Showing papers on "Sessile drop technique published in 2003"

Extended methodology for determining wetting properties of porous media

Abstract: [1] Because most methods for assessing the wettability of porous materials are restricted in their applicability, we developed two new methods for measuring contact angles and particle surface energy. The proposed methods (the Wilhelmy plate method (WPM) and the modified capillary rise method (MCRM)) were tested on 24 soils. For comparison, the water drop penetration time test (WDPTT) and the sessile drop method (SDM) were also applied. It was found that advancing contact angles, measured either with WPM or MCRM, agreed well in the range of 0° to 142°. Sessile drop contact angles were within the domain enclosed by the range of advancing and receding contact angles as determined with WPM. WDPTT, however, was only sensitive in the narrow range of 85° to 115°. We conclude that both WPM and MCRM are reliable methods for determining contact angles and particle surface energy over a wide range of porous material wettabilities.

Influence of the Electrical Double Layer in Electrowetting

Abstract: Electrowetting (wetting under the influence of an applied electric field) of three fluoropolymer surfaces (amorphous Teflon, DuPont) by electrolyte solutions was studied with the sessile drop method. The electrowetting curve (contact angle/potential) is analogous to the electrocapillary curve (surface tension/potential) and may be described by a combination of the Young and Lippmann equations. The influence of the electrical double layer at the polymer/solution interface has been neglected in the past because the overall interfacial capacitance is mainly determined by the capacitance of the insulating polymer layer. We demonstrate that for some surfaces a systematic deviation occurs at positive potentials. This departure from the constant capacitance regime is attributed to double layer effects, namely, the adsorption of hydroxide and halide anions. The pH, ionic strength, and polymer composition can all influence electrowetting behavior.

Stable and unstable surface evolution during the drying of a polymer solution drop.

Abstract: Drying of a sessile drop of a complex liquid can lead to intriguing complex shapes. We report here a study dealing with a model system, made of a hydrosoluble polymer that is glassy when pure. Under solvent evaporation, polymers accumulate near the vapor/drop interface and may form a glassy skin, which bends as the volume of liquid it encloses decreases. The conditions for the occurrence of this buckling instability have been investigated; the experimental results are well explained by a model that compares the characteristic times for drying and for the formation of a glassy skin. Depending on the experimental conditions, different types of shape distortion take place; secondary instabilities that break the axisymmetry are also observed.

A thermodynamic model for wetting free energies from contact angles

Abstract: A simple thermodynamic model for determining the wetting free energy of a small, sessile drop spreading on a solid surface has been derived by assuming that the process can be treated as adsorption. According to the model, free energies are expected to be small for large contact angles and increase exponentially as contact angles tend toward zero. Wetting free energies calculated using contact angles taken from the literature for various liquid−solid pairs agreed reasonably well with the bond strengths measured by other experimental techniques.

Wetting of Solid Al2O3 with Molten CaO-Al2O3-SiO2

Abstract: The wetting behavior of solid Al 2 O 3 with molten CaO-Al 2 O 3 -SiO 2 was investigated at 1 873 K using the sessile drop method. A new model was developed to represent the time dependence of the contact angle, i.e., the spreading behavior of a liquid drop on a solid substrate. The model takes into consideration chemical interactions which continually take place at the interface between the solid Al 2 O 3 and molten CaO-Al 2 O 3 -SiO 2 . By applying the model to the experimental results of the present study the equilibrium contact angle between the liquid slag and solid alumina was determined for a number of different slag compositions, and an iso-contact angle diagram was constructed. The equilibrium contact angle was greatly affected by the slag composition, and it was found that the interfacial tension was the major factor governing the equilibrium contact angle. In the region of low SiO 2 content, the slag with higher CaO content exhibits a smaller contact angle, i.e., better wettability with alumina. For slag with a given CaO/SiO 2 ratio, an increase in Al 2 O 3 results in a corresponding increase in the contact angle, i.e., decrease in wettability. For a given CaO/Al 2 O 3 ratio, the variation of the contact angle with SiO 2 content shows a minimum. The contact angle decreases by increasing the surface roughness of the alumina substrate.

