Figure 4.

Model and example of a full fit for the measured force-evolution. (A) (Red) While approaching, the bead deforms the full actin cloud between the two beads leading to the measured forces that are fitted by the model of polymer elasticity including the discussed nonlinear Young’s modulus. (B) (Violet) Subsequent viscoelastic relaxation of the network between the two beads, following the theory of polymer relaxation of sparse actin networks given in Eq. 9. To understand the negative forces in the retracting phase, we imply that the network behind the bead is starting to relax into the void created by the bead during the approach phase (red arrows). (C) (Green) This leads to an additional elastic constraint while the bead returns to the initial position, which explains the negative force during the return phase.