A unified model description of mobile and localized adsorption: I. MFA with nonadditive lateral interactions — an application to disordered adsorbed monolayer on a structureless substrate

A simple model is used to treat mobile adsorption on a structureless substrate. The influence of temperature on the state of motion of adsorbed particles is described by means of an interpolation formula used earlier in the theory of hindered rotation, whereas the influence of the nearest-neighbour potential with varying degree of coverage is accounted for in terms of a “free area”, available for individual particle motion, for which a simple analytical expression, based on an harmonic oscillator approximation, is obtained. An irregularity in the long range order of the adsorbed monolayer is introduced in terms of “varying distances”, resp. of varying energy bonds between the particles in dependence of the number of nearest neighbours actually present in the first coordination sphere of each particle (nonadditivity of the bonds).     

Fat fractals in the lung? A comment on a paper by grebogi et al.

An exponent characterising the scaling behaviour of a branching structure occupying volume (an example of a “fat” fractal), introduced recently by Grebogi et al., is examined using data on the arteries, veins and airways of mammalian lungs. The results cast doubt on the usefulness of the exponent in this context.     

First-principles studies: Thiolated Au2Cr and Au6Cr clusters

The structural, energetic and magnetic properties of thiol-passivating Au₂Cr and Au₆Cr clusters are investigated by performing first-principles calculation based on density functional theory. We find that the adsorption of thiolate is energetically more favorable than thiol and that the thiolates favor “top” site adsorption on Cr atom in one methanethiolate adsorbed Au₂Cr cluster while they favor “bridge” site adsorption on top of the middle point of Cr–Au bonds in three methanethiolates adsorbed Au₂Cr cluster. In thiol-passivating Au₆Cr cluster, the thiol favor “top” site adsorption on top of atom Au while the thiolate favor “bridge” site adsorption on top of the middle point of Au–Au bonds. The energetics of the reactions indicates that these thiol-passivating Au₂Cr or Au₆Cr clusters can be used as hydrogen storage materials. There are large and positive spin populations on atom Cr. The spin populations of atoms Au, S and H are attributed to both the spin delocalization and the spin polarization mechanisms.     

Adsorption of interacting monomers on spanning clusters of polyatomic species

The adsorption-desorption process occurring on heterogeneous surfaces is studied by considering a special case where a fractal is used as adsorbent. The fractal surface is the spanning cluster corresponding to the random deposition of objects that occupy more than one site (k-mers) on a square lattice. Such a surface is characterized according to the deposited k-mer. Then, the adsorption of repulsively interacting particles adsorbed on the fractal surface is studied by using Monte Carlo simulations. Different thermodynamic quantities (adsorption isotherms, coverage susceptibility, etc.) are calculated and explained in terms of the characteristics of the substrate. A scheme to characterize the structure of the substrate by just considering the adsorption isotherm is presented and discussed.     

LCAO studies of hydrogen chemisorption on nickel: II. Cluster models for adsorption sites

The adsorption of single hydrogen atoms, investigated by means of cluster calculations, has been compared with the adsorption of hydrogen monolayers on periodic crystals (paper I). From the similarity of the adsorption energy curves we conclude that the (direct and indirect) interactions between adsorbed hydrogen atoms are relatively small up to monolayer coverage. For adsorption on different sites of ideal low index surfaces the stability decreases in the order Atop > Bridge > Centred. For Atop adsorption it increases with a decreasing number of nearest neighbours to the nickel atom in the NiH “surface molecule”, thus leading to especially strong adsorption sites at the edges of a stepped surface and to low stability in the notches. In general, we find that the Ni_nH “surface molecule” with n = 1, 2, 3 or 4 determines the equilibrium positions for H adsorption; the inclusion of one shell of neighbours to the nickel atoms is sufficient to explain the differences in adsorption energy. The Extended Hückel method is not well suited to study dissociative chemisorption of H₂, although some qualitative trends are correct.     

Cluster structure of fullerene-containing soot and C60 fullerene powder

Electron microscopy and electron diffraction methods were employed in a study of the structure of a fullerene-containing soot produced in gas discharge and of a C₆₀ fullerene powder. The data obtained were analyzed with the use of fractal geometry concepts. It has been shown that, in the structure of the objects studied, several spatial scales can be identified. The effective radius of the structure’s “elementary particles” calculated using scaling relationships is equal to 6 Å for the soot and 4.5 Å for the C₆₀ fullerene. The “elementary particles” combine into associates. The number of particles in an associate in both the soot and the C₆₀ powder is not large (about 10). The associates form fractal nanoclusters 30–80 nm in size having a fractal dimension of 1.60±0.05 in the soot and of 1.8±0.05 in the C₆₀ fullerene. The structure of the soot nanocluster is unstable and can be significantly modified by externally applied factors (e.g., as a result of treatment with toluene). The nanoclusters combine into aggregates having the form of branching cross-linked filaments. Eventually, these aggregates combine to form macroparticles of soot.     

