Twisted results on interior packing and surface energy for filament bundles

Twisted filament bundles are a common structural motif found in both natural and synthetic systems. Examples range from protein assemblies such as collagen and fibrin, to artificial structures such as carbon nanotube ropes and microfabricated materials. They are oftentimes found to self-assemble from fibers via various adhesive interactions, be they depletion, capillary, or other such forces. In these assemblies, twist frustrates the perfect crystalline order of the fibers, requiring the presence of defects in the packing. Through numerical simulations, we discover defect organizations ranging from dislocations and grain boundaries for low twist, to multi-five-fold disclination clusters for high twist. And furthermore, by developing and employing an analytical continuum model, we find that for sufficiently long fibers, twist reduces the surface energy of the assembly. Together, this suggests that the equilibrium lowest energy state of a filament bundle may be twisted regardless of any intrinsic chirality present in the system.