Characterizing cell motions in flowing blood

Finding a way to fully characterize the probabilistic motions of cells is an important step in developing 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. The dispersive motions of 0.5-micrometer beads and 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 erratically moving particles were tracked in the axial direction and in a moving reference flame. The effective diffusion coefficient of the particles was estimated to be in good agreement with published work. Using a continuous time random walk model (CTRW), the squared displacement was plotted versus the average time and a power law fit exponent was used to characterize the particles' random motions in terms of their diffusion behavior in a shear field. Values consistent with Brownian motion were found for the bead suspensions and an anomalous diffusion was found for the RBC suspensions.