Topics in surface structure and crystal growth

Crystal growth occurs following widely different modes; in particular the transition from layer-by-layer to continuous growth, and the transition from stable to dendritic growth have attracted much theoretical attention. The former transition (corresponding, for equilibrium surfaces, to the roughening transition) is considered from the point of view of atom-surface scattering used as a tool to probe the state of the surface, and the effects of roughness on the height, width and shape of the incoherent peaks are studied. As an approach to the latter transition, simulations of cluster growth are considered where dendritic shapes are obtained by letting “atoms” fall along random straight trajectories from a “vapour” onto the growing cluster. Two geometries are considered: either the “atoms” fall along the cords from an outer large circle, or they fall obliquely on to a line from points of a parallel line. In both cases ramified structures are obtained, although less open that from diffusion-limited aggregation (DLA), and their fractal nature is studied.     

Focusing of an oscillating shock wave emitted by a toroidal bubble cloud

An analysis of pressure-field dynamics is performed for an axially symmetric problem of interaction between a shock wave and a “free” bubble system (toroidal cluster) giving rise to a steady oscillating shock wave. The results of a numerical study of near-axis wave structure are presented for a focusing shock wave emitted by a bubble cluster. It is shown that the wave reflected from the axis has irregular structure. The Mach disk developing on the axis has a core of finite thickness with a nonuniform radial pressure distribution. The evolution of the Mach-disk core is analyzed, and the maximum pressure in the core is computed as a function of the gas volume fraction in the cluster. The effect of geometric parameters of the toroidal bubble cloud on the cumulative effect is examined.     

Thermodynamically stable fractal-like domain structures in magnetic films

A fractal-like structure of the domain boundaries was revealed in “overcritical” uniaxial Permalloy magnetic films. The fractal dimension of domain boundaries at the film surfaces was determined as a function of the film thickness. It is shown that the phase transition between the two possible types of fractal-like structures is accompanied by a jump in fractal dimension.

     

Relationship Between Physical and Mechanical Properties of a Polymer Composite and Fractal Dimension of Structural Elements of its Surface

Russian Physics Journal - The paper shows the promising use of the “fractal dimension” parameter for qualitative and quantitative analysis of the surface structure of samples based on...     

Far-infrared absorption in gas-evaporated Al particles: Effects of a fractal structure

We analyze far-infrared absorption in gas evaporated Al particles. Electron microscopy showed that the samples have a fractal structure for length scales below a correlation length. By comparing the experimental complex dielectric function with Bergman-Milton bounds for Al₂O₃-coated particles we extracted a structural parameter which could be reconciled with computations based on the fractal structure. Thus we can explain the allegedly “anomalous” far-infrared absorption semi-quantitatively in terms of particle aggregation.     

Effect of pyrogallol-poly(ethylene glycol) on stabilization,structure, and interparticle potential of concentrated nickel nanoparticle suspensions

Nickel nanoparticles with an average diameter of 90 nm have been dispersed in de-ionized water with addition of pyrogallol-poly(ethylene glycol) polymers, hereafter termed Gallol-PEG, of different molecular weights as a surfactant. Measurement of zeta potential, infrared spectrum, and adsorption isotherm confirms the preferential anchoring from polar end of the surfactant molecules on the particle surface, forming a Langmuir-typed adsorption layer (adlayer) to provide an electrosteric stabilization. Concentrated nanoparticle suspensions with a solids loading up to 40 vol.% and an apparent viscosity lower than 10 Pa s at a shear rate of 100 s⁻¹ have been obtained, indicating that the Gallol-PEG adsorption is effective in facilitating the suspension flow under stress. The suspensions are yet fractal in structure with an experimentally determined fractal dimension of 2.1, revealing that a reaction-limited cluster–cluster aggregation is operative. This weakly coagulated fractal structure stems primarily from the shallow interparticle attraction operative over a moderate interparticle separation (~5–10 nm), and is prone to the adlayer thickness and the molecular conformation of the surfactant.     

Flory theory of polymeric fractals - intersection,saturation and condensation

We consider swelling effects of polymeric fractals, recently introduced by Cates, by usual simple Flory arguments for the free energy. The Flory arguments can be formulated to give a unified view for all polymers, linear, branched, or percolation clusters, as long they are of fractal connectivity.If the size of solvent molecules, being fractals themselves, is comparable to the given cluster, new values of the fractal dimensions can be found. The upper critical dimension is reduced. This is due to usual screening of the excluded volume. By standing overlap and repulsive energies of fractals of different fractal dimensions we find condensation to non-fractal objects depending on the value of the fracton dimension. A melt of polymeric fractals of the same fractal dimension and the same size becomes compact if the spectral dimension exceeds a “critical” value.These considerations are of relevance concerning recent experiments, considering static and dynamic properties of mixtures of microgels and linear polymers of different or equal sizes.     

