Aqueous Wetting Films Research Articles

Electric-Field-Controlled Flow in Nanoscale-Thin Wetting Films

A novel nanofluidic system based on electroosmotic flow in nanoscale-thin aqueous wetting films is reported. The water films formed spontaneously on mica substrates in a saturation humidity environment. The film thickness was determined to be a few tens of nanometers by optical interference and fluorescence intensity measurements and was consistent with a theoretical evaluation of the thickness of a film based on the competing forces of electrostatic repulsion and capillary pressure. Lateral flow was induced by applying a dc electric field tangential to the film and characterized by tracking the position of a fluorescent probe. The mobilities of the thin fluid layer and the flow marker were lower than the predictions of the electrokinetic theory, which may be a result of adsorption of the fluorescent molecules to the mica. Confinement of the film to two-dimensional "channels" was achieved by microcontact printing of patterned hydrophobic monolayers onto the substrate. This system has the advantage of simple and inexpensive fabrication in comparison to nanofluidic devices made by traditional lithography techniques.

Negative charges at the air/water interface and their consequences for aqueous wetting films containing surfactants

Negative charges at the air/water interface have been proven previously by the authors via disjoining pressure measurements of wetting films. In the present study the surface charges of wetting film are modified in order to control the film stability. Studies at different pH values support the assumption that the negative charges are caused by an excess of hydroxide ions. Addition of different types of salt anions seems to lead to an increase in surface charges with decreasing hydration shell. Both water-soluble and water-insoluble amphiphiles replaces the charges. The air/water surface remains negatively charged in case of non-ionic surfactant (dodecyl-beta maltoside, C12G2) and anionic ampihiphiles (stearic acid and SDS). The charge will be reversed in case of cationic amphiphiles. Under all of those conditions, films against likely charged solid surfaces are stable and unstable against oppositely charged solid substrates. For this purpose, the charge of silicon wafers was precoated with polyelectrolytes before. At concentrations well below the CMC cationic surfactants are not able to reverse the charges at the air/water interface. Respective wetting films are less stable and show a partial dewetting even on likely charged surfaces. These results lead to the conclusion that the films are stabilised by electrostatic repulsion. Potential adsorption of water-soluble surfactants on oppositely charged surfaces is also taken into account and is studied by contact-angle measurements.

Rupture of Wetting Films Caused by Nanobubbles

It is now widely accepted that nanometer sized bubbles, attached at a hydrophobic silica surface, can cause rupture of aqueous wetting films due to the so-called nucleation mechanism. But the knowledge of the existence of such nanobubbles does not give an answer to how the subprocesses of this rupture mechanism operate. The aim of this paper is to describe the steps of the rupture process in detail: (1) During drainage of the wetting film, the apex of the largest nanobubble comes to a distance from the wetting film surface, where surface forces are acting. (2) An aqueous "foam film" in nanoscale size is formed between the bubble and the wetting film surface; in this foam film different Derjaguin-Landau-Verwey-Overbeek (DLVO) forces are acting than in the surrounding wetting film. In the investigated system, hydrophobized silica/water/air, all DLVO forces in the wetting film are repulsive, whereas in the foam film the van der Waals force becomes attractive. (3) The surface forces over and around the apex of the nanobubble lead to a deformation of the film surfaces, which causes an additional capillary pressure in the foam film. An analysis of the pressure balance in the system shows that this additional capillary pressure can destabilize the foam film and leads to rupture of the foam film. (4) If the newly formed hole in the wetting film has a sufficient diameter, the whole wetting film is destabilized and the solid becomes dewetted. Experimental data of rupture thickness and lifetime of wetting films of pure electrolyte and surfactant solutions show that the stabilization of the foam film by surfactants has a crucial effect on the stability of the wetting film.

Stability and rupture of aqueous wetting films.

The rupture of aqueous wetting films on solid surfaces is an important step in a lot of coagulation processes in colloidal systems. Particularly, the interaction of an air bubble with a solid particle in the industrial flotation process [1,2], which is widely used in mineral processing, in paper recycling and waste water treatment, the forming, thinning and rupture of thin intervening water films play a crucial role. Whether the film rupture takes place in the short contact time of the particle and the bubble (millisecond scale) or not is of decisive consequence for flotation success and yield.

