几种复杂流体的物性研究

【作者】 高勇；

【导师】 黄吉平；

【作者基本信息】 复旦大学 ， 理论物理， 2010， 博士

【摘要】 “软物质(Soft matter)"这个词,是由诺贝尔奖获得者、法国物理学家德热纳(P. G. de Gennes)于1991年明确提出来。之前“软物质”也叫“复杂流体”软物质(Soft matter)或称软凝聚态物质(Soft condensed matter)是由固、液、气集团或大分子等为基本组元组成,是处于理想流体和固体之间的复杂体系。其结构单元(building blocks)间相互作用弱(约为kT),热涨落和熵主导其运动和变化。这类物质与普通固体、液体和气体很不相同。流体热涨落和固态的约束共存导致了软物质的新行为,体现了软物质组成、结构和相互作用的复杂性及其特殊性。21世纪被认为是生命科学的世纪,从物质划代角度来看,这也是软物质的世纪。如果没有软物质,生命也不复存在。任何生物结构(包括DNA、蛋白质和生物膜)都是建筑在软物质的基础上。软物质的许多奇特的行为、丰富物理内涵和广泛应用背景,引起越来越多物理学家的兴趣,成为具挑战性和迫切性的重要研究方向,成为凝聚态物理研究的重要前沿领域。我们主要研究铁磁流体、逆铁磁流体和电流变液等几种复杂流体的结构、性质和应用。之后我们介绍了特异材料和软光学特异材料的概念和一些应用。第二章,在外加直流磁场作用下,我们用有限元数值模拟方法展示了铁磁流体中光学负折射现象。该铁磁流体是由四氧化三铁颗粒外面包裹银壳层分散在水中形成的。在外加直流磁场的情况下,颗粒会聚集成链状,随着磁场的增大链会聚集成柱状结构。磁场可控的全角和宽频光学负折射来源于磁场诱导的链／柱状结构,在当前研究体系中传播的横磁波(TM)的等频率曲线是双曲线,根据边界条件,我们可以确定能流是负折射。软光学特异材料的提出,使得我们多了一个自由度(外场)来调节材料的结构,因此材料的光学性质会随着外场的变化而变化。此外,我们还介绍了多频可见光波段隐身斗篷的理论基础。基于变换光学和特异材料,我们可以设计一些光学器件,几乎任意地调控电磁波。但在实际应用中,材料吸收会相对比较大,实验中可以用一些增益材料来尽量减少吸收。另一挑战是拓宽高频下的工作频率。第三章,我们研究了逆铁磁流体的基态和磁泳现象。利用偶极-多极相互作用模型,我们发现逆铁磁流体的基态为体心正方晶格。了解体系的基态,有利于我们进一步研究体系的基本性质。在外加非均匀磁场作用下,考虑到结构转变和颗粒之间的相互作用,我们利用Ewald-Kornfeld公式和Maxwell-Garnett有效媒质理论求出作用在非磁性颗粒上的磁泳力。研究结果表明,非磁性颗粒的磁泳力受到逆铁磁流体具体的晶格结构、非磁性颗粒的体积分数、几何形状以及外磁场的频率等因素的调控。磁泳现象广泛应用于选矿和分离生物细胞中。我们的研究结果对磁泳现象的实际应用有着一定的指导意义。第四章,考虑到多极相互作用,我们利用三种方法研究了在电场作用下三颗粒系统的动力学行为,其中两个颗粒对称地固定于第三个自由颗粒的两侧。研究发现,第三个自由颗粒横向被拉开并释放,它会沿垂直于两固定颗粒的球心连线振荡。当横向振幅很小时,颗粒的运动是简谐运动。简谐运动的周期受颗粒的介电常数、密度、半径、颗粒间的距离和外电场调控。我们的研究结果有助于理解在外场作用下复杂流体中颗粒链间的相互作用的微观机理。更多还原

【Abstract】 "Soft matter" was named firstly by P. G. de Gennes, the winner of Nobel Prize in 1991, and physical scientist in France. "Soft matter" is also called "Complex fluid" "Soft matter" or "Soft condensed matter" is a kind of complex system between the ideal fluid and solid. Generally, soft matter is composed of group (solid, liquid, gas) or the large-molecule. The interaction between building blocks is week (about KT), so the thermal fluctuation and entropy control the motion of the system. This kind of materials is much different from solid, liquid and gas. Thermal fluctuation of fluid and the constraint of solid cause the new behavior in soft matter, which exhibits the complexity and particularity of composition, structure and interaction of soft matter.The 21st century is considered as the century of life sciences, from the point of view of matter division, this is also the century of soft matter. If there is no soft matter, life can not exist. Any biological structure (including DNA, proteins and biomembrane) are built on the basis of soft matter. For the peculiar behavior, rich physical connotation, and various applications, soft matter attracts more and more attention of physicists, and has become an important research direction with challenge and urgency, and an important research frontier in condensed matter physics. We mainly investigate the structures, properties, and applications of some complex fluids, such as ferrofluids, inverse ferrofluids, and electrorheological fluids. Then, we introduce the concept and some applicatios of metamaterials and soft metamaterials.In Chapter 2, we numerically demonstrate optical negative refraction by using finite element simulations in ferrofluids containing isotropic Fe3O4 nanoparticles, each hav-ing an isotropic Ag shell suspended in water, in the presence of an external dc magnetic field H. Under external magnetic field, the particles will form chains, as the magnetic increases, the chains will form columns. The all-angle broadband optical negative re-fraction with magnetocontrollability arises from H-induced chains or columns. They result in hyperbolic equifrequency contour for transverse magnetic waves propagating in the system. From the boundary conditions, we found that the refraction of energy is negative. The proposed concept of soft optical matamaterials add us an external free-dom (external fields) to control the structure of the materials, so the optical properties of the materials will change as the external field change. Besides, we introduce the theory about multifrequecy optical cloak. On the basis of transformation optics and metamaterials, we can design some optical devises, which can arbitrarily manipulate electromagnetic field. In real applications, the absorption of the materials is big. We can use the gain materials to compensate for loss. Another challenge is to extend the working band at high frequencies.In Chapter 3, we investigate the ground state and magnetophoresis of inverse fer-rofluids. We found that the ground state of inverse ferrofluids is body-centered tetrag-onal (bct) lattices by using the dipole-multipole interaction model. Understanding the ground state of the system, we can further investigate the basic properties of the sys-tem. On the basis of the Ewald-Kornfeld formulation and the Maxwell-Garnett theory, we theoretically investigate the magnetophoretic force exerting on the nonmagnetic particles in inverse ferrofluids due to the presence of a nonuniform magnetic field, by taking into account the structural transition and long-range interaction. We numerically demonstrate that the force can be adjusted by choosing appropriate lattices, volume fractions, geometric shapes, and conductivities of the nonmagnetic particles, as well as frequencies of external magnetic fields. Magnetophoresis has been widely used in mill run and separation of biological cells. Our results have some significance in practical application of magnetophoresis.In Chapter 4, taking into account the multipole interaction, we utilize three methods to investigate the dynamic behavior of a chain of three microparticles in an electric field, two of which are fixed and symmetrically located in the two opposite sides of the third free microparticle. We reveal that, if the free microparticle is laterally dragged and released, it can oscillate perpendicular to the line joining the centers of the two fixed microparticles, being in simple harmonic oscillation with a fixed period for small oscillation amplitudes. The period of the harmonic oscillation can be controlled by the permittivity, density, radius, the distance of the particles, as well as the external electric field. Our results help us to understand the mechanism of the interaction of the particle chains in complex fluids under external fields.更多还原