Growth of rough surfaces

The growth of objects with rough surfaces is a common phenomenon. We describe several models used in the study of the temporal evolution of fluctuating interfaces and then we explain the dynamical scaling in this class of growth models with self-affine surfaces. The recent progress in this field is reviewed. Much emphasis is put on roughness properties and in particular on a possible kinetic roughening transition — a nonequilibrium analog of the thermal roughening transition. Both analytical and numerical results for scaling exponents are summarized and indications of a phase transition in some models are discussed.     

Fractal structure of digital speckle patterns produced by rough surfaces

We report on the fractal analysis of digital speckle patterns experimentally generated using an optical setup to record the light scattered from metallic rough surfaces in the normal direction. Using the differential box counting technique, we have calculated the fractal dimension of digital speckle patterns for six samples with different roughness. Our results show a quadratic dependence between the surface roughness and the fractal dimension of the corresponding digital speckle pattern. As an application a method to determine the surface roughness of metallic surfaces is proposed.     

Adhesion of elastic films on mound rough surfaces

In this work, we have investigated the adhesive behaviour of elastic films in contact with solid substrates, which are bounded by mound surface roughness. This type of roughness is described by the rms roughness amplitude w, the average mound separation Λ, and the system correlation length ζ. It is shown that both lateral roughness parameters Λ and ζ strongly influence adhesive characteristics. Indeed, with increasing elastic film modulus E, film adhesion is only possible for sufficiently large mound separations Λ. Moreover, the critical elastic modulus E_c (for which spontaneous film decohesion takes place for E>E_c) is shown to increase fast with increasing mound separation Λ when Λ?ζ, while as a function of the system correlation length ζ it increases relatively fast when ζ?Λ.     

Turbulent plane wall jets over smooth and rough surfaces

Large-eddy simulations were carried out to study the effects of surface roughness on a plane wall-jet using the Lagrangian dynamic eddy-viscosity subgrid-scale model, at Re = 7500 (based on the jet bulk velocity and height). Results over both smooth and rough surfaces were validated by experimental data at the same Reynolds number. As the jet is injected into the still environment, large-scale rollers are generated in the shear layer between the high-momentum fluid of the jet and the surrounding and are convected downstream with the flow. To understand the extent to which the outer-layer structures modify the flow in the inner layer and the extent to which the effect of roughness spreads away from the wall, both instantaneous and mean flow fields were investigated. The results revealed that, for the Reynolds number and roughness height considered in this study, the effect of roughness is mostly confined to the near-wall region of the wall jet. There is no structural difference between the outer layer of the wall jet over the smooth and rough surfaces. Roughness does not affect the size of the outer-layer structures or the scaling of the profiles of Reynolds stresses in the outer layer. However, in the inner layer, roughness redistributes stresses from streamwise to wall-normal and spanwise directions toward isotropy. Contours of joint probability-density function of the streamwise and wall-normal velocity fluctuations at the bottom of the logarithmic region match those of the turbulent boundary layer at the same height; while the traces of the outer-layer structure were detected at the top of the logarithmic region, indicating that they do not affect the flow very close to the wall, but still modify a major portion of the inner layer. This modification must be taken into consideration when the inner layer of a wall jet is compared with the conventional turbulent boundary layer.     

Investigation of light scattering from rough periodic surfaces — numerical solutions

The Beckmann scalar scattering model based on the Kirchhoff approximation was used to investigate the scattering of light from periodic surfaces whose roughness amplitude is comparable to, and greater than, the illumination wavelength. Solutions by numerical integration were obtained for surfaces of different profiles and of different roughnesses. It was found that the scattering patterns from these surfaces were very different at large roughness amplitudes. As the incident angle was varied on a given surface, it was also observed that the intensity of any individual diffraction order oscillated and the degree of oscillation was directly related to roughness. By utilizing this property, a new procedure could be developed for surface roughness assessment.     

Large-eddy simulation of impinging jets on smooth and rough surfaces

We performed large-eddy simulations (LES) of forced impinging jets over smooth and rough surfaces, containing large-scale, azimuthal vortices generated by the enhanced primary instability in the jet shear layer. The interaction between these vortices and the turbulence in the wall jet that is formed downstream of the impingement region determines their rate of decay. To explore the surface-roughness effects on the evolution of the vortices, sand-grain-like surfaces are generated using uniformly distributed but randomly oriented ellipsoids. The flow is compared to our previous LES of jets impinging on a smooth surface. In spite of the severe modification caused by the roughness on the near-wall flow, the vortex development is not significantly altered. Slightly faster decay of the primary vortices is observed in the rough-wall case compared to the smooth-wall one; the secondary vortex that detaches from the wall and is lifted up has larger vorticity. The highly disturbed near-wall flow is advected outward and affects the evolution of the primary vortex for a longer period during the vortex interaction. The robust turbulent generation mechanism in the outer shear layer, however, mitigates the changes in vortex behaviour. The momentum deficit and the enhancement of turbulence due to the surface roughness play a key role during this process.     

