The near-wall excess of platelet-sized particles in blood flow: its dependence on hematocrit and wall shear rate.

Methods involving microscopy were used to obtain concentration profiles of platelet-sized beads during flow through glass channels. Suspensions of fluorescent latex beads (2.38 microns diam) and washed red blood cells were made from an isotonic albumin-dextrose solution. A syringe pump regulated the suspension flow through glass channels, which were either 50 or 100 microns wide; most experiments used a wall shear rate of 1630 sec-1. Via stroboscopic epifluorescence microscopy, photographs were collected on image planes parallel to the channel wall. Profiles of the bead concentration in the narrow channel direction were made by assembling counts of the focused bead images in the photographs. The results showed that a near-wall excess of the beads occurred when the suspension contained a significant fraction of red cells (over 7%). For hematocrits of 15 to 45% (the highest studied), the excess was above five times the concentration in the central region. Experiments with channels of both widths showed the region of excess beads was 5 to 8 micron thick. A series of experiments with 50-micron channels, with a suspension hematocrit of 15%, and with wall shear rates from 50 to 3180 sec-1 showed that near-wall excesses were large only for wall shear rates of 430 sec-1 and above. This work demonstrated the effects of wall shear rate (flow rate) and hematocrit on the number of platelet-sized beads near a surface and hence illustrated physical (rheological) factors that act in blood-surface interaction.