The root causes for the mechanisms for margination and near-wall platelet accumulation have previously been investigated by numerical simulations and the results pointed to two main contributors. First, the behavior of the red blood cells (RBCs), namely, traveling around the centerline and formation of the cell-free layer (CFL). Second, the multiple interactions between RBCs and platelets. However, these mechanisms remain to be experimentally verified. In this work, we focus on the dynamics of platelet-size particles inside RBC flows through straight-square microchannels. We used rigid particles with nominal diameter of $2.47\mu \mathrm{m}$ to mimic platelets. The three-dimensional (3D) coordinates ($x,y,z$) and velocity components ($v_x,v_y,v_z$) of the platelet-size particles inside the RBC-suspension flows were measured by means of the general defocusing particle tracking method (GDPT). From the 3D particle trajectories, we were able to investigate the root causes for margination and near-wall particle accumulation. The overall picture points to the RBCs as the dictator of the near-wall particle accumulation and margination. We show that the phenomenon of margination is an irreversible, fast, unpredictable and discontinuous process, and more importantly it can be an opportunity-based event.

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• Blood flow
• Microfluidics
• Platelets
• Red blood cells
• In-vitro methods
• Lagrangian particle tracking