Platelet aggregation precedes stable adhesion. Blood containing the anticoagulant PPACK (93 μM) and the fluorescent dye mepacrine (10 μM) was perfused over collagen type 1 fibrils (2.5 mg/mL coating concentration). After 7 seconds of flow, single platelets adhere when the shear rate is 3000 s^–1 (A), but an aggregate has formed at 24 000 s^–1 (C). After 4 additional seconds, few more platelets are present on the surface exposed to the lower shear rate (B), and those already adherent have remained stationary. In contrast, in the aggregate exposed to the higher shear stress (D), hydrodynamic forces have stretched a group of platelets, identified by brackets of different length in panels C and D. Images from Video S1.