Complexity of precipitation patterns: Comparison of simulation with experiment

Numerical simulations show that a simple model for the formation of Liesegang precipitation patterns, which takes into account the dependence of nucleation and particle growth kinetics on supersaturation, can explain not only simple patterns like parallel bands in a test tube or concentric rings in a petri dish, but also more complex structural features, such as dislocations, helices, "Saturn rings," or patterns formed in the case of equal initial concentrations of the source substances. The limits of application of the model are discussed. (c) 1994 American Institute of Physics.     

Comparison of models and lattice-gas simulations for Liesegang patterns

For more than a century Liesegang patterns – self-organized, quasi-periodic structures occurring in diffusion-limited chemical reactions with two components – have been attracting scientists. The pattern formation can be described by four basic empirical laws. In addition to many experiments, several models have been devised to understand the formation of the bands and rings. Here we review the most important models and complement them with detailed three-dimensional lattice-gas simulations. We show how the mean-field predictions can be reconciled with experimental data by a redefinition of the distances suggested by our lattice-gas simulations.     

复杂流体的聚集态结构,性质及应用

综合报道了油－水、油－油分散体系中中各种聚集体，如胶团，微乳状液、凝胶、Ｌｉｅｓｅｇａｎｇｒｉｎｇ及分形图形等的形成机理，结构和特性，以及流体中聚集结构的形成在实际生产中的意义。     

Wave optics of Liesegang rings

Liesegang rings refract and reflect at the interface between the regions of the same gel but of different thickness. The incident and the refracted rings obey a refraction law analogous to the Snell's law of classical optics, with a reverse of the spacing coefficient being a counterpart of the refraction index. The wavelike behavior of the rings at the interface is explained by geometrical arguments derived from the Jablczynski's spacing principle, and is reproduced in numerical simulations based on a three-dimensional minimalistic version of the nucleation-growth model.     

Bifurcation analysis of Liesegang ring pattern formation

Bifurcation analysis is introduced to a prototype Liesegang ring (LR) model to explain pattern formation as an instability of a propagating plane reaction front. A theoretical criterion for the onset of patterning is derived and numerically tested. The uneven spacing law of LR bands is explained as a consequence of the time varying velocity of the moving reaction front. Suggestions for controlling pattern formation are provided.     

传统糯米灰浆碳化过程中Liesegang环的形成机理研究

采用FTIR,XRD,SEM等技术,对传统糯米灰浆碳化过程中出现的Liesegang环现象进行了初步研究。结果表明,使用陈化石灰和糯米浆制备的糯米灰浆在碳化过程中能形成较明显的Liesegang环;陈化石灰-糯米灰浆固化后的极小孔隙,是传统糯米灰浆中Liesegang环形成的本质原因,其形成机理符合延迟成核理论。     

Liesegang rings as the common gross property of oxyhydrate and other gel polymer systems: another look at the problem of periodicity

In this work the authors attempt to develop the Liesegang operator theory, which describes the formation of the Liesegang concentration periodicity in gel systems and enables one to understand quantitatively the autowave pulsating nature of the concentration attractor walls' interaction in gel oxyhydrate and other heavy metal systems. We suggest that a new antinode is formed under the influence of the double electric layer fields. The formation of the Liesegang concentration periodicity in gel systems is conditioned by laws and is supposed to be their general property.     

Recent Theoretical Developments on the Formation of Liesegang Patterns

When an electrolyte A diffuses into a gel containing another electrolyte B, the eventual formation of a rhythmic pattern of precipitate by the moving chemical reaction front is known as the Liesegang phenomenon. Although the Liesegang phenomenon has been studied for a century, the mechanisms responsible for these structures are still under discussion. However, recently, important theoretical progresses have been made towards a theoretical understanding of this phenomena. A critical analysis of the present state of the art as well as a discussion of some open problems is presented.     

