Wetting by solid helium,a model system

This is a review of the wetting properties of solid helium on various solid substrates. Due to its extreme purity and to its very fast growth dynamics, solid helium 4 is often considered as a model system in materials science. Several wetting phenomena have been studied with helium 4 crystals, namely contact angles on solid substrates with variable roughness, wetting on graphite where epitaxial growth takes place, the roughening transition as a function of film thickness, the wetting of grain boundaries by the liquid phase.     

Mechanisms for liquid slip at solid surfaces

One of the oldest unresolved problems in fluid mechanics is the nature of liquid flow along solid surfaces. It is traditionally assumed that across the liquid-solid interface, liquid and solid speeds exactly match. However, recent observations document that on the molecular scale, liquids can slip relative to solids. We formulate a model in which the liquid dynamics are described by a stochastic differential-difference equation, related to the Frenkel-Kontorova equation. The model, in agreement with molecular dynamics simulations, reveals that slip occurs via two mechanisms: localized defect propagation and concurrent slip of large domains. Well-defined transitions occur between the two mechanisms.     

Laser-induced coherent modulation of solid and liquid surfaces

The interaction of a single spatially coherent, polarized laser beam with an interface possessing microscopic roughness leads to inhomogeneous energy deposition due to the interference between the incident beam and scattered interface fields. At high intensities this can lead to spatially periodic melting or vaporization of solid or liquid layers, and temporary or permanent modification of the interface. We review our experimental and theoretical work and discuss the detailed electrodynamics, spatial spectra, surface structure and temporal growth characteristics of this phenomenon.     

Wetting transitions of simple liquid films adsorbed on self-assembled monolayer substrates: an ellipsometric study

We report on an ellipsometric experimental study designed to explore the relevance of the wetting phase diagram predicted by liquid state physics of basic models, to the wide class of simple organic liquid films that adsorb from saturated vapour onto planar substrates at room temperature. The wetting properties are explored by measuring adsorption isotherms in the approach to saturation, in particular, for adsorption of n-hexane on a variety of specially constructed substrates (self-assembled monolayers) spanning a wide range of surface energy, and by carrying out the microscopic equivalent of contact angle experiments at saturation. We locate a wetting transition, which in our case is continuous, and then study its properties in detail. The general prediction of the wetting phase diagram, that wetting transitions should be ubiquitous in nature and readily located via control over the substrate field, is supported by our data, but the quantitative nature of the thick film adsorption regime is not in agreement with Lifshitz theory. This conclusion supports the work of a variety of earlier related studies, but contrasts with recent results for adsorption onto the surface of water. In addition, the correlation length determined from our complete wetting adsorption isotherms is mesoscopic, suggesting that equilibrium statistical mechanics of simple models of inhomogeneous fluids cannot explain the data.     

Structure and excitations in liquid and solid surfaces

After a short summary of the thermodynamics of surfaces, attention is focussed on recent progress in determining the static structure of liquid surfaces, and the connection with surface energy. This is discussed fully for simple insulating liquids. For simple metals, both in liquid and solid phases, a different approach is necessary, because the density profiles of both electrons and ions enter the discussion. This area is reviewed and the importance of experiments to measure electron and atomic density profiles near surfaces is stressed. Additionally, a discussion of the surface tension of transition metals is included.Electronic excitations associated with metal surfaces are then considered, as well as the atomic dynamics of both liquid and solid surfaces.     

Wetting modifications of uhmwpe surfaces induced by ion implantation

Ultra-high-molecular-weight-polyethylene (UHMWPE) surfaces are characterized in terms of roughness and wetting. Changes in the surface morphology of the polymer were induced macroscopically by mechanical friction and microscopically by ion implantation. The ion irradiation was obtained by using 300?keV Xe⁺ beams with doses ranging between 10¹⁴ and 10¹⁵?ions/cm².

Roughness and wetting measurements were performed in order to investigate the UHMWPE surface properties before and after the surface treatments. The wetting angle of the polymeric surface increases with the decrease of the roughness and with the increase of the absorbed dose. Results are discussed from the point of view of the biological reactions that could degrade the UHMWPE biocompatible surfaces employed in different mobile prostheses.     

Shear stress fluctuations in the granular liquid and solid phases

We report on experimentally observed shear stress fluctuations in both granular solid and fluid states, showing that they are non-Gaussian at low shear rates, reflecting the predominance of correlated structures (force chains) in the solidlike phase, which also exhibit finite rigidity to shear. Peaks in the rigidity and the stress distribution's skewness indicate that a change to the force-bearing mechanism occurs at the transition to fluid behavior, which, it is shown, can be predicted from the behavior of the stress at lower shear rates. In the fluid state stress is Gaussian distributed, suggesting that the central limit theorem holds. The fiber bundle model with random load sharing effectively reproduces the stress distribution at the yield point and also exhibits the exponential stress distribution anticipated from extant work on stress propagation in granular materials.     

Invited paperFourier-transform heterodyne spectroscopy of liquid and solid surfaces

Fourier-transform heterodyne spectroscopy is a simple but powerful technique to study narrow line shapes and small frequency shifts. We review the technique and its application to the study of hydrodynamic fluctuations. We introduce a simple scheme for obtaining the full spectrum of light scattered from liquid and solid surfaces. Using this scheme we obtained a spectral resolution of 200 Hz. Two applications of the technique, one involving the detection of counter-propagating capillary waves, the other measurement of the epitaxial crystal-growth speed of a crystalline silicon interface, are presented. Received: 21 August 1996     

Elliptical multielectron dimples on liquid and solid helium surfaces

The structure of multielectron dimples on helium substrates is investigated for conditions deviating from axial symmetry. Main sources for an ellipticity of dimples are an anisotropy of the surface tension, as for the liquid-solid He interface, or anisotropic boundary conditions, as for liquid He between vertical parallel plates.     

Effect of phase fluctuations of a wave on the deformation of caustic surfaces

The article discusses the evolution of caustic surfaces formed by radiation with a chaotically modulated phase front as a function of the parameters of this modulation. It is shown that the well-known decrease in the mean-statistical characteristics of the field near caustics can take place either with only an insignificant change in the focusing factor in an actual case, or with a considerable decrease in it. A parameter is introduced determining the type of caustic surface and, consequently, also the value and structure of the field near it. Conditions are determined with which the maximal value of the field in an actual individual case is close to the unperturbed value, exceeds it, or is considerably less.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 11, pp. 1603–1609, November, 1978.In conclusion we note that, in the general case of the-dimensional systems, the pictures of fluctuating caustic surfaces will be more complex, and cannot be characterized by a single dimensionless parameter . However, in many cases, where the focusing has a quasicylindrical character, the results obtained here can be used directly.     

Enhanced observation of number fluctuations by use of structured light

The point statistics of light scattered from structured, incoherent illumination of a random and fluctuating population of identical scatterers are derived. Theoretical predictions for a two-beam illumination pattern in which the relative beam intensity is modulated are compared with computer simulations and experiments performed by illumination of a rotating mask of identical but random apertures. The use of structured light is found to enhance the size of fluctuations, and quantitative comparisons between theory and practical measurement are made. The structured-light technique introduces flexibility in fluctuation measurement, permitting optimal control of the dynamic range, and is relevant to the development of instrumentation for particle and flow analysis.     

Wetting behavior of magnesite and dolomite surfaces

Magnesite and dolomite are salt-type minerals that show similar chemical composition and flotation behavior due to same crystal structure, and sparingly soluble nature. The surface properties of minerals play a major role in determining their separation from each other in processes such as flotation. During flotation process, selectivity problem arises between magnesite and associated gangue minerals such as dolomite. There is a close relationship between floatability of minerals and their contact angles. Therefore, surface hydrophobicity of magnesite and dolomite minerals was investigated by contact angle measurements in the absence and presence of flotation reagents.Magnesite and dolomite show hydrophilic properties and they have got a small contact angle (magnesite ∼10.4° and dolomite ∼6.6°) in distilled water in the absence of any surfactant. The contact angle values at the magnesite and dolomite surfaces remained at 9.7°-10.9° in the presence of petroleum sulphonates (R825 and R840) while sodium oleate affected hydrophobicity of magnesite, and the contact angle value increased up to 79°. The contact angle value of 39° at dolomite surface was obtained in the solution of sodium oleate, respectively.     

Theory of simple fluids near a structured surface

Convenient theoretical methods for calculating structure and thermodynamic properties of fluids near an irregular hard wall are developed. Using the diagrammatic expansion, we prove that: Firstly, a fluid system in contact with an irregular surface can be treated as two coupling fluids. The first one is defined by the studied fluid interacting with a planar hard wall. It is called the reference system. The second, which is referred to as the fictitious system, is confined to the surface cavities. Secondly, these developments can be used to find a self-consistent approximation.     

Multiphonon atomic scattering by solid surfaces

A multiphonon solution for the problem of atom-surface scattering at thermal energies is proposed. The scattering equations are solved using the assumption of low inelastic scattering intensities, and the theoretical formalism is basically an improvement of a previous work on one-phonon scattering. Present results reduce to the one-phonon expression of the previous work when the appropriate limit is taken.