Studies of the wetting characteristics of liquid iron on dense alumina by the X-ray sessile drop technique

Abstract: In the present work, the reaction between a molten iron drop and dense alumina was studied using the X-ray sessile-drop method under different oxygen partial pressures in the gas atmosphere. The changes in contact angles between the iron drop and the alumina substrate were followed as functions of temperature and varying partial pressures of oxygen in the temperature range 1823 to 1873 K both in static and dynamic modes. The results of the contact angle measurements with pure iron in contact with dense alumina in extremely well-purified argon as well as under different oxygen partial pressures in the gas atmosphere showed good agreement with earlier measurements reported in the literature. In the dynamic mode, when argon was replaced by a CO-CO2-Ar mixture with a well-defined PO, in the gas, the contact angle showed an initial decrease followed by a period of nearly constant contact angle. At the end of this period, the length of which was a function of the P-O2 imposed, a further steep decrease in the contact angle was noticed. An intermediate layer of FeAl2O4 was detected in the scanning electron microscope (SEM) analysis of the reacted substrates. An interesting observation in the present experiments is that the iron drop moved away from the site of the reaction once the product layer covered the interface. The results are analyzed on the basis of the various forces acting on the drop.

Influence of ash on interfacial reactions between coke and liquid iron

Abstract: Interfacial reactions occurring between molten iron and carbonaceous materials are of great significance in the steel industry, and specifically, the reaction of iron with metallurgical coke is one of the key phenomena occurring during blast furnace ironmaking. Major operating parameters such as hot metal composition will be directly influenced by the reactions occurring between liquid iron and coke. In the current investigation, the interfacial reactions occurring between coke and liquid iron were studied at a temperature of 1550 °C using the sessile drop method to further the understanding of the fundamental reactions occurring at the interface between coke and iron. The formation of interfacial reaction products was observed, and time-dependent reactions were identified. The transfer of elements such as carbon, sulfur, and silicon was determined. The reduction of silica was determined as having a major influence on the transfer of both silicon and carbon into liquid iron.

An analytical solution for determination of small contact angles from sessile drops of arbitrary size.

Abstract: An analytical solution to the capillary equation of Young and Laplace is derived that allows determination of the static contact angle based on the volume of a sessile drop and the wetted area of the substrate. This solution does not require numerical integration to determine the drop profile and accounts for surface deformation due to gravitational effects. Calculation of the static contact angle by this method is remarkably simple and accurate when the contact angle is less than 30°. A natural scaling arises in the solution, which provides indication of when a drop is small enough so as to neglect gravitational influences on the surface shape which, for small contact angles, is generally less than 1 μl. The technique described has the simplicity of the spherical cap approximation but remains accurate for any size of sessile drop.

Effect of evaporation on the contact angle of a sessile drop on solid substrates

Abstract: The effect of evaporation on the contact angle of a sessile drop of water on glass and polycarbonate substrates has been investigated. It has been observed that the contact radius depends on the initial mass of the drop and remains constant throughout the progress of evaporation. The evaporation rates in both systems are proportional to the contact radii of sessile drops. The contact angle, however, decreases as evaporation proceeds. In the case of water-glass system, the contact angle decreases almost linearly with time. On the other hand, in water-polycarbonate system, the contact angle shows a considerable departure from linearity. It has been shown that the non-linear behavior of contact angle becomes more pronounced with increasing initial contact angle.

Comparison between five experimental methods to evaluate interfacial tension between molten polymers

Abstract: In this work, an experimental comparison between five different techniques to measure interfacial tension between molten polymers is presented. The five techniques include two equilibrium methods: the pendant drop (PD) and the sessile drop (SD); two dynamic methods: the breaking thread (BT) and imbedded fiber retraction (IF); and a rheological method based on linear viscoelastic measurements of the blend (RM). The polymer pairs studied were polystyrene/polypropylene (PS/PP); and PP/high density polyethylene (PP/HDPE). It was possible to determine the interfacial tension between PP/PS with all the methods tested and the results corroborated within 20%. However, the interfacial tension between PP and HDPE could be evaluated only using rheological methods because of a too-small difference of index of refraction between both polymers. The experimental precision increased in the following order: RM < SD < BT < IF < PD. The rheological method had the advantage of being simpler and faster than dynamic and equilibrium methods. However, when using the rheological method, care should be taken because the results obtained may depend upon the concentration of the blend used for the measurements. It was observed that the pendant drop and breaking thread methods cannot be used for polymers with high viscosity (above 5 x 10 5 Pa.s).

