Schematic model depicting the hydrodynamic forces encountered by surface-interacting platelets versus microspheres. The force (F_s) acting on a platelet and a 7-μm bead in shear flow is depicted. Representative hard spheres with diameters proportional to the thickness (A) or volume of a discoid platelet (B) are also shown. The equations of Goldman (F_s = 6πRhC_F τ; T_s = 4πR³C_F τ) were used to estimate the forces (F_s) acting on a platelet or bead based on a motionless hard sphere in shear flow near a wall, where τ is the shear stress, R is the sphere radius, h is the distance from the center of the sphere to the wall, and C_F and C_T represent numerical factors that depend on h/R.³⁷  For the case where a cell is tethered to the wall, h = R, C_F = 1.7005, and C_T = 0.943 99. Because the geometry of a resting platelet is discoid and not spherical, a low and high estimate of the shear force was determined for spheres with either a diameter (1 μm) or a volume (8 fL) equal to that of a platelet.