Showing papers on "Sessile drop technique published in 1995"

Young-Dupre Revisited

Abstract: The Young-Dupre equation for the work of adhesion of a liquid drop to a solid surface, where the solid surface is in equilibrium with the vapor of the liquid, is given as W = γ L (1 + cos θ), where γ L is the surface tension of the liquid and θ the contact angle. This work (W) has generally been identified with the free energy of adhesion. It is shown here that it constitutes the total work of adhesion only under the artificial condition that the sessile drop retains its shape after detaching from the solid surface. Under real conditions, W represents only one component of the total free-energy change taking place when a drop is separated from, or attached to, a vapor-equilibrated smooth solid surface. In the present work, a Net Free Energy of Adhesion, ΔF N , is derived which gives the total free energy necessary to separate a sessile drop from a smooth solid surface to form a free sphere (its negative, of course, is the free energy of attachment of the sphere). It is given by ΔF N = πr 2 γ L [(2α/sin θ) 2/3 - α], where r is the radius of the solid-liquid interface and α, called the effective area, is [2/(1 + cos θ)] - cos θ. The Net Free Energy of Adhesion and Young-Dupre work of adhesion are compared as functions of the contact angle. This is done for systems of constant solid-liquid interfacial area and for systems of constant drop volume.

Wettability and interfacial interactions in bioceramic-body-liquid systems

Abstract: Wetting experiments, by the sessile drop technique, were carried out at 37°C in air to determine the surface and interfacial interactions that take place in various solid bioceramics based on Al2O3, ZrO2(YPZ), SiO2, and TiO2 in contact with water, Ringer solution, artificial synovial fluid, calf serum, human plasma, and whole blood (+ EDTA). The surface energy of the liquids was measured by the ring method. The calculated values of the energy of interaction (work of adhesion) reveal that intermolecular forces act across the solid-liquid interfaces. The contribution of the dispersion and polar interactions to the surface energy of the polar liquids and the pure or mixed oxides was determined assuming that in the system of Mn-steel-liquids only dispersion forces act at the interface. It was found that the contribution of the polar interactions to the energy of interaction at the solid-liquid interface increases with the glassy phase content of the oxide that causes reduction of the measured contact angle. © 1995 John Wiley & Sons, Inc.

Wetting behaviour in the Al-Si/SiC system: interface reactions and solubility effects

Abstract: The sessile drop technique was used to study the wetting behavior of Al-Si alloys on SiC sintered ceramic substrates under vacuum in the 700--1,100 C temperature interval. Al-Si alloys with Si concentrations up to 50% were tested. An expected non-wetting/wetting transition was observed at 900--1,000 C due to the presence of an alumina film surrounding the molten alloy. At higher temperatures wetting was observed and the Si concentration of the alloy has a marked effect on the measured contact angles, {theta}. At 1,100 C {theta} decreases from 55 to 25{degree} when instead of pure Al an Al12.3%Si or an Al16.6%Si alloy is used. The suppression of the formation of a continuous Al{sub 4}C{sub 3} layer at the interface and a process of dissolution and reconstruction of the SiC surface, due to the increased Si concentration of the Al-Si alloys, are the key factors to explain the observed behavior.

Relationship between wettability and reactivity in Fe/SiC system

Abstract: In this work, both surface and bulk interactions between Fe and SiC are studied. Surface interactions are quantified by contact angles measured by the sessile drop technique under high vacuum. Products of Fe-SiC bulk reactions are characterized by scanning electron microscopy and microprobe analysis. The interpretation of the experimental results on reactivity is based on classical thermodynamics applied to equilibria of bulk phases. The results are used to discuss the general relationship between bulk reactivity and wettability.

Wetting and adhesion of Ni-Al alloys on α-Al2O3 single crystals

Abstract: The effect of Al additions on the wetting and adhesion of Ni on an α-Al2O3 single crystal was studied. Contact angles were measured by the sessile drop technique under vacuum or in He atmosphere. The morphological and chemical features of metal-vapour and metal-oxide interfaces were determined by scanning electron microscope (SEM), microprobe analysis and profilometry. The work of adhesion of Ni-Al alloys on Al2O3 substrates was significantly higher than for pure Ni and Al components. This result was explained by co-operative adsorption of aluminium and oxygen atoms at the Ni-Al2O3 interface. The influence of oxidation of the alloy on wetting and bonding is also discussed.

