means of a separation analyser (LUMiFuge 114). The instrument detects the light transmission over the total length of the cell containing the suspension. During centrifugation the analyser automatically determine s the position of the particle free fluid/suspension interface or the sedim ent by mans of a special algorithm. The data obtained with sedimentation of rigid spherical RBC at different volume concentrations demonstrate that, i n the case of suspensions rotated in containers of constant cross section, there is good agreement between the theory of kinematic waves developed by Anestis and Schneider (1983) and the results of the experiments. Such good agreement was obtained even though a restrictive one-dimensional model was used to obtain the theoretically derived sedimentation time course. In addi tion, we describe an algorithm enabling the experimental determination of t he viscosity and related Aux density function to be made for any suspension . Through this approach, we investigated in detail the rheological behavior of suspended rigid spheres at low Reynolds numbers ranging from 10(-6) to 10(-3). The method here introduced also enabled us to investigate RBC suspe nsions with respect to the deformability and interactions of the cells by m eans of the separation analysis. Normal, rigid as well as aggregating RBC e xhibited marked differences in the sedimentation kinetics, which were quant ified by means of the flux and viscosity functions based on the theory of k inematic waves.