Orientational dynamics governs the pathways of entropic crystallization of Brownian squares

Abstract: In dense systems of hard-interacting colloidal particles with anisotropic shapes, crystallization pathways represent an interesting frontier since directional entropic forces often cause fascinating variations in the equilibrium crystal structures. At increasing densities, when the orientational excluded volumes of anisotropic particles start overlapping, their translational and rotational dynamics become coupled, introducing complexities in the kinetics of their configurational reorganization, which facilitates ordering. To elucidate this, we have studied a two-dimensional system of osmotically compressed corner-rounded Brownian square platelets, which are known to equilibrate into hexagonal and rhombic crystalline phases as the osmotic pressure is increased. By analyzing the translational and orientational dynamics of the particles and calculating their corresponding contributions to minimize the free energy, we have shown that the accessible range of orientational diffusion of particles governs the pathways of structural evolution and consequent optimal equilibrium ordering at a given osmotic pressure. As the accessible orientational excursion becomes wider, the rotational contribution to configurational entropy minimizes the total free energy, leading to hexagonal ordering. At higher osmotic pressures, the long collective translational fluctuations of side-aligned particles with restricted rotational diffusion maximize entropy, thereby inducing a free energetically favored rhombic crystalline structure. Intriguingly, the density, which solely governs the crystallization of hard spheres, does not have any direct effect on this process. Complementary Brownian dynamics simulations further corroborate these experimental observations and interpretations. Our findings are also relevant to other systems of hard interacting anisotropic shapes in two and three dimensions.