A comparison of the wettability of copper-copper oxide and silver-copper oxide on polycrystalline alumina

Abstract: The contact angles of liquid silver-copper oxide/alumina and liquid copper-copper oxide/alumina systems were determined using the sessile drop method. Copper oxide (CuO) additions of 1.5–10.0 wt% were made. Temperatures of 970–1250 °C for the silver-based alloys and 1090–1300 °C for the copper-based alloys were studied. Minimum contact angles of 42±8 and 64±7 ° were obtained for the copper-copper oxide alloys and the silver-copper oxide alloys, respectively. The contact angle was approximately constant for the silver-copper oxide alloy within the immiscible liquid composition range. While the contact angles were higher for the silver-based alloys relative to the copper-based alloys, successful infiltration of a porous alumina sample was achieved at only 1050 °C for a Ag-10 wt% CuO alloy. Compression tests on infiltrated samples revealed similar compressive strengths for alumina samples infiltrated with silver-copper oxide alloys, silver-copper-copper oxide alloys and copper-copper oxide alloys. The compressive fracture strength for the infiltrated samples was an order of magnitude higher than the fracture strength of the porous alumina body without infiltration. Although silver-based alloys are more expensive than comparable copper-based alloys, in many applications the additional cost may be offset by lower processing or brazing temperatures, improved thermal and electrical conductivity, and improved toughness.

Surface Characterization of 2-Hydroxyethyl Methacrylate/Styrene Block Copolymers by Transmission Electron Microscopy Observation and Contact Angle Measurement

Abstract: The surface structures of poly(HEMA-block-styrene-block-HEMA)s and poly(HEMA-ran-styrene) (where HEMA denotes 2-hydroxyethyl methacrylate) are characterized by TEM observation and contact angle measurements using sessile drop method and Wilhelmy plate technique under the dry and wet conditions. One (T-60) of the triblock copolymers is produced by the addition reaction of telechelic polystyrene with isocyanate terminals and semitelechelic PHEMA with an amino group at one chain end, and the others (L-50, L-60) are synthesized by anionic living polymerization. The random copolymer (R-50) is prepared by radical polymerization. L-50, L-60, T-60, and R-50 contain 45, 61, 63, and 47 wt % of PHEMA segments, respectively. From the TEM observation, the outermost surface of the as-cast L-50 film is found to be completely covered with polystyrene layer, while the microdomains of polystyrene and PHEMA are mixed in the top surface area of the as-cast T-60 film. By use of the staining technique, the wet surface structures of L-60 and T-60 are also observed. At the hydrated top surface, the polystyrene microdomains wrapped with PHEMA segments are stretched toward the water side, while the microphase separated structure in bulk remains intact. The contact angle measurements elucidates that the hydrophobic surfaces are replaced with hydrophilic ones by soaking the L-50 and T-60 films in water and that the hydrated surface of L-50 reverts to the hydrophobic one again by annealing. The surface of R-50 does not show a drastic restructuring in response to the environmental change.

Interfacial tensions of liquid Fe-Ni alloys and stainless steels in contact with CaO-SiO2″AI2O3-based slags at 1550 °C

Abstract: In the present work, the interfacial tensions of Fe-Ni alloys in contact with slags of the CaO-Al2O3-SiO2 system were measured at 1550 °C. Nickel additions to the alloy were found to decrease interfacial tension. The effects of alumina and titania additions to the slag on the interfacial tension of the Fe-20 wt pct Ni alloy were determined: alumina was found to increase the interfacial tension by a small amount, while titania was found to decrease it drastically. Using the present interfacial tension data for the CaO-Al2O3-SiO2 system and the ones measured by Jimbo and Cramb, Girifalco and Good’s interaction coefficient (ϕ) was determined as a function of the slag composition using regression analysis and was found to be a useful means of correlating interfacial tension data. The interfacial tension of an Fe-20 wt pct Ni-2.39 wt pct Al alloy in contact with a CaO-Al2O3-SiO2 slag was found to decrease drastically in the first 60 to 75 minutes of the experiment due to the dynamic effects of mass transfer. Slight lowering of interfacial tensions of industrial stainless steels due to sulfur transfer from liquid metal to slag was also observed. The equilibrium interfacial tensions of type 304 stainless steels were found to be more dependent on the slag chemistry than on the nickel and chromium content of the alloy.

