Thomas G. Mason, University of California, Los Angeles
March 26, 2013

We observe the rich phase behavior and dynamics of Brownian systems of hard polygons that are concentrated to high densities in two dimensions. We lithographically fabricate large numbers of uniform regular pentagonal, square, and triangular polymer microplatelets, stably disperse these microplatelets in water, confine them so they diffuse in a plane, and concentrate them by applying a weak gravitational osmotic compression. Using video particle tracking optical microscopy, we directly measure the translational and rotational trajectories of many interacting microplatelets. Interestingly, a dense system of pentagons displays an order-disorder transition under increasing compression, as well as a glassy ergodic-nonergodic transition in rotational dynamics. By contrast, a system of squares undergoes a hexagonal-to-rhombic crystal-crystal transition under 2D compression. Triangles have altogether different behavior and self-organize into a liquid crystal phase, which we call a triatic phase. At higher particle densities, this triatic phase clearly exhibits local chiral symmetry breaking. We show that maximizing a combination of rotational and translational entropy at a fixed particle area fraction provides a fundamental basis for explaining a wide variety of self-organized structures in dense 2D Brownian systems of hard complex shapes.

 

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