气体-表面相互作用的分子动力学模拟研究

采用分子动力学模拟方法研究了气体分子Ar在光滑和粗糙Pt表面上的散射规律.提出了一种速度抽样方法,计算了不同温度条件下气体分子对光滑和粗糙表面的切向动量适应系数和吸附概率.结果显示：光滑表面条件下,气体分子的切向动量系数和吸附概率都随着温度的升高而降低;粗糙度对气体分子切向动量与表面的适应具有极大的促进作用,当粗糙度足够大时,切向动量适应系数的大小趋近于1.0,对温度的敏感性也逐渐降低.采用粒子束方法对气体分子在光滑和粗糙表面上的散射规律进行了定量分析.总结了散射过程中气体分子的典型轨迹和动量变化规律,将气体分子在光滑表面的散射分为两种类型：单次碰撞后散射和多次碰撞后散射.单次碰撞后散射的气体分子平均切向动量有所减小,而经过多次碰撞后散射的气体分子则倾向于保持原有的平均切向动量.对于粗糙表面,粗糙度的存在使气体分子与表面间的动量和能量适应更加充分,导致气体分子在较粗糙表面上散射后的平均切向动量大幅减小并接近于0,且气体分子在表面上经历的碰撞次数越多,其散射后的能量损失越严重.     

Step dynamics in relaxation of sharp corners on crystal surfaces

The dynamic behavior of steps involved in the relaxation of sharp corners in microfabricated structures on crystalline surfaces have been studied. We find that during the early stages of relaxation of slightly tapered trenches on Si(0 0 1), wide (1 1 0) terraces perpendicular to the substrate are formed near the corners of the trench sidewalls. The evolution of a step profile around the corner region, where step density abruptly changes, is analyzed using one-dimensional step models. It is found that, in case that mass transport occurs through surface diffusion, the preexisting steps on the trench sidewall are accumulated in the corner region, and extensive terraces are formed near the corners.     

Incomplete similarity of a plane turbulent wall jet on smooth and transitionally rough surfaces

This study assesses the hypothesis of incomplete similarity for a plane turbulent wall jet on smooth and transitionally rough surfaces. Typically, a wall jet is considered to consist of two regions: an inner layer and an outer layer. The degree to which these two regions reach equilibrium with each other and interact to produce the property of self-similarity remains an open question. In this study, the analysis of the outer and inner regions indicates that each region is characterised by a half-width which exhibits its own distinct dependence on the streamwise distance x from the slot, and a single self-similar structure for both regions does not exist. More specifically, the inner and outer layers of the wall jet exhibit different scaling laws, which results in two self-similar mean velocity profiles, both of which retain a dependence on the slot height H. As such, incomplete similarity of the wall jet on smooth and transitionally rough surfaces is confirmed by this study. In addition, comparison of the experimental results for the transitionally rough surface with the smooth wall case indicates that the surface roughness modifies the development of the mean velocity profile in both the inner and outer regions, although the effect on the outer region is relatively small and close to the experimental uncertainty.     

Density functional study of initial HfCl4 adsorption and decomposition reactions on silicon surfaces with SiON interfacial layer

The initial adsorption and decomposition of HfCl₄ on silicon surfaces with different types of SiON interfacial layers are investigated using density functional theory. We find that the reactions of HfCl₄ on both the hydroxylated and nitrided silicon surfaces proceed through similar reaction pathways. By comparison of the reaction energies of HfCl₄ with the hydroxyl and amino surface sites, we find that it is both kinetically and thermodynamically favorable for the reactions of HfCl₄ on hydroxyl site of silicon substrates. Comparing with the adjacent bridging oxygen, we also find that the neighboring hydroxyl can facilitate the adsorption of HfCl₄ on the amido surface site. Also, it is more kinetically and thermodynamically favorable for the reaction of HfCl₄ with bridging NH site than that with NH₂ site.     

