Phase behavior of dispersions of hard spherical particles

A series of experiments on concentrated dispersions of hard colloidal spheres is discussed. The observed phase behavior is analogous to that of simple atomic systems: colloidal fluid, crystal and glass phases are found. The structure of the crystals, revealed by light diffraction, is a strongly faulted stacking of hexagonally-packed layers of particles. Dynamic light scattering confirms that the concentration of the metastable fluid phase for which long-ranged particle diffusion ceases coincides with the concentration where the glass transition is observed macroscopically. In studies of a binary mixture of colloidal spheres with a size ratio 0.61 eutectics, glass formation and the AB₁₃ type alloy structure have been identified.     

亚微米聚苯乙烯微球在气-液界面组装的机理研究

系统地研究了亚微米聚苯乙烯微球在气-液界面的组装机理.聚苯乙烯微球在介质对流的带动下会到达悬浮液的表面并在气-液界面组装，气-液界面处聚苯乙烯微球间由弯液面产生的毛细管力是组装的推动力.界面处聚苯乙烯微球在干燥过程中其润湿性发生了转变，由完全润湿到部分润湿并最终变成不润湿，相应的聚苯乙烯微球与分散介质间接触角也逐渐增大.研究表明，只有接触角达到或超过某数值θ′_critical时，才能够出现气-液界面组装现象.考虑到PS胶粒晶体的表面是“规则”粗糙的表面，由Wenzel公式知θ′_critical大于测量值θ＝22°.聚苯乙烯微球润湿性的转变是界面组装发生和持续进行的关键性因素. 关键词： 自组装 胶粒晶体 聚苯乙烯微球 润湿性     

亚微米聚苯乙烯微球在气-液界面组装的机理研究

系统地研究了亚微米聚苯乙烯微球在气-液界面的组装机理.聚苯乙烯微球在介质对流的带动下会到达悬浮液的表面并在气-液界面组装，气-液界面处聚苯乙烯微球间由弯液面产生的毛细管力是组装的推动力.界面处聚苯乙烯微球在干燥过程中其润湿性发生了转变，由完全润湿到部分润湿并最终变成不润湿，相应的聚苯乙烯微球与分散介质间接触角也逐渐增大.研究表明，只有接触角达到或超过某数值θ′_critical时，才能够出现气-液界面组装现象.考虑到PS胶粒晶体的表面是“规则”粗糙的表面，由Wenzel公式知θ′_critical大于测量值θ＝22°.聚苯乙烯微球润湿性的转变是界面组装发生和持续进行的关键性因素. 关键词： 自组装 胶粒晶体 聚苯乙烯微球 润湿性     

Entropic wetting and many-body induced layering in a model colloid-polymer mixture

We develop an efficient simulation scheme to study a model suspension of equally sized colloidal hard spheres and nonadsorbing ideal polymer coils, both in bulk and adsorbed against a planar hard wall. The many-body character of the polymer-mediated effective interactions between the colloids yields a bulk phase diagram and adsorption phenomena that differ substantially from those found for pairwise simple fluids; e.g., we find an anomalously large bulk liquid regime and, far from the bulk triple point, three layering transitions in the partial wetting regime prior to a transition to complete wetting by colloidal liquid.     

Small angle X-ray scattering of the colloidal crystal

The monodisperse polystyrene spheres are assembled into the colloidal crystal on the glass substrate by vertical deposition method, which is aimed at the so-called photonic crystal applications. The structural information of the bulk colloidal crystal is crucial for understanding the crystal growth mechanism and developing the various applications of colloidal crystal. Small-angle X-ray scattering (SAXS) technique was used to obtain the bulk structure of the colloidal crystal at Beamline 1W2A of BSRF. It is found that the SAXS pattern is sensitive to the relative orientation between the colloidal sample and the incident X-ray direction. The crystal lattice was well distinguished and determined by the SAXS data.     

