Modeling laser-assisted particle removal using molecular dynamics

Laser-assisted particle removal, a method of cleaning nano- to micro-scale particles from surfaces, was modeled using molecular dynamics. A two-dimensional molecular model consisting of substrate, particle, and adsorbed fluid was used. In order to obtain statistical accuracy of cleaning efficiencies, over 1200 particle-removal simulations were conducted. The effects of fluid thickness and substrate temperature were both considered, and good qualitative agreement with experimental results was obtained. The molecular dynamics approach is shown to be an effective way to study these systems. PACS 81.65.Cf; 79.20.Ds     

Order parameter modeling of fluid dynamics in narrow confinements subjected to hydrophobic interactions

A novel phase-field approach is developed for quantitative modeling of the complex thermophysics over reduced length scales in narrow fluidic confinements, as induced by the surface roughness-hydrophobicity coupling and the consequent hydrodynamic interactions. The method is tested for flows on micro- and nano- corrugated surfaces in narrow confinements, and the agreement with molecular dynamics and lattice Boltzmann simulations is found to be quantitative.     

Melting and resolidification of gold film irradiated by nano- to femtosecond lasers

Rapid melting and resolidification of a free-standing gold film subject to nano- to femtosecond laser pulses are investigated using the two-temperature model in conjunction with an interfacial tracking method. The interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, in the ultra-fast phase-change process are obtained by considering the interfacial energy balance and nucleation dynamics. A nonlinear electron heat capacity and a temperature-dependent electron–lattice coupling factor for the rapid phase change are taken into account. Effects of laser pulse width and fluence on melting and resolidification are also studied. PACS 42.62.Eh; 63.20.Kr; 64.70.Dv     

Bright, coherent, ultrafast soft X-ray harmonics spanning the water window from a tabletop light source

We demonstrate fully phase-matched high harmonic emission spanning the water window spectral region important for nano- and bioimaging and a breadth of materials and molecular dynamics studies. We also generate the broadest bright coherent bandwidth (≈300 eV) to date from any light source, small or large, that is consistent with a single subfemtosecond burst. The harmonic photon flux at 0.5 keV is 103 higher than demonstrated previously. This work extends bright, spatially coherent, attosecond pulses into the soft x-ray region for the first time.     

On the use of slow manifolds in molecular and geophysical fluid dynamics

Constraints typically arise from the elimination of high frequency oscillations in mechanical systems. Examples are provided by bond constraints in molecular simulations and incompressibility constraints in fluid dynamics. A key issue is the accuracy of constrained dynamics with regard to the full dynamics. In this review we focus on the smooth solution components and discuss the concept of slow manifold and soft constraints in molecular and geophysical fluid dynamics. While the formal mathematical derivation of constraints is the same for both molecular and fluid dynamics, the predominant numerical techniques for dealing with constraints are different in the two fields. Semi-implicit time- stepping methods are often used in geophysical fluid dynamics while explicitly enforced constraints are more common in molecular dynamics.     

Different crystallization processes of as-deposited amorphous Ge2Sb2Te5 films on nano- and picosecond single laser pulse irradiation

The crystallization dynamics of as-deposited amorphous Ge₂Sb₂Te₅ films induced by nano- and picosecond single laser pulse irradiation is studied using in situ reflectivity measurements. Compared with nanosecond laser pulse, the typical recalescence phenomenon did not appear during the picosecond laser pulse-induced crystallization processes when the pulse fluence gradually increased from crystallization to ablation threshold. The absence of melting and recalescence phenomenon significantly decreased the crystallization time from hundreds to a few tens of nanoseconds. The role of pulse duration time scale on the crystallization process is qualitatively analyzed.     

Surface roughness-hydrophobicity coupling in microchannel and nanochannel flows

An approach based on a lattice version of the Boltzmann kinetic equation for describing multiphase flows in nano- and microcorrugated devices is proposed. We specialize it to describe the wetting-dewetting transition of fluids in the presence of nanoscopic grooves etched on the boundaries. This approach permits us to retain the essential supramolecular details of fluid-solid interactions without surrendering--actually boosting--the computational efficiency of continuum methods. The method is used to analyze the importance of conspiring effects between hydrophobicity and roughness on the global mass flow rate of the microchannel. In particular we show that smart surfaces can be tailored to yield very different mass throughput by changing the bulk pressure. The mesoscopic method is also validated quantitatively against the molecular dynamics results of [Cottin-Bizonne, Nat. Mater. 2, 237 (2003)].     

Production of nano- and microdiamonds with Si-V and N-V luminescent centers at high pressures in systems based on mixtures of hydrocarbon and fluorocarbon compounds

A new method has been proposed for the synthesis of nano- and microdiamonds with various contents of luminescent silicon-vacancy (Si-V) and nitrogen-vacancy (N-V) centers at high static pressures in growth systems based on mixtures of hydrocarbon, fluorocarbon, and organic silicon compounds without catalyst metals. The high efficiency of the proposed method of the doping of diamonds at nano- and microlevels has been demonstrated by analyzing the spectra of photoluminescence and absorption of the diamonds obtained.     

