Equilibrium Film Research Articles

Interfacial Films and Wetting Behavior of Hexadecane on Aqueous Solutions of Dodecyltrimethylammonium Bromide

The interfacial tensions and dihedral angles of hexadecane lenses on aqueous solutions of dodecyltrimethylammonium bromide (DTAB) were measured in air as a function of the molality of DTAB at 298.15 K and atmospheric pressure. The air/water interfacial tension of DTAB solution shows a break point that shifts to a lower concentration in the presence of a hexadecane lens. Ellipsometric measurements showed a continuous variation in ellipticity with concentration for pure DTAB solutions, but a discontinuous change in the ellipticity of the air/water interface in the presence of a hexadecane lens. This behavior can be viewed either as a first-order wetting transition driven by the adsorbed surfactant, or as a gaseous-expanded phase transition in the surfactant adsorbed film induced by the hexadecane. The formation of a microscopic oil film in equilibrium with a lens is rationalized by an attractive long-range potential across the thin film. From the measurement of dihedral angles, it was found that there was a concentration region where a lens exists stably even when complete wetting was expected from measured values of interfacial tensions.

Free thin liquid films (foam films) from rhamnolipids: type of the film and stability

For the first time the foam film model has been used to study the interaction behavior of microbial type biosurfactants adsorbed at the air/solution interface. The studies were carried out with microscopic foam films formed from solutions of anionic type rhamnolipid biosurfactants at a constant concentration of 10−4 M. Measurements of the equilibrium film thickness (h) as a function of the electrolyte (NaCl) concentration (Cel) show that with the increase of Cel from 5×10−4 to 1 M, h gradually decreases from about 100 to 5 nm. Formation of common films (h>30 nm), common black films (CBF) with h∼6–20 nm and 5 nm thick Newton black foam films (NBF) was found. The experimentally determined critical electrolyte concentration of CBF–NBF transition (Cel,cr) is 0.8 M NaCl. The direct measurements of disjoining pressure isotherms at different Cel corroborate the type of the common films and demonstrate the existence of an aqueous core in the common and CBF, and the bilayer structure of the NBF. The experimental studies show the role of the Derjaguin–Landau–Verwey–Overbeek (DLVO) surface forces for the stability of the common films. A possible action of additional structural (hydration) and eventual short-range steric forces and their effect for the stability of the thin CBF and NBF are discussed. For thick common films the surface electric parameters—potential of the diffuse electric layer (ϕ0) and surface charge density (σ0) are assessed by the equilibrium film method using the equations of the DLVO theory. The comparison of the obtained ϕ0=71 mV with the previous results obtained for films stabilized with the salt of a fatty acid—Na oleate, demonstrates the role of the carboxylic group of the rhamnolipid molecule for the surface charge of the studied foam films. The investigations can contribute to gaining new knowledge not only for the different type of interactions in thin films stabilized with microbial type biosurfactants, but also for the interactions between two surfaces through a thin film.

Molecular dynamics simulation of defect formation during energetic Cu deposition

The deposition of energetic Cu atoms from 5 to 80 eV onto (0 0 1) Cu was simulated with molecular dynamics. The Cu–Cu interaction potential was a spline of the embedded atom potential developed from equilibrium data, and the universal scattering potential. Incident Cu atoms substituted for first layer substrate atoms by an exchange process at energies as low as 5 eV. Incident Cu atoms of 20 eV penetrated to the second substrate layer, and 20 eV was sufficient energy to produce interstitial defects. Incident atoms of 80 eV penetrated to the third atomic layer, produced interstitials 12 atomic layers into the substrate by focused replacement collision sequences, and produced sputtered atoms with a 16% yield. Interstitial clusters of up to 7 atoms were observed. The observed mechanisms of film growth included: the direct deposition of atoms into film equilibrium atom positions, the exchange of substrate atoms to equilibrium film atoms positions, and the migration of interstitials to equilibrium film atom positions. The relative frequency of each process was a function of incident energy. Since all observed growth mechanisms resulted in film atoms in equilibrium atomic positions, these simulations suggest that stresses in homoepitaxial Cu thin films are due to point defects. Vacancies would produce tensile strain and interstitial atoms would produce compressive strain in the films. It is proposed that immobile interstitial clusters could be responsible for retaining interstitial atoms and clusters in growing metal thin films.

