Freezing transition and correlated motion in a quasi-two-dimensional colloid suspension.
Freezing transition and correlated motion in a quasi-two-dimensional colloid suspension.
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准二维胶体悬浮液中的冻结转变和相关运动。
DOI：
10.1103/physreve.68.061508
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发表时间：
2003
期刊：
影响因子：
--
通讯作者：
S. Rice
中科院分区：
文献类型：
--
作者：
R. Zangi;S. Rice
最近的实验表明，致密准二维胶体悬浮液中单粒子位移分布与高斯形式的偏差是涉及相邻胶体粒子协同运动的非均质动力学的结果[J.化学。物理。 47, 9142 (2001)]。在本文中，我们报告了近硬球胶体颗粒的准二维组装的分子动力学（MD）模拟结果。我们使用的胶体-胶体相互作用是短程的并且到处都是排斥的；它与先前研究中使用的 Marcus-Rice (MR) 和修改后的 MR 相互作用有关 [Phys.修订版 E 58, 7529 (1998)]。与这些系统的情况一样，我们研究的系统支持液相、六相和固相。我们的计算表明，单粒子位移分布与高斯形式的偏差存在于液相中，并且其大小在液相线密度处急剧增加并延伸到结晶相。对于大于液相线密度的密度，我们发现三个动态弛豫过程，其中包括在中间时间由于笼效应而减慢颗粒扩散位移的增长速率。随着密度向固相线密度增加，均方位移对时间的依赖性在中间时间从次线性变为零。长时弛豫模式的开始对应于粒子位移分布与高斯形式的偏差最大的时间。此时，随着密度呈指数增加，van Hove 函数的自部分相对于 r 表现出多个最大值，而 van Hove 函数的独特部分在原点处达到最大值，从而发出跳跃动力学信号。长期以来，颗粒均方位移在所有密度（包括固相密度）下都具有扩散特征。我们的发现的一个显着特征是颗粒从液相通过六方相并进入固相的位移特征的连续性。导致晶体中扩散过程的协同跳跃可以用一种机制来解释，该机制涉及随机位置和随机方向（但沿着晶体轴）的许多此类相关跳跃，从而产生有效的随机游走行为。我们认为，我们发现的集体运动是由瞬时简正模式振动沿扩散路径的叠加产生的。扩散路径沿着具有强键方向相关性的方向，并且在进入六方相时开始幅度快速增长。
Recent experiments have demonstrated that the deviation of the single-particle displacement distribution from Gaussian form in a dense quasi-two-dimensional colloid suspension is a result of heterogenous dynamics that involves cooperative motions of neighboring colloid particles [J. Chem. Phys. 47, 9142 (2001)]. In this paper, we report the results of molecular dynamics (MD) simulations of a quasi-two-dimensional assembly of nearly hard-sphere colloid particles. The colloid-colloid interaction we use is short ranged and everywhere repulsive; it is related to the Marcus-Rice (MR) and modified MR interactions used in a previous study [Phys. Rev. E 58, 7529 (1998)]. As is the case for those systems, the one we study supports liquid, hexatic, and solid phases. Our calculations show that the deviation of the single-particle displacement distribution from Gaussian form is present in the liquid phase, and that a sharp increase in its magnitude occurs at the liquidus density and extends into the crystalline phase. For densities greater than the liquidus density we find three dynamical relaxation processes that include, at intermediate times, a slowing down in the rate of growth of the diffusive displacement of a particle due to the cage effect. As the density increases toward the solidus density, the dependence of the mean squared displacement on time, at intermediate times, changes from sublinear to zero. The onset of the long-time relaxation mode corresponds to the time at which the deviation of the particle displacement distribution from Gaussian form is a maximum. At this time, which increases exponentially with the density, the self-part of the van Hove function exhibits multiple maxima with respect to r while the distinct part of the van Hove function is a maximum at the origin, thereby signaling jump dynamics. At long times the particle mean square displacement has diffusive character at all densities including solid phase densities. A remarkable feature of our findings is the continuity of character of the particle displacement from the liquid phase through the hexatic phase and into the solid phase. Cooperative jumps that lead to diffusive process in crystals can be explained by a mechanism that involves many such correlated hops in random locations and random directions (but along the crystallographic axes) thereby generating effective random walk behavior. We argue that the collective motion we have found is generated by superpositions of instantaneous normal mode vibrations along diffusive paths. The diffusive paths are along the directions with strong bond orientation correlation, and start to grow in amplitude rapidly on entry into the hexatic phase.