Showing papers on "Sessile drop technique published in 2001"

The influence on surface free energy of PVD-coatings

Abstract: Surface free energy is a characteristic factor which affects the surface properties and interfacial interactions such as adsorption, wetting and adhesion. Surface free energy is thus of interest in the field of adhesive technologies, biomedical applications, cleaning procedures or for the wettability of tribological systems. One method of determining the polar and dispersive terms of the free surface energies of solids is based on measurements of the contact angles of pure liquids on solid surfaces. This paper describes the use of dynamic contact angle analysis for the evaluation of surface tension.

Effect of Undulations on Surface Energy: A Quantitative Assessment

Abstract: Contact angle measurements have been widely used to estimate the surface energy of various materials. Such measurements are severely limited with substrate surfaces that exhibit surface restructuring, are contaminated, and/or are porous. Although the captive bubble/drop method addresses the capillarity problem, surface undulations have not previously been accounted for in a quantitative way. We do so here with a series of 8 different pore size synthetic polymer membranes, all fabricated from poly(ether sulfone), as model rough porous surfaces of the same surface chemistry. Also, 3 of the 8 different pore size membranes were rendered hydrophilic through photoinduced graft polymerization producing 17 different modified membranes that are similarly tested. By incorporation of roughness parameters obtained from AFM measurements, corrections to the captive bubble/drop constant angle measurements were successfully made using a simple model of the surface. The predicted average value for the sessile drop contact...

Wetting behavior of nanodroplets: The limits of Young's rule validity

Abstract: We study the contact angle of polymer melt sessile droplets on a flat structureless substrate subject to long-range van der Waals forces when the droplet size is diminished. For this purpose, droplets containing 4096 to 512 monomers for chains of length N = 32 are carefully equilibrated at 80% of the Θ-temperature using a coarse-grained bead spring model of flexible polymers in a Monte Carlo computer experiment. The spherically averaged density profile of these droplets, both in the z-direction perpendicular to the substrate surface and in the radial direction, is obtained. At weak adhesion we find that the contact angle grows steadily with the decrease of drop size so that below a critical size these drops dewet the surface. This suggests a strong contribution of a positive line tension κ in the determination of the equilibrium drop shape. At much stronger adhesion, however, the contact angle does not change with drop size, indicating κ ≈ 0. In general, our simulational results outline the limits of validity of Young's relation for the contact angle of sessile nanodroplets and support the more general Gretz rule which takes into account the role of line tension in the balance of surface forces.

Measurement of liquid surface properties by laser-induced surface deformation spectroscopy

Abstract: We developed a technique of picking up the liquid surface in a noncontact manner by a cw-laser radiation. The momentum change of light at the laser transmission through the air-liquid interface appears as the radiation pressure, which deforms the liquid surface into the shape determined by the balance between the Laplace force of the curved surface and the radiation pressure. The displacement of the liquid surface is inversely proportional to the surface tension, which was measured by an optical probe. The dynamic response of the liquid surface deformation was theoretically derived under the periodical modulation of the radiation pressure. The experimentally observed spectra were in good agreement with the theory giving the dynamic properties of the liquid surface. The technique of the laser induced surface deformation has potential as a measurement tool of the surface dynamic properties, such as the time-dependent surface tension and surface viscoelasticity.

Contact angles and spreading kinetics of Al and Al–Cu alloys on sintered AlN

Abstract: Wetting (both final contact angles and spreading kinetics) and bonding ( W a ) of pure Al and Al–Cu alloys on sintered AlN are studied by sessile drop experiments under high vacuum at 1100°C. At this temperature, the process which controls the spreading rate is shown to be the deoxidation of AlN surface. The mechanical behaviour of solidified droplets on cooling can be explained satisfactorily taking into account two parameters: the work of adhesion of the alloys on AlN and the yield stress of the alloys.

