Showing papers on "Sessile drop technique published in 2005"

Anisotropy in the wetting of rough surfaces.

Abstract: Surface roughness amplifies the water-repellency of hydrophobic materials. If the roughness geometry is, on average, isotropic then the shape of a sessile drop is almost spherical and the apparent contact angle of the drop on the rough surface is nearly uniform along the contact line. If the roughness geometry is not isotropic, e.g., parallel grooves, then the apparent contact angle is no longer uniform along the contact line. The apparent contact angles observed perpendicular and parallel to the direction of the grooves are different. A better understanding of this problem is critical in designing rough superhydrophobic surfaces. The primary objective of this work is to determine the mechanism of anisotropic wetting and to propose a methodology to quantify the apparent contact angles and the drop shape. We report a theoretical and an experimental study of wetting of surfaces with parallel groove geometry.

The coalescence speed of a pendent and a sessile drop

Abstract: When two liquid drops come into contact, they coalesce rapidly, owing to the large curvature and unbalanced surface-tension forces in the neck region. We use an ultra-high-speed video camera to study the coalescence of a pendent and a sessile drop, over a range of drop sizes and liquid viscosities. For low viscosity, the outward motion of the liquid contact region is successfully described by a dynamic capillary-inertial model based on the local vertical spacing between the two drop surfaces. This model applies even when the drops are of different sizes. Increasing viscosity slows down the coalescence when the Reynolds number m. For the largest viscosities, the neck region initially grows in size at a constant velocity. The neck curvature also becomes progressively sharper with increasing viscosity. The results are compared to previously predicted power laws, finding slight, but significant deviations from the predicted exponents. These deviations are most probably caused by the finite initial contact radius.

Measurement of contact angle and work of adhesion at high temperature

Abstract: A critical review is given of the present state of knowledge and future perspectives in high-temperature contact angle measurement. Experimental results obtained by the different versions of the sessile drop method and by various procedures are given in order to illustrate the two main sources of scatter in wettability data, the first being related to the quality of the substrates and the second to control of the furnace atmosphere.

Wetting and infiltration of carbon by liquid silicon

Abstract: The wettability of carbon materials by molten silicon was investigated at 1430∘C under vacuum by using the dispensed drop variant of the sessile drop technique. The results are compared with data in the literature and used to contribute to a comprehensive understanding of wetting in the liquid-Si/solid-C system. Consequences on the dynamics of Liquid Silicon Infiltration (LSI) processes are discussed.

Determination of the surface tension of liquid stainless steel

Abstract: The surface tensions (σ) and temperature dependencies (dσ/dT) of several commercial 4-series ferritic stainless steels have been measured using the sessile drop technique on an Al2O3 plate over the temperature range 1789 to 1883 K in an atmosphere of high purity (P O 2 < 10−19 MPa) argon gas. Precise densities of liquid stainless steels have also been obtained using the modified sessile drop method in order to calculate accurate values of the surface tension. The surface tensions of liquid stainless steels decreased markedly with increasing sulphur concentration in the steels. The variation of surface tensions of liquid stainless steels can be described by the following equation σ = 1790 − 182 ln (1 + 260a S) (mN/m) when only S is considered or σ = 1820 − 304 ln (1 + 383a O) − 182ln (1 + 260a S) (mN/m) when both S and O are considered. The equations apply to the following compositional ranges: mass%O = 0.0022–0.0064, mass%S = 0.0008–0.05. The temperature coefficient of the surface tension (dσ/dT) of liquid stainless steel was found to change from negative to positive at a sulphur concentration of about 30 mass ppm in the steel. Nitrogen was found to have little effect on the surface tension of liquid stainless steel.

Simple analytical model of capillary flow in an evaporating sessile drop.

Abstract: An analytical expression of hydrodynamic potential inside an evaporating sessile drop with pinned contact line is found. The problem is considered for a hemispherical drop (with the contact angle of 90\ifmmode^\circ\else\textdegree\fi{}) at the very early stages of the evaporation process when the shape of the drop is still a hemisphere and the evaporation field is uniform. The capillary flow carries a fluid from the drop apex to the contact line. Comparison with the published calculations performed using lubrication approximation (very thin drop) suggests that qualitative picture of the capillary flow is insensitive to the ratio of initial drop height to the drop radius.

