Showing papers on "Sessile drop technique published in 2004"

Adhesion and nonwetting-wetting transition in the Al/α-Al 2 O 3 interface

Abstract: Using a reactive force field (ReaxFF), we investigated the structural, energetic, and adhesion properties, of both solid and liquid Al/alpha-Al2O3 interfaces. The ReaxFF was developed solely with ab initio calculations on various phases of Al and Al2O3 and Al-O-H clusters. Our computed lattice constants, elastic constants, surface energies, and calculated work of separation for the solid-solid interface agree well with earlier first-principles calculations and experiments. For the liquid-solid system, we also investigated the nonwetting-wetting transition of liquid Al on alpha-Al2O3(0001). Our results revealed that the evaporation of Al atoms and diffusion of O atoms in alpha-Al2O3 lead to the wetting of liquid Al on the oxide surface. The driving force for this process is a decrease in interfacial energy. The nonwetting-wetting transition was found to lie in the 1000–1100 K range, which is in good agreement with sessile drop experiments.

Experimental and Numerical Investigation of the Evaporation into Air of a Drop on a Heated Surface

Abstract: Understanding the wetting and evaporation behaviour of volatile droplets on heated surfaces is very important for many industrial applications. In this paper the behaviour of a sessile drop evaporating on a heated surface is investigated both experimentally and numerically. Results are reported for the evaporation of water drops on two different substrates at various temperatures. A numerical model, based on a finite element method, has been developed to describe the hydrodynamics inside the evaporating drop and the effect of the humidity on the evaporation process, assuming the droplet to be a spherical cap. The energy and Navier–Stokes equations are solved within the droplet and the vapour concentration is computed using the diffusion equation. The drop volume and flow and temperature fields within the drop are obtained and the evolution of the volume in time is compared with the experimental results.

Critical Factors Affecting the Wettability of α‐Alumina by Molten Aluminum

Abstract: The wetting behaviors of α-Al2O3 single crystals with three different faces—R(01[Onemacr]2), A(01[Twomacr]0), and C(0001)—and polycrystals (PC) by molten aluminum were studied over a wide temperature range using both a conventional and an improved sessile-drop method. The critical factors affecting the wettability, such as temperature, atmosphere, substrate surface roughness, and crystallographic orientation, and the influence from the experimental technique, were thoroughly investigated. The results show that the aluminum surface oxidation and the thickness of the oxide film have a pronounced effect on the wettability, especially at low temperatures. To eliminate this effect, the experimental temperature must be over a critical value. Vacuum favors lowering this value compared with atmosphere, and the improved sessile-drop method, particularly using an impingement-dropping mode (I-mode), helps to weaken this effect by mechanical disruption and removal of the oxide film. However, the dropping distance and the dropping force must be controlled to prevent a receding contact angle. The effects of the substrate surface roughness and temperature are not significant in the case of a clean and a fine-prepared aluminum surface. On the other hand, the effect of the aluminum crystallographic orientation is noticeable and the wettability is in the order of R > A > PC > C. The intrinsic contact angles of the Al/α-Al2O3 system in the temperature range 1000°–1500°C were estimated to be 76°–85° for the R and A faces, 88°–100° for the C face, and 77°–90° for the polycrystal, depending on the temperature.

A Leidenfrost Point Model for Impinging Droplets and Sprays

Abstract: This study presents, for impinging droplets and sprays, a model of the Leidenfrost point (LFP); the minimum liquid/solid interface temperature required to support film boiling on a smooth surface. The present model is an extension of a previously developed sessile drop model, based on bubble nucleation, growth, and merging criteria, as well as surface cavity size characterization. The basic concept of the model is that for liquid/solid interface temperatures at and above the LFP, a sufficient number of cavities are activated and the bubble growth rates are sufficiently fast that a continuous vapor layer is established nearly instantaneously between the liquid and the solid. For impinging droplets, the influence of the rise in interfacial pressure created by the impact of the droplet with the surface, must be accounted for in determining fluid properties at the liquid-solid interface. The effect of droplet impact velocity on the LFP predicted by the model is verified for single impinging droplets, streams of droplets, as well as sprays

