The Isotropic to Nematic Liquid Crystalline Phase Transition of F-actin Varies from Continuous to First Order

We report that the properties of the isotropic to nematic liquid crystalline phase transition of F-actin depend critically on the average filament length. For average filament lengths longer than 2 $\mu$m, we confirm previous findings that the phase transition is continuous in both alignment and concentration. For average filament lengths shorter than 2 $\mu$m, we show for the first time a first order transition with a clear discontinuity in both alignment and concentration. Tactoidal droplets of coexisting isotropic and nematic phases, differing in concentration by approximately 30%, form over the course of hours and appear to settle into near equilibrium metastable states.