Dewetting of ultrathin surfactant-covered films

Many industrially and technologically important situations involve thin films covered with either pre-existing or introduced surfactant, which can potentially affect dewetting and spreading processes. The two-dimensional dewetting dynamics of ultrathin liquid films are studied here in the presence of insoluble surfactant; surfactants can drive a flow due to surface tension gradients and additionally the coefficients of the intermolecular potential, which are usually assumed to be constant, can depend on the surfactant concentration. Coupled evolution equations for the film height and surfactant concentration are derived using lubrication theory. These equations are parameterized by a Marangoni parameter, ℳ, and the equilibrium film thickness, lc, obtained by setting the intermolecular potential to zero. A linear stability analysis of these equations shows that the presence of surfactant can widen the band of unstable wave numbers and that, for relatively large lc, the selected wave number is minimized for a certain value of ℳ. Numerical simulations of the evolution equations show that initially nonuniform distributions of surfactant can destabilize clean spinodally stable films. Our results also show that it is possible to destabilize these films using uniform initial surfactant distributions, although this is only possible when the intermolecular potential coefficients are concentration dependent.

Influence of dispersion forces on phase equilibria between thin liquid films and their vapour

Heat transfer and nucleation processes in nucleate boiling strongly depend on the phase equilibrium at the liquid–vapour interface. In a certain region between heated wall and a vapour bubble where a thin liquid film is adsorbed, phase equilibria are considerably influenced by dispersion forces acting on the liquid film. As shown in the paper in such systems the chemical potential, decisive for phase equilibria between liquid films and their vapour, contains an additional term for the action of dispersion forces, and differs from the chemical potential of dispersion-free systems, though their chemical potential is usually taken in the literature for systems with dispersion forces. With the aid of the chemical potential the Kelvin equations for the pressures at the liquid–vapour interface were derived. It turned out that the Gibbs assumption of a geometrical interface between extremely thin liquid films in equilibrium with its vapour does not hold. Instead, following the ideas of van der Waals junior, the small but finite transition interlayer between both phases had to be introduced. As numerical examples illustrate, the dispersion forces considerably influence the pressures at the liquid–vapour interface. In nucleate boiling processes the driving pressure difference for evaporation undergoes a maximum within a tiny area underneath vapour bubbles. As could be shown the maximum driving pressure difference between gas-side interface and gas-core is a considerable fraction of the vapour pressure itself and contributes significantly to the high heat fluxes in nucleate boiling.

Effect of Ionic Surfactants on the Dimple Relaxation in Wetting Films

A procedure for simulation of the dimple relaxation in wetting films is developed based on a precise treatment of the effects of ionic surfactants. The theory is compared with experimental data for the drainage of films of 1 mM solution of sodium dodecyl sulfate. An important effect of the interfacial electrostatics to reduce substantially the Marangoni effect is discovered. It is shown that the adsorption-pressure coupling is also important for ionic surfactants, which, enhanced by the decreased Marangoni number, increases the thinning rate close to the equilibrium. The latter could be a plausible explanation of the discrepancy in the mobility determined by drainage experiments and by dynamics of artificial waves on an equilibrium film from ionic surfactant solution.

An Imaging Reflectometry Study of the Effect of Electrolyte on the Drainage and Profile of an Aqueous Film between an Oil Droplet and a Hydrophilic Silica Surface

Imaging reflectometry was used to study the effect of electrolyte concentration on the profile and drainage of an aqueous film confined by an oil droplet and a hydrophilic silica surface. Droplets were observed to adopt a dimpled profile at the film's center on approach to the silica surface due to hydrodynamic effects. The intervening film eventually evolves to an equilibrium film of uniform thickness stabilized by the double layer repulsion between the drop and the silica surface, which also determines the film thickness at equilibrium. The range over which double layer interactions are detectable is much larger than measurable by mechanical force measuring techniques such as SFA and AFM, indicating the high sensitivity of this system to the detection of surface interactions. In effect, the deformable oil/water interface acts as an infinitely weak spring. The drainage rate at the film's periphery (barrier ring) is controlled initially by hydrodynamic effects. However, as the film thins and double layer overlap occurs, the drainage rate becomes hindered, since double layer repulsion acts to stabilize the draining film, thus opposing drainage. Drainage at the film's center is driven by the Laplace pressure across the oil/water interface while spreading of the film at the barrier ring leads to a large pressure drop across the film from its center across the barrier ring and hence hindered fluid flow.

