Wetting Phenomena Research Articles

Interfacial wetting transitions in ionic and metallic fluid mixtures

AbstractRecent experimental investigations of interfacial wetting phenomena in binary liquid mixtures characterized by Coulomb or screened Coulomb short range interactions are discussed. Wetting transitions occurring at the fluid‐wall and the fluid‐vapour interface are considered focusing on the examples of fluid alkalimetal‐alkalihalide mixtures and liquid gallium‐based binary alloys at elevated temperatures. Different spectroscopic methods have been employed to probe the respective interfaces, spectroscopic ellipsometry and second harmonic generation (SHG). In the case of K‐KCl solutions the SHG results exhibit changes characteristic of a prewetting transition in metalrich solutions implying a first order wetting transition at coexistence. A wetting transition is observed for liquid Ga‐Bi and Ga‐Pb alloys approaching the monotectic point at coexistence.

Studies of the adsorbed water layers on solid surfaces by means of the thermal analysis special technique

The paper presents the thermogravimetric studies of water film properties on the solid surfaces using the programmed desorption under the quasi-isothermal conditions. The samples wetted in a different way before the measurements were heated under the suitable quasi-isothermal conditions of temperature increase in the derivatograph using a platinum labyrinth crucible in order to evaporate water films from the surface. During the measurements, the mass loss Q-TG and the differential Q-DTG curves were registered as a function of temperature and/or time which are characterized by the inflections showing individual stages of water desorption from the samples. On the basis of experimental data, the surface wetting mechanism and kinetics of the thermodesorption of water films, discontinuous properties of adsorbed water layers on porous surfaces, adsorption capacity of samples and nature of active sites (e.g. hydroxyl groups) have been presented. It was stated that this method could be successfully applied in the studies of adsorbed water films and surface wetting phenomena.

A novel contact angle measurement technique by analysis of capillary rise profile around a cylinder (ACRPAC)

A new experimental technique and its computational scheme to determine the contact angles of capillary rise profiles around a cylinder are presented. In the experiment, a carefully coated conic glass cylinder was inserted vertically and slowly into a tested liquid. Then the precise image of the partial capillary rise profile of the liquid around the conic cylinder was acquired and digitized by applying computer image processing and analysis techniques. From the digitized profiles of the liquid-vapour interface and the conic cylinder, the local inclination angle β and the local radius Rc, of the conic cylinder at the three-phase contact circle were calculated directly. Furthermore, an objective function was constructed, which expresses the discrepancy between the physically observed capillary rise profile and the theoretically predicted curve, i.e. the curve representing a solution of the Laplace equation of capillarity. The contact angle of the capillary rise profile on the conic cylinder was used as an adjustable parameter in optimizing the objective function and determined once the minimum objective function had been achieved. The accuracy of the measured contact angles is approximately 0.1°. In addition to the local gravity, the densities of the liquid and vapour phases and the liquid-vapour surface tension, the input requirement is the digital information of the partial capillary rise profile which is provided by a specially designed computer image analysis program. This method was tested by measuring contact angles of four n-alkane liquids around cylindrical glass fibres coated with FC725. The measured contact angles are in very good agreement with those determined by the Wilhelmy plate technique. Finally, the present technique was also applied to study the dependence of contact angles on the geometry of the conic cylinder, i.e. on cos ó/Rc. Contact angles of the four n-alkane liquids on a conic glass cylinder coated with FC725 were measured at different positions along the cylinder. The results were interpreted in terms of the line tension effect. The calculated line tensions were positive and of the order of 1 μJ m−1, which is consistent with the published data for similar solid-liquid systems obtained by using the sessile drop method. In particular, the contact angle without line tension effect θ∞ for a given solid-liquid system can be measured directly by this method. The validity of the derived contact angle θ∞ and line tension σ was also confirmed by means of the axisymmetric drop shape analysis (ADSA) technique. This novel technique is particularly suitable to the study of the wetting and spreading phenomena of a liquid on fibres, as most fibre surfaces are rough and their shapes may deviate considerably from those of right circular cylinders. A general user-oriented computer program to implement the technique was developed.