Selective triggering of phase change in dielectrics by femtosecond pulse trains based on electron dynamics control

In this study we experimentally reveal that the phase change mechanism can be selectively triggered by shaping femtosecond pulse trains based on electron dynamics control (EDC), including manipulation of excitations, ionizations, densities, and temperatures of electrons. By designing the pulse energy distribution to adjust the absorptions, excitations, ionizations, and recombinations of electrons, the dominant phase change mechanism experiences transition from nonthermal to thermal process. This phenomenon is observed in quadruple, triple, and double pulses per train ablation of fused silica separately. This opens up possibilities for controlling phase change mechanisms by EDC, which is of great significance in laser processing of dielectrics and fabrication of integrated nano- and micro-optical devices.     

纳米MgO掺杂聚乙烯微观特性的分子动力学模拟研究

电力系统高压电缆的主要绝缘材料为聚乙烯,为了提升聚乙烯的热稳定性以及减弱水分对其的渗透能力,采用纳米MgO掺杂聚乙烯,利用分子动力学模拟方法建立包含低密度聚乙烯(LDPE)、不同颗粒半径的MgO纳米团簇以及相同质量分数水分的复合模拟模型.研究结果表明,水分会降低复合体系的玻璃化温度,MgO的掺杂则会提高复合体系的玻璃化温度,减弱聚乙烯分子链的运动并减小复合体系的自由体积,使得复合体系结构更加稳定,从而增强了聚乙烯材料的热稳定性能.此外发现水分子的扩散随着温度的上升而增大,纳米MgO的添加会与水分子形成氢键抑制水分子的扩散,同时自由体积的缩减使水分子的溶解度系数与扩散系数都减小,导致水分子的渗透能力减弱,更难以渗透进聚乙烯材料破坏其结构.研究结果可为聚乙烯的水树枝生长以及老化过程的抑制提供有益的参考.     

Laminar flame and acoustic waves in two-dimensional flow

The complete system of fluid dynamics equations describing the development of instability of a reaction front in a two-dimensional flow in reversed time are reduced to a closed system of equations of front dynamics by using Lagrangian variables and integrals of motion. The system can be used to analyze processes behind the front without solving the complete system of fluid dynamics and chemical kinetics equations. It is demonstrated how the gas density disturbances induced by the moving front can be described in the adiabatic approximation.     

Fabrication of PbS thin films composed of highly (200)-oriented nano-/micro-rods in deep eutectic solvent

PbS thin films composed of highly (200)-oriented shuttle-like nano-/micro-rods were successfully fabricated on glass substrates by the environment friendly ionothermal method at 140 °C in deep eutectic solvent (DES). The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), ultraviolet-visible (UV–vis) and photoluminescence (PL) spectra, respectively. The possible mechanism of the oriented growth of PbS nano-/micro-rods was discussed. The PbS thin films composed of shuttle-like nano-/micro-rods exhibited a large absorbance property in the wavelength range of 350–1100 nm, and moreover, the PL spectrum had a broad emission band centered at 490 nm. The shuttle-like PbS nano-/micro-rods-based thin films might have potential application in solar cells.     

Surface-directed spinodal decomposition and enrichment in fluid mixtures: Molecular dynamics simulations

We present results from molecular dynamics simulations for the non-equilibrium evolution of a binary fluid in the presence of a wetting surface. We study the pattern dynamics which results when a homogeneous fluid mixture is quenched to temperatures both above and below the critical temperature. Our extensive computer simulation results are in agreement with arguments based on Ginzburg-Landau theory.     

Systematic parametric investigation on the CVD process of polysiloxane nano- and microstructures

Amorphous polysiloxane nano- and microstructures with different shapes can be synthesized from trifunctional organosilane precursors. In the present study, various polysiloxane nano- and microstructures have been produced via a chemical vapor deposition process using ethyltrichlorosilane as precursor. The structure formation and shape are the result of a delicate interplay between temperature, absolute amount of water, and relative humidity. The impact of these reaction parameters during a chemical vapor deposition process has been examined. Experiments have been performed to find a correlation between the reaction conditions and the final shape. Scanning electron microscopy data show that different structures like polysiloxane microrings, microrods, sprouts, nanofilaments, and mixtures of them can be synthesized depending on the reaction conditions. Furthermore, the in-depth comparison of the nanofilament diameters illustrates the dominating influence of relative humidity on structure formation. There is a general trend that at a higher value of relative humidity, structures with a larger diameter are formed independent from the temperature. Here, we clearly differentiate between relative humidity as major and absolute amount of water and temperature as minor important adjusting screws defining the thickness and shape of the resulting nano- and microstructures. Based on these observations, we proof the mechanism of the initial step of structure formation. It is shown that nano- and micro-sized water droplets formed on the substrate surface are likely to act as starting points for structure formation. All results described here strongly confirm the recently published droplet assisted growth and shaping mechanism.     

