有限尺寸胶体体系的二维结晶

从工业上的大尺寸晶体生长到实验室中受限小体系的结晶,结晶是普遍存在的物理现象,也一直是物理学中的重要研究课题.与大尺度结晶相变的研究相比,对于有限小尺度体系结晶过程的研究相对较少.本文通过设计具有吸引相互作用的胶体体系,在实验上研究了有限小尺寸胶体体系的二维结晶相变.通过计算和分析径向分布函数、泰森多边形以及取向序参量,发现有限小尺寸体系的结晶过程是从中央高密度区域开始,随着结晶的进行,周围液相减小而晶相增加,最后完全转变为晶态的过程.体系结晶速率呈现两个阶段:在结晶初期中央区域是高密度的亚稳态液体,会降低结晶自由能能垒,使得体系快速结晶;随后晶相长大,亚稳态液体消失,体系结晶速率变慢.此外,通过统计有序度参量的分布发现:在结晶过程中,序参量出现双峰分布,分别对应液相和晶相,与大尺度胶体体系的二维结晶行为一致,说明序参量分布的变化规律是二维结晶相变的重要特征.     

Scaling in colloidal crystallization

Summary The process of colloidal crystallization has already been separately investigated by light scattering mearurements of the crystalline order (Bragg peaks) and of long-ranged density fluctuations (small-angle scattering). We present the firstsimultaneous small- and large-angle light scattering experiment. Our measurements have been performed on hard-sphere colloids of different particle densities. We show results for the temporal evolution of crystallization and find two distinct regimes: homogeneous nucleation and diffusion- or reaction-limited growth at early times, followed by ripening of the Lifshitz-Slyozov or Lifshitz-Allen-Cahn type. To our preliminary experience, we can already state that the results are always consistent in both partial experiments. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. With deep regret we have to announce that Prof. Klaus Sch?tzel, who inspired and guided the work presented here, has died in October 1994.     

High-resolution small-angle x-ray diffraction study of long-range order in hard-sphere colloidal crystals

The long-range order parameters in single crystals of hard colloidal spheres grown in sediments of colloid-polymer mixtures are determined using synchrotron small-angle x-ray diffraction with a resolution of 10(-6) of the wave vector. The interplanar positional order derived from the width of lattice reflections extends over at least 500 lattice planes. The lattice planes are orientationally correlated within approximately 0.1 degrees throughout the crystals, whereas the stacking of hexagonal planes remains random.     

Boundary-driven colloidal crystallization in simple shear flow

Using confocal microscopy, we directly observe that simple shear flow induces transient crystallization of colloids by wall-normal propagation of crystallization fronts from each shearing surface. The initial rate of the front propagation was 1.75±0.07 colloidal layers per unit of applied strain. The rate slowed to 0.29±0.04 colloidal layers per unit of applied strain as the two fronts approached each other at the midplane. The retardation of the front propagation is caused by self-concentration of shear strain in the growing bands of the lower-viscosity crystal, an effect that leads to a progressive reduction of the shear rate in the remaining amorphous material. These findings differ significantly from previous hypotheses for flow-induced colloidal crystallization by homogeneous mechanisms.     

Connecting structural relaxation with the low frequency modes in a hard-sphere colloidal glass

Structural relaxation in hard-sphere colloidal glasses has been studied using confocal microscopy. The motion of individual particles is followed over long time scales to detect the rearranging regions in the system. We have used normal mode analysis to understand the origin of the rearranging regions. The low-frequency modes, obtained over short time scales, show strong spatial correlation with the rearrangements that happen on long time scales.     

Kossel line analysis on crystallization in colloidal suspensions

The processes of crystallization in salt-free aqueous suspensions of highly-charged latex particles were observed by laser diffraction experiments for semi-dilute specimens of concentration 0.1–10 vol%. The Kossel diffraction patterns provide accurate three-dimensional information on the suspensions enabling the time evolution of the crystal structure to be recorded in detail. The results showed that the ordering formation evolved by way of the following intermediate processes: two-dimensional hep structure ← random layer structure ← layer structure with one sliding degree of freedom ← stacking disorder structure ← stacking structure with multivariant periodicity ← fcc structure with (111) twin ← normal fcc structure. For less concentrated suspensions (< 2 vol %), the phase transition proceeded further from the normal fcc structure to the normal bcc structure via the bcc twin structure with twin plane (112) or (112).     

Entropically driven colloidal crystallization on patterned surfaces

We investigate the self-assembly of colloidal spheres on periodically patterned templates. The surface potentials and the surface phases are induced entropically by the presence of dissolved, nonadsorbing polymers. A rich variety of two-dimensional fluidlike and solidlike phases was observed to form on template potentials with both one- and two-dimensional symmetry. The same methodology was then used to nucleate an oriented single fcc crystal more than 30 layers thick. The general approach provides a new route for directed self-assembly of novel mesoscopic structures.     