Modeling of Microporous Hollow Fiber Membrane Modules Operated under Partially Wetted Conditions

Gas−liquid mass transfer has been studied theoretically in a microprous hollow fiber membrane module operated under partially wetted conditions in the laminar flow regime. Dissolved oxygen removal has been used as an example for the simulation study. The mathematical model developed consists of nonlinear partial differential equations and is solved using the orthogonal collocation technique. The effect of membrane wetting pressures on the overall mass transfer performances of the module has been examined. The results indicate that under partially wetted operating mode, a maximum overall mass transfer coefficient is attainable with respect to the water velocity, which is completely different from the results obtained under both wetted and nonwetted conditions where the overall mass transfer coefficient is generally increased with water velocity. The phenomena of partial wetting of the membrane may provide an explanation to the observation that the overall mass transfer coefficient is increased to a maximum value and then is decreased with water velocity in a gas−liquid contactor.

Enhanced hydrogel adhesion by polymer interdiffusion: Use of linear poly(ethylene glycol) as an adhesion promoter

The fracture energy required to separate layers of hydrogel films was investigated to evaluate the impact of bulk polymer diffusion on hydrogel/hydrogel adhesion and to obtain molecular information on the fracture energy in polymer mucoadhesion. Poly(ethylene glycol) (PEG) was incorporated in a hydrogel and was used as an adhesion promoter. The influence of PEG molecular weight and contact time on PEG diffusion across the hydrogel/hydrogel interface was investigated by using tensiometric studies and near-field FTIR microscopy. These experiments indicated that linear PEG diffusion enhanced the adhesion between the two hydrogel layers. The enhanced adhesion could not be explained by surface wetting phenomena alone. These results indicated that bulk diffusion of linear polymers such as PEG (adhesion promoter) incorporated into polymer networks (hydrogels) was an effective technique for enhancing gel/gel adhesion in various applications including polymer/mucus interactions in mucoadhesion and development of mucoadhesive controlled drug delivery systems.

Realization of silicon quantum wires by selective chemical etching and thermal oxidation

Ultra-fine silicon quantum wires with SiO2 boundaries were successfully fabricated by combining SiGe/Si heteroepitaxy, selective chemical etching and subsequent thermal oxidation. The results are observed by scanning electron microscopy. The present method provides a very controllable way to fabricate ultra-fine silicon quantum wires, which is fully compatible with silicon microelectronic technology. As one of the key processes of controlling the lateral dimensions of silicon quantum wires, the wet oxidation of silicon wires has been investigated, self-limiting wet oxidation phenomenon in silicon wires is observed. The characteristic of the oxidation retardation of silicon wires is discussed.

Spreading of a macroscopic lattice gas.

We present a simple mechanical model for dynamic wetting phenomena. Metallic balls spread along a periodically corrugated surface simulating molecules of liquid advancing along a solid substrate. A vertical stack of balls mimics a liquid droplet. Stochastic motion of the balls, driven by mechanical vibration of the corrugated surface, induces diffusional motion. Simple theoretical estimates are introduced and agree with the results of the analog experiments, with numerical simulation, and with experimental data for microscopic spreading dynamics.

Stability of Wetting Fronts in Dry Homogeneous Soils under Low Infiltration Rates

AbstractOur understanding of unstable wetting phenomena in soils is limited. Therefore, lysimeter experiments were conducted in the laboratory to validate current wetting front instability theories. Four different grades of sieved and air‐dried perlite and quartz sand were used as the experimental material. Water was applied by a sprinkler system at rates within the range of natural precipitation rates in New Mexico. Experiments were conducted in small lysimeters (diameter 30 cm, height 50 cm) as well as a large one (diameter 100 cm, height 150 cm). The experimental results show that wetting front instability will cause fingering phenomena in a homogeneous soil system. This observation confirms experimental and theoretical results of other workers. The diameter of fingers was observed to be a function of the grain size of the sand. Small fingers (3–4 cm diameter) were found in coarse sand (grain size 1.41–0.84 mm); large diameter fingers (12 cm diameter) were observed in fine sand (grain size 0.42–0.25 mm). Our experimental results in the coarse sand show that, for infiltration rates varying between 0.3 and 12 cm/h, finger diameters remain more or less constant. This observation also agrees with existing theories. However, at infiltration rates lower than 0.12 cm/h, the coarse sand experiments show that the wetting fronts became stable. For rates between 0.3 and 0.12 cm/h, the wetting is semistable; that is, there is incomplete wetting without distinct development of fingers. A similar trend was observed in the experimental results of sands with grain sizes of 0.841 to 0.594 and 0.594 to 0.42 mm. This phenomenon has not been observed in previous experimental studies and is not predicted by current wetting front instability theories. Our experimental data under infiltration rates similar to natural precipitation intensities may explain why unstable wetting has rarely been observed in wettable field soils.