大口径粗糙球面的检测

 为了解决大口径光学元件在精磨阶段的检测问题,基于电磁场散射理论,分析了在红外干涉仪测量粗糙表面过程中,所产生干涉条纹的对比度与粗糙表面粗糙度之间的关系,并通过测量不同粗糙表面对这一结果进行了实验验证。利用红外干涉仪测量了280#精磨粗糙球面（Φ=1130mm,R=3600mm）的面形,讲述了测量大型粗糙球面时干涉仪的调整方法。从测量结果可以看出,此粗糙球面具有比较明显的球差,面形峰谷（PV）值为0.526λ(λ=10.6μm),均方根（RMS）值为0.117λ(λ=10.6μm)。     

Growth in a restricted solid on solid model with correlated noise

We introduce correlated growth into a restricted solid on solid model (RSOS), a stochastic deposition model with a constraint on neighboring height differences. A two-dimensional lattice model is used in which particles are deposited via horizontal Levy flight steps with a step length distribution exponentf. Though RSOS is in the same universality class as ballistic deposition for uncorrelated deposition, it appears to depart from it for strong correlations. Forf=1, the short-range limit is reached and both exponents and, which describe the dependence of surface width on time and strip length, tend to 1. Forf>1 we retreat to an enhanced algorithm, searching for growth sites which become excessively rare. We find an unusual short-time dependence, but still tends to 1. The number of growth sitesG shows saturation forf<1, while forf1 we observeG/L0 as the strip lengthL increases. Finally, we test directly the relationship of noise-noise correlation strength tof, and find that a direct comparison between correlated growth models and theoretical predictions for growth with correlated noise is so far unjustified.     

Droplets on inclined rough surfaces

The behaviour of liquid droplets on inclined heterogeneous surfaces was simulated by the lattice-Boltzmann method using the Shan-Chen multiphase model. The effect of topography of the surface on the contact angle hysteresis was investigated. It is shown in particular, by using anisotropic rough surfaces, how surface topography and thereby the continuity of the three-phase contact line, affect this hysteresis. Our results clearly indicate that the superhydrophobicity of a surface cannot be judged by the contact angle alone.     

Stochastic analysis of different rough surfaces

This paper shows in detail the application of a new stochastic approach for the characterization of surface height profiles, which is based on the theory of Markov processes. With this analysis we achieve a characterization of the scale dependent complexity of surface roughness by means of a Fokker-Planck or Langevin equation, providing the complete stochastic information of multiscale joint probabilities. The method is applied to several surfaces with different properties, for the purpose of showing the utility of this method in more detail. In particular we show evidence of the Markov properties, and we estimate the parameters of the Fokker-Planck equation by pure, parameter-free data analysis. The resulting Fokker-Planck equations are verified by numerical reconstruction of the conditional probability density functions. The results are compared with those from the analysis of multi-affine and extended multi-affine scaling properties which is often used for surface topographies. The different surface structures analysed here show in detail the advantages and disadvantages of these methods.Received: 5 April 2004, Published online: 12 October 2004PACS: 02.50.-r Probability theory, stochastic processes, and statistics - 02.50.Ga Markov processes - 68.35.Bs Surface structure and topography of clean surfaces     

Stiffness of contacts between rough surfaces

The effect of self-affine roughness on solid contact is examined with molecular dynamics and continuum calculations. The contact area and normal stiffness rise linearly with the applied load, and the load rises exponentially with decreasing separation between surfaces. Results for a wide range of roughness, system size, and Poisson ratio can be collapsed using Persson's contact theory for continuous elastic media. The compliance due to atomic-scale motion at the interface between solids has little effect on the area and normal stiffness, but can reduce the total transverse stiffness by orders of magnitude. The scaling of this effect with system size is discussed.     

Nanoscale observations of the epitaxial growth of hashemite on barite (0 0 1)

The heteroepitaxial growth of hashemite BaCrO₄ on barite BaSO₄(0 0 1) from supersaturated aqueous solutions was observed in situ using an atomic force microscope (AFM). It was shown that the first hashemite layer grows via two-dimensional nucleation easily forming a complete epitaxial layer, which is likely to have a low level of intrinsic stress. Two-dimensional nucleation of the second and subsequent layers proceeds with significantly lower rates, and growth occurs with lower step velocities. These layers seem to have significant level of intrinsic stress and tend to reduce it via the formation of free surface normal to the growth layer (holes in the layer, dendrite-like shape of nuclei and steps, preferable formation of nuclei at the step edges). As a result, the initially flat surface becomes rough. The process described corresponds to the Stranski-Krastanov epitaxial growth mode, which is well known for growth of semiconductor and metal films but not previously recognised for crystals grown from aqueous solutions.