Effect of Alkyl Chain Length on the Fluorescence of 9-Alkylfluorenyl Thiols as Self-Assembled Monolayers on Gold

Abstract: Spectral characterization indicates that SAMs of 9-alkylfluorenyl thiols 1 form well-ordered monolayers on polycrystalline gold. Sessile drop contact angle measurements demonstrate the hydrophobic nature of these self-assembled thin films. The thicknesses of the films as measured by optical ellipsometry correspond well to the calculated thickness of monolayer films in which the extended alkyl chains are oriented at a 30° angle to the Au surface. The films are densely packed and are passivating to electron transfer between the Au surface and K4Fe(CN)6 in a contacting aqueous KCl solution. Grazing angle reflectance FTIR spectra show that the aromatic fluorenyl groups are fixed at a defined distance from the Au surface as determined by alkyl chain length disposed in an all-trans conformation. Surface fluorescence spectra for the SAMs are red-shifted and broadened compared with the corresponding molecule's fluorescence when measured in dilute solution. Lifetimes for the terminal fluorenyl groups when bound as...

Dynamic wettability of different machined wood surfaces

Abstract: Wetting dynamics on machined wood surfaces is of great interest for the adhesive bonding technology of wood. In this work, the change of apparent contact angles with time of phenolresorcinol-formaldehyde (PRF), polyvinyl-acetate (PVAc), and a series of probe liquids on sawed, planed, sanded, and razor-blade-cut wood surfaces of Southern pine was studied by the sessile drop method using a CCD camera technique. The results indicate that the fastest wetting of the probe liquids occurs on the sanded surfaces because of higher surface roughness and the resulting increase in capillary forces as compared with the sawed, planed and razor blade cut surfaces. The fastest wetting of the PRF and PVAc resins occurred on the comparably smooth planed and razor-blade-cut wood surfaces. A smoother wood surface seems to provide better wetting and penetration properties for high-viscosity liquids such as adhesives, which probably can be attributed to less entrapment of air between the resin and the wood structure.

Use of thermodynamic data to calculate surface tension and viscosity of Sn-based soldering alloy systems

Abstract: A thermodynamic database for the Pb-free soldering alloy systems, which include Sn, Ag, Cu, Bi, and In, has been made using the CALPHAD method. The resulting thermodynamic properties of the Sn-based binary alloy systems were used to determine the surface tensions and viscosities. The surface tensions were calculated using Butler’s monolayer model and the viscosities by Hirai’s and Seetharaman’s models. Butler’s model was also used to determine the surface active element. The results for binary systems were extended to the Sn-based ternary systems (Sn-Ag-Cu, Sn-Ag-Bi). The surface tensions of commercial eutectic Sn-Pb and Sn-Pb-Ag solder alloys were measured by the sessile drop method. The measured values and other researchers’ results were compared with the calculated data.

Wetting Characteristics of Aqueous Surfactant-Laden Drops

Abstract: Strategies to promote or control droplet spreading on solid surfaces is important to many industrial and biological processes. The manipulation of surface tension and contact angle by the addition of surface-active compounds to the bulk liquid of a drop offers one strategy for altering the adhesion tension maintaining drop integrity on a solid surface. Here, we present results of an experimental study of surfactant-laden drop behavior on solid surfaces. Wetting experiments were performed to determine how contact angles and contact angle hysteresis change with varying soluble surfactant concentration in the bulk liquid phase. Static, advancing, and receding contact angles of aqueous solutions of two different surfactants were measured over a wide range of surfactant concentrations on three different solids using an inclining surface. In general, adhesion tension increased and then plateaus as the surfactant concentration increased from very dilute values to higher concentrations. At surfactant concentrations above the critical micelle concentration, a continued decrease in contact angle was observed, leading to an increase in the adhesion tension. Subsequent experiments confirmed adsorption of surfactants to the solid substrate. Contact angle hysteresis was calculated from the experimental data. The behavior of the contact angle hysteresis was substrate and surfactant concentration dependent, but only at lower surfactant concentrations. Material combinations that led to partial wetting could be converted to complete wetting by the addition of sufficient surfactant in the bulk phase, but at the cost of increasing adsorption onto the solid, which restricts drop motion.

Relationship between wetting and electrical contact properties of pure metals and alloys on semiconducting barium titanate ceramics

Abstract: In this work, the relationship between the thermodynamic properties of metal–ceramic interfaces and their electrical contact properties was explored for semiconducting BaTiO3 ceramics. The role of chemical reactivity at the interface was quantified through sