Modeling of nanostructuring burnishing on different scales

The operating characteristics of machine parts and units are defined in many respects by the physical and mechanical properties of their surface layers. Unfortunately, it is still not quite clear what parameters and mechanisms are responsible for one or another modification of surface layer properties. In this context, computer modeling techniques can be a useful tool in studying the variation of surface properties in contact interaction and run-in. Of fundamental importance is the possibility to consider the processes occurring on nanoscales and scales of individual atoms. In the work, loading conditions in plastic surface deformation was reproduced on the macroscale (traditional approach), atomic scale, and mesoscale by computer modeling with the finite element method, movable cellular automata method, and molecular dynamics method. The modeling results are in good qualitative agreement with data of experimental measurements.     

Modeling fast and slow earthquakes at various scales

Earthquake sources represent dynamic rupture within rocky materials at depth and often can be modeled as propagating shear slip controlled by friction laws. These laws provide boundary conditions on fault planes embedded in elastic media. Recent developments in observation networks, laboratory experiments, and methods of data analysis have expanded our knowledge of the physics of earthquakes. Newly discovered slow earthquakes are qualitatively different phenomena from ordinary fast earthquakes and provide independent information on slow deformation at depth. Many numerical simulations have been carried out to model both fast and slow earthquakes, but problems remain, especially with scaling laws. Some mechanisms are required to explain the power-law nature of earthquake rupture and the lack of characteristic length. Conceptual models that include a hierarchical structure over a wide range of scales would be helpful for characterizing diverse behavior in different seismic regions and for improving probabilistic forecasts of earthquakes.     

Capturing deviation from ergodicity at different scales

We address here the issue of quantifying the extent to which a given dynamical system falls short of being ergodic and introduce a new multiscale technique which we call the “ergodicity defect”. Our approach is aimed at capturing both deviation from ergodicity and its dependence on scale. The method uses ergodic theory of dynamical systems and applies harmonic analysis, in particular the scaling analysis is motivated by wavelet theory.We base the definition of the ergodicity defect on the Birkhoff characterization. We systematically exploit the role of the observation function by using characteristic functions arising from a dyadic equipartition of the phase space. This allows us to view the dependence of the defect on scale. In order to build intuition, we consider the defect for specific examples with known dynamic properties and we are able to explicitly compute the defect for some of these simple examples. We focus on three distinctive cases of the dependence of the defect on scale: (1) a defect value that increases as the scale becomes finer, (2) a defect value decreasing with scale and (3) a defect value independent of scale, which occurs for instance when a map is ergodic. We explain the information contained in these three scenarios. We see more complicated behavior with an example which has invariant subsets at various scales.     

Calculation of the effective diffusion coefficient for random wear surface migration on different scales

The paper considers the contact interaction of crystalline solids under shear deformation in the context of molecular dynamics. The interatomic interaction is specified by a potential calculated using the embedded atom method. The peculiarities of structural changes in a contact zone are studied for various materials of the contact pair. Based on the data extracted, the effective diffusion coefficient was estimated for random migration of the contact zone in a direction perpendicular to applied shear strains. The calculation results agree well with data of a microscopic contact model built around the method of movable cellular automata.     

Micrometer-scale deformation jumps at different stages of creep in solids

Laser interferometry is used to study micrometer-scale creep-strain nonuniformities (jumps) that occur during compression of metals (Ag, Al, Bi, Cu, Pb, Sn, Zn) and LiF: Mg crystals. The strain rate is found to vary periodically. The average magnitude of deformation L over one period and the variation of L with the total strain are determined. Correlations are found to exist between L and the Mg content in the LiF crystals, between L and the grain size in the metals, and between the magnitude of small jumps and the Burgers vector in the metals.     

Relaxation dynamics at different time scales in electrostatic complexes: time-salt superposition

In this Letter we show that in the rheology of electrostatically assembled soft materials, salt concentration plays a similar role as temperature for polymer melts, and as strain rate for soft solids. We rescale linear and nonlinear rheological data of a set of model electrostatic complexes at different salt concentrations to access a range of time scales that is otherwise inaccessible. This provides new insights into the relaxation mechanisms of electrostatic complexes, which we rationalize in terms of a microscopic mechanism underlying salt-enhanced activated processes.     

An empirical model for free surface energy of strained solids at different temperature regimes

We have developed an empirical formulation, based on the elastic theory, to calculate the variation of the surface free energy when a crystal is strained in the elastic regime. The model permits to obtain the variation of the surface energy at different strains and temperatures when are known the thermal dependence on the bulk and surface elastic constants. Molecular dynamics (MD) simulations were performed using the three low index surfaces of Al, to validate the accuracy of the model. The comparison between the empirical model and the MD simulations shows a good agreement for temperatures ranging between 0 and 900 K, and for deformation between −2% and 2%.     

