Search results for "spheres"

Two-Step Crystallization Kinetics in Colloidal Hard-Sphere Systems

2005

The crystallization kinetics of colloidal hard spheres was studied using a special Bragg spectrometer with high sensitivity. In contrast with the classical scenario we observe a two-step nucleation process: the number of crystallites increases slowly at early times, followed by a dramatic reduction at intermediate times, prior to undergoing a rapid increase at late times. We explain these results in terms of a polydispersity limited growth of crystallites, where the crystallization at early times is governed by local fractionation processes, leading to a long delay prior to final crystallization.

2015

In many technical applications, but also in natural processes like ice nucleation in clouds, crystallization proceeds in the presence of stresses and flows, hence the importance to understand the crystallization mechanism in simple situations. We employ molecular dynamics simulations to study the crystallization kinetics of a nearly hard sphere liquid that is weakly sheared. We demonstrate that shear flow both enhances and suppresses the crystallization kinetics of hard spheres. The effect of shear depends on the quiescent mechanism: suppression in the activated regime and enhancement in the diffusion-limited regime for small strain rates. At higher strain rates crystallization again become…

ChemInform Abstract: Ta1.09Fe2.39Te4, a New Non-Stoichiometric Ternary Tantalum Telluride.

2010

Abstract Ta1.09Fe2.39Te4 was prepared by chemical transport from the elements in sealed silica tubes in a temperature gradient from 700 to 600 °C. It crystallizes in the monoclinic space group P2/m with a = 6.162(2) A , b = 7.852(3) A , c = 7.250(3) A , β = 95.32(3)° and Z = 2 . Its structure can be derived from a hexagonal close packing of tellurium atoms with tantalum and iron atoms in octahedral voids and additional iron atoms in tetrahedral voids. The structure is closely related to the structures of MM'Te2 (MNb, Ta; M′Fe, Co, Ni) and MxFeγTe2 (MNb, x = 0.89, γ = 0.93; MTa, x = 0.77, γ = 0.90).

Blending pieces of Dupin cyclides for 3D modeling and reconstruction : study in the space of spheres

2013

The thesis deals with the blending of canal surfaces in geometric modeling using pieces of Dupin Cyclides. We try to solve a problem of reconstructing real parts manufactured and controlled by the CEA of Valduc. Using the space of spheres in which we can manipulate both points, spheres and canal surfaces, we simplify some problems. This space is represented by a 4-dimensional quadric in a 5-dimensional space, equipped with the Lorentz form, it is the Lorentz space. In the space of spheres, problems of blending canal surfaces by pieces of Dupin cyclides are simplified in linear problems. We give algorithms to make such blends using the space of spheres and after we come back to 3 dimensions …

Chińska aktywność na Bałkanach w XXI wieku i jej implikacje

2022

Celem artykułu jest szkicowe przedstawienie, na podstawie analiz i materiałów prasowych, wielosektorowej obecności Chin na Bałkanach i jej wpływu na sytuację ekonomiczną regionu. Bałkany są ważne dla mocarstw ze względu na położenie geopolityczne i geostrategiczne na skrzyżowaniu szlaków międzykontynentalnych. Chiny znacząco wpływają na ekonomikę poszczególnych państw region, wykorzystując dyplomację publiczną celem ograniczenia wpływów ze strony innych ważnych graczy: Unii Europejskiej, Rosji i USA. Autorka przedstawia hipotezę, że Chiny, tak jak USA pod koniec XX wieku, zmierzają do stworzenia sfery wpływów na Półwyspie Bałkańskim, w kontekście swoich interesów globalnych. Bałkany są post…

Hard-sphere fluids in annular wedges: density distributions and depletion potentials.

2009

We analyze the density distribution and the adsorption of solvent hard spheres in an annular slit formed by two large solute spheres or a large solute and a wall at close distances by means of fundamental measure density functional theory, anisotropic integral equations and simulations. We find that the main features of the density distribution in the slit are described by an effective, two--dimensional system of disks in the vicinity of a central obstacle. For large solute--solvent size ratios, the resulting depletion force has a straightforward geometrical interpretation which gives a precise "colloidal" limit for the depletion interaction. For intermediate size ratios 5...10 and high sol…

Crossing the boundary between face-centred cubic and hexagonal close packed: the structure of nanosized cobalt is unraveled by a model accounting for…

2014

The properties of nanostructured cobalt in the fields of magnetic, catalytic and biomaterials depend critically on Co close packing. This paper reports a structural analysis of nanosized cobalt based on the whole X-ray diffraction (XRD) pattern simulation allowed by the Debye equation. The underlying structural model involves statistical sequences of cobalt layers and produces simulated XRD powder patterns bearing the concurrent signatures of hexagonal and cubic close packing (h.c.p. and f.c.c.). Shape, size distribution and distance distribution between pairs of atoms are also modelled. The simulation algorithm allows straightforward fitting to experimental data and hence the quantitative …

Vibrations of weakly coupled nanoparticles

2009

The vibrations of a coupled pair of isotropic silver spheres are investigated and compared with the vibrations of the single isolated spheres. Situations of both strong coupling and also weak coupling are investigated using continuum elasticity and perturbation theory. The numerical calculation of the eigenmodes of such dimers is augmented with a symmetry analysis. This checks the convergence and applicability of the numerical method and shows how the eigenmodes of the dimer are constructed from those of the isolated spheres. The frequencies of the lowest frequency vibrations of such dimers are shown to be very sensitive to the strength of the coupling between the spheres. Some of these mod…

The β-relaxation amplitudes for a dipolar hard sphere glass

1998

Abstract The β-relaxation amplitudes h m l , l ′ ( q ) for a dipolar hard sphere model are calculated in the diagonalization approximation up to l =1. The relevance to experimental difficulties to detect the characteristic β-relaxation minimum in dielectric loss measurements is discussed. Additionally, the microscopic nature of the β-relaxation of the rotational components is investigated by discussing the amplitudes in real space at different temperatures and densities.

Non-Monotonic Concentration Dependence of the Electro-Phoretic Mobility of Charged Spheres in Realistic Salt Free Suspensions

2020

Using super-heterodyne Doppler velocimetry with multiple scattering correction, we extend the opti-cally accessible range of concentrations in experiments on colloidal electro-kinetics. We here meas-ured the electro-phoretic mobility and the DC conductivity of aqueous charged sphere suspensions covering about three orders of magnitude in particle concentrations and transmissions as low as 40%. The extended concentration range for the first time allows the demonstration of a non-monotonic con-centration dependence of the mobility for a single particle species. Our observations reconcile previ-ous experimental observations made on other species over restricted concentration ranges. We com-par…