Active microrheology of networks composed of semiflexible polymers: computer simulation of magnetic tweezers

We have simulated the motion of a bead subjected to a constant force while embedded in a network of semiflexible polymers which can represent actin filaments. We find that the bead displacement obeys the power law x approximately t(alpha). After the initial stage characterized by the exponent alpha1 approximately 0.75, we find a different regime with alpha2 approximately 0.5. The response in this regime is linear in force and scales with the polymer concentration as c(-1.4). We find that the polymers pile up ahead of the moving bead, while behind it the polymer density is reduced. We show that the force resisting the bead motion is due to steric repulsion exerted by the polymers on the front hemisphere of the bead.