The kinetics of albumin adsorption to the air/water interface measured by automatic axisymmetric drop shape analysis

Abstract: The adsorption of human serum albumin (HSA) on the air/water interface was investigated by means of the sessile bubble technique. The surface tension as a function of time was measured analyzing the shape of axisymmetric bubbles by computer-controlled video image analysis. Surface pressure–time data was converted to surface concentration–time curves by means of area–pressure data obtained by a Langmuir surface balance from spread films. Adsorption was measured at HSA concentrations from 10−4to 1% and at pH 2.2, 4.0, 5.8, 7.5, and 10.0. Adsorption up to the initial surface concentration of 0.5–1.0 mg m−2was found to be essentially irreversible and controlled by a combination of diffusion and convection, while further adsorption was activation controlled with an activation energy term proportional to the surface concentration and one term independent of the surface concentration. The pH dependency of the adsorption rate was relatively small.

Wetting and infiltration of graphite materials by molten silicon

Abstract: The wetting and infiltration of various graphite materials by liquid silicon has been studied with the sessile drop method in argon at 1430 °C. Wetting and spreading of silicon on the graphite substrates is enhanced as the substrate roughness is increased, as a result of the accelarated dissolution of graphite in liquid silicon by the substrate roughening. The infiltration of molten silicon into the graphite materials is observed on the cross-section of the silicon/graphite interface. The vertical infiltration depth seems to be independent of the contact angles, but increases as the porosity (or pore size) of the graphite materials is increased. The lateral infiltration is also observed and seems to be dependent on the wetting behaviour.

The cause of surface tension increase with temperature in multicomponent aluminosilicates derived from coal-ash slags

Abstract: An explanation is proposed for the increase of surface tension with temperature in multicomponent aluminosilicate systems such as those derived from coal-ash slags. Two major factors are considered: (1) depolymerization of aluminosilicates caused by rearrangements of intermediate structures in the surface layers, and (2) the increase in surface entropy caused by evaporation of some ash slag components. Electron spectroscopy for chemical analysis spectra were recorded for oxygen 1s photoelectrons on quenched bulk slags and on sessile drops to gain insight into the depolymerization of coal-ash slags with temperature. The tests performed on quenched bulk slags indicated replacement of bridging oxygen [Si-O] with non-bridging oxygen atoms [Si-O−] as a function of increasing temperature. Mossbauer spectra showed an increase in ferrous iron from 4% to 12% of total iron as temperature rose from 1400 °C to 1500 °C. The increase in non-bridging oxygens resulted from the reduction of tetrahedrally coordinated Fe3+ to octahedrally coordinated Fe2+. Also, the intensity of the non-bridging oxygen 1s photoelectron peak was higher when detected on the surface of a sessile drop than when detected from the bulk of the drop.

Wetting kinetics of liquid aluminum on an al2o3 surface

Abstract: The wetting process of a liquid aluminium drop on a solid Al2O3 surface has been investigated. Two different models have been proposed: a hydrodynamic model based on the effect of viscosity as well as a diffusion model which considers diffusional effects. The prediction from the models was compared with experiments. It was found that the wetting process of a liquid aluminium drop on an Al2O3 surface can be represented by the diffusion model rather than by the hydrodynamic model. In contrast, the hydrodynamic model could be employed to describe the wetting process of a liquid polymer drop on a solid.

The effect of large oxygen additions on the wettability and work of adhesion of copper-oxygen alloys on polycrystalline alumina

Abstract: The apparent contact angle, the solid-liquid interfacial energy, and the work of adhesion were determined for liquid copper and copper-oxygen alloys on polycrystalline alumina using a sessile drop technique. The oxygen contents were varied from 1 to 11 wt.% and the temperature was varied from 1135 to 1300°C. Most studies of this system have centered on considerably lower oxygen contents. In this study, three regions of wetting behavior were observed. These regions correlated with three liquid regions on the copper-oxygen binary phase diagram. For 0.001–3 wt.%, the contact angle decreased and the work of adhesion increased significantly as the oxygen content increased. Within the immiscible liquid composition range (3–7.5 wt.%), the contact angle and work of adhesion were approximately constant. At higher oxygen contents (8–10.5 wt.%), only slight changes in the contact angle and work of adhesion were observed. At 10.5 wt.%, the contact angle achieved a minimum of 12 ± 4° and the work of adhesion reached a maximum value of 1340 ± 60 mJ m −2 .

