Three-dimensional laser scanning confocal microscopy has revealed that filamentous actin, when complexed with the cross-linking protein alpha-actinin, will spontaneously assemble on a micron scale into a structure comprised of a relatively rigid, frequently branching, 3D network of bundles with characteristic mesh size of the order of the persistence length of F-actin. In contrast, additional nanoscale ordering is observed, as synchrotron x-ray diffraction has revealed a disordered, distorted square lattice of actin fibers within the individual bundles.

Vanadium pentoxide (V2O5) gels are suspensions of V2O5 ribbonlike particles dispersed in water. At volume fractions larger than 0.7%, they form a lyotropic nematic liquid-crystalline phase. We investigate here three different methods of synthesis (by ion-exchange, from peroxovanadic species, and by dissolution of V2O5 powders) and suggest a common chemical mechanism giving rise to these V2O5 ribbons. In addition, we investigate in detail the ion-exchange synthesis process by optical microscopy, time-resolved synchrotron small-angle X-ray scattering, rheology, and conductivity measurements. In particular, we relate the flocculation occurring during synthesis and the subsequent dispersion of V2O5 condensed moieties to their electric surface charge. The fractal structure of the transient flocculate is demonstrated. We then give a complete physical description of the formation of the ribbons and their organization in a nematic phase. Finally, we try to describe this polymerization process by borrowing concepts devised to understand the self-assembly of amphiphilic molecules.