Wettability and reactivity of molten silicon with various substrates

Abstract: Contact angles of molten silicon on various substrates have been determined using the sessile drop method and reactivity has been investigated by examining cross sections between silicon and substrates with an electron-probe microanalyzer (EPMA). The contact angles between molten silicon and oxide substrates, such as SiO2(s), Al2O3(s) and MgO(s), are in the range 85° to 88°. The reaction zone is composed of forsterite (2MgO·SiO2) and clinoenstatite (2MgO·2SiO2) on the MgO(s)-side of the interface between the Si and MgO. The contact angle between molten silicon and Si3N4 is about 90°. Molten silicon spreads over the SiC plate and the contact angle is estimated to be 8°. Large contact-angle values (around 145°) have been observed on BN substrates. At the interface between Si(l) and the BN substrate, a discontinuous Si3N4 layer is believed to form and might retard the dissolution of BN into molten silicon. The BN substrate is regarded as being the most suitable substrate for supporting a molten silicon drop during surface tension measurements, due to the large contact angle and low contamination.

Water contact angles of vertically aligned Si nanorod arrays

Abstract: The equilibrium water contact angles of vertically aligned nanorod Si films were measured by the sessile drop method. For as-deposited hydrophilic films, there was a contact angle transition from a rough surface to a 'hemi-wicking' porous surface at normal film thickness d = 500 nm; while for the HF treated hydrophobic films, a transition from a partially composite to a composite surface was observed at the same film thickness. The observed results can be reasonably interpreted within the framework of the classical Young theory.

Contact Angle Measurements of Sn-Ag and Sn-Cu Lead-Free Solders on Copper Substrates

Abstract: In this study, the contact angles of four lead-free solders, namely, Sn-3.5Ag, Sn-3.5Ag-4.8Bi, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu (wt.%), were measured on copper substrates at different temperatures. Measurements were performed using the sessile-drop method. Contact angles ranging from 30° to 40° after wetting under vacuum with no fluxes and between 10° and 30° with rosin mildly activated (RMA) and rosin activated (RA) fluxes were obtained. The Sn-3.5Ag-4.8Bi exhibited the lowest contact angles, indicating improved wettability with the addition of bismuth. For all soldering alloys, lower contact angles were observed using RMA flux. Intermetallics formed at the solder/Cu interface were identified as Cu6Sn5 adjacent to the solder and Cu3Sn adjacent to the copper substrate. The Cu3Sn intermetallic phase was generally not observed when RMA flux was used. The effect of temperature on contact angle was dependent on the type of flux used.

Constrained sessile drop as a new configuration to measure low surface tension in lung surfactant systems.

Abstract: Existing methodology for surface tension measurements based on drop shapes suffers from the shortcoming that it is not capable to function at very low surface tension if the liquid dispersion is opaque, such as therapeutic lung surfactants at clinically relevant concentrations. The novel configuration proposed here removes the two big restrictions, i.e., the film leakage problem that is encountered with such methods as the pulsating bubble surfactometer as well as the pendant drop arrangement, and the problem of the opaqueness of the liquid, as in the original captive bubble arrangement. A sharp knife edge is the key design feature in the constrained sessile drop that avoids film leakage at low surface tension. The use of the constrained sessile drop configuration in conjunction with axisymmetric drop shape analysis to measure surface tension allows complete automation of the setup. Dynamic studies with lung surfactant can be performed readily by changing the volume of a sessile drop, and thus the surface area, by means of a motor-driven syringe. To illustrate the validity of using this configuration, experiments were performed using an exogenous lung surfactant preparation, bovine lipid extract surfactant (BLES) at 5.0 mg/ml. A comparison of results obtained for BLES at low concentration between the constrained sessile drop and captive bubble arrangement shows excellent agreement between the two approaches. When the surface area of the BLES film (0.5 mg/ml) was compressed by about the same amount in both systems, the minimum surface tensions attained were identical within the 95% confidence limits.

Wetting of Rough Surfaces

Abstract: This paper focuses on effects of roughness on wettability. According to Wenzel's equation, the transition of theoretical wetting contact angles is 90°, whereas many experimental results have indicated that such a transition takes place at contact angles smaller than 90°. A new model of wetting on roughness surface is established in this paper. The model indicates that the influencing factors of wetting on roughness surface include not only equilibrium contact angle θ0 and surface roughness, but also the system of liquids and solid substrates. There is a corresponding transition angle for every surface roughness, and the transition angle is lower than 90°. Surface roughness is propitious to improve the contact angle only when θ0 is lower than the transition angle. The effect of surface roughness on the contact angle increases with the increase of rE. To engineer the surface with different roughnesses, a Ti test sample is polished with sandpaper with abrasive number 350, 500, 1000 and 2000; the contact angles of water on Ti are measured by the sessile drop method. The results of the theoretical analysis agree with experimental ones.

