Sedimentation of single red blood cells

The sedimentation of red blood cells (RBCs) due to gravitation or centrifugation at low Reynolds numbers is considered via computer simulations using multi-particle collision dynamics (MPC) and triangulated membranes. The RBCs assume different shapes depending on the elastic moduli of the cell membranes and the strength of the gravitation; these include parachutes, teardrops, and fin-tailed spheres. We present a phase diagram of the shapes and study the transition between parachutes and teardrops in detail by quantifying the orientation and the shape of the cells. The sedimentation velocity is found to be essentially independent of the cell shape. The sedimentation is shown to lack an observable horizontal drift component of the motion even for asymmetric cell shapes. The consequences of our results for future studies of collective effects and the clinical erythrocyte sedimentation rate (ESR) test are discussed.