Glassy colloidal systems

This review focuses on recent developments in the theoretical, numerical and experimental study of slow dynamics in colloidal systems, with a particular emphasis on the glass transition phenomenon. Colloidal systems appear to be particularly suited for tackling the general problem of dynamic arrest, since they show a larger flexibility compared to atomic and molecular glasses because of their size and the possibility of manipulating the physical and chemical properties of the samples. Indeed, a wealth of new effects, not easily observable in molecular liquids, have been predicted and measured in colloidal systems. The slow dynamic behavior of three classes of colloidal suspension is reviewed – hard colloids, short-range attractive colloids and soft colloidal systems – selecting the model systems among the most prominent candidates for grasping the essential features of dynamic arrest. Emphasis is on the possibility of performing a detailed comparison between experimental data and theoretical predictions based on the mode coupling theory of the glass transition. Finally, the importance of understanding the system's kinetic arrest phase diagram, i.e. the regions in phase space where disordered arrested states can be expected, is stressed. When and how these states are kinetically stabilized with respect to the ordered lowest free energy phases is then examined in order to provide a framework for interpreting and developing new ideas in the study of new materials.

Glassy colloidal systems

All authors

F. Sciortino &; P. Tartaglia

https://doi.org/10.1080/00018730500414570

Published online:

19 February 2007

Table     

Aggregation kinetics in a model colloidal suspension

We present molecular dynamics simulations of aggregation kinetics in a colloidal suspension modeled as a highly asymmetric binary mixture. Starting from a configuration with largely uncorrelated colloidal particles the system relaxes by coagulation-fragmentation dynamics to a structured state of low-dimensionality clusters with an exponential size distribution. The results show that short-range repulsive interactions alone can give rise to so-called cluster phases. For the present model and probably other, more common colloids, the observed clusters appear to be equilibrium phase fluctuations induced by the entropic intercolloidal attractions.     

NMR velocimetry studies of the steady-shear rheology of a concentrated hard-sphere colloidal system

NMR velocimetry has been used to observe the steady-shear rheological behaviour of a concentrated suspension of hard-sphere like 370 nm diameter PMMA core-shell latex particles at the volume fraction Φ = 0.46, the liquid core of the spheres rendering possible NMR observation of the particles themselves. Rheological measurements in a cone-and-plate geometry indicate that when aged (i.e. left at rest for two weeks), the material exhibits yield stress behaviour at very low shear rates. For shear rates greater than 1 s ^{- 1} a transition to liquid-like behaviour was observed, leading to a rejuvenated fluid state which exhibits shear-thinning behaviour over a wide range of shear rates. A similar yield stress behaviour was reflected in NMR velocimetry measurements in a Couette geometry, where the solid-to liquid transition could be clearly observed. Under steady-state flow, the fluid state inside the radius at which yield stress was observed, exhibited shear-thinning behaviour with a power law exponent n slowly approaching unity with increasing shear rate. This behaviour has some similarities with a model of Derec et al. in which aging and rejuvenation effects compete. Substantial wall slip was observed both at the inner and at the outer wall, an effect which disappeared as the shear rate was increased. No radial particle migration from the high-shear region at the inner wall was observed.     

利用胶体系统研究玻璃态

玻璃态是一种无序结构的固体,组成单元可以是原子、分子、高分子、胶体粒子等等。尽管玻璃态材料在生活中很常见,有广泛的工业应用,但相关理论,尤其是液体到玻璃态转变的理论是物理学中争议最多的领域之一。溶液中的胶体粒子可以形成晶体、液体、玻璃等各种态。在光学显微镜下可以直接看到三维内部单个微米胶体粒子,通过图像处理还可以得到粒子的布朗运动轨迹,从而得到玻璃化转变过程中的微观动力学信息,这是原子分子玻璃系统中难以测量的。文章介绍了胶体作为模型系统对玻璃态的研究,主要包括传统的过冷液体到玻璃态的转变,另外还涉及气相、凝胶、多晶等其他无序或半无序态与玻璃态之间的过渡或转变。     

Microstructure identification during crystallization of charged colloidal suspensions

We use a polarized light microscope in its orthoscopic and conoscopic arrangements and laser light diffraction to study the effect of particle volume fraction and cell thickness on the microstructure of crystallizing suspensions of negatively charged polystyrene microspheres. Deionized suspensions of these particles nucleate at random sites in the bulk of the suspension to give a variety of structures, orientations and sizes. Orthoscopic observation of the Bragg diffraction colors between crossed polars and conoscopic inspection of the interference figures reveal structural details. We find that the crystallites grow by parallel stacking of the (111) layers to single and twin fcc structures. At moderate volume fractions, Φ ≈ 0.09, the structures are essentially “frozen” in space by their neighbors. At lower concentrations, Φ ≈ 0.05, the crystallites are larger with smoother boundaries and exhibit a range of colors. In thick cells, L ≥ 200 μm, and Φ ≤ 0.05, the colored crystallites become dark with time as they align with the (111) planes parallel to the cell walls. In thin, 50μm cells and Φ ≤ 0.05, this alignment is enhanced. We demonstrate that striated crystallites with lamellae of alternating colors and varying width are polysynthetic fcc twins with (111) twin plane. The number density of twin crystals and the frequency of striations decrease with decreasing volume fraction.     

