UCLA

This dissertation investigates the mechanics of semiflexible filament network and the effects of polydispersity, anisotropy, and labile crosslinks through numerical simulations.

This study starts with exploring the effects of filament length polydispersity on the mechanical properties of semiflexible network. Extending previous studies on monodisperse network, the responses of networks of bimodal and exponential length distributions are tested numerically. It is found that in polydisperse networks, mixtures of long and short filaments interact cooperatively to generally produce mechanical response closer to the affine prediction than comparable monodisperse networks of either long or short filaments. Overall, length polydispersity has the effect of sharpening and shifting the affine/nonaffine(A/NA) transition to lower network densities.

The effect of adding long, stiff filaments to a semiflexible network is studied next. It is shown that the addition of a small fraction of longer and stiffer filaments (microtubules) to a nonaffine network (actin filaments) leads to a significant increase in its overall elastic moduli, even though the long filaments do not form a stress bearing network by themselves. Moreover, there is a strong negative correlation between long filament density and local geometric measures of nonaffinity.

Anisotropic, monodisperse network is then investigated and like isotropic networks, it undergoes an A/NA transition controlled by the ratio of the filament length to the nonaffinity length $\lambda$. Deep in the nonaffine regime, however, these anisotropic networks exhibit a vanishing linear response regime for highly ordered networks and a dependence of the shear modulus on shear direction. These features can be understood in terms of a generalized floppy modes analysis of the nonaffine mechanics and a type of cooperative Euler buckling, which are discussed in this study, too.

Lastly, by allowing crosslinks to break and rebind, network's dynamic response is studied. It is shown that force-induced crosslink breakage leads to significant creeping and the network retains part of its elastic modulus even after significant plastic flow. The spatial correlation of crosslink breakage is also studied.