Van der Waals Interaction between Two Truncated Spheres Covered by a Uniform Layer (Deformed Drops, Vesicles, or Bubbles)

In this paper, we derive explicit expressions for the van der Waals interaction energy and force between two truncated spheres covered by a thin layer of uniform thickness. These expressions can be used for description of the interactions in dispersions of deformable particles (emulsions, vesicle suspensions, and foams). As an illustration of the applicability of these formulas to real systems, we consider the interaction between micrometer-sized oil drops, covered by a protein adsorption layer, in oil-in-water emulsions. The Hamaker constants for the interactions between protein, oil, and water are calculated from literature data. The equilibrium radius and thickness of the liquid film, which forms upon the collision of two emulsion drops, are determined as functions of the drop size and electrolyte concentration. From the parameters of the equilibrium films, we calculate the total interaction energy between the drops. The numerical results show that both factors, the presence of the protein layer and the drop deformation, have a significant effect on the pair interaction energy.

Equilibrium Amorphous Silicon–Calcium–Oxygen Films at Interfaces in Copper–Alumina Composites Prepared by Melt Infiltration

The microstructure of copper–alumina (Cu‐Al2O3) composites that have been prepared via the melt infiltration of liquid copper into porous alumina preforms was studied in detail, using various transmission electron microscopy (TEM) techniques. Two different samples—with open pore diameters of 0.2 and 0.8 μm—were investigated. For both specimens, a single crystalline copper network that extended throughout the open porosity of the alumina preform was observed. An amorphous glass phase that contained silicon and calcium was observed at the Al2O3/Cu/Al2O3 triple junctions. The diameters of these amorphous pockets, which were strongly faceted along the Al2O3 grains, were up to 20 and 100 nm for the initial pore sizes of 0.2 and 0.8 μm, respectively. A glass phase that contained silicon and calcium also was present at the Cu/Al2O3 interfaces, whereas the Al2O3 boundaries remained dry. Detailed high‐resolution transmission electron microscopy investigations have shown that the interfacial glass phase at the Cu/Al2O3 interfaces exhibited a uniform equilibrium film thickness along the interface region. However, the interfacial film thickness was dependent on the orientation of the Al2O3 grain, and its value varied from 0.4 nm for Al2O3 rhombohedral‐plane termination ((1¯012)) up to 1 nm for Al2O3 basal‐plane termination ((0001)).

Effect of Fructose, Sucrose, and Dimethyl Sulfoxide on the Equilibrium Thickness of DMPC Foam Films

Foam films stabilized by phospholipids are applied for model studies of interactions between lipid layers. The thickness of the free-standing foam films, obtained from aqueous lipid dispersions of DMPC in the presence of low molecular weight sugars fructose and sucrose, and dimethyl sulfoxide (DMSO), was measured by means of the microinterferometric technique. The equilibrium film thickness is determined by the interplay of the different components of the disjoining pressure and it is influenced by the composition of the film forming dispersion. In the absence of solutes and electrolyte DMPC forms Newton black films. The addition of sugars and DMSO results in an increase in the film thickness, which is attributed to an increase in the surface potential. In this case, the electrostatic disjoining pressure determines the film thickness, while the van der Waals attraction is not significantly influenced by the solutes. The effective Hamaker constants are obtained from the velocity of film thinning. The doubl...

Foam and Wetting Films from Aqueous Cetyltrimethylammonium Bromide Solutions: Electrostatic Stability1

Foam films and wetting films on quartz formed from aqueous solutions of cetyltrimethylammonium bromide (CTAB) are investigated in a wide range of surfactant concentrations in the presence of background electrolyte (5 × 10–4 mol dm–3 NaCl). Foam and wetting films are convenient models for the study of symmetric (free thin liquid films) and asymmetric (thin liquid films on solid substrate) films with the same air/solution interface. Microinterferometric methods of assessment of foam and wetting films are used which allow precise determination of the film thickness. Determined are the values of the potential ϕ0 of the diffuse electrical layer at the solution/air interface (applying the method of “equilibrium foam films”) and the potential ϕ1 at the solution/quartz interface (applying the method of capillary electrokinetics). These values are used to analyze the stability of the films studied in terms of the DLVO theory. A conclusion drawn is that both kinds of films studied are stabilized by electrostatic interaction forces. It is shown that with increasing CTAB concentration, a charge reversal occurs at both the solution/air and solution/quartz interfaces which determines the stability/instability conditions of the foam and wetting films. Concentration ranges where both kinds of films produce stable (equilibrium) films are found. There are also concentration ranges where the films either rupture or are metastable (quasi-equilibrium). The CTAB concentration ranges, which provide the formation of unstable (rupturing and metastable) and stable films, are different for symmetric (foam) and asymmetric (wetting) thin liquid films. It is only at high CTAB concentrations (higher that > 2 × 10–4 mol dm–3) that both cases render formation of stable equilibrium films. These studies give direct experimental indications that the electrostatic interactions between identical or different interfaces can differ when the surfactant concentration is varied.