Improvements in Surface Tension Measurements of Liquid Metals Having Low Capillary Constants by the Constrained Drop Method

Abstract: Accurate measurements of surface tension of liquid metals having low capillary constants (the ratio between density (r) and surface tension (s), r/s) have been attempted using the constrained drop method. High accuracy of surface tension measurements was obtained by making a large axi-symmetric liquid drop and adopting a developed image capturing system composed of a high-resolution charge-coupled device (CCD) camera, an additional CCD camera to adjust the level of the metal drop and a He‐Ne laser.

Measurement of the surface tension of liquid Ga, Bi, Sn, In and Pb by the constrained drop method

Abstract: The effect of the droplet size on the accuracy of surface tension measurement by the sessile drop method is discussed for liquid metals through a simulation by using the Laplace equation. It is found that with increasing size of the droplet, a higher accuracy of the measured value of the surface tension can be obtained. In order to make a large droplet of liquid metals, the constrained drop method with a special crucible shape was applied to measure the surface tension of liquid Ga, Sn, Bi, In, and Pb. The uncertainty of the measured surface tension was within 1%. The temperature dependences of the surface tension of liquid Ga, Sn, Bi, In, and Pb were obtained in the present experiment as follows: Ga: σGa = 737 −0.062T mN/m (823 ≤ T≤ 993K)Sn: σSn = 579 −0.062T mN/m (723≤ T≤ 993K)Bi: σBi = 417 −0.070T mN/m (773 ≤ T≤ 873K)In: σIn = 600−0.082T mN/m (673 ≤ T≤ 993K)Pb: σPb = 499 −0.089T mN/m (757 ≤ T≤ 907K)

Determining Contact Angles of Powders by Liquid Penetration

Abstract: The wettability of a powder is important, especially in chemical and pharmaceutical technologies when considering the manufacturing process and the properties of the final product. However, contact angle methods for powders are problematic. The common measurement techniques are the sessile drop method and the liquid penetration method. In this work the determination of contact angles of powders by liquid penetration is considered. In particular, we investigated the influence of the sample preparation and sampling device on the wetting process. A bulk with constant properties during the measurement must be guaranteed. We postulate a packing procedure to obtain reproducible measurements by ensuring constant powder properties.

Reactive wetting of SiO2 substrates by molten Al

Abstract: The reactive wetting behavior of SiO2 substrates by molten Al was investigated at temperatures between 800 °C to 1250 °C in a purified Ar-3 pct H2 atmosphere of about 0.11 MPa using an improved sessile drop method. The time dependence of the changes in contact angle and droplet geometry was monitored and the wetting kinetics was identified. The initial equilibrium or quasi-equilibrium contact angles are generally larger than 90 deg and do not significantly vary with temperature. The subsequent remarkable decrease in the contact angle mainly results from the progressive decrease in the droplet volume rather than the advance of the solid-liquid interfacial front. The significant effect of temperature on the wetting kinetics is essentially related to its effect on the reaction and molten Al penetration progress. For systems with a considerable decrease in the droplet volume during reactive wetting, a criterion for evaluation of the true wetting improvement was proposed.

Morphologies Adopted by Al2O3 Single‐Crystal Surfaces in Contact with Cu Droplets

Abstract: The morphologies of α-Al2O3 single-crystal substrates, annealed in contact with Cu droplets ranging in size from 1 to 10 μm, are observed by high-resolution scanning electron microscopy. After 45 min at 1373 K in a He-5% H2 atmosphere, the alumina substrates display different shapes depending on their orientation and on the addition of small amounts of titanium to the copper. The alumina surface may remain flat, display a hollow of faceted spherical shape at the solid/liquid interface, or develop anisotropic ridges at the triple line. The shape changes result from the diffusion of the components of alumina that takes place to achieve a minimum interfacial energy of the drop–substrate system. Explanations of these features are proposed with a discussion of the conditions under which the Young equation can be used for characterizing wetting at high temperature.

