Search results for " sphere"
showing 10 items of 404 documents
Prehnite: Structural similarity of the monoclinic and orthorhombic polymorphs and their Si/Al ordering
1990
Abstract Two modifications of the naturally occurring mineral prehnite, Ca 2 Al(Si 3 AlO 10 )(OH) 2 , are characterized by precise single crystal diffraction methods. The two forms are topologically identical. They differ only in the siting of one Al and one Si atom per unit cell: these switch their places in two of the tetrahedrally coordinated sites. This appears to be the only known case where a silicate occurs with two differently ordered tetrahedral distributions of Al and Si atoms. The two forms are not polytypes according to the current definition of polytypes. The two modifications are so similar to each other that they differ only in the eighth coordination sphere.
Crystallization in suspensions of hard spheres: a Monte Carlo and molecular dynamics simulation study
2011
The crystallization of a metastable melt is one of the most important non-equilibrium phenomena in condensed matter physics, and hard sphere colloidal model systems have been used for several decades to investigate this process by experimental observation and computer simulation. Nevertheless, there is still an unexplained discrepancy between the simulation data and experimental nucleation rate densities. In this paper we examine the nucleation process in hard spheres using molecular dynamics and Monte Carlo simulation. We show that the crystallization process is mediated by precursors of low orientational bond-order and that our simulation data fairly match the experimental data sets.
Hard sphere fluids at a soft repulsive wall: A comparative study using Monte Carlo and density functional methods
2011
Hard-sphere fluids confined between parallel plates at a distance D apart are studied for a wide range of packing fractions including also the onset of crystallization, applying Monte Carlo simulation techniques and density functional theory. The walls repel the hard spheres (of diameter σ) with a Weeks-Chandler-Andersen (WCA) potential V(WCA)(z) = 4ε[(σ(w)/z)(12) - (σ(w)/z)(6) + 1/4], with range σ(w) = σ/2. We vary the strength ε over a wide range and the case of simple hard walls is also treated for comparison. By the variation of ε one can change both the surface excess packing fraction and the wall-fluid (γ(wf)) and wall-crystal (γ(wc)) surface free energies. Several different methods t…
Nucleation in suspensions of anisotropic colloids
2005
We report Monte Carlo studies of liquid crystal nucleation in two types of anisotropic colloidal systems: hard rods and hard ellipsoids. In both cases we find that nucleation pathways differ strongly from the pathways in systems of spherical particles. Short hard rods show an effect of self-poisoning. This part of the article is based on a previous publication [T. Schilling, D. Frenkel, Self-poisoning of crystal nuclei in hard-rod liquids, Phys. Rev. Lett. 92 (2004) 085505]. When a crystallite forms, its surfaces are covered preferentially by rods which align perpendicular to the surface. Therefore subsequent growth is stunted. Hard, almost spherical ellipsoids can be compressed to very hig…
Heterogeneous and homogeneous crystal nucleation in colloidal hard-sphere like microgels at low metastabilities
2011
We studied the crystallization kinetics of the homogeneous bulk as well as of the heterogeneous wall crystallization in a colloidal hard sphere model system within the fluid crystal coexistence region. Using time resolved static light scattering and Bragg microscopy we determined several parameters characterizing the crystallization process (induction times, growth coefficients, nucleation rate densities,…). By performing further analysis according to classical nucleation theory we can show that wall crystallization is favored by a vanishing nucleation barrier height leading to the conclusion that a hard sphere crystal completely wets a structureless hard wall.
Crystallization kinetics of colloidal binary mixtures with depletion attraction.
2014
In this work the crystallization kinetics of colloidal binary mixtures with attractive interaction potential (Asakura–Oosawa) has been addressed. Parameters such as fraction of crystals, linear crystal dimension and crystal packing have been quantified in order to understand how the crystal formation is driven in terms of the depth of the attractive potential and the composition of the binary mixture (described by the number ratio). It was found that inside the eutectic triangle, crystallization is mainly governed by nucleation and the crystal packing is close to the close-packing of hard spheres. Moving out from the eutectic triangle towards small component results in the crystallization o…
Solidification of a colloidal hard sphere like model system approaching and crossing the glass transition
2014
We investigated the process of vitrification and crystallization in a model system of colloidal hard spheres. The kinetics of the solidification process was measured using time resolved static light scattering, while the time evolution of the dynamic properties was determined using time resolved dynamic light scattering. By performing further analysis we confirm that solidification of hard sphere colloids is mediated by precursors. Analyzing the dynamic properties we can show that the long time dynamics and thus the shear rigidity of the metastable melt is highly correlated with the number density of solid clusters (precursors) nucleated. In crystallization these objects convert into highly…
Crystallizing polystyrene microgel colloids
2007
Spherical microgel particles of sufficiently high degree of internal cross-linking and swollen in a good solvent in many respects behave quite similarly to hard-sphere colloids. Due to solvent uptake they can be refractive-index-matched and density-matched in suitable organic solvents. We present preliminary measurements of the crystallization kinetics of 1:10 cross-linked polystyrene microgel particles. We measured Bragg and small-angle light scattering of the solidifying shear melt. Two different scattering patterns, a set of Debye-Scherrer rings and a second ring pattern at small angles could be observed. We check for similarities and differences compared to previously investigated collo…
Solvent hydrodynamics speed up crystal nucleation in suspensions of hard spheres
2014
We present a computer simulation study on the crystal nucleation process in suspensions of hard spheres, fully taking into account the solvent hydrodynamics. If the dynamics of collodial crystallization were purely diffusive, the crystal nucleation rate densities would drop as the inverse of the solvent viscosity. However, we observe that the nucleation rate densities do not scale in this way, but are enhanced at high viscosities. This effect might explain the large discrepancy between the nuclation rate densities obtained by simulation and experiment that have reported in the literature so far.
Precursor-mediated crystallization process in suspensions of hard spheres.
2010
We report on a large scale computer simulation study of crystal nucleation in hard spheres. Through a combined analysis of real and reciprocal space data, a picture of a two-step crystallization process is supported: First dense, amorphous clusters form which then act as precursors for the nucleation of well-ordered crystallites. This kind of crystallization process has been previously observed in systems that interact via potentials that have an attractive as well as a repulsive part, most prominently in protein solutions. In this context the effect has been attributed to the presence of metastable fluid-fluid demixing. Our simulations, however, show that a purely repulsive system (that ha…