Particle wettability and line tension

For a spherical solid particle with a uniform surface at rest in a fluid/fluid interface, it is shown that if the line tension associated with the three-phase contact line around the particle is increased (conceptually), the contact angle between the particle and interface does not change continuously to 0 or 180°. Rather, when a critical value of line tension is attained a discontinuous wetting–drying transition can occur. Such a transition corresponds to the condition where the free energy of the particle at local equilibrium in the interface is equal to that of the particle completely immersed in one of the bulk phases. However, local equilibrium in the interface is possible for line tensions greater than that required for the transition. This means that an ‘activation’ energy is required for the irreversible removal of the particle into one of the bulk phases. Literature values of line tensions span at least five orders of magnitude. If the larger of the reported values operate in systems with the geometry of interest here, interesting wetting phenomena should be apparent for particles with diameters of the order of 1 mm.

Studies of silica gel surface wetting phenomena by means of controlled-rate thermal analysis

The paper presents the controlled-rate thermal analysis studies of water film properties on silica gel samples using thermogravimetry and differential scanning calorimetry techniques. On the basis of experimental data the adsorption capacity of silica gel sample, concentration of hydroxyl groups, kinetic of desorption process of water films, surface wetting mechanism and behavior of water films in subambient and elevated temperatures have been investigated. It was stated that this method could be successfully applied in the studies of adsorbed water films and surface wetting phenomena.

A critical review of wetting and adhesion phenomena in the preparation of polymer-mineral composites

The wetting behavior of liquid polymers at solid surfaces and the adhesion forces involved at polymer-filler interfaces should always be considered in the preparation of high-quality polymer composites, including those made with mineral fillers. Spontaneous polymer spreading over the filler surface is a basic condition for the design of polymer-mineral composites. In this regard, the wetting and adhesion characteristics of polymer-mineral systems are reviewed. Based on this review it is clear that several aspects of wetting phenomena at mineral-filler surfaces require further systematic study. Specifically, efforts should be made to determine the wetting characteristics of molten polymers at mineral-filler surfaces in greater detail. Also, the effects of mineral-surface contamination (including surface hydration) on wetting and adhesion phenomena need specific investigation. It is expected that an improved fundamental understanding of wetting and adhesion phenomena for polymer-mineral composites will provide the basis for further technological development.

Chemical thermodynamics as a predictive tool in the reactive metal brazing of ceramics

Thermodynamics have long been applied to our understanding of the reactive wetting phenomena in metal-ceramic joining. We postulate the existence of a “solvent effect” due to the interaction between the reactive element addition and the brazing alloy. This effect plays a significant role in reactive wetting. By taking this effect into account, more realistic reactivities of different reactive element additions into a given brazing base alloy are predicted. Irreversible thermodynamics are also used to characterize the driving forces for reactive metal-ceramic joining.

Optical interferometry in phase separated 3He 4He mixture films

We report on interferometric studies of 3He 4He liquid mixtures. Anomalous wetting phenomena were observed in phase separated mixture films: the upper 3He-rich phase did not wet the lower 4He-rich phase under a small continuous feed of 4He atoms to the vapor phase. Instead, under this dynamic situation, floating pools of the 3He-rich phase were stabilized with a contact angle on the order of 10 mrad. In equilibrium, the concentrated 3He-rich phase was found to nucleate uniformly on the free surface, consistent with Antonow's rule.

Molecular dynamics studies of microscopic wetting phenomena on self-assembled monolayers

Molecular dynamics calculations have been used to investigate the behavior of overlayers of water or n-alkane fluids on solid surfaces formed from “self-assembled” monolayers of long-chain hydrocarbons. A microscopic analog of the wetting contact angle is used to measure the surface wetting characteristics. On a nonpolar surface, formed by close packed chains having -CH3 tailgroups, the water molecules aggregate to form a compact droplet. The calculated contact angle of the droplet is similar to experimental values for macroscopic water droplets. Contrary to intuition, the overlayers of hexadecane or decane form droplets with smaller contact angles on the same surface. However, the calculated contact angles are again in reasonable accord with experimental values.