Casimir effect in critical films of binary liquid mixtures

We present experimental evidence for the Casimir effect within critical films of binary liquid mixtures possessing opposite boundary conditions (+-) by studying the thickness of these vapor-adsorbed films on a silicon wafer as a function of temperature near the critical temperature. Our results for two different critical mixtures demonstrate that the critical Casimir pressure scaling function vartheta(+-)(y) scales with y=L/xi, where L is the equilibrium film thickness and xi is the bulk correlation length. Additionally, on approaching the critical temperature T(c) an increase in the film thickness L is observed, implying that the sign of the universal Casimir amplitude Delta(+-)=vartheta(+-)(0)/2 at T(c) is positive, consistent with theoretical predictions. However, the magnitude of the Casimir amplitude that we measure is approximately two orders of magnitude smaller than that given by prevailing theories. In the two-phase region of the liquid mixture, preliminary evidence suggests that the adsorbed film undergoes a surface phase transition from a film near the critical composition at T less, similarT(c) to a film near one of the bulk phases at T<<T(c). This low temperature film composition most likely corresponds to the bulk phase rich in the component that preferentially adsorbs at the silicon surface.

Phospholipid foam films studied by contact angle measurements and fluorescence microscopy

Foam films drawn from suspensions of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) in water/ethanol mixtures were used for the investigation of the relation between the properties of the monolayers and the interaction between the film surfaces. The film thickness and the contact angle between the film and the meniscus were measured as a function of the temperature in a range around the temperature of the main phase transition for the lipid. Additionally, fluorescence microscopy was applied to investigate the distribution of a fluorescent lipidlike dye in the surface of the film and the meniscus. From the contact angle the free energy of film formation was calculated. At the temperature of the chain-melting phase transition the film thickness decreases by 0.7 nm. This can be related to a decrease in the thickness of the hydrocarbon layers of the lipid monolayers at this temperature. The decrease in the film thickness leads to a reduction in the free energy by increasing the van der Waals attraction between the film surfaces. No structures were observed in the monolayers of the film in the fluorescence investigation. However, on formation of the very thin equilibrium film the dye was expelled from the film area, indicating an increase in the packing density of the lipid, if the monolayers are in adhesive contact in the film.

Morphology of Ultrathin Supported Diblock Copolymer Films: Theory and Experiment

An analysis of the morphological behavior of substrate-supported diblock copolymer films for thicknesses t below the equilibrium period L0 of the copolymer is presented. Substrate-supported films generally exhibit dissimilar interactions between the copolymer block components and the free and substrate surfaces. Accordingly, in this study, self-consistent-field calculations that incorporate asymmetric surface energetics were used to assess equilibrium film morphologies. Phase diagrams were constructed as a function of film thickness, surface interaction energies, the segmental interaction, and the chain length. In conjunction, experiments were conducted on a series of polystyrene-b-poly(n-alkyl methacrylate) copolymer films supported by silicon substrates. These employed a novel atomic force microscopy technique that allowed for the precise tracking of morphology as a function of film thickness. Comparison of the experimental results and calculations revealed several common trends. In particular, hybrid m...

The dependence of equilibrium film thickness on grain orientation at interphase boundaries in ceramic–ceramic composites

High-resolution transmission electron microscope observations of hexagonal boron nitride – 3C silicon carbide interphase boundaries suggest that where one or more phases is highly anisotropic, an orientation dependence on equilibrium film thickness can arise. Theoretical considerations of this phenomenon in terms of the equilibrium thickness of an amorphous film between two crystalline media are consistent with the trend seen experimentally.

Heat exchange processes near the boundary of a film boiling nucleus

Results are presented of an experimental investigation of the change in temperature, heat flux to the liquid, and rate of displacement of the isotherms near a film boiling nucleus propagating over a plane surface. The experiment was carried out in a liquid nitrogen bath at atmospheric pressure on the saturation line. The heater was a sapphire plate 1.2 mm thick having a heat transfer surface area of 77×22 mm2. The following facts were established: 1) near the boundary of the film boiling nucleus a new heat exchange mechanism takes place caused by the instability of the liquid microlayer; 2) the maximum heat flux to the liquid is considerably greater than the critical heat flux; 3) the vapor film in the film boiling region grows gradually with increasing distance from the boundary, i.e., there is a smooth transition in terms of heat exchange intensity before the equilibrium film boiling level is reached.

