Blood flow dynamics: A Lattice Boltzmann-immersed boundary approach

We develop a novel Lattice Boltzmann-Immersed Boundary method to simulate the motion of whole blood. This method is applied to explore the experimentally-observed redistribution of platelets and the enhanced mixing of chemicals in concentrated suspensions of red blood cells undergoing channel flow. Simulations capture red-blood-cell-induced lateral platelet motion and the consequent development of a platelet concentration profile that includes an enhanced concentration within a few microns of the channel walls. The concentration of platelets near the blood vessel wall is important because platelets survey the condition of the vessel wall and respond to injuries to it. Under arterial flow conditions platelets are non-uniformly distributed across the vessel lumen and have a high concentration within a few microns of the vessel wall. We track the development of a red blood cell-free layer near the wall and the later development of the platelet near-wall excess and find that the latter develops more quickly at high wall shear rates and that the magnitude of the excess and its proximity to the wall are dependent on hematocrit. Treating the simulation data as if it were generated by a drift-diffusion process, we find that the effective lateral platelet diffusivity depends strongly on lateral position; it has a magnitude of order 10^(-6) cm2/sec over much of the lumen but drops to almostzero close to the wall. This large effective diffusivity over the core of the lumen combined with reduced space for platelets in this region because of the inward migration of red blood cells largely but not completely account for the observed platelet concentration profiles. We present evidence for a highly localized red blood cell-induced platelet drift at the edge of the red cell-free layer and suggest a physical mechanism that may generate it. We then extend our model to explore how the motion of red blood cells and platelets changes in the vicinity of a blood clot that protrudes into the vessel lumen. We extract information about the nature of the platelets' interactions with the clot in order to assess the effect of red-blood-cell-induced motion on clot growth.