Aqueous Wetting Films on Fused Quartz

Using an image analyzing interferometer, IAI, the interfacial characteristics of an isothermal constrained vapor bubble, CVB, in a quartz cuvette were studied as a precursor to heat transfer research. The effects of pH and electrolyte concentration on the meniscus properties (curvature and adsorbed film thickness) and the stability of the aqueous wetting films were evaluated. The surface potential in the electric double layer was a function of the cleaning and hydroxylation of the quartz surface. The disjoining pressure isotherm for pure water was very close to that predicted by the Langmuir equation. For aqueous solutions of moderate electrolyte concentration, the Gouy–Chapman theory provided a good representation of the electrostatic effects in the film. The effect of temperature on the film properties of aqueous solutions and pure water was also evaluated: The meniscus curvature decreased with increasing temperature, while Marangoni effects, intermolecular forces, and local evaporation and condensation enhanced waves on the adsorbed film layer. Pure water wetting films were mechanically metastable, breaking into droplets and very thin films (less than 10 nm) after a few hours. Aqueous wetting films with pH 12.4 proved to be stable during a test of several months, even when subjected to temperature and mechanical perturbations. The mechanical stability of wetting films can explain the reported differences between the critical heat fluxes of pure water and aqueous solutions. The IAI-CVB technique is a simple and versatile experimental technique for studying the characteristics of interfacial systems.

On the forces of hydrophobic attraction in wetting films of aqueous solutions

On the basis of the isotherms of disjoining pressure of aqueous wetting films the contact angles, θ were calculated as a function of the electrical potentials of the substrate, φ 1, and film air interface, φ 2 and on the degree of its hydrophobicity. When calculating θ values, dispersion repulsion and electrical and hydrophobic attraction forces are taken into account. In the case of high values of φ 1 and φ 2, which differ in sign, formation of high contact angles is caused by the electrostatic attraction forces only. Hydrophobic attraction forces play an important role when both interfaces of a wetting film are hydrophobic. This may be realized in the absence of surfactants or when the surfactant molecules are hydrodynamically shifted to the rear part of the rapidly approaching bubble. This indicates in particular the importance of hydrodynamic effects during flotation. These effects influence not only the kinetics of the bubble-to-particle approach but also the forces of attachment.

Influence of cationic surfactant on the surface charge of silica and on the stability of aqueous wetting films

Interference and ellipsometric methods were used to measure the equilibrium thicknesses of wetting films (50–70 nm) of aqueous solutions of CTAB and NaCI. The experimental isotherms of disjoining pressure of wetting films agree with the results of calculations performed according to the theory of electrostatic forces acting between film surfaces whose potentials differ in value. The ranges of the stability, metastability, and instability of the films were determined. The rupturing of metastable films is accompanied by a transition from complete to partial wetting.

Profile of the transition region between aqueous wetting films on quartz and the adjacent meniscus

Between an equilibrium wetting film and the adjacent meniscus a transition region exists, its shape being determined by both capillary and surface forces. A detailed experimental study of the profile of the transition region has been performed. Using a special device, an aqueous meniscus is pressed through a tube against a quartz glass plate. The resulting circular microscopic wetting film as well as the interference pattern of the surrounding meniscus are photographed through a microscope in reflected light. The local thicknesses are evaluated from the corresponding optical densities in the photographs and used to construct the profiles of both the transition region and the meniscus. The profiles obtained for aqueous solutions of 0.1, 0.5 and 1 mmol dm −3 KCl and those theoretically calculated on the basis of a suitable disjoining pressure isotherm are found to be in good agreement.

The transition region between aqueous wetting films on quartz and the adjacent meniscus

Aqueous wetting films on a plane quartz plate are studied experimentally to elucidate some of the details of the transition region between the film and

Stability of Aqueous Wetting Films in Athabasca Tar Sands

Abstract The existence of thin films of water that completely wet the sand grains has long been regarded as an important feature of the Athabasca oil sands deposit. Direct microscopic evidence, however, cannot be relied on to establish whether such films are present, The existence and stability of such films, therefore, must be inferred from the relevant surface chemical forces for the oil/brine/rock system. A detailed analysis of these forces shows that the stability of these thin wetting films is critically dependent on whether the zeta potentials (and charge densities) for the two electrical double layers bounding the film are of like sign. The zeta potential and charge density for the rock/brine interface will in almost all cases be negative in sign. Therefore, a requirement for the stability of a wetting film will be that these quantities are also negative at the brine/oil interface. New measurements of the electrophoretic mobility of small particles of Athabasca bitumen suspended in an aqueous phase are reported. These data show that the zeta potential at the bitumen/water interface is strongly negative. Consequently, these results support the hypothesis that wetting films will be stable in this instance.