The Wettability of Titanium Diboride by Molten Aluminum Drops

Abstract: Titanium diboride is widely accepted to be completely wet by liquid aluminum, yet few published wetting studies demonstrate this behavior, and reported contact angles vary widely. Sessile drop substrates from four different sources were selected and their microstructures and chemistries characterized. The results of sessile drop experiments using four techniques to modify oxide film behavior were compared. The Al-TiB2 interfaces were examined in metallographic sections or after chemical removal of the Al drop. Al wets a material containing 5.5 wt% Ni in vacuum experiments before the hold temperature of 1025∘ C is reached. The other TiB2 substrates are completely wet by Al at 1025∘ C, but only after prolonged holds under vacuum. Elimination of boron oxide from the TiB2 surface leads to a spreading condition. The role of the substrate microstructure (porosity, grain size, roughness, and carbon content) in altering the wetting kinetics is discussed.

The Surface Tension and Density of Liquid Ag–Bi, Ag–Sn, and Bi–Sn Alloys

Abstract: The sessile drop method has been used to measure density and surface tension for pure Ag, Bi, Sn, and their mixtures. For pure metals and Bi–Sn alloys negative temperature coefficients of surface tension have been obtained. In case of Ag–Bi and Ag–Sn alloys the temperature coefficients of surface tension take negative or positive values depending on composition. Experimental values of the surface tension for Ag–Bi, Ag–Sn, and Bi–Sn are compared with those computed from Butler’s model. A relatively good agreement is observed.

Hemocompatibility of Poly(acrylonitrile-co-N-vinyl-2-pyrrolidone)s: Swelling Behavior and Water States

Abstract: Hemocompatibility is an essential aspect of blood contacting polymers. Knowledge of the relationship between polymer structure and hemocompatibility is important in designing such polymers. In this work, the effect of swelling behavior and states of water on the hemocompatibility of poly(acrylonitrile-co-N-vinyl-2-pyrrolidone) (PANCNVP) films was studied. Platelet adhesion and plasma recalcification time tests were used to evaluate the hemocompatibility of the films. Considering the importance of surface properties on the hemocompatibility of polymers, static water contact angles were measured by both sessile drop and captive bubble methods. It was found that, on the film surface of PANCNVP with a higher NVP content, adhered platelets were remarkably suppressed and the recalcification time was longer. The total water content adsorbed on the PANCNVP film was determined through swelling experiments performed at temperatures of interest. Differential scanning calorimetry and thermogravimetric analysis were used to probe the states of water in the films. Based on the results from these experiments, it was hypothesized that the better hemocompatibility of PANCNVP films with higher NVP contents was due to their higher free water content, because water molecule exchange at the polymer/liquid interface, facilitated by a high free water content, is unfavorable for the formation of surface bound water, which causes poor hemocompatibility. [diagram in text].

Water Induced Hydrophobic Surface

Abstract: A polyurethane coating is described that has hydrophilic wetting behavior when dry and hydrophobic when wet. A difference of ∼25° in advancing contact angles for dry (83°) and wet (108°) states is found by sessile drop and dynamic methods. The term “contraphilic” is suggested for this reversible change opposite customary amphiphilic behavior. Contraphilic behavior results from a soft block containing semifluorinated and 5,5-dimethyhydantoin segmers. Amide inter/intramolecular hydrogen bonding is proposed for the hydrophilic (dry) state, while surface-confined, amide−water hydrogen bonding “releases” semifluorinated groups, giving the hydrophobic state. Water-induced hydrophobic surfaces may lead to applications for easily switched wetting, such as in microfluidics.

Effects of roughness pitch of surfaces on their wettability

Abstract: The surface roughness factors, such as the Wenzel roughness factor and so on, are very interrelated to each other. Therefore, it makes a precise discussion difficult on how the surface roughness affects the wettability. We already reported the effect of the surface roughness on the wettability at a constant Wenzel roughness factor using two kinds of models, the hemisphere close packing model and the hemiround rod close lining model. Nevertheless, the pitch is proportional to the height in these models. Therefore, we could not independently discuss the influence of roughness height and roughness pitch on the wettability. We developed our new models which can independently describe the influence of the surface roughness height and the roughness pitch on the wettability. We simulated loose packing sphere models by periodically placing small ball bearings and the loose lining round rod models by winding fine wires. The wettability was measured by the sessile drop method for the non-wetting system using paraffin coated samples and aqueous solutions. These results show that there is a critical pitch which determines the maximum contact angle in both systems. These results can be explained by the ratio of the solid/liquid/vapor and liquid/vapor line length at the three phase line.

