Stochastic analysis and regeneration of rough surfaces

We investigate the Markov property of rough surfaces. Using stochastic analysis, we characterize the complexity of the surface roughness by means of a Fokker-Planck or Langevin equation. The obtained Langevin equation enables us to regenerate surfaces with similar statistical properties compared with the observed morphology by atomic force microscopy.     

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.     

Nanoadhesion between rough surfaces

A model is developed to describe the adhesion between deformable fractal surfaces over the mesoscopic realm that covers the familiar range of interest in nanotechnology from atomic dimensions to microns. This model helps us gain a quantitative understanding of the variation of adhesion with surface energy, with microstructure of rough surfaces, and with bulk deformability. The present analysis goes beyond the Gaussian distribution of asperity heights by investigating the influence of the microstructure of self-affine fractal surfaces. Our calculation reveals that orders of magnitude increase in adhesion are possible as the roughness exponent decreases.     

Scattering from rough surfaces

Scattering from rough surfaces is studied using a perturbative treatment of the Ewald-Oseen extinction theorem. Expressions for the first and second order fields in the roughness parameter are presented for arbitrary incident fields and used for the calculation of scattering and extinction cross sections. The cross sections are shown to have contributions from diffuse scattering as well as from surface polariton emission and include the hitherto studied effects such as Smith-Purcell radiation, Wood anomalies and reflectance drops at rough surfaces.     

Growth of rough epitaxial surfaces

We present here a set of coupled continuum equations to describe atomic deposition. We take into account evaporation due to thermal and mechanical disturbances as well as subsequent accretion at favourable grooves.     

Speckle correlation for the analysis of random processes at rough surfaces

The importance of technological processes like corrosion, ablation or deposition causes interest in the quantitative monitoring of changes at rough surfaces. Thus, there is a need for effective methods to measure the statistical parameters characterizing changes in the profile or the material composition of such objects. The speckle field scattered from the surface is used as information carrier and its change is measured by correlation. This is realized by sophisticated data acquisition and digital processing techniques. An important issue is the interpretation of the correlation output in terms of statistical parameters describing the surface change. For many random surfaces a geometrical relation between surface profile and optical phase proves satisfactory. This allows to determine the standard deviation of the profile change. For a verification, speckle decorrelation in model surfaces of known deviation is measured. The paper introduces the speckle correlation concept, outlines some history and current setups and describes methods for data evaluation. The reliability of the quantitative interpretation of the speckle decorrelation is demonstrated. The method is illustrated by studies of metal corrosion and material removal in the cleaning of historical objects by laser ablation.     

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.     

Transition radiation from rough surfaces

The light emission of normally incident 77 keV electrons on silver surfaces of different roughness has been investigated. The roughness was quantitatively determined by measuring the scattered light of a normally incident laser beam. In addition to the transition radiation, which is expected for smooth surfaces, light emission of surface plasmons via surface roughness occurs. The shape and the intensity of the spectral distribution of this emission show a characteristic change with the surface roughness.     

Cantilever bending with rough surfaces

We show that the response of the curvature of cantilevers to changes in their surface stress depends significantly on the topology of the surface. The origin is the transverse coupling between the out-of-plane and in-plane components of the surface-induced stress. Deliberate structuring of the surface allows the magnitude and even sign of the cantilever response to be tuned, and experiments measuring the surface stress on nominally planar surfaces require a correction when there is roughness.     

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.     

Wetting failure condition on rough surfaces

Wetting states and processes attract plenty of interest of scientific and industrial societies. Air entrainment, i.e.,wetting failure, on smooth plate in wetting process has been investigated carefully before. Liquid bath entries of "rough"silicon wafers are studied experimentally in the present work, and the air entrainment condition is analyzed specially with the lubrication theory. The roughness effects on the moving contact lines are therefore explored. The contact line pinning is found to be the main reason for the dynamically enhanced hydrophobicity of rough surface, which implies an effective microscopic contact angle of θ_e = θ_Y + 90° where θY is the Young's contact angle of the material. Our results suggest that the solid surfaces can be considered as hydrophobic ones for a wide range of dynamic process, since they are normally rough. The work can also be considered as a starting point for investigating the high-speed advancing of moving contact line on rough surfaces.     

Elastoplastic contact between randomly rough surfaces

I have developed a theory of contact mechanics between randomly rough surfaces. The solids are assumed to deform elastically when the stress sigma is below the yield stress sigma(Y), and plastically when sigma reaches sigma(Y). I study the dependence of the (apparent) area of contact on the magnification. I show that in most cases the area of real contact A is proportional to the load. If the rough surface is self-affine fractal (Hurst exponent H) the whole way up to the lateral size L of the nominal contact area, then (assuming no plastic deformation) A approximately L(H).     

Enhanced transmission through randomly rough surfaces

We present an experimental and theoretical study of two enhancement effects that occur in the transmission of light through a thin metal film whose illuminated surface is a one-dimensional random surface while its back surface is planar. The first is a well defined peak in the antispecular direction in the angular distribution of the intensity of the incoherent component of the transmitted light (enhanced transmission). The second is an additionally well defined peak in the forward direction in the angular distribution of the intensity of the incoherent component of the transmitted light, when the illuminated surface is not only randomly rough but has even symmetry as well (enhanced refraction). A fully automated bidirectional reflectometer has been used to measure the intensity of the incoherent component of He-Ne laser light transmitted through gold and silver films of these two types and the results are compared with the predictions of theoretical calculations of the enhancement effects.