探索软芯系统的相行为：二维熔化及准晶的自组装

发布时间：2019-05-30 05:29

【摘要】：软物质最突出的特点-柔软性和复杂性,使软物质呈现尤为丰富有趣的物理现象,尤其是软物质的自组装行为。软物质的复杂性使其在一定条件下自组装成更为复杂的物质结构,其柔软性导致自组装结构的最终物质形式由多种相互作用决定。对软物质自组装的深入研究不但有助于我们理解不同物相之间的相变行为,而且为设计具有特殊功能的材料提供了一条新的途径。在众多的研究方向中,二维熔化和软准晶的形成一直是软凝聚态物理的热门内容。在木文中,我们主要研究软芯体系的二维固体-液体相变性质和准晶的自组装行为。在第一章的绪论中,我们首先介绍了软物质的重要特性及其主要研究方向。接着我们从软物质自组装的分类和自组装形式两方面着重介绍软物质的自组装及其研究现状。在众多的自组装研究方向中,我们重点研究二维熔化和准晶的自组装。KTHNY理论是当前理解和分析二维熔化的重要理论基础。我们从二维固体,KTHNY理论的假设和结论,以及一些研究成果详细地介绍二维熔化的重要内容,并且从二维熔化的结构特征和动力学特征两方面列出常用的辨别二维熔化相变性质和各个不同物态的方法。最后我们跟随着准晶的发展,分别介绍了准晶的高维空间模型,重要物理特征以及两种重要的准晶结构。将准晶和软物质联系起来组装成软准晶,是当前研究和解决准晶中一系列问题的重要途径。不同物理特性的组分粒子对软物质的宏观性质有很大的影响。在第二章中,我们首先介绍两种常用的软粒子模型,硬芯软壳层模型和超软模型,及其典型的相行为。然后,我们针对一种典型的超软体系及其相行为进行了粗浅地分析。在第三章中,我们研究三种不同的能够出现再熔化现象的软芯系统的二维熔化。这类软芯系统的相图中存在一个最大熔化温度Tm,对应的密度③ρm。通过分析等温状态方程及其有限尺度效应,以及用方向关联函数和位置关联函数确定各个物态,我们发现在ppm 一侧,六角相-液体的转变是非连续的,存在相分离,并且共存区密度间隔随温度的升高不断减小,在最大熔化温度Tm 处趋于消失。在ppm一侧,六角相和液体之间的相变性质是连续的。更进一步,通过分析液体的最大方向关联长度,我们确定最大熔化温度Tm是两种相变性质类型的分水岭。更直接地,我们通过直观的瞬时位型图清楚地看到共存相和纯的六角相。这些结果表明,具有最大熔化温度的软芯系统既能够出现非连续的六角相-液体的转变,也存在连续的六角相-液体转变。在传统的准晶形成的方法中,多种相互竞争的长度尺度被认为是不可或缺的,无论是相互作用势直接提供的,还是暗藏在粒子大小或者形状上。在第四章中,我们沿用第三章中的一种纯排斥的,各向同性的软芯模型,惊奇地发现八轴对称和十二轴对称准晶的存在。通过分析准晶的位型结构,我们发现五边形是形成准晶序的基本要素。通过分析准晶的动力学性质,以及固体形成前的液体的结构,进一步肯定了五边形的重要性。然后,我们通过研究准晶形成对路径依赖与否,以及不同固体结构的势能的比较确定我们的准晶是稳定存在的。最后,我们简单地分析了准晶的相变性质及其振动特性。我们的结果为形成准晶提供了不可思议的简单路径,而且对准晶的理论理解提出了挑战。在第五章,我们对本论文进行了总结,并对未来的工作做了展望。
[Abstract]:The most outstanding characteristics of the soft matter-flexibility and complexity make the soft material present especially interesting physical phenomena, in particular the self-assembly behavior of the soft material. The complexity of the soft material allows it to self-assemble into a more complex material structure under certain conditions, the flexibility of which results in the final substance form of the self-assembled structure being determined by a variety of interactions. The in-depth study of the self-assembly of soft material not only helps us to understand the phase-change behavior between different phases, but also provides a new way for designing materials with special functions. In many research directions, the formation of two-dimensional melting and soft quasicrystal has been a popular content of soft condensed matter physics. In this paper, we mainly study the properties of the two-dimensional solid-liquid phase transition and the self-assembly behavior of the quasicrystal. In the introduction of the first chapter, we first introduce the important characteristics of soft matter and its main research direction. Then we focus on the self-assembly and self-assembly of the soft material from the two aspects of the self-assembly classification and the self-assembly form of the soft material. In many self-assembly research directions, we focus on the self-assembly of two-dimensional melting and quasicrystal. The KTHY theory is an important theoretical basis for the current understanding and analysis of two-dimensional melting. Based on the assumption and conclusion of the two-dimensional solid and the KTHY theory, and some research results, the important contents of the two-dimensional melting are introduced in detail, and the two-dimensional melting phase-change properties and the different material states are listed from the two aspects of the structural and dynamic characteristics of the two-dimensional melting. In the end, with the development of the quasicrystal, the high-dimensional space model, the important physical characteristics and the two important quasi-crystal structures of the quasi-crystal are introduced. It is an important way to study and solve a series of problems in the quasi-crystal by linking the quasi-crystal and the soft material to the soft quasicrystal. The component particles of different physical properties have a great effect on the macroscopic properties of the soft material. In the second chapter, we first introduce two commonly used soft-particle models, hard-core soft-shell model and super-soft model, and its typical phase behavior. Then, we analyze the typical hypersoft system and its phase behavior. In the third chapter, we study the two-dimensional melting of three different soft-core systems capable of re-melting. In the phase diagram of this kind of soft-core system, there is a maximum melting temperature, Tm, and the corresponding density is 1. m. By analyzing the isothermal state equation and its finite-scale effect, and using the directional correlation function and the position correlation function to determine the state of each object, we find that on the ppm side, The transition of the hexagonal phase-liquid is non-continuous, there is phase separation, and the density of the coexistence zone decreases with increasing temperature and tends to disappear at the maximum melting temperature Tm. The phase-change properties between the hexagonal phase and the liquid are continuous on one side of the ppm. Further, by analyzing the maximum direction correlation length of the liquid, we determine that the maximum melting temperature Tm is a watershed of two phase change property types. More directly, we clearly see the coexistence phase and the pure hexagonal phase by an intuitive instantaneous bit pattern. These results show that the soft-core system with the maximum melting temperature can exhibit both a discontinuous hexagonal phase-liquid transition and a continuous hexagonal phase-liquid transition. In the conventional quasi-crystal formation method, a variety of competing length scales are considered indispensable, whether directly provided by the interaction potential or hidden in the particle size or shape. In the fourth chapter, we follow a purely repulsive and isotropic soft-core model in the third chapter, and the existence of the eight-axis and the twelve-axis symmetric quasicrystal is found. By analyzing the position-type structure of the quasicrystal, we find that the pentagonal shape is the basic element for forming the quasi-crystal order. The importance of the pentagons is further confirmed by analyzing the dynamics of the quasicrystal and the structure of the liquid before the formation of the solid. We then determine that our quasicrystal is stable by studying the dependence of the quasi-crystal on the path, and the comparison of the potential energy of the different solid structures. In the end, we simply analyze the phase-change properties of the quasicrystal and its vibration characteristics. Our results provide an inconceivable simple path for the formation of the quasicrystal, and the theoretical understanding of the alignment crystal presents a challenge. In the fifth chapter, we sum up the thesis and look forward to the future work.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469

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