Drop Size Dependence of the Contact Angle of Nanodroplets

Abstract: The contact angle of nanosized non-polarized argon sessile droplets on a solid substrate is studied by using molecular dynamics simulations. It is found that the drop size dependence of the contact angle is sensitive to the interaction between the liquid molecules and solid molecules. The contact angle decreases with the decreasing drop size for larger interaction between the liquid molecules and the solid substrate, and vice versa. This observation is consistent with most of the previous theoretical and experimental results.

Effect of humidity on the adsorption kinetics of lung surfactant at air-water interfaces.

Abstract: The in vitro adsorption kinetics of lung surfactant at air-water interfaces is affected by both the composition of the surfactant preparations and the conditions under which the assessment is conducted. Relevant experimental conditions are surfactant concentration, temperature, subphase pH, electrolyte concentration, humidity, and gas composition of the atmosphere exposed to the interface. The effect of humidity on the adsorption kinetics of a therapeutic lung surfactant preparation, bovine lipid extract surfactant (BLES), was studied by measuring the dynamic surface tension (DST). Axisymmetric drop shape analysis (ADSA) was used in conjunction with three different experimental methodologies, i.e., captive bubble (CB), pendant drop (PD), and constrained sessile drop (CSD), to measure the DST. The experimental results obtained from these three methodologies show that for 100% relative humidity (RH) at 37 degrees C the rate of adsorption of BLES at an air-water interface is substantially slower than for low humidity. It is also found that there is a difference in the rate of surface tension decrease measured from the PD and CB/CSD methods. These experimental results agree well with an adsorption model that considers the combined effects of entropic force, electrostatic interaction, and gravity. These findings have implications for the development and evaluation of new formulations for surfactant replacement therapy.

Temperature dependence of surface tension of liquid Sn–Ag, In–Ag and In–Cu alloys

Abstract: It is usually known that the surface tension of liquid metals and alloys decreases with increasing temperature, i.e., the temperature dependence of the surface tension is negative. We found, however, that some liquid alloys, which have large difference of the surface tension of pure components, show positive temperature dependence in certain composition ranges. Some Pb-free alloys, for which information on the surface tension is indispensable to be developed as environmental-friendly material, can be listed in this special category. The experimental results and the thermodynamic analysis of the temperature dependence of those alloys are discussed in the paper.

Wetting and spreading of liquid metals on ZrB2-based ceramics

Abstract: The possibility to exploit commercially the peculiar characteristics of refractory metallic and ceramic materials and in particular of Zirconium diboride ceramics—a class of promising materials for high temperature applications—often depends to a great extent on the ability to join different ceramics one to the other or to special metallic alloys. As the behaviour of a metal-ceramic joint is ruled by the chemical and the physical properties of the interface, the knowledge of wettability, interfacial tensions and interfacial reactions is mandatory to understand what happens at the liquid metal-ceramic interface during joining processes. In the framework of an extensive study aimed at evaluating the wettability and the interfacial characteristics of different metal-ceramic systems, the behaviour of ZrB2 in contact with liquid Ag and its alloys (Cu, Ti, Zr, Hf) has been studied. ZrB2 pure, with different sintering aids or “alloyed” with other ceramic materials (SiC, Si3N4), have been used. The wetting and spreading experiments have been performed by the sessile drop technique under controlled atmospheres. The wetting and spreading characteristics and the interfacial reactions are discussed as a function of time, compositions of the ceramic and of the alloy involved. The interfacial morphologies, analysed by SEM and EDS, show the presence of regular interfaces and adsorption layers and of different bulk phases which are interpreted in terms of the relevant phase-diagrams.

On the behavior of liquid drops on a solid surface. 1. The sliding of drops on an inclined surface

Abstract: The idea of contact angle was generalized by using the principle of minimum total energy. The problems of the shape of the two-dimensional sessile drop and the drop on an inclined surface are considered. The differential equations describing the drop shapes are found in both cases. A case of good wetting is considered to determine the profiles of the sessile and inclined drops. The generalization of the boundary conditions for the contact angle hysteresis of the drop on an inclined surface is discussed. The limit of the inclination angle at which the drop begins to run down the inclined plane is found. The models of the drop detachment from the solid and the motion of liquid inside of the drop are discussed. This is not an original text.