Ceramic-metal interaction and wetting phenomena in the B4C/Cu system

Abstract: The experimental study of the wetting phenomena in the boron carbide–copper system, using the sessile drop method at 1150 °C shows that molten Cu attacks boron carbide substrates forming a crater below the contact area. Boron additions prevent crater formation, improve wetting and, for a 10 at.% B alloy, reduce the equilibrium wetting angle to 40°. Thermodynamic analysis of the Cu–B–C system in the Cu-rich corner confirms the experimental results. These results and similar ones obtained in the TiC–Cu system suggest that the presence of a carbide phase over a concentration range is a key feature that governs wetting phenomena and chemical interaction at these metal–carbide interfaces.

Influence of surface morphology on the wettability of cluster-assembled carbon films

Abstract: We have evaluated the surface energy and characterized the wettability of cluster-assembled carbon films deposited on polyethylene, Si and Al substrates by a sessile-drop method using water and glycerol. The effective surface energy of nanostructured carbon is of the order of 30 mJ/m2 as calculated using the Fowkes approach. The advancing contact angle (66–83°) is found to increase with surface roughness, i.e. the surface becomes more hydrophobic as the roughness increases. This behavior is ascribed to gas trapping in pores of the film with a self-affine surface. It is argued that the wettability of nanostructured carbon and other cluster-assembled coatings can be tuned simply by controlling the surface topography, through a suitable control of the deposition parameters, such as the cluster size distribution.

Production of Al–Ti–B grain refining master alloys

Abstract: In the present work the interfacial phenomena observed when an Al-Ti-B master alloy is produced have been studied using a modified sessile drop technique.The effect of emulsification has been demonstrated and the influence of small levels of CaF2 and MgF2 seen. The Al-Ti-B master alloys were made by the addition of potassium fluotitanate, K2TiF6, and potassium fluoborate, KBF4, to molten aluminium at 750°C. The product was an Al-5Ti-1B(wt-%) master alloy and a KF-AlF3 flux of eutectic composition. Problems can occur with the production of such alloys by first, emulsification of the liquid Al-Ti-B alloy and KF-AlF3 flux and second, agglomeration of titanium diboride particles by the wetting and engulfment of the KF-AlF3 flux. It has been found that levels of CaF2 and MgF2 in the fluoride salts greater than 50 ppm and 70 ppm, respectively, can prevent the above emulsification and boride agglomeration occurring.

Wetting of silicon carbide by copper alloys

Abstract: This work reports on a study of the wetting of SiC by some Copper alloys. The work is intended to gain a deeper understanding on the weeting behaviour of selected CuTi alloys in contact with a polycristalline silicon carbide substrate. Although the main body of this work deals with the CuTi/SiC system a few observations are also reported on the interaction between a CuZr alloy and a silicon infiltrated SiC grade. The contact angles established between the liquid alloys and silicon carbide at 1200°C were measured by the sessile drop method using a vacuum between 10−4 and 10−5 Pa. Since the interface generated between the metallic and ceramic phases in a composite material is a region that plays a critical role in determining its mechanical properties the obtained specimens were subsequently characterised using a SEM fitted with EDS facilities. X-ray diffraction was used to determine the phases formed while DSC analyses were employed to determine the temperatures for the formation of reaction products.

Investigation of the reactions between oxygen-containing iron and SiO2 substrate by X-ray sessile-drop technique

Abstract: The X-ray sessile-drop method was employed in the present investigation to measure the contact angle between liquid iron and a silica substrate under argon as well as CO-CO2-Ar atmospheres in the temperature range of 1823 to 1833 K. In the latter case, the measurements were carried out in the dynamic mode, and the contact-angle changes were followed as a function of time as oxygen in the gas dissolved in the metal. The static measurements in argon showed that the contact angles in the experimental temperature range are of the order of 135 deg, similar to those observed in the case of the alumina substrate. In the dynamic mode, oxygen partial pressures varying between 9.9.10(-4) and 1.5.10(-2) Pa were imposed on the system. In these experiments, the contact angle decreased in two stages, with an intermediate steady-state region. Fayalite slag, formed due to the reaction between the metallic phase and the substrate, was found to accumulate around the drop. The results are of relevance in understanding the mechanism of corrosion of silica-containing refractories by molten iron.