Metastability and multistability: Gelation and liquid crystal formation in suspensions of colloidal rods

The competition between gelation and isotropic-nematic (I-N) phase separation in dispersions of moderately attractive sterically stabilized boehmite (AlOOH) rods is studied. Two different scenarios are observed. In one system I-N phase separation proceeds by slow sedimentation (syneresis) of a gel resulting in a weakly birefringent, highly viscous nematic phase. In another system, closely related to the first, eventually two distinctly different nematic phases appear. One of them is again weakly birefringent and highly viscous such as in the first system. The other one is strongly birefringent and has a lower viscosity. We present a possible explanation for the observed three-phase isotropic-nematic-nematic coexistence in terms of a combination of short-range attraction and polydispersity.