Observations of the reduction of FeO from slag by graphite, coke and coal char

Abstract: The reduction of FeO from iron-saturated FeO-CaO-Al2O3-SiO2 slags by graphite, coke and coal char at 1 673 K has been investigated using a sessile drop technique. Metallographic analysis of samples quenched from the reaction temperature, and in situ observations of the reaction interface, reveal significant differences in the slag/carbon contact, and in the morphologies of the product iron and its composition; these differences were found to depend on the carbon type used in the reduction. In particular it has been shown that, in the case of graphite and coke, liquid Fe-C droplets were rapidly formed at the slag/C interface. Reactions of the slag with coal chars, in contrast, result predominantly in the formation of solid iron. These observations indicate that the reaction pathways, and hence reaction kinetics, are dependent on carbon type.

Thermodynamic study on the melting of nanometer-sized gold particles on graphite substrate

Abstract: Surface tension plays an important role in lowering the melting temperature of nanometer-sized particles, but whether the surface tension determined in macro scale is valid for the nanometer-sized particles is unclear. Moreover, the melting of the nanometer-sized particles formed on solid substrates can be affected by interfacial tension, but no research has been reported on the effect of substrates on the melting temperature. Therefore, in order to predict the melting temperature of nanometer-sized metallic particles on solid substrates, thermodynamic parameters such as surface tension and interfacial tension should be properly estimated. In the present work, thermodynamic assesment is given on the melting temperature of gold particles in nanometer-size placed on a graphite substrate. Surface tension of liquid gold and the contact angle between liquid gold and the graphite substrate are measured by the constrained drop method and the sessile drop method in macro scale, respectively. Then, the effect of the graphite substrate on the melting temperature of nanometer-sized gold particles are examined by thermodynamic calculations minimizing the total Gibbs free energy, the sum of bulk, surface and interface energies. It is found that the graphite substrate has negligible effect on the melting temperature of nanometer-sized gold particles. Thermodynamic assessments provide that the surface tension of solid gold is 1.339 N/m at 1373 K and that the decrease in the surface tension of liquid gold with size will be considerable for the particles smaller than ~5 nm.

Elliptic solution to the Young-Laplace differential equation.

Abstract: The Young–Laplace equation differential form can be solved under the elliptic representation for a fluid–fluid interface in the range 0 ⩽ ϕ ⩽ 90 ° . For a characteristic point ( X max , Y = 90 ° ) we find a simple analytical relation between the curvature radius and the elliptic parameters that yields the surface tension in the range 0.125 β 100 . The solution to the differential equation gives an error lower than 2.5% for published normalized data. The origin of the coordinates of the profile drop is the maximum diameter and the distance from the equator to the drop apex. Through this mechanism, the need for numerical methods and published algorithms prior to this work is eliminated. To challenge the method, the procedure is used with published data of numerical solutions. The dimensions of the parameters used are invariant with respect to the coordinate system. For practical applications, this useful equation can be used in pendant drop, sessile drop, rising bubble, spinning drop, and capillary methods. Finally, to increase the sensitivity of the procedure, the elliptic segment data under the maximum diameter can be treated with a linear regression ( y 2 vs x 2 ) to obtain the elliptic parameters (a and b) in order to apply the final equation.

Atomistic simulations of reactive wetting in metallic systems

Abstract: Atomistic simulations were performed to investigate high temperature wetting phenomena for metals. A sessile drop configuration was modeled for two systems: Ag(l) on Cu and Pb(l) on Cu. The former case is an eutectic binary and the wetting kinetics were greatly enhanced by the presence of aggressive interdiffusion between Ag and Cu. Wetting kinetics were directly dependent upon dissolution kinetics. The dissolution rate was nearly identical for Ag(l) on Cu(100) compared to Cu(111); as such, the spreading rate was very similar on both surfaces. Pb and Cu are bulk immiscible so spreading of Pb(l) on Cu occurred in the absence of significant substrate dissolution. For Pb(l) on Cu(111) a precursor wetting film of atomic thickness emerged from the partially wetting liquid drop and rapidly covered the surface. For Pb(l) on Cu(100), a foot was also observed to emerge from a partially wetting drop; however, spreading kinetics were dramatically slower for Pb(l) on Cu(100) than on Cu(111). For the former, a surface alloying reaction was observed to occur as the liquid wet the surface. The alloying reaction was associated with dramatically decreased wetting kinetics on Cu(100) versus Cu(111), where no alloying was observed. These two cases demonstrate markedly different atomistic mechanisms of wetting where, for Ag(l) on Cu, the dissolution reaction is associated with increased wetting kinetics while, for Pb(l) on Cu, the surface alloying reaction is associated with decreased wetting kinetics.