Augmentation of diffusion coefficients of solutes in flowing erythrocyte suspensions

Diffusive mass transport in laminar-flowing suspensions is a complex subject found in various biological and industrial applications. Previous experimental work done in this area has used techniques that have limitations and have not yielded conclusive results. This research includes the development and verification of a new device to determine diffusion coefficients for solutes in flowing mediums. The device was tested and successfully validated for the measurement of diffusivities of urea and bovine serum albumin in flowing aqueous medium at different shear conditions, demonstrating the absence of inertial forces within the device for the entire range of conditions. With this validation, the device was used to determine diffusion coefficients of urea and bovine serum albumin in bovine erythrocyte suspensions at different values of shear rate and erythrocyte volume fractions.;Diffusion coefficients obtained in the absence of erythrocytes under different flow conditions both for urea and bovine serum albumin were all close to the reported aqueous values determined in static systems. Experiments performed with non-zero erythrocyte volume fractions and different flow conditions allowed the determination of the influence of each of these two variables on the mass transfer properties of solutes in the suspension. At zero or low shear rate, erythrocyte concentration decreased the observed diffusivity. For higher shear rates, an increase of diffusivity was observed. This increase exhibited quadratic behavior with increasing erythrocyte volume fraction. Diffusivity augmentation in high shear flows and at high erythrocyte volume fraction suspensions was found to be also a function of the molecular diffusivity; solutes with lower molecular diffusivity exhibited greater augmentation.;Mathematical models developed to predict diffusivity augmentation in flowing suspensions were compared with the data obtained. Each of the selected models was derived from different assumptions about the cause of the augmentation: localized convection caused by rotating particles or particle-displaced fluid convection caused by diffusing particles. All models lacked predictive power, suggesting that augmentation is caused by a combination of the two phenomena mentioned.