Reduction Of Surface Free Energy Research Articles

金属薄膜上における表面自由エネルギーに駆動された拡散・偏析と偏析表面の材料としての利用

金属薄膜上における表面自由エネルギーに駆動された拡散・偏析と偏析表面の材料としての利用

The clinical meaning of the surface roughness and the surface free energy of intra-oral hard substrata on the microbiology of the supra- and subgingival plaque: results of in vitro and in vivo experiments

In the oral cavity, which may be considered as an open growth system, most bacteria can only survive if they adhere to the hard surfaces (teeth, filling materials, dental implants, or prostheses). Such bacterial adhesion occurs in four phases: transport to the surface, initial adhesion with a reversible and irreversible stage, attachment by specific interactions and finally colonization. During this process the roughness and the free energy of the surfaces play a key role. The reduction in roughness of a surface will result in a dramatic retardation of plaque formation and maturation. A reduction in surface free energy of the substratum will result in a decrease in plaque growth rate, a decrease in plaque retention capacity of the surface and in the selection of specific organisms. Although both parameters interact with each other, the influence of surface roughness is dominant. The importance of both parameters justifies the demand for smooth surfaces with a low surface free energy in order to prevent plaque formation, thereby reducing the occurrence of gingival inflammation.

Influence of surfactants on the growth-kinetics of Si on Si(111)

We compare the Si/Si(111) homoepitaxy with the growth modified using Sb, As and Ga as surfactants. STM measurements show an increased density of 2D-islands for surfactant-mediated epitaxy, i.e. the surface diffusion of Si is reduced. The surfactant-modified epitaxy which is energetically driven by the reduction of surface free energy has a strong influence on growth-kinetics. This kinetically limited growth can explain the suppression of 3D-islanding in the case of heteroepitaxial growth. The size of the critical nucleus and activation energy for Si/Si(111) surface diffusion are determined.

Influence of surfactants on the growth-kinetics of Si on Si(111)

We compare the S1/S1(111) homoepitaxy with the growth modified using Sb, As and Ga as surfactants STM measurements show an increased density of 2D-islands for surfactant-mediated epitaxy, i e the surface diffusion of S1 is reduced. The surfactant-modified epitaxy which is energitacally driven by the reduction of surface free energy has a strong influence on growth-kinetics. This kinetically limited growth can explain the suppression of 3D-islanding in the case of heteroepitaxial growth. The size of the critical nucleus and activation energy for S1/S1(111) surface diffusion are determined.

Characterization of nanoporosity in polymer-derived ceramic fibres by X-ray scattering techniques

Ceramic fibres with Si-C-O and Si-N-C-O compositions, prepared by pyrolysis of polymer precursors, generally have densities lower than those calculated from a volume additivity rule. However, techniques often used to detect porosity such as electron microscopy methods, surface area and porosimetry measurements show that little surface-connected porosity is present. X-ray scattering measurements, both wide-angle (WAXS) and small-angle (SAXS), show considerable scattering in the range 1° < 2θ < 10° (CuKα). Treatment of the scattering data by the classical Guinier (low angle limit) and Porod (high angle limit) methods indicate that closed, globular, nanometre-scale porosity (1 to 3 nm diameter) is present in all ceramic fibres examined. X-ray scattering power correlates quantitatively with the volume fraction porosity, as expected if porosity is the dominant facto affecting X-ray scattering. Nano-particles of excess carbon and ofβ-SiC nanocrystallites, though present, are minor contributors to the scattering of X-rays in these ceramic fibres. Fibres are three-dimensional, not of fractal dimension, and are not oriented. As density increases with increasing pyrolysis temperature, average pore size increases and pore volume fraction decreases. This results from a thermodynamically favourable reduction of surface free energy and apparently occurs by a viscous flow process.

Self-similarity and growth kinetics driven by surface free energy reduction

We consider a configuration of interfaces (area S) moving under the driving force of interfacial free energy reduction in a large volume V. The time dependence of the linear scale l = V S of the configuration is deduced for systems with a single or multicomponent order parameter, conserved or non-conserved, by use of the hypothesis of statistical self-similarity: specifically, we assume that statistical parameters that are invariant under uniform magnification are also independent of time and that local equilibrium of the intersection angles of interfaces holds along lines of intersections (if any). This hypothesis, together with the scaling characteristics of interface velocities in the system under consideration, is sufficient to yield the result l( t) ~ t n with n > 0; in particular, we show that n = 1 2 for curvature driven growth and n = 1 3 for diffusion limited coarsening. We conclude that these values are independent of many of the approximations and restrictions that characterize specific models (e.g. over-simplifications of geometry, mean field approximations, small volume fraction of precipitates). Free energy functional models that exhibit the above growth laws are also discussed.

Anisotropic surface energy reduction of Zr by chemisorption

The anisotropy of the surface free energy reduction of Zr due to gas adsorption was studied by calculating the surface energies of crystal planes at solid-vapour interfaces. The Langmuir adsorption isotherm was adopted for the calculation, whereby the anisotropy was introduced from the nearest-bond energy approximation based on a pairwise interaction model. The (101̄0) plane was found to have the lowest relative surface energy in the unembrittled state, that of the (0001) plane being slightly higher. Gas adsorption, however, influences the energy state significantly, and the effective surface free energy after adsorption becomes lowest for the (0001) plane, followed by the ( 10 1 ̄ 1 ) plane family. This tendency became more pronounced as the coverage increased. Energy maxima occurred in the planes that were inclined about 30–50° and 70–75°, respectively, to the basal plane. The mode of spatial distribution of the surface energies at 350°C on stereographic triangles was sensitive to a coverage higher than 0.5; the effect of temperature in the range of 250–450°C, however, was relatively insignificant.

The effect of surfactants on the motility and adhesion of gliding bacteria

Surfactants markedly inhibited swarming and gliding motility but not other movements nor the growth of Cytophaga U67. The swarming of several other gliding bacteria was also diminished. Inhibition of translocation may result from the fact that the surfactants blocked adhesion of these bacteria. In contrast, the gliding of Oscillatoria princeps was unaffected by reduction of surface free energy.

The effects of aqueous chemical environments on crack propagation in quartz

The chemical role of water and other aqueous environments in crack propagation was examined in a series of crack propagation tests in natural and synthetic quartz. The potential roles of surface free energy reduction and zeta potential (surface electrostatic potential) in chemical weakening were specifically examined by running the crack propagation tests in the presence‐of six surface active aqueous environments which varied sub‐stantially in these parameters with respect to quartz. Calorimetric and electrophoresis tests were also undertaken in order to determine the degree of reduction of the surface energy of quartz produced by each chemical environment and the zeta potential between each environment and quartz. It was found that there was a moderate correlation between reduction of the crack propagation stress of quartz and the degree by which the surface energy was reduced in the presence of a particular environment. No such correlation was found with respect to zeta potential. It was also observed that some of the chemical environments appeared to produce highly branched cracks in quartz. This branching effect is probably related to the velocity of crack propagation.

Alloying, chemisorption and hydrogen embrittlement of steel

The chemisorption activity of iron was related to the embrittlement character of hydrogen. Theoretical and experimental considerations indicate that substitutional (and interstitial) impurities are unlikely to modify the heat of adsorption to any significant degree. This implies that the reduction in surface free energy induced via the adsorption of hydrogen is essentially the same in iron and its alloys. A Griffith analysis then suggests that alloying will be ineffective in ameliorating this aspect of hydrogen embrittlement. This is contrasted with the indigenous influence of alloying (which may itself change the surface free energy or modify the dominant fracture mode). The modification of the dominant fracture mode, in concert with the essentially unchanged embrittling effect of hydrogen, may strongly influence the net severity of the embrittlement problem.

Changes in morphology of ionic crystals due to capillarity

The problem of mass transport in ionic crystals to reduce surface free energy is discussed. The treatment follows the previous work of Mullins on one-component systems, for surfaces which are nearly planar. The present analysis includes the existence of two interdependent ionic fluxes and of the space charge layer in the vicinity of the surface.

Faceting of copper surfaces at 1000°C

Polished copper surfaces were observed to facet on annealing at 1000°C in wet hydrogen atmospheres. It was found that the quantity hi p H 2O p H 2 must be greater than zero for extensive faceting to occur, though some faceting, particularly at the sample edges, can occur for In p H 2O p H 2 as small as −3.5. The criteria for faceting to occur by the reduction of surface free energy are discussed and it is shown that facets near the (111) orientation are thermodynamically stable. The effect of anisotropy of surface free energy on facet nucleation is discussed. The outlines of lens-shaped facets were measured. The shapes and sizes of the lens-shaped facets are consistent with a volume diffusion mechanism of facet formation. The observed profiles of the curved surfaces near separated linear facets are compared to theoretically calculated profiles. The calculations by Mullins of facet profiles are extended slightly to facilitate the comparison. The observed profiles are in reasonable agreement with the calculated profiles for facet formation by surface diffusion with the surface diffusion coefficient the same on both the low index and the complex surfaces.

ADSORPTION OF BENZENE AND ETHANOL UP TO HIGH RELATIVE PRESSURES ON FINELY DIVIDED SODIUM CHLORIDE: PART I. PARTICLE SIZE EFFECTS AND THE ADSORPTION OF BENZENE: PART II. THE LOWERING OF SURFACE FREE ENERGY BY ETHANOL

Adsorption of benzene has been studied using four samples of fine sodium chloride. The range of relative pressures employed extended to 0.99. The data were employed to show that a correction to the relative pressure should be applied because of the curvature of the surface. The correction was made in the form of a reduction of the apparent relative pressure by application of the Kelvin equation, since the relative pressure over a convex surface would be less than over a plane surface. The adsorption data at high relative pressures for several samples of salt could then be represented by a common curve. It was further concluded that the thick-layer theory of adsorption due to Frenkel, Halsey, and Hill was applicable to adsorption on salt. Adsorption data for ethanol were then obtained and the reduction of surface free energy of the salt by the saturated vapor was evaluated. This figure was then combined with van Zeggeren's and Benson's value of the solid–liquid interfacial free energy for salt and ethanol to provide a provisional value of the surface free energy of sodium chloride of 227 ergs cm−2.

The thermal etching of silver

The mechanism whereby grain boundaries are delineated and striations formed on polished surfaces of heated metal specimens has been examined. Experiments on electrolytically polished silver show that grooves form at grain boundaries at temperatures as low as 300° C and striations at 500° C in air. Striations only appear in the presence of oxygen and may be removed by heating in nitrogen. A furnace for high-temperature photomicrography, suitable for specimen temperatures up to about 950° C, is described. Previous theories are found inadequate to explain the effects observed in silver, and a theory which regards the surface etching as an approach to equilibrium by the reduction of surface free energy is suggested. Thus the equilibrium condition of the boundary is a groove whose shape is determined by the relative magnitudes of the free energy per unit area of the boundary and the surface energies of the crystalline faces meeting the boundary in the surface of the specimen. The striations are caused by the development of those crystalline planes having the lowest free energy; the relative surface energies of different planes being modified by the presence of oxygen. It is suggested that the chief means whereby the boundary grooves are formed is that of surface migration of ions: both surface migration of ions and evaporation of silver oxide molecules may be expected to play major parts in the formation of the striations.