Coupling ability of silane grafted poly(propene) at glass fibers/poly(propene) interface

Abstract: Isotactic copolymers of propene and dienes were synthesized through Ziegler ‐ Natta catalysis and functionalized with silanes through Speier catalysis, either at one end of linear chains or at the end of short ramifications. Fibers/matrix coupling can be obtained with such polymers in one hand, through chemical bonding with the glass surface OH functions and in the other hand by cocrystallization with the poly(propylene) matrix. The grafting ability of the coupling agents (CAs) onto glass surface was studied by the sessile drop technique after solvent washing. For ramificated poly(propene)s it is possible to adjust the surface free energy with the silane content. Adhesion of the poly(propene) matrix on the sized fibers was evaluated by the microtension test. The CAs characteristics required to promote adhesion between the fibers and matrix are discussed as a function of their chemical nature and structure. The more important interfacial shear stress was obtained with poly(propene-co-8.5%methyloctadiene-g-chlorodimethylsilane) showing that a compromise must be found between the silane content and polymer crystallinity. Published by Elsevier Ltd.

Oxygen adsorption behavior on the surface of liquid Cu-Ag alloys

Abstract: The behavior of oxygen adsorption on the surface of liquid Cu-Ag alloys was investigated by measuring their surface tension (a) with the sessile drop method in the oxygen partial pressures (pO 2 ) between 2.5 x 10 - 1 1 and 2.5 x 10 - 3 Pa. The oxygen adsorption (the surface excess concentration of oxygen) was calculated from the slope of dσ/d In p O 2 by applying Gibbs adsorption isotherm, for liquid Cu, Cu-5 at%Ag, Cu-10 at%Ag, Cu-20 at%Ag and Ag. It was found that the oxygen adsorption increased with the oxygen partial pressure, up to saturation on the surface. The oxygen adsorption on the surface of liquid Cu-20 at%Ag alloys exhibited almost the same behavior as that of pure liquid Ag, because surface saturation was not achieved for the Cu-20 at%Ag alloys, even at high oxygen partial pressures. Thermodynamic calculations using Butler's model indicated that the mole fraction of Ag in the surface of liquid Cu-Ag alloys drastically increases to 0.81 when the mole fraction of Ag in the bulk is only 0.2. Thus, it is considered that the outermost surface of liquid Cu-20 at%Ag alloys contains an enhanced level of Ag, which determines the oxygen adsorption behavior on liquid Cu-20 at%Ag alloys.

The effect of temperature, matrix alloying and substrate coatings on wettability and shear strength of Al/Al2O3 couples

Abstract: A fresh approach has been advanced to examine in the Al/Al2O3 system the effects of temperature, alloying of Al with Ti or Sn, and Ti and Sn coatings on the substrate, on contact angles measured using a sessile-drop test, and on interface strength measured using a modified push-off test that allows shearing of solidified droplets with less than 90 deg contact angle. In the modified test, the solidified sessile-drop samples are bisected perpendicular to the drop/Al2O3 interface at the midplane of the contact circle to obtain samples that permit bond strength measurement by stress application to the flat surface of the bisected couple. The test results show that interface strength is strongly influenced by the wetting properties; low contact angles correspond to high interface strength, which also exhibits a strong temperature dependence. An increase in the wettability test temperature led to an increase in the interface strength in the low-temperature range where contact angles were large and wettability was poor. The room-temperature shear tests conducted on thermally cycled sessile-drop test specimens revealed the effect of chemically formed interfacial oxides; a weakening of the thermally cycled Al/Al2O3 interface was caused under the following conditions: (1) slow contact heating and short contact times in the wettability test, and (2) fast contact heating and longer contact times. The addition of 6 wt pct Ti or 7 wt pct Sn to Al only marginally influenced the contact angle and interfacial shear strength. However, Al2O3 substrates having thin (<1 µm) Ti coatings yielded relatively low contact angles and high bond strength, which appears to be related to the dissolution of the coating in Al and formation of a favorable interface structure.

Effect of oxygen on surface tension of liquid Ag-Sn alloys

Abstract: The effect of oxygen on the surface tension of liquid Ag-Sn alloys used as one of the main components in lead-free soldering alloys was measured by the sessile drop method. The surface tension of liquid Ag-Sn alloys decreased with increasing tin content. The effect of oxygen was investigated for oxygen partial pressure ranging from about 10 - 1 7 to 10 - 1 2 atm. The effect of oxygen on the surface tension of liquid Ag-Sn alloys is closer to pure tin rather than pure silver. Thermodynamic simulation using Butler's equation showed that the surface of liquid Ag-Sn alloys is enriched with tin. Accordingly, the effect of oxygen adsorption on the surface tension of liquid Ag-Sn alloys was considered to be closer to that of tin than silver.