Dynamic broadening of the crystal-fluid interface of colloidal hard spheres

We investigate the structure and dynamics of the crystal-fluid interface of colloidal hard spheres in real space by confocal microscopy. Tuning the buoyancy of the particles allows us to study the interface close to and away from equilibrium. We find that the interface broadens from 8-9 particle diameters close to equilibrium to 15 particle diameters away from equilibrium. Furthermore, the interfacial velocity, i.e., the velocity by which the interface moves upwards, increases significantly. The increasing gravitational drive leads to supersaturation of the fluid above the crystal surface. This dramatically affects crystal nucleation and growth, resulting in the observed dynamic broadening of the crystal-fluid interface.     

Quantum corrections to thermodynamics of polar hard sphere fluids

The quantum corrections to the thermodynamic properties of polar hard sphere fluids and fluid mixtures are estimated taking into account the influence of dipole and quadrupole moments. Expressions are given for the second virial coefficient, free energy and pressure and results are given for different values ofμ* andϑ*. The first order quantum correction arises due to the translational contribution only. The quantum effect increases with density,μ* andϑ*. Numerical results are also estimated for binary mixtures of (i) hard spheres and dipole hard spheres and (ii) hard spheres and quadrupole hard spheres. The ‘excess’ free energy for dipole hard sphere binary mixture is also reported. It is found that the ‘excess’ quantum effect depends on the concentration and the particle diameter ratio and increases with increase ofμ* andϑ*.     

Theory of polar liquids

Two recent contributions to the statistical theory of polar fluids, namely the perturbation theory of Stell, Rasaiah and Narang (SRN) and the meanspherical-approximation (MSA) results of Wertheim, and of Nienhuis and Deutch, are compared and contrasted for the conceptually simple model of hard spheres, diameter R, with central point dipoles, of strength μ (dipolar hard spheres). It is shown that the MSA approach replaces correlation functions which enter correctly into the SRN theory by their low-density limits : to this extent it is unsatisfactory. On the other hand the MSA work does suggest reasons why the naive Padé approximant featuring in SRN theory may be expected to do reasonable justice to the physics of the problem. Numerical comparisons of the excess free-energy (as compared with non-polar hard spheres) as a function of reduced density, ρ* = ρR ³, are given at two temperatures, T* = 2 and T* = 0·25, where T* = kTR ³/μ². Similar curves, for T* = 1 and T* = 0·5, are available from the authors. The gas-liquid (T*, ρ*)-phase boundary is located, near the critical point, on both theories, as are the vapour pressure curves. These are calculated using the Carnahan-Starling equation of state for hard spheres ; and critical comment is made in justification of employing this in the context of MSA results for the excess quantities. The two theories are found to have appreciably different numerical consequences.     

Depletion effects in smectic phases of hard-rod-hard-sphere mixtures

It is known that when hard spheres are added to a pure system of hard rods the stability of the smectic phase may be greatly enhanced, and that this effect can be rationalised in terms of depletion forces. In the present paper we first study the effect of orientational order on depletion forces in this particular binary system, comparing our results with those obtained adopting the usual approximation of considering the rods parallel and their orientations frozen. We consider mixtures with rods of different aspect ratios and spheres of different diameters, and we treat them within Onsager theory. Our results indicate that depletion effects, and consequently smectic stability, decrease significantly as a result of orientational disorder in the smectic phase when compared with corresponding data based on the frozen-orientation approximation. These results are discussed in terms of the τ parameter, which has been proposed as a convenient measure of depletion strength. We present closed expressions for τ, and show that it is intimately connected with the depletion potential. We then analyse the effect of particle geometry by comparing results pertaining to systems of parallel rods of different shapes (spherocylinders, cylinders and parallelepipeds). We finally provide results based on the Zwanzig approximation of a fundamental-measure density-functional theory applied to mixtures of parallelepipeds and cubes of different sizes. In this case, we show that the τ parameter exhibits a linear asymptotic behaviour in the limit of large values of the hard-rod aspect ratio, in conformity with Onsager theory, as well as in the limit of large values of the ratio of rod breadth to cube side length, d, in contrast to Onsager approximation, which predicts τ ∼ d ³. Based on both this result and the Percus-Yevick approximation for the direct correlation function for a hard-sphere binary mixture in the same limit of infinite asymmetry, we speculate that, for spherocylinders and spheres, the τ parameter should be of order unity as d tends to infinity.     