Hydrogen adsorption on alkali metal surfaces: Wetting, prewetting and triple-point wetting

We report the results of a quartz microbalance study of the multilayer adsorption of H2 and D2 on sodium and on rubidium. On sodium H2 and D2 display the well known phenomenon of triple-point wetting. Interestingly low temperature adsorption isotherms of H2 on the evaporated Na surface show sharp layering transitions. On rubidium a weaker substrate H2 and D2 undergo a wetting transition in the liquid phase followed by sharp presetting transitions away from liquid-vapor coexistence. The prewetting phase diagram of H2 on Rb has been quantitatively mapped out.

Effect of yield strength on the basic fatigue strength of welded joints: Ohta, A., Maeda, Y. and Suzuki, N. Fatigue Fract Eng Mater Struct (May 1993) 16 (5), 473–479

Chemical and hydrodynamic aspects of wetting and interfacial phenomena during the solidification processing of metal-matrix composites are reviewed. Significant experimental results on fibre-matrix interactions and wetting under equilibrium and non-equilibrium conditions in composites of engineering interest have been compiled, based on a survey of the recent literature. Finally, certain aspects of wetting relevant to stir-casting and infiltration processing of composites are discussed.

Asymptotic decay of correlations in liquids and their mixtures

We consider the asymptotic decay of structural correlations in pure fluids, fluid mixtures, and fluids subject to various types of inhomogeneity. For short ranged potentials, both the form and the amplitude of the longest range decay are determined by leading order poles in the complex Fourier transform of the bulk structure factor. Generically, for such potentials, asymptotic decay falls into two classes: (i) controlled by a single simple pole on the imaginary axis (monotonic exponential decay) and (ii) controlled by a conjugate pair of simple poles (exponentially damped oscillatory decay). General expressions are given for the decay length, the amplitude, and [in class (ii)] the wavelength and phase involved. In the case of fluid mixtures, we find that there is only one decay length and (if applicable) one oscillatory wavelength required to specify the asymptotic decay of all the component density profiles and all the partial radial distribution functions gij(r). Moreover, simple amplitude relations link the amplitudes associated with the decay of correlation of individual components. We give explicit results for the case of binary systems, expanding on and partially correcting recent work by Martynov. In addition, numerical results for g(r) for the pure fluid square-well model and for gij(r) for binary hard sphere mixtures are presented in order to illustrate the fact that the asymptotic forms remain remarkably accurate at intermediate range. This is seen to arise because the higher order poles are typically well-separated from the low order ones. We also discuss why the asymptotics of solvation forces for confined fluids and of density profiles of inhomogeneous fluids (embracing wetting phenomena) fall within the same theoretical framework. Finally, we comment on possible modifications to the theory arising from the presence of power-law attractive potentials (dispersion forces).

Interfacial and capillary phenomena in solidification processing of metal-matrix composites

Chemical and hydrodynamic aspects of wetting and interfacial phenomena during the solidification processing of metal-matrix composites are reviewed. Significant experimental results on fibre-matrix interactions and wetting under equilibrium and non-equilibrium conditions in composites of engineering interest have been compiled, based on a survey of the recent literature. Finally, certain aspects of wetting relevant to stir-casting and infiltration processing of composites are discussed.

Helium prewetting and nonwetting on weak-binding substrates.

New wetting and nonwetting phenomena for $^{4}\mathrm{He}$ adsorbed on weak-binding substrates such as the alkali metals are predicted. With a nonlocal density-functional model, we calculate the density profile of $^{4}\mathrm{He}$ films adsorbed on surfaces characterized by an attractive van der Waals potential tail of strength ${\mathit{C}}_{3}$ and a well depth D. As a function of ${\mathit{C}}_{3}$ and D, $^{4}\mathrm{He}$ films exhibit both nonwetting and wetting, the latter being accompanied by a prewetting transition. Specifically, we find that $^{4}\mathrm{He}$ will not wet all the alkali metals; only on Li and Na is wetting behavior, with prewetting, expected.

Critical Behavior of Interfaces: Roughening and Wetting Phenomena

ABSTRACTThe critical behavior of interfaces is discussed from a theoretical point of view. Two classes of critical phenomena will be considered: (i) Roughening phenomena related to changes in the interfacial morphology; and (ii) Wetting phenomena related to changes in the interfacial structure. In two dimensions, the critical behavior can be determined exactly for a variety of models. As a result, one obtains different universality classes depending on the nature of the inter molecular forces.