Instability of Ultra-Thin Water Films and the Mechanism of Droplet Formation on Hydrophilic Surfaces

This paper presents a new theory of droplet formation during condensation of water on a hydrophilic surface. The theory uses hydration, electrostatic, van der Waals, and elastic strain interactions between a hydrophilic solid surface and a water film, and shows that contributions to the disjoining pressure are dominated by hydration forces for films thinner than 3 nm. The equilibrium film thickness is found to remain almost constant at about 0.5 nm for a wide range of relative humidity, although it increases sharply as the relative humidity approaches unity. The competition between strain energy on one hand, and hydration, van der Waals, and liquid-vapor surface tension on the other, induces instability for films thicker than a critical value. The critical wavelength of instability, Lcr is also predicted as a function of film thickness. The theory proposes that as the relative humidity increases, nucleation initially occurs in monolayer fashion due to strong hydration forces. Using nucleation thermodynamics it predicts a critical nucleus size, d*, and internuclei spacing, I, as a function of subcooling, ΔT, of the solid surface and shows that both length scales decrease with increasing subcooling. Since these monolayer nuclei are formed on the adsorbed water film, it is shown that when the internuclei spacing is larger than the critical wavelength, l &gt; Lcr instability occurs in the film resulting in droplet formation. The theory predicts that beyond a certain value of subcooling, the interdroplet spacing is “choked” and cannot decrease further.

Analysis of Liquid Crystalline Fiber Coatings

A phenomenological model for the spreading of nematic liquid crystals on fibers of circular cross section is presented. The model is used to calculate the wetting thresholds of nematics on fibers for three chracteristic director orientations: axial (undistorted), radial (splay), and azimuthal (bend): the wetting thresholds are given in terms of critical film thickness hc and critical spreading parameter Sc. When the spreading parameter S is greater than the critical value Sc, nematic liquid crystal fiber wetting is similar to that of viscous fluids when director distortions are absent. In other cases when nematic liquid crystals spread on fibers the distortion due to the cylindrical geometry gives rise to Frank elasticity. For symmetric director boundary conditions the characteristic modes of splay and bend increase the magnitude of critical spreading parameter Sc. It is found that when spreading occurs (S>Sc) the equilibrium film thickness decreases with increasing elastic storage. The film thickness for rod-like and disc-like nematics differs because the splay-bend elastic anisotropy is reversed.

A Diffuse Interface Description of Intergranular Films in Polycrystalline Ceramics

The thermodynamic equilibrium between a thin liquid phase and adjoining grains of a co‐existing solid phase is treated, using a diffuse interface formulation applied to a multilayer of liquid and solid phases. The analysis leads to spatial variation in composition, which minimizes the overall free energy. The overall energy includes a gradient‐energy contribution. For a fixed overall composition, the equilibrium concentration profile is dependent on the thicknesses of the two phases, relative to the interface width (a characteristic‐length scale in the analysis). When the thickness of the liquid phase approaches the characteristic length, its composition can deviate markedly from the bulk liquid‐phase composition. In the absence of any stabilizing interactions, such as repulsive structural and electrostatic forces, the analysis indicates that there is a driving force for thinning of the film and a minimum thickness exists at which it becomes favorable for the film to dissolve. Therefore, the observations of equilibrium film thicknesses in certain ceramics imply that, in those materials, additional stabilizing forces must exist between the grains.

Imaging Ellipsometry/Reflectometry for Profiling the Shape of a Deformable Droplet as It Approaches an Interface

The application of imaging ellipsometry/reflectometry to study the system of an oil droplet approaching a silica surface in a continuous aqueous medium is described for the first time. Imaging ellipsometry/reflectometry is shown to be capable of accurately determining changes in the profile of a droplet as film drainage occurs between a droplet and an interface. The technique has the advantages that it is sensitive to the measurement of both thick and thin films and yields a film profile in real time without the need to scan across the surface. It is shown that imaging ellipsometry/reflectometry provides a valuable alternative to interferometric and photometric techniques, which have previously been employed to study a system of this type. The approach of a squalene droplet to a silica surface in a continuous aqueous phase was investigated. The droplet was observed to dimple on approach to the silica plate. The shape of the droplet evolved from a dimpled to flattened profile. The equilibrium film thicknes...