Assessing the Exact Muffin-Tin Orbitals method for the Bain path of metals

Abstract

We scrutinise the muffin-tin approximation and the screening within the framework of the Exact Muffin-Tin Orbitals method in the case of cubic and tetragonal crystal symmetries. Systematic total energy calculations are carried out for the Bain path including the body-centred cubic and face-centred cubic structures for a set of simple and transition metals. The present converged results in terms of potential sphere radius (S) and hard sphere radius (b) are in good agreement with previous theoretical calculations. We demonstrate that for all structures considered here, potential sphere radii around and slightly larger than the average Wigner–Seitz radius (w) yield accurate total energy results whereas S values smaller than w give large errors. It is shown that for converged total energies hard spheres with radii b = 0.7–0.8w should be used for an efficient screening within real space clusters consisting typically of 70–90 lattice sites. The less efficient convergence of the total energy in the case of small hard spheres is ascribed to the delocalisation of the screened spherical waves, which leads to inaccurate interstitial overlap matrix. The above conclusions are not significantly affected by the volume of the system.     

Dense packing in the monodisperse hard-sphere system: A numerical study

We report a numerical study of the close packing of monodisperse hard spheres. The close packings of hard spheres are produced by the Lubachesky-Stillinger (LS) compression algorithm and span the range from the disordered states to the ordered states. We provide quantitative evidence for the claim that the density and structural order of the arrested close packing can be determined by the compression rate, i.e., with slower rates producing denser and more ordered structures. Through deeply analyzing the structure of the resulting arrested close packings, a transition region has been identified in the plane of density and reciprocal compression rate, in between what have been historically thought of as amorphous and crystalline packings. We also find clear system size dependences in studying the structural properties of the packings from the disordered ones to the ordered ones. These detailed investigations, on the structure of the arrested close packings, may provide a link between the glassy states and the crystalline states in the hard spheres.     

On the freezing and structure of hard spheres under spherical confinement

The equation of state and the structure of hard spheres confined in spherical pores have been investigated via molecular dynamics for different pore radii ranging from 5.0 to 10.0?σ, where σ is the particle diameter. The hard boundary is chosen to capture the pure geometric effect of spherical confinement. A discontinuity in the equation of state was observed, indicating the onset of a freezing-like phase transition, which was similar to that of the bulk hard-sphere fluids. The behaviour of confined particles resembles that of the bulk with increase in the pore size, while its deviation from the bulk is found to be larger at the solid-like phase. For the pore radius below 5.0, FCC-like crystal clusters are not formed in spherically confined hard spheres.     

Influence of interatomic repulsion on the structure of liquids at melting

Radial distribution functions and liquid structure factors for fluids of particles interacting through 1/rⁿ repulsive potentials are computed ‘exactly’ and systematically compared at crystallization for several values of n ranging between the two extremes of hard spheres (n = ∞) and Coulomb forces (n = 1). Striking similarities are pointed out in the large r behaviour (beyond the first peak) of the various radial distribution functions, and in the small k behaviour (including the first peak) of the various structure factors. Some information about the steepness of the repulsive potential in liquid rare gases and in liquid alkalis is gained from a comparison of the damping of successive peaks of the structure factors with experimental data.     

The measurement of solid–liquid interfacial energy in colloidal suspensions using grain boundary grooves

Interfacial energy is a fundamental physiochemical property of any multi-phase system. Among the most direct approaches for determining solid–liquid interfacial energy is a technique based on measuring the shape of grain boundary grooves in specimens subjected to a linear temperature gradient. This technique was adapted to crystallizing colloids in a gravitational field. Such colloids exhibit a freezing–melting phase transition and are important not only as self-assembling precursors to photonic crystals, but also as physical models of atomic and molecular systems. The grain boundary groove technique was tested using suspensions of sterically stabilized poly(methyl methacrylate) spheres, which have been shown to closely approximate the hard sphere potential. Whereas isotropic models did not fit grain boundary groove data well, the capillary vector model, which is suitable for both isotropic and anisotropic surface energies, produced γ₁₁₀?=?0.58?±?0.05 k _B T/σ². This value of interfacial energy is in agreement with many of the published values for hard spheres, supporting the validity of our grain boundary groove technique adaptations to colloidal systems in a gravitational field. Finally, kinks observed in groove profiles suggest a minimum anisotropy parameter of ε?=?0.029 for hard spheres.     

