Studies on platelet margination have shown that the platelets can effectively marginate at the microvessel wall in a multi-file flow of red blood cells (RBCs), whereas axially migrated RBCs push platelets toward the wall. However, it is unclear whether these results can be extended to capillaries, which potentially cause a single-file line of RBCs, or a so-called bolus flow. Our previous numerical results ( Takeishi and Imai, 2017) showed that microparticles with a diameter of 1 µm (1-µm-MPs)

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... re captured by a bolus flow of RBCs, instead of being marginated in capillaries. Herein we perform numerical simulations to clarify whether platelets are captured or escape from the vortex-like flow structures between RBCs. We demonstrate that platelets are also captured in a capillary whose diameter is 25% larger than that of RBCs at a physiologically-relevant hematocrit (Hct ∼ 0.2), but the number of captured platelets is smaller than that of 1-µm-MPs. When the capillary diameter is comparable to that of RBCs, however, many platelets flow near the wall due to an unstable bolus flow resulting in a less number of captured platelets. These results suggest that the size effect reduces platelet capture events compared to 1-µm-MPs. We also investigate the effect of Hct and the non-dimensional shear rate (capillary number) on capture events. These findings may help not only to understand platelet adhesion in capillaries but also to develop therapeutic drug carriers.