dc.description.abstract | Liquids composed of elongated particles such as dumbbells, ellipsoid, spherocylinders, or rod-like bio-molecules and nano-particles, widely exist in nature. The anisotropic shape of elongated particles gives additional rotational degree of freedom, which breaks isotropic symmetry and enriches micro-packing and motion under thermal agitation. In previous works, the 2D cold liquid composed of spherocylinders with shape aspect ratio (SAR) greater than 3 exhibits tetratic ordered domains with parallel or perpendicular aligned particles, and separation of rotational and translational hopping time scales. In this work, we numerically investigate the micro-structures and motions of the 2D cold liquids composed of short rods by reducing SAR from 3 to 1.35. It is found that the quasi-long range tetratic alignment order and the hexagonal bond-orientation order, respectively at the two SAR ends, are both destroyed in the intermediate SAR regime around SAR=2. The poor particle packing of the disordered structure makes the translational hopping time reach minimum. However, long rods give a strong rotational topological constraint with rod alignment ordering, so the rotational hopping time monotonically increases with increasing SAR, and the rotational and the translational hopping time scales are separated at the small aspect ratio end, after the cross-over of their rate in the intermediate regime.
At the high aspect ratio end, the good local tetratic order has the same positive effects on suppressing rotational and translational hoppings. At the low aspect ratio end, the local hexagonal order has opposite effects on rotational and translational hoppings. The smaller minimum bond length to the nearest neighbors from a site with poor hexagonal ordering is the key for making tighter rotational caging to suppress rotational hopping.
In a 2D system, the packing density and temperature are also significant factors. With increasing packing density or decreasing temperature, the crystalline ordered domains with 4-fold tetratic alignment order increases for SAR > 2. Furthermore, the separation of translational and rotational hopping time scales is weakened for decreasing packing density and increasing temperature for SAR < 2. | en_US |