Finite Contact Angle Research Articles

Dynamics of stick-slip jumps

The dynamics of stick-slip jumps on heterogeneous surfaces is analysed with a simple experimental model system. The heterogeneity of the surface is controlled by the application of thin low-energy ink lines on a high-energy glass surface. The liquid spreads completely on the bare glass surface but has a finite contact angle on the ink. The ink lines serve to pin the contact line in a controllable way. The stick-slip jumps from one pinning position to another manifest themselves as “waves” propagating along the contact line with well-defined shapes and constant velocity. The important findings are: (1) the propagation velocity of a stick-slip wave is to first order independent of the wave amplitude, (2) the velocity depends on the instantaneous contact angle, reminiscent of “Tanner's Law”, and (3) above a certain critical width of the pinning line, the waves halt completely.

Residual water saturation, electrical conductivity, and rough rock/pore interfaces

In this paper, we model the roughness of the pore/rock interface in terms of a random surface; we construct the surface by assigning random heights to the vertices of a triangular lattice. A completely wetting fluid preferentially lies along the concave creases of the surface and forms connected pathways that span the surface. A wetting fluid with finite contact angle does not necessarily form continuous pathways in this model. Electrical conductivity for the completely wetting case is simply proportional to the wetting-fluid saturation at very low saturations; this result agrees with recent experimental studies.

Capillary flow phenomena and wettability in porous media: II. Dynamic flow studies

The dynamics of capillary penetration of liquids into packs of monodisperse spherical beads of varying surface energy have been studied. Square-root-of-time dependence has been shown to be obeyed in horizontal one-dimensional flow for all media. The STVF model of Miller and Miller was shown to be valid for similar media of varying particle sizes and for liquids with different physical properties but exhibiting the same contact angle. The rates of horizontal one-dimensional flow of liquids exhibiting finite contact angles with polymethylmethacrylate-coated and siliconized beads, reported as reduced sorptivities and reduced penetrabilities, were found to deviate from values calculated for known contact angles using the cylindrical tube model of Washburn.

Nucleation and Growth of Ultrathin Epitaxial Metal Silicides on Silicon

ABSTRACTGrowth of epitaxial single crystal nickel and cobalt disilicide films on silicon is performed under UHV conditions by deposition of Co or Ni on silicon followed by a high temperature reaction to form the silicide. The uniformity and perfection of ultrathin epitaxial layers has been studied using transmission electron microscopy. Mechanisms controlling island growth are discussed. Island nucleation observed in pseudomorphic films of NiSi2 /Si(100) (∼60Å thick) is shown to be the result of the difference in symmetry between the NiSi2 and Si. Islands related by a translation vector a/4<111> show an equilibrium island separation of 15±1.5Å. The boundary between islands is described as a coreless defect. In comparison,, pseudomorphic layers of CoSi2 /Si(111) are observed up to thicknesses ∼30Å. Pinholes are commonly observed in CoSi2 /Si(111) thin films. Finite contact angles (∼50) between substrate and deposit suggest a desire for three-dimensional growth under equilibrium conditions.

Imperfect Hele - Shaw cells

The covering surfaces of a Hele-Shaw cell may be roughened mechanically, or modified by random chemical patches which change the wetting properties. We discuss first the quasi-static injection of a single fluid (with a finite contact angle θ e ) in an imperfect cell. The shape of the injected region depends critically on a dimensionless number τ (ratio of the boundary energy τ over the capillary energy fluctuations in one patch). At τ Analyse de l'injection quasistatique d'un fluide partiellement mouillant dans une cellule de Hele shaw

Geometry of clusters of strongly coagulated fluid drops and the occurrence of collapsed Plateau borders

A detailed geometric analysis is presented of stabilized clusters of strongly coagulated fluid drops in a liquid medium. These systems are characterized by a finite contact angle θ at the perimeter of each thin liquid film separating adjacent drops. When the number of drops in the cluster is at least three and θ > 30°, three such films meet along a line, the “collapsed linear Plateau border.” Two types or termination points of collapsed linear borders are defined: “Q-points” and “T-points.” A Q-point is the point of intersection of a collapsed linear Plateau border with three film perimeters. A T-point, or “collapsed tetrahedral Plateau border,” is the point where four collapsed linear borders meet at tetrahedral angles of 109.47°. Depending on the nature of its endpoints, a collapsed linear border can be classified as being of the QQ-, TT-, or QT-type. When 30° < θ < 35.26°, the border is always of the QQ type. When θ > 35.26°, it can be of any of the above three types, depending on the number of drops in the cluster and the location of the border within the cluster. At a Q-point, the angle α between the collapsed Plateau border and each of the three film perimeters is given by cos α = cot θ/√3.

