Raft Instability of Biopolymer Gels

Following recent x-ray diffraction experiments by Wong, Li, and Safinya on biopolymer gels, we apply Onsager excluded volume theory to a nematic mixture of rigid rods and strong “ $\pi /2\mathrm{}$” cross-linkers obtaining a long-ranged, highly anisotropic depletion attraction between the linkers. This attraction leads to breakdown of the percolation theory for this class of gels, to breakdown of Onsager’s second-order virial method, and to formation of heterogeneities in the form of raftlike ribbons.