Electrohydrodynamic wave-packet collapse and soliton instability for dielectric fluids in (2 + 1)-dimensions

In this paper we introduce a modified lattice Boltzmann model (LBM) with the capability of mimicking a fluid system with dynamic heterogeneities. The physical system is modeled as a one-dimensional fluid, interacting with finite-lifetime moving obstacles. Fluid motion is described by a lattice Boltzmann equation and obstacles are randomly distributed semi-permeable barriers which constrain the motion of the fluid particles. After a lifetime delay, obstacles move to new random positions. It is found that the non-linearly coupled dynamics of the fluid and obstacles produces heterogeneous patterns in fluid density and non-exponential relaxation of two-time autocorrelation function.Received: 19 March 2004, Published online: 29 June 2004PACS: 47.11. + j Computational methods in fluid dynamics - 05.70.Ln Nonequilibrium and irreversible thermodynamics     

Luminescence,vibrational and XANES studies of AlN nanomaterials

The paper reports comparative studies on synthesized aluminium nitride nanotubes, nanoparticles and commercially available micron-sized AlN powder using different spectroscopic techniques: cathodoluminescence measurements (CL), X-ray absorption near edge spectroscopy (XANES) and Fourier-transform infrared spectroscopy (FTIR). Crucial distinctions in CL spectra are observed for nano- and microsized aluminium nitride powders; systematic shift of the IR absorption maximum has been detected for nanostructured aluminium nitride as compared to commercial samples. Through XANES experiments on Al K-edge structural differences between nano- and bulk AlN are revealed, intensity of features in absorption spectra has been found to be a function of wurtzite and zincblend phases amount in nanostructured samples.     

Numerical simulation of the dynamics of current channel microstructure formation in atmospheric nanosecond discharges in a uniform electric field

The results of simulation of the current channel microstructure formation in atmospheric nano- second discharges in a uniform electric field due to the development of instability of the ionization process in the avalanche stage followed by cycling breakdowns of the avalanche are considered. It is shown that the enhancement of the electric field at the ionization front due to the intrinsic field of the avalanche leads to the contraction of the path length between consecutive avalanche breakups; after several breakups, the ionized gas passes to the plasma state. The effect of small electric field perturbations on the dynamics of microstructure formation is investigated; as a result, the possibility of “induced” avalanche breakup at the instant of action of perturbations is established.     

锂离子电池用具有分级三维离子电子混合导电网络结构的纳微复合电极材料

文章评述了分级三维离子电子混合导电网络结构和具有该结构的纳微复合电极材料在锂离子电池中的应用等方面的最新研究工作进展.首先介绍了纳微复合电极结构相关概念及其优缺点,然后列举了一些运用此概念设计并构筑出的电极材料实例.研究证明,此新型电极结构能够大幅提高锂离子电池电极材料的储锂性能,并且该结构设计还可推广到其他电化学储能...     

Confined dynamics, forms and transitions in colloidal systems: from clay to DNA

Colloidal suspensions are a classic example of confining systems developing large specific surfaces, presenting a rich variety of shapes and exhibiting complex organization on a length scale ranging from 1 nm to several micrometers. Two distinct confined dynamics are generally considered in such systems: (1) the embedded fluid dynamics entrapped in the pore network with two main contributions, surface interaction and long-range connectivity, and (2) the dynamics of the host matrix, associated with a time evolution of the interfacial geometry. This last contribution is particularly important during dynamic and structural transitions of colloidal suspensions such as jamming, glass transition, phase separations and flocculation. It is generally believed that the characteristic time scale needed to describe colloidal movement and interfacial geometrical reorganization is much slower than the dynamics of the embedded fluid (except in the trivial situation where the fluid molecule is irreversibly adsorbed to a colloidal surface). Thus, few connections are made between these two distinct dynamics. In this presentation, we show how the slow and confined water dynamics at proximity of a colloidal surface provides an original way to probe colloidal shape and colloidal orientation dynamics. Two topics are presented. First of all, water field-cycling NMR relaxometry is used to probe the glass transition and the strong rotational slowing down of a colloidal system made of plate-like particles, a synthetic clay (laponite). Second, we analyze the case of long colloidal thin rods (either mineral or biologic such as DNA cylinders) dispersed in very diluted suspensions. At large distance and/or long time, these particles appear as a portion of a line. We discuss how the embedded fluid dynamics can be sensitive to this morphological crossover and may provide information about the particle shape. Some comparisons with recent experiments are presented.