Liquid/liquid/solid contact angles measured by interference microscopy

Interference microscopy has been used to measure contact angles in aqueous/organic/graphite systems. For contact angles < 15° this technique is of high precision. The solid substrate, highly oriented pyrolytic graphite (HOPG), was prepared for contact-angle measurements by cleaving under various media and by high-temperature outgassing. Differences in the measured contact angles depending on the surface pretreatment are reported. For the system n-heptane/water/HOPG in which the liquids were outgassed and the graphite cleaved under heptane the mean receding contact angle was 7.25 ± 1°. A similar result is obtained by high-temperature outgassing of the solid. Preliminary experiments show that n-decane and n-undecane show finite (though smaller) contact angles, while n-hexadecane, n-octanol, benzene and toluene displace water from the surface, which is then completely wetted by the organic phase.The overall mean receding contact angle for the heptane system is 7.25 ± 1.5°.

Rheology of foams and highly concentrated emulsions: I. Elastic properties and yield stress of a cylindrical model system

Expressions are derived for certain rheological properties, such as the stress vs strain relationship, yield stress, and shear modulus, of monodisperse foams and highly concentrated emulsions for the model of infinitely long cylindrical drops (or bubbles). The variables considered are the volume fraction of the dispersed phase, the drop radius, the interfacial tension, the thickness of the films separating adjacent drops, and the films' associated contact angle. Both the yield stress and the shear modulus are proportional to the interfacial tension and inversely proportional to the drop radius. The yield stress increases sharply with increasing volume fraction, while the shear modulus varies as its square root. The effect of a finite contact angle, θ, is to decrease the shear modulus and, in most cases, to increase the yield stress. Finally, the effect of a finite film thickness is to always increase both the yield stress and the shear modulus. The implications of these results to real emulsions and foams are discussed.

The interfacial profile in the contact line region and the Young—Dupré equation

The Young-Laplace equation of capillarity augmented by the disjoining pressure concept of Deryagin is used to obtain the liquid-vapor interfacial profile in the contact line region of a finite contact angle system. The calculated apparent contact angle agreed reasonably well with experimental results for the alkane-PIFE system. This work demonstrates that a modified and simpler version of the model and procedures leads directly to the Young-Dupre equation.

Adsorption and capillary condensation at the contact line in change of phase heat transfer

A model based on adsorption and capillary condensation which describes the wetting characteristics of the contact line in change of phase heat transfer is proposed. A relationship between the equilibrium shape of a thin film at the contact line, superheat, and interfacial forces is derived for both zero and finite contact angle systems on “smooth” surfaces. Contact line boundary conditions which are a function of temperature, superheat and interfacial forces are presented. Homogeneous and heterogeneous nucleation at the minimum film boiling temperature is discussed.

Interfacial profile in the contact line region of a finite contact angle system

The Young-Laplace equation of capillarity augmented by the disjoining pressure concept of Deryagin has been used to obtain the interfacial profile in the contact line region of a finite contact angle system. An initial interfacial slope identical to zero and an initial effective thin-film thickness of the order of interatomic spacing lead to a finite apparent contact angle and substantial initial curvature. The results agree with published data on the apparent contact angles of the alkane-PTFE system.

Highly concentrated emulsions. II. Real systems. The effect of film thickness and contact angle on the volume fraction in creamed emulsions

Several aspects of a previously developed two-dimensional theory are extended to real, though monodisperse, emulsions. Specifically, the volume fraction φ 0 of the dispersed phase in close-packed, uncompressed emulsions is predicted as a function of the thickness of the film of continuous phase separating neighboring droplets, and of the contact angle between the film and its adjacent Plateau border. When the film thickness and contact angle are both zero, φ 0 equals 0.7405. A finite film thickness tends to reduce this value, while a finite contact angle tends to raise it as a result of spontaneous deformation of the drops. In principle, φ approaches unity as the contact angle increases to 35.26°. Experiments with cream layers formed at the top of initially more dilute oil-in-water emulsions, stabilized with anionic surfactants such as sodium dodecyl sulfate, confirm these theoretical predictions, at least semiquantitatively. The effects of the type of anionic emulsifier, counterion, and oil phase were studied. In some systems, contact angles as high as 65° were observed. Whenever the contact angle is large, it is difficult to realize the high volume fractions predicted theoretically, because of the formation of a rather rigid floc network, trapping pockets of continuous phase that are not readily expelled, even by centrifugation. For compressed emulsions, the relationship between the volume fraction φ (> φ 0) on the one hand and the drop shape and the disjoining pressure in the films on the other (which could be developed rigorously for cylindrical systems) could be extended to real systems only for the limiting case of extremely high volume fractions, where the continuous phase is present in the form of straight Plateau borders bounded by cylindrical surfaces.