Wettability of Calcium Phosphate Ceramics by Water

Abstract: To evaluate the wettability of phosphate ceramics by water, contact angles of hydroxyapatite-tricalcium phosphate composite ceramics were measured by the sessile drop and adhesion tension methods. It was difficult to obtain an equilibrium contact angle by the sessile drop method. On the other hand, reproducible values were obtained by the adhesion tension method if the sample was ground to less than 0.6mm in thickness. The contact angles of hydroxyapatite and β-tricalcium phosphate ceramics were 30.7° and 23.5°, respectively. The contact angle of the composite ceramics obtained by mixing of hydroxyapatite with tricalcium phosphate increased suggesting that the hydrophilicity of the surface decreased. The wettability of the composite ceramics increased with increasing surface hydroxyl group density measured by X-ray photoelectron spectroscopy.

Static wetting of a liquid drop on a solid

Abstract: A variational treatment of sessile drop shape is provided. The surface energy and the driving force for drop spreading are also analysed. This analysis demonstrates that the Young equation does indeed give the equilibrium contact angle in a gravity-free environment.

Generation of isothermal spreading data and interfacial energy data for liquid reactive metals on ceramic substrates: The copper-titanium/alumina system

Abstract: The sessile drop technique is frequently used to evaluate the wettability and spreadability of liquid metals on ceramic substrates. In this study, the spreading kinetics of copper-20 wt% titanium alloys on polycrystalline alumina were evaluated based on measurements of spreading radius versus time. The process of spreading was monitored by anin situ video recording system. The tests were performed using three different initial metal configurations. It was found that conventional sessile drop testing configurations cannot be used to generate isothermal spreading kinetics data because of significant spreading during the heat-up cycle from the solidus temperature to the test temperature. An improved sessile drop technique was developed which eliminated the non-isothermal experience by introducing the liquid copper to the solid titanium/alumina at the desired testing temperature. Using this technique, only a few seconds of data were lost (while the liquid copper dissolved the solid titanium). Because very limited interfacial energy data exist for the copper-titanium/alumina system, especially at higher titanium concentrations, the equilibrium contact angle, the solid-liquid interfacial energy, and the work of adhesion from 1000 to 1300 °C are also presented.

Wettability of Al2O3 by liquid Cu as influenced by additives and partial transient liquid-phase bonding of Al2O3

Abstract: The effect of additions (Cr, Ni and 80Ni.20Cr alloy) on the wetting characteristics of liquid Cu on Al 2 O 3 has been studied using a sessile drop method in a vacuum at 1150°C. Through the use of microdesigned multilayer Cu/80Ni.20Cr/Cu interlayers, PTLP (Partial Transient Liquid-Phase) bonding of Al 2 O 3 has been achieved in a vacuum at 1150°C. 80Ni.20Cr alloy and Cr additions reduced the contact angle of liquid Cu on an Al 2 O 3 substrate. For Cu with 10 mass%80Ni.20Cr alloy and with 6 mass%Cr added, contact angles decreased with time reaching values of ≤90° and ≤70°, respectively, at the end of the measurements. From SEM and EPMA observations, dissolution of the additions occurred, and thereby introduced Ni and Cr into liquid Cu droplets. The uniform distribution of Ni throughout the Cu droplet and the segregation of Cr at the Cu droplet/Al 2 O 3 interface were observed in the solidified specimen. The average strength for Cu/80Ni.20Cr/Cu interlayer bonded Al 2 O 3 was 137 MPa with a standard deviation of ±33 MPa. Failure of these samples occurred along the Al 2 O 3 /interlayer interface. From SEM and EPMA observations, some dissolution of 80Ni.20Cr core layer into liquid Cu film and the diffusion of liquid Cu into 80Ni.20Cr core layer occurred, however, because of the thicker Cu layers used, homogenization was incomplete. The results demonstrate that at the bonding temperature, the dissolution of Cr into liquid Cu film improved the wettability of liquid Cu film on Al 2 O 3 and facilitate the formation of reliably strong joint of PTLP bonded Al 2 O 3 using Cu/80Ni.20Cr/Cu interlayer.