Figure 4. Reactive microsphere motion illustrates the discrete nature of bond formation events and force loading.

Reactive spheres were simulated with S = 50 s⁻¹, n_L° = 100 sites/µm², n_R° = 50 sites/µm², association kinetics governed by (1), and dissociation kinetics governed by (3). (A) Results for a 6 µm-diameter sphere with a rope model of bond deformation, (5). Sampled flow-direction velocity (V_S,X, red), perpendicular velocity (V_S,Y, maroon), gap size (dark blue) and number of bonds (light blue) are shown. (B) Results for a 6 µm-diameter sphere with a freely-jointed chain model of force deformation, (6). An increase in the magnitude of the fluctuations of V_S,X was observed with the decrease in stiffness relative to (A). (C) Results for a 10 µm-diameter sphere with a freely-jointed chain model of force deformation, (6). The motion was higher frequency in nature with shorter pauses and more frequent pause events. (D) A comparison of simulated bond loadings for the rope tether results in (A), top, and the freely-jointed chain in (B), bottom. Each sampled point on the inset is spaced by 1 ms. In both cases, the sphere's pause tended to be supported by a singly-loaded bond. The decreased stiffness of the freely-jointed chain tether resulted in a smaller standard deviation of the supported force. A lower maximum initial force loading was predicted for the freely-jointed chain model than for the rope model (insets).