Electric field effect on crystallization kinetics

The effect of a dc electric field on the kinetics of dielectric crystallization is considered. It is shown that the electric field has a significant effect on the rate of nucleation, the linear rate of crystallization, and the time for complete crystallization of a given volume of mother liquor, and also leads to structural changes in the crystallizing solid phase.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 63–67, February, 1979.     

A branching model for crystallization kinetics

In order to test whether the branching probability of crystallites plays an important role in determining the structure and the mutual interaction of growing bodies in a bulk crystallization system, mathematical model experiments for dendrite growth on a hexagonal lattice were performed. It was shown that when the branching frequency is high enough, a dendrite contour developed for each growing body from different primary nuclei, giving rise to a sharp boundary. As the branching frequency decreased, however, the boundary became blurred as the result of the interaction of growing units from different origins, suggesting the inadequacy of the Avrami-type kinetic model, i.e., the expanding-and-overlapping sphere model. For the latter case a calculation of kinetic equations was made (with simple assumptions) assuming growth of a one-dimensional unit which branches in succession. Crystallization curves similar to those derived from current theories were obtained without restriction on the constant (i.e., without integer constant).     

Transport and crystallization of colloidal particles in a thin nematic cell

In a thin planar nematic cell, the application of an AC electric field induces a macroscopic transport of micrometer-sized colloidal particles along the nematic director. We have analyzed the dependence of particle velocities on the electric-field amplitude and frequency and found that it decreases exponentially with increasing frequency. Using specially designed electrodes we have observed that colloidal particles could be pumped and accelerated across the field-no-field interface, and measured the structural force and the corresponding potential, which is of the order of 10000 k_BT for 4μm particles. We demonstrate that spatially periodic close-packed crystalline colloidal structures can be obtained, which are thermodinamically metastable for many days after turning off the electric field and slowly decay into linear chains. Above the nematic-isotropic phase transition, such crystalline structures are non-stable and decay in few minutes.     

Raman scattering study of glass crystallization kinetics

Laser induced glass-crystalline transition is studied by light scattering. Three significant effects are observed depending on the incident laser energy density: (i) Spectral band narrowing indicating cluster enlargement constitutes a precursor effect, (ii) an intensity increase effect indicates a rapid rise of the density of clusters attaining microcrystalline size and (iii) a dynamical reversal effect indicative of glass-crystalline instability. Cluster volume and crystallization appear as separate but related threshold phenomena.     

Influence of shear on polypropylene crystallization kinetics

Isothermal crystallization kinetics under shear in the melt of iPP was investigated by optical microscopy. It appears that shearing from 200 to the crystallization temperatures enhanced the kinetics, but the shear effect was not obvious if the melt of iPP was sheared only at 200. The experiment results show that relaxation plays an important role during crystallization, and that spherulite growth rates increased with shear rates and were governed by relaxation. The effect of flow on the crystallization kinetics can be understood by considering that the increase of the degree of order due to flow results is an effective change of the melt free energy. The Laurizen-Hoffman theory and the DE-IAA model were used to describe the shear-induced crystallization kinetics of iPP excellently.Received: 30 August 2004, Published online: 19 October 2004PACS: 61.41. + e Polymers, elastomers, and plastics - 61.50.-f Crystalline state - 64.70.-p Specific phase transitions     

Locked-floating-solid to locked-smectic transition in colloidal systems

We investigate two-dimensional melting of a colloidal system in the presence of a one-dimensional periodic substrate potential created by two interfering laser beams. We study the commensurability ratio p=sqrt[3]a/2d=2 with a the mean particle distance and d the period of the periodic potential. In contrast with the previously investigated case p=1, here we observe that melting of the locked-floating solid occurs via a novel locked-smectic phase, predicted by recent theoretical studies.     

Effect of the stabilizing coating and the presence of free polymer on the rate of crystallization of colloidal systems

Visual observations and preliminary light scattering experiments on the crystallization process in various colloidal systems are reported. In the first place we studied the influence of the stabilizing coating of the colloidal particles on the rate of crystallization. The various coatings give rise to different ranges of the repulsive interaction. This is found to have a pronounced effect on the rate of crystallization. Furthermore we investigated the effect of free polymer added to the dispersion. It appears that addition of free polymer has a dramatic effect on the crystallization phenomena.