Aspect Ratio Dependence of Isotropic-Nematic Phase Separation of Nanoplates in Gravity

We studied isotropic-nematic (I-N) phase separation via gravity sedimentation in suspensions of plate-like colloidal particles of identical thickness but different lateral sizes (diameters). It is well-known that I-N phase transition occurs at a higher concentration for particles with larger aspect ratio (thickness/diameter) than for particles with smaller aspect ratio. Here we report that for the larger aspect ratios of nanoplates, gravity-driven I-N phase separation is faster. In a homogenously mixed I-N biphasic suspension of nanoplates, nematic tactoids nucleate, grow, and then undergo sedimentation in gravity, leading to the formation of a clear horizontal interface between the I and N phase. For I-N coexistent suspension of nanoplates with different aspect ratios but the same amount of nematic fractions, the larger the aspect ratio, the faster the formation of nematic tactoids and interface between isotropic liquid and nematic liquid crystal phase. The tactoid formation rate is governed by the rotational and translational diffusion rates, which are faster at larger aspect ratios. The time required for I-N separation (t*, seconds) varies inversely with the mean aspect ratio (< ξ >) of nanoplates and follows the relation, t^* = α < ξ >ⁿ, where α = 0.97 ± 1.30 s and n = −2.1 ± 0.2. The phase separation kinetics studied in our experiments offers guidance for the selection of aspect ratio of nanoplates for samples to be studied at the International Space Station (ISS).