Surface forces in adsorbed multilayers of water on quartz

The adsorption of water vapor on smooth crystalline quartz at pressures close to saturation has been determined by ellipsometry, particular attention being paid to excluding gel layers and contamination. Contrary to some literature reports, such surfaces proved to be perfectly hydrophilic, showing thick adsorption multilayers and zero (or very low) contact angle. Quartz dehydroxylated by heating, or slightly contaminated, or deliberately methylated, showed thinner films and finite contact angles ranging from 20 to 80°. Films on clean quartz at saturation are consistent with calculated electrical double layer repulsion. But below saturation they are much thicker than expected on current theory of DLVO-type forces. Contact angles apparently arise from changes in short-range forces of a “structural” type.

Highly concentrated emulsions. I. Cylindrical systems

Expressions are derived relating the geometric and thermodynamic properties of highly concentrated, monodisperse emulsions to the volume fraction of the dispersed phase for the model of infinitely long cylindrical drops. The equilibrium thickness of the thin films separating the deformed droplets, as well as the associated contact angle, are introduced as additional variables. Thermodynamic properties considered are the pressure exerted on the films, the “osmotic pressure” of the emulsion, the work of compression, the relative vapor pressure of the continuous phase, etc. The theory is capable of predicting the maximum attainable volume fraction as a function of drop size, equilibrium film thickness, contact angle, and the maximum disjoining pressure that can be developed within the films. The presence of a finite contact angle leads to spontaneous deformation of the drops and contraction of the emulsion to a volume fraction in excess of that corresponding to hexagonally close-packed cylinders.

The contact angle of phospholipid monolayer on a wilhelmy plate

Contact angles of phospholipid monolayers with polished glass plate were measured by a Wilhelmy method. On monolayers of dipalmitoyl phosphatidyl choline and sphingomyelin with choline residue, we obtained finite contact angles which increased with the increase of surface pressure. On the other hand, on monolayers of dipalmitoyl phosphatidyl ethanolamine without choline residue and fatty acids, such as myristic acid and stearic acid, we had nearly zero contact angles in the surface pressure range of 0–40 dyn cm −1.

An Evaporating Ethanol Meniscus—Part II: Analytical Studies

A model of capillary flow heat transfer is used to analyze the data obtained in Part I. The results indicate that fluid flow resulting from a change in the meniscus profile replenishes the liquid evaporated in a stationary evaporating meniscus. Local evaporative heat fluxes can be very high near the interline region of a finite contact angle meniscus. Therefore, a small stationary evaporating meniscus can be used as a very effective local heat sink.

The corrosion and wetting behaviour of copper in the presence of some thiol reagents

The corrosion and wetting behaviour of copper in aqueous solutions of potassium diethyldithiophosphate and sodium diethyldithiocarbamate has been studied by measurement of the limiting current density for oxygen attack of the metal and contact angles. In dilute solutions of the reagent, bulk precipitation of the metal-reagent salt is observed; there is no effect on the limiting current density; and the contact angle is zero. In concentrated solutions there is no bulk precipitate; the limiting current density decreases with increasing reagent concentration; the contact angle is finite. Finite contact angles appear to result from the presence of a hydrophobic metal-reagent salt on the surface. The behaviour of the system, which could not be explained on the thermodynamic basis, is explained in terms of a simple model that takes into consideration kinetic factors.

Adsorption on low energy surfaces: III. n-Octane, benzene, and carbon tetrachloride on modified quartz

Prior work on low energy surfaces has suggested that the adsorbed film is submonolayer at saturation pressure in those cases for which the liquid phase of the adsorbate vapor exhibits a finite contact angle on the solid surface. This study reports on the modification of particulate quartz by high temperature- and pressure-induced esterification of surface hydroxyl sites with n-amyl alcohol and with 2,2,2-trifluoroethanol and on subsequent vapor adsorption studies using n-octane, benzene, and carbon tetrachloride as adsorbates. The adsorption data were analyzed in the two-dimensional van der Waals model framework. Spreading pressures at saturation vapor pressure ranging from 5 to 7.2 erg/cm 2 were obtained with surface coverages near one-half monolayer. The values are compared with those obtained from various treatments of data provided by other experimental techniques.

The hydrophilic nature of gold and platinum

Clean gold and platinum surfaces have been shown to be hydrophilic in borate and sulphate media at all potentials between hydrogen and oxygen evolution. Finite contact angles were observed at the nitrogen/metal/solution interface immediately after polishing with diamond paste, but electrochemical evidence demonstrated that such surfaces were contaminated. After cleaning the surface chemically or electrochemically, zero contact angles were observed.

The hydrophilic nature of gold and platinum

Clean gold and platinum surfaces have been shown to be hydrophilic in borate and sulphate media at all potentials between hydrogen and oxygen evolution. Finite contact angles were observed at the nitrogen/metal/solution interface immediately after polishing with diamond paste, but electrochemical evidence demonstrated that such surfaces were contaminated. After cleaning the surface chemically or electrochemically, zero contact angles were observed.