Effect of shear rate gradients on activation-independent platelet aggregates. Blood containing PPACK and PG E₁ (see legend to Figure 2) was perfused over immobilized VWF. The drawing between the single-frame images shows the shape of the flow path and the direction of flow (black arrow) in the perfusion chamber used for this experiment. The highest shear rate was either at the inlet (top images) or at the outlet (bottom images), and positions where the wall shear rate was 5000 s^–1 (left images) or 20 000 s^–1 (right images) are shown for each flow condition. Both images on the right show activation-independent aggregates at 20 000 s^–1 (position indicated by a black dot in the drawing). When higher shear rates are upstream of the position shown (top right image), aggregates are elongated and stationary on the surface for prolonged periods of time. When lower shear rates are upstream of the position shown (bottom right image), aggregates are predominantly rolling with varying shape. Only single platelets are seen at 5000 s^–1 (position indicated by “x” in the drawing), whether downstream of higher shear rates (top left image) or lower shear rates (bottom left image). Images from Video S4. Bar graphs: Quantitative evaluation of the number of activation-independent aggregates with cross-sectional area more than 100 μM² (left) and surface arrest time more than 30 seconds (right) present at positions exposed to the indicated shear rates in a flow field with velocity increasing linearly in the direction of flow (lower shear rates upstream). Measurements performed in a field of view = 120 000 μM².