Two dimensional chemical pattern formation in gels. Experiments and computer simulation

Precipitate forming chemical reactions have been studied in chemically cross-linked poly(vinyl-alcohol) gel layers and in thin silica films. One of the reactive components was incorporated into the gel, the other was allowed to diffuse into the film. This reaction-diffusion process resulted in patterns that were different in some respects from that of the regular Liesegang phenomena often occurring in such systems. Instead of stripes, tree-like precipitate structures have developed. Digital image analysis provided evidence that the tree-like structures involve a periodicity of the Liesegang type. Occurrence of this periodicity was found not only parallel but also perpendicular to the direction of the diffusion. Computer simulations were carried out in order to find possible mechanisms for these variants of the Liesegang phenomenon and to establish the influence of various factors on the chemical pattern formation involved in the model.     

Fully coordinated silica nanoclusters: (SiO2)N molecular rings

A new form of finite silica with edge-sharing SiO2 units connected in a ring is proposed. High-level density-functional calculations for (SiO2)(N), N=4-14, show the rings to be energetically more stable than the corresponding (SiO2)(N) linear chains for N>11. The rings display frequency modes in remarkable agreement with infrared bands measured on dehydrated silica surfaces indicating their potential as models of strained extended silica systems. Silica rings, if synthesized, may also be useful precursors for new bulk-silica polymorphs with tubular or porous morphologies.     

α-异噁唑偶氮基-β-二酮类衍生物及其互变异构体的光谱

研究了5种新型的具有不同结构的aα-异噁唑偶氮基-β-二酮类衍生物的红外光谱和紫外光谱，讨论了它们的酮式-烯醇式以及偶氮式．腙式之间的互变异构化现象。结果表明，所有化合物无论以固体形式或在溶液中均以腙式二酮体和偶氮式烯醇体的混合形式存在。各化合物的紫外吸收谱均呈双峰状，峰值分别处于246～262nm和326～339nm之间，其中，高波长吸收峰的强度明显高于低波长吸收峰强度，说明了各化合物中偶氮式烯醇体形式均多于腙式二酮体形式，其原因是由于各化合物中偶氮式烯醇体的异构体较多，且易以分子内氢键形式形成六元环，有利于结构稳定。     

Effect of the symmetry of H2 molecules on their rotations around an OCS molecule in superfluid 4He droplets

The infrared spectra of OCS-(H2)(n) clusters in cold (0.15 K) superfluid 4He droplets coated with 3He exhibit resolved rotational bands for each n up to n=8 para-H2 (pH(2)) or ortho-D2 (oD(2)) molecules. An analysis of the different Q-branch intensities based on the different spin symmetries of pH(2) and oD(2) indicates the formation of symmetric 5- or 6-membered rings around the linear carbonyl sulfide (OCS) chromophore. The rings of distinguishable oD(2) are found to undergo axial rotations, whereas for 6 pH(2) molecules the symmetry-allowed rotational levels lie too high to be excited at the 0.15 K droplet temperatures.     

Resonant transport and giant persistent currents in double-asymmetric rings

Occurrence of resonant transport due to Fabry-Perot like effect through cascade double asymmetric rings is proposed. We find that high performance reflectors with wide bands can be generated by two asymmetrical open rings with an arm ratio of 1/3. At the resonances, giant persistent currents exist in both rings of the system. The resonance energy is variant and dependent on the length of the connected wire. However, the confined space of the system at the resonance is not equal to the integral multiple of a half-wavelength of traditional Fabry-Perot etalons.     

Two-dimensional electron gas in a periodic magnetic field

We study the energy spectrum and electronic properties of a two-dimensional (2D) spinless electron gas in a periodic magnetic field which has the symmetry of a triangular lattice. We show that the energy bands depend strongly on the value of the magnetic field. For large field the low-energy electrons are localized on closed rings where the magnetic field vanishes. This results in the appearance of persistent currents around these rings. We also calculate the intrinsic Hall conductivity, which is quantized when the Fermi level is in a gap.     

红外吸收光谱法研究磁控溅射沉积SiOx非晶薄膜的过程

利用磁控溅射技术,在单晶Si衬底上生长了SiOx非晶薄膜.傅里叶变换红外吸收光谱(FTIR)显示,SiOx非晶薄膜存在3个吸收谱带.研究发现,随着溅射功率的提高,薄膜中先后形成Si-Oy-Si4-y(0＜y≤4),Si6环以及无桥氧空位中心(NBOHC)缺陷等结构,这几种结构对应的Si-O-Si键的伸缩振动吸收、非对称伸缩振动吸收以及O-Si-O键的振动吸收是导致薄膜的FTIR光谱出现3个吸收谱带的根本原因.     

Optical spectroscopy on Se clusters and chains confined in zeolites

The incorporation of selenium into the supercages of zeolite Y leads to the formation of Se ₈ rings and distorted Se chains in a ratio which is influenced by the nature of the zeolitic cations. Here we review Raman (including resonance Raman) and UV/vis absorption spectroscopy results on Se encapsulated into a number of cation-exchanged faujasite zeolites. Both rings and chains give rise to characteristic Raman bands. In particular, low-frequency Raman bands are attributed to localized vibrations in ordered segments of distorted chains. The UV/vis absorption spectra indicate an opening of the band gap of selenium upon confinement in these zeolites. This can be reversed through electronic interaction with zeolite cations.Received: 1 January 2003, Published online: 14 October 2003PACS: 78.30.-j Infrared and Raman spectra - 82.75.Mj Measurements and simulation of properties (optical, structural) of molecules in zeolites     

Liesegang patterns: Complex formation of precipitate in an electric field

Formation of 1D Liesegang patterns was studied numerically in precipitation and reversible complex formation of precipitate scenarios in an electric field. The Ostwald’s supersaturation model reported by Buki, Kárpáti-Smidróczki and Zrínyi (BKZ model) was extended further. In the presence of an electric field the position of the first and the last bands (X _n) measured from the junction point of the outer and the inner electrolytes can be described by the functionX _n =a ₁τ _n ^1/2 +a ₂τ_n +a ₃, where τ_n is the time elapsed until the nth band formation,a ₁,a ₂ anda ₃ are constants. The variation of the total number of bands with different electric field strengths (ε) has a maximum. For higher ε one can observe a moving precipitation zone that becomes wider due to precipitation and reversible complex formation.     

Ultrasound velocimetry in a shear-thickening wormlike micellar solution: Evidence for the coexistence of radial and vorticity shear bands

We carried out pointwise local velocity measurements on 40 mM cetylpyridinium chloride-sodium salicylate (CPyCl-NaSal) wormlike micellar solution using high-frequency ultrasound velocimetry in a Couette shear cell. The studied wormlike solution exhibits Newtonian, shear-thinning and shear-thickening rheological behavior in a stress-controlled environment. Previous rheology, flow visualization and small-angle light/neutron scattering experiments in the shear-thickening regime of this system showed the presence of stress-driven alternating transparent and turbid rings or vorticity bands along the axis of the Couette geometry. Through local velocity measurements we observe a homogeneous flow inside the 1mm gap of the Couette cell in the shear-thinning (stress-plateau) region. Only when the solution is sheared beyond the critical shear stress (shear-thickening regime) in a stress-controlled experiment, we observe inhomogeneous flow characterized by radial or velocity gradient shear bands with a highly sheared band near the rotor and a weakly sheared band near the stator of the Couette geometry. Furthermore, fast measurements performed in the shear-thickening regime to capture the temporal evolution of local velocities indicate coexistence of both radial and vorticity shear bands. However the same measurements carried out in shear rate controlled mode of the rheometer do not show such rheological complexity.     

LiKSO4单晶体内体位相光栅引起的光衍射现象

本文报道光通过LiKSO₄单晶体产生的有规律性的衍射现象,衍射环和衍射带。观察了不同实验条件对衍射现象的影响,可以肯定这些现象是波矢k垂直于c轴的体位相光栅所产生。伴随衍射环内外各出现副环的来源尚未能理解。 关键词：