Synthesis of gold nanostars with fractal structure: application in surface-enhanced Raman scattering

Multi-branched gold nanostars with fractal feature were synthesized using the Triton X-100 participant seed-growth method. By increasing the amount of ascorbic acid, the branch length of gold nanostars could be greatly increased. It has been interesting to find that the secondary growth of new branches takes place from the elementary structure when the aspect ratio of the branches is greater than 8.0 and the corresponding plasmon absorption wavelength is greater than 900 nm. Raman activity of the gold nanostar films has been investigated by using the 4-mercaptobenzoic acid (4-MBA) as Raman active probe. Experimental results show that the surface-enhanced Raman scattering (SERS) ability of the gold nanostars could be efficiently improved when the fractal structure appears. The physical mechanism has been attributed to the intense increased secondary branch number and the increased “hot spots”. These unique multi-branched gold nanostars with fractal feature and great SERS activity should have great potential in sensing applications.     

Experimental study of different modes of block sliding along interface. Part 2. Field experiments and phenomenological model of the phenomenon

The paper reports the results of field experiments on studying different modes of gravitational sliding of a block on the natural fault surface. Various materials were used as interface filler to model the whole range of deformation events that can be arbitrarily divided into three groups: accelerated creep, slow slip, and dynamic slip. The experiments show that the type of modeled deformation events is defined by both structural parameters of contact between blocks and material composition of the contact filler.Foundations for a new geomechanical model of occurrence of different-type dynamic events were developed. The model is based on the idea that “contact spots” form subnormally to the crack edges during shear deformation; the “spots” are clusters of force mesostructures whose evolution governs the deformation mode. The spatial configuration of “contact spots” remains unchanged during the entire “loading-slip” cycle but changes after the dynamic event occurrence. The destroyed force mesostructures can be replaced by similar structures under intergranular interaction forces when the external influence is fully compensated. Unless “contact spots” are incompletely destroyed, the deformation process dynamics is defined by their rheology. The migration of “contact spots” during deformation of a crack filled with heterogeneous material causes changes in deformation parameters and transformation of the mode itself due to changing rheology of local contact areas between blocks.It is found by fractal analysis that in order for dynamic slip to occur, spatially structured “contact spots” characterized by low fractal dimension must be formed; slow slip events can exist only in a certain parametric domain called the “dome of slow events”. It is found that the probability of slow slip occurrence is higher on fault regions characterized by maximum fractal dimension values: fault tips, fault branching and fault intersection zones.     

Étude par la diffraction des électrons lents et la spectroscopie auger de l'adsorption du soufre sur le platine

The adsorption of sulphur on the (111), (100) and (110) faces of platinum has been studied by LEED and Auger spectroscopy. By the combined use of radioactive S³⁵ the contents of the surface unit meshes were determined “in situ” for each state of adsorption. On the (111) face one observes a preliminary adsorption with localization of sulphur atoms at sites of maximum coordination of the metal. At higher pressures and temperatures, S-S distance contracts and a compact hexagonal arrangement, with sulphur atoms outside sites, is obtained. The thermal stability of the various structures was studied as a function of sulphur coverage. During adsorption on the (100) and (110) faces, two steps may be distinguished: the first one seems to correspond to an adsorption in sites, the second corresponds to the formation of a two-dimensional compound whose square mesh is slightly different on the two faces.     

Simulation of silicon nanoparticles stabilized by hydrogen at high temperatures

The stability of different silicon nanoparticles are investigated at a high temperature. The temperature dependence of the physicochemical properties of 60- and 73-atom silicon nanoparticles are investigated using the molecular dynamics method. The 73-atom particles have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene. They are surrounded by a “coat” from 60 atoms of hydrogen. The nanoassembled particle at the presence of a hydrogen “coat” has the most stable number (close to four) of Si–Si bonds per atom. The structure and kinetic properties of a hollow single-layer fullerene-structured Si₆₀ cluster are considered in the temperature range 10 K ≤ T ≤ 1760 K. Five series of calculations are conducted, with a simulation of several media inside and outside the Si₆₀ cluster, specifically, the vacuum and interior spaces filled with 30 and 60 hydrogen atoms with and without the exterior hydrogen environment of 60 atoms. Fullerene surrounded by a hydrogen “coat” and containing 60 hydrogen atoms in the interior space has a higher stability. Such cluster has smaller self-diffusion coefficients at high temperatures. The fullerene stabilized with hydrogen is stable to the formation of linear atomic chains up to the temperatures 270-280 K.