Fluctuations in settling velocity of red blood cell aggregates

Sedimentation of red blood cell aggregates was experimentally investigated by optical imaging. Suspensions of red blood cell at low hematocrit were obtained from blood of healthy donors. The velocity of three-dimensional red blood cell aggregates was measured using particle image velocimetry. The magnitude and spatial correlation functions of the velocity fluctuations of the settling aggregates were determined. It is shown that the fluctuations in the settling velocity exhibit characteristic correlations in the form of swirls. The formation of 3-D red blood cell aggregates leads to a large initial swirl. The growth of the aggregates and their sedimentation diminishes the swirls size.