2D isotropic-nematic transition in colloidal magnetic ellipsoids

Two-dimensional (2D) liquid crystal phase transition is one of the most important issues in condensed matter physics. In contrast to computational work reported in the last few decades, little research has been done in experiments. Anisotropic colloids provide an excellent experimental model system to study phase transitions, such as crystallization and glass transition in condensed matter physics with single particle resolution. However, the corresponding study for the two-dimensional liquid crystal transition remains a challenge, since the condensed anisotropic colloids usually stuck into the metastable glass phase rather than their equilibrium liquid crystal phase. Here we report a colloidal system composed of paramagnetic anisotropic particles that can overcome the free energy barrier from the metastable states towards the equilibrium phase by applying an external magnetic field. The experiments show a 2D isotropic-nematic transition with increasing packing density of the colloidal particles. The transition process is experimentally identified and found to be dependent on the anisotropy of the colloidal particles.