Sheu Alice Shu-Yao, Rice Stuart
Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, USA.
J Chem Phys. 2008 Jun 28;128(24):244517. doi: 10.1063/1.2949797.
We report the results of a theoretical study of locally ordered fluctuations in a quasi-two-dimensional colloid fluid. The fluctuations in the equilibrium state are monitored by the aperture cross-correlation function of radiation scattered by the fluid, as calculated from molecular dynamics simulations of near hard spheres with diameter sigma confined between smooth hard walls. These locally ordered fluctuations are transient; their decay can be monitored as a function of the time between the cross-correlated scattered radiation signals, but only the single-time cross-correlated signals are discussed in this paper. Systems with thicknesses less than two hard sphere diameters were studied. For wall separation H in the range 1 sigma</=H</=1.57 sigma, the liquid freezes into a single hexagonal layer and the locally ordered fluctuations only exhibit hexagonal symmetry that extends well into the liquid region. For H>/=1.57 sigma, hexagonal fluctuations persist in the dense liquid up to H=1.75 sigma, and fluctuations with square ordered symmetry, that of the solid to which the liquid freezes, only emerge at densities approximately 2% below freezing. For H=1.8 sigma and 1.85 sigma, hexagonal ordered flucuations are no longer found, and the square ordered fluctuations dominate the dense liquid region as the system freezes into a two layer square solid. For H=1.9 sigma and 1.95 sigma, where the liquid freezes into a two layer hexagonal solid, both square and hexagonal ordered fluctuations are observed. At lower densities, the ordered fluctuations only exhibit square symmetry. Hexagonal ordered fluctuations appear at densities approximately 7% below freezing and become more dominant as the density is increased, but the square ordered fluctuations persist until the system is converted into the solid.
我们报告了对准二维胶体流体中局部有序涨落的理论研究结果。通过流体散射辐射的孔径互相关函数来监测平衡态下的涨落，该函数是根据直径为σ的近硬球在光滑硬壁之间受限的分子动力学模拟计算得出的。这些局部有序涨落是瞬态的；它们的衰减可以作为互相关散射辐射信号之间时间的函数进行监测，但本文仅讨论单次互相关信号。研究了厚度小于两个硬球直径的系统。对于壁间距H在1σ≤H≤1.57σ范围内，液体冻结成单一的六边形层，局部有序涨落仅呈现延伸至液体区域深处的六边形对称性。对于H≥1.57σ，六边形涨落在稠密液体中持续存在直至H = 1.75σ，而具有方形有序对称性（即液体冻结成的固体的对称性）的涨落仅在密度比冻结密度低约2%时出现。对于H = 1.8σ和1.85σ，不再发现六边形有序涨落，随着系统冻结成两层方形固体，方形有序涨落在稠密液体区域占主导。对于H = 1.9σ和1.95σ，液体冻结成两层六边形固体，观察到方形和六边形有序涨落。在较低密度下，有序涨落仅呈现方形对称性。六边形有序涨落在密度比冻结密度低约7%时出现，并随着密度增加变得更加占主导，但方形有序涨落一直持续到系统转变为固体。