Adsorption mode of neurotensin family peptides onto a colloidal silver surface: SERS studies

Kinetensin (KN) and its amino acids 1–8 fragment ([des‐Leu⁹]KN), neuromedin N (NMN), and xenopsin (XP) and its two analogs (human XP‐1/xenin‐8 and XP‐2) belong to the neurotensin family of peptides and are known to stimulate the growth of human tumors. In this work, we report surface‐enhanced Raman scattering (SERS) studies of these peptides and discuss their structures, orientation, and mode of adsorption onto a colloidal assembly of apparently randomly adhering Ag spheres with diameters of approximately 20 – 25 nm. We show that small alternations in both the amino acid composition and tertiary structure, which induce striking biological in vitro, were responsible for the observed spectroscopic changes. Copyright © 2012 John Wiley & Sons, Ltd.     

Steps and facets at the surface of soft crystals

We consider the shape of crystals which are soft in the sense that their elastic modulus μ is small compared to their surface tension γ, more precisely μa < γ where a is the lattice spacing. We show that their surface steps penetrate inside the crystal as edge dislocations. As a consequence, these steps are broad with a small energy which we calculate. We also calculate the elastic interaction between steps a distance d apart, which is a 1/d ² repulsion. We finally calculate the roughening temperatures of successive facets in order to compare with the remarkable shapes of lyotropic crystals recently observed by Pieranski et al. [#!Pieranski!#,#!EPJ!#]. Good agreement is found. Received 25 June 2001     

The stability limit of the fluid phase of polydisperse sticky spheres

It has been shown by Stell (1991, J. statist. Phys., 63, 1203) that at low temperature mono-disperse sticky spheres collapse to form coexisting close-packed solid and infinitely dilute gases. We show that polydisperse sticky spheres also collapse and calculate the collapse temperature. The polydisperse spheres separate into fractions with narrower polydispersities which can then solidify. This is perhaps the first example of a single-peaked polydisperse mixture phase solidifying and separating. It implies that a mixture of polydisperse large hard spheres with much smaller hard spheres does not show fluid—fluid coexistence.     

SiO_2胶体晶体自组装技术

介绍了垂直沉降法和旋涂法制备SiO2胶体晶体,并对两种方法制备的胶体晶体在宏观形貌、微观结构及光子带隙性质进行了比较。采用改进的Stober法在乙醇介质中合成粒径不同、单分散性较好的SiO2微球,用垂直沉积法和旋涂法制备出有序性较好的密排结构的SiO2胶体晶体。宏观照片表明,用旋涂法制备的SiO2胶体晶体经白光照射出现的光柱呈6次对称,而垂直沉降法制备的胶体晶体表面出现条纹。SEM分析表明,选用不同溶剂在同等旋涂工艺下制备SiO2胶体晶体,用乙醇和乙二醇混合溶液作溶剂制备出的SiO2胶体晶体质量最好。透射光谱表明,垂直沉降法所制备的胶体晶体在(111)方向具有明显的光子带隙性质,而旋涂法制备的胶体晶体则不明显。     

沉积法自组装三维有序的Eu(DBM)3Phen/SiO2胶体球

采用修饰的St ber法合成了300 nm的Eu(DBM)3Phen/SiO2胶体杂化球,并通过沉积法将这种胶体杂化球组装成厚度为5 mm,面积为12 cm2的三维有序结构。通过扫描电子显微镜观察发现这些胶体球在垂直于烧杯底面的所有层面中都显示了立方密堆积的结构。元素分析进一步证实了荧光分子被包埋在SiO2胶体球中。在355 nm的激发下,这种三维有序结构具有铕离子的特征发射。