Contribution of polarimetric imaging for the characterization of fibrous surface properties at different scales

The point in using polarimetric imaging for surface characterization is highlighted in this paper. A method for the evaluation of nonwoven surface properties at microscopic and macroscopic scales is described. This method is based on a polarimetric apparatus and various image processing operations are then performed depending on the studied scale. Polarimetric imaging applied to nonwovens, particularly degree of polarization imaging, highlights texture inhomogeneities. At both scales, image processing techniques were designed to analyze surface zones of different textures. At the macroscopic scale, a basic image processing was developed in order to detect the nonwoven manufacturing process defects. Moreover at the microscopic scale, i.e. at the fiber scale, image processing was adapted to evaluate fiber orientation within nonwovens, which is known to be an important information for mechanical behavior prediction.     

Mechanical heterogeneity of dentin at different length scales as determined by AFM phase contrast

In this study we sought to gain insights of the structural and mechanical heterogeneity of dentin at different length scales. We compared four distinct demineralization protocols with respect to their ability to expose the periodic pattern of dentin collagen. Additionally, we analyzed the phase contrast resulting from AFM images obtained in tapping mode to interrogate the viscoelastic behavior and surface adhesion properties of peritubular and intertubular dentin, and partially demineralized dentin collagen fibrils, particularly with respect to their gap and overlap regions. Results demonstrated that all demineralization protocols exposed the gap and overlap zones of dentin collagen fibrils. Phase contrast analyses suggested that the intertubular dentin, where the organic matrix is concentrated, generated a higher phase contrast due a higher contribution of energy dissipation (damping) than the highly mineralized peritubular region. At increasing amplitudes, viscoelasticity appeared to play a more significant contribution to the phase contrast of the images of collagen fibrils. The overlap region yielded a greater phase contrast than the more elastic gap zones. In summary, our results contribute to the perspective that, at different length scales, dentin is constituted of structural features that retain heterogeneous mechanical properties contributing to overall mechanical performance of the tissue. Furthermore, the interpretation of phase contrast from images generated with AFM tapping mode appears to be an effective tool to gain an improved understanding of the structure and property relationship of biological tissues and biomaterials at the micro- and nano-scale.     

Modelling transient heat conduction in solids at multiple length and time scales: A coupled non-equilibrium molecular dynamics/continuum approach

A method for controlling the thermal boundary conditions of non-equilibrium molecular dynamics simulations is presented. The method is simple to implement into a conventional molecular dynamics code and independent of the atomistic model employed. It works by regulating the temperature in a thermostatted boundary region by feedback control to achieve the desired temperature at the edge of an inner region where the true atomistic dynamics are retained. This is necessary to avoid intrinsic boundary effects in non-equilibrium molecular dynamics simulations. Three thermostats are investigated: the global deterministic Nosé–Hoover thermostat and two local stochastic thermostats, Langevin and stadium damping. The latter thermostat is introduced to avoid the adverse reflection of phonons that occurs at an abrupt interface. The method is then extended to allow atomistic/continuum models to be thermally coupled concurrently for the analysis of large steady state and transient heat conduction problems. The effectiveness of the algorithm is demonstrated for the example of heat flow down a three-dimensional atomistic rod of uniform cross-section subjected to a variety of boundary conditions.     

Force of attraction between solids with different temperatures

The methods of fluctuation electrodynamics based on the theory of molecular Van der Waals forces are used to obtain an expression for the density of attraction force between two absorbing media with different temperatures, which are separated by a nonabsorbing plane-parallel layer. The spectral density of the force was calculated as a projection of the Maxwell stress tensor on the outer normal to the solid surface. The mean square characteristics of the fluctuction field of the solids are obtained using the generalized Kirchhoff’s law and Green’s function of the corresponding regular problem. The solution versions are obtained depending on the relation between the temperatures of interacting solids. It is shown that for equal temperatures of the solids the expression obtained yields the formula for the attraction force in the case of equilibrium. Institute of Physics of Microstructures, Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 12, pp. 1495–1510, December, 1997.     

Probing nanoscale solids at thermal extremes

We report a novel nanoscale thermal platform compatible with extreme temperature operation and real-time high-resolution transmission electron microscopy. Applied to multiwall carbon nanotubes, we find atomic-scale stability to 3200 K, demonstrating that carbon nanotubes are more robust than graphite or diamond. Even at these thermal extremes, nanotubes maintain 10% of their peak thermal conductivity and support electrical current densities approximately 2 x 10{8} A/cm{2}. We also apply this platform to determine the diameter dependence of the melting temperature of gold nanocrystals down to three nanometers.     

Jet noise prediction using different turbulent scales

The turbulent energy dissipation rate time-scale and length-scale has been routinely used for the prediction of noise from turbulent flows, particularly jet streams. However, this is not the only possible choice. In general, scales evolving in a turbulent medium are threefold. First, those associated with the mean flow; second, those attributed to the turbulence and the mean flow interactions; and third, scales related to the turbulence-turbulence interactions. In this paper, special attention will be paid to further study of the underlying physics of aerodynamic noise by examining various time-scales. To do so, three time scales, namely, dissipation, production, and strain rate time scales, are defined and used in the source modelling to emphasis the effect of the turbulence structures at different jet regions on the jet noise production mechanism. The required mean value and turbulence parameters are obtained using a modified k − ∈ turbulence model, and Lighthill’s Acoustic Analogy is used for the prediction of the emanated noise. The text was submitted by the authors in English.     

Elastic constants of solids at high pressures

The isothermal and adiabatic nth-order (n ?? 2) elastic constants of a loaded crystal are defined. These constants fully determine the behavior of solids at an arbitrary load and are controlled by both an interatomic interaction and an applied load. Expressions that relate these constants (of the second, third, and fourth order) to Brugger elastic constants of the corresponding order, which are only determined by an inter-atomic interaction, are found for cubic symmetry crystals under hydrostatic pressure. These expressions are used to calculate the equation of state and the second- and third-order elastic constants of bcc tantalum at T = 0 K over a wide pressure range (0?C600 GPa) using an electron density functional method. The results of calculating the equation of state and the second-order elastic constants agree with available experimental data and the calculation results obtained in other works.     

Microphase separation at two length scales

The possibility of microphase separation at two different length scales in monodisperse AB block copolymer melts consisting of a homopolymer A block and either a linear alternating AB copolymer block (poly(A)_m-block-poly(B-alt -A)_n) or an AB comb copolymer block poly(A)_m-block-poly(A-graft-B)_n, is investigated. An analysis of the structure factor reveals that in the parameter space of n and m three different cases can be distinguished: I) The structure factor has only one minimum corresponding to the short length scale (i.e. the characteristic length of the repeating unit of the alternating or comb block). II) The structure factor has only one minimum corresponding to the long length scale (the characteristic length of the blocks). III) Two minima are present leading to a competition between microphase separation at the short and the long length scale. Depending on the choice of n and m, one of these three possibilities will occur. Received 25 August 2000     

Brownian motion at short time scales

Brownian motion has played important roles in many different fields of science since its origin was first explained by Albert Einstein in 1905. Einstein's theory of Brownian motion, however, is only applicable at long time scales. At short time scales, Brownian motion of a suspended particle is not completely random, due to the inertia of the particle and the surrounding fluid. Moreover, the thermal force exerted on a particle suspended in a liquid is not a white noise, but is colored. Recent experimental developments in optical trapping and detection have made this new regime of Brownian motion accessible. This review summarizes related theories and recent experiments on Brownian motion at short time scales, with a focus on the measurement of the instantaneous velocity of a Brownian particle in a gas and the observation of the transition from ballistic to diffusive Brownian motion in a liquid.     

Modeling static charge dissipation on solids: An historical perspective

Gilbert was the first to recognize the specific character of electrics, materials able to attract a needle when rubbed. Four centuries later, the detailed understanding of this experiment remains delicate, even concerning one aspect only: charge dissipation after charging. The double nature of the material, dielectric and allowing charge transport, identified by Faraday, will participate. We examine here both aspects, following an historical perspective. Dielectric absorption, involving slow polarization mechanisms, can be related to a viscoelastic behavior of the material, as long as superposition principle applies. From Kohlrausch to modern spectroscopy, dielectric functions were proposed, and attempts were made to account for the general behavior, involving time power laws and stretched exponentials.Charge transport in insulating solids may be modeled using the concepts of carrier mobility and trapping. In disordered materials, dispersive transport has to be considered, due the broad distribution in trapping energies. This leads also to time power laws in the decay process. Hence both faces of the insulator, dielectric and conductive, often lead to the same dispersion in the time response of the signal. It may be related to intrinsic parameters of the material, like its fractal nature. It has also important practical consequences.     

Electronic transfer processes studied at different time scales by selective resonant core hole excitation of adsorbed molecules

Core hole spectroscopy has been shown to provide information on ultrafast charge transfer processes on a time scale of the core hole lifetime. In this paper we present high-resolution autoionization studies of SF₆ and thiophene molecules adsorbed on a Ru(001) surface to further illustrate the potential of the so-called core hole clock method. Resonance states with very different core hole lifetimes can be excited with these sulfur-containing molecules. The selection of a specific resonant core hole excitation enables a variation of the time scale for probing electronic as well as nuclear dynamics. PACS 32.80.Dz; 32.80.Hd; 73.20.Jc     

Modeling multiple time scales during glass formation with phase-field crystals

The dynamics of glass formation in monatomic and binary liquids are studied numerically using a microscopic field theory for the evolution of the time-averaged atomic number density. A stochastic framework combining phase-field crystal free energies and dynamic density functional theory is shown to successfully describe several aspects of glass formation over multiple time scales. Agreement with mode coupling theory is demonstrated for underdamped liquids at moderate supercoolings, and a rapidly growing dynamic correlation length is found to be associated with fragile behavior.     

Atmospheric motions of different scales and their transport capacities

A brief review is given of the various motion systems in the atmosphere, with emphasis on those which are judged most important for transport of airborne material.