Characterizing the probabilistic motions of cells is a prerequisite for the development of a general model of transport and surface deposition of white cells and platelets (WBC/P). These phenomena differ greatly when red blood cells (RBC) are present at normal levels, but follow convective diffusion in dilute suspensions. A critical need is to understand and characterize dispersion and diffusion as they apply to red cells in suspension flow. The dispersive motions of 0.5-micrometer beads and fluorescently labeled human RBC flowing in dilute (0.003%) and concentrated (25%) RBC suspensions, respectively, were characterized using fluorescence videomicroscopy methods, and times for individual tracer particles to move fixed distances were measured. The particles were tracked in the axial direction and in a moving reference frame. The experimentally estimated effective diffusion coefficient of the particles was in good agreement with published work (RBC ~ 1x10 -8 and beads ~ 4x10 -9 [cm 2 /sec]). Using a continuous time random walk model (CTRW) to characterize the particles’ random motions in a shear field, the average time was plotted versus the squared displacement and a power law fit exponent was used to quantitatively distinguish between diffusion and dispersion. Values consistent with Brownian motion were found for the bead suspensions and an anomalous diffusion was found for the RBC suspensions, which indicated that beads random motions were diffusive and the RBC ones were dispersive. The methods developed in this work could be used to study dispersion events at different lateral locations along the channel’s height and investigate the effects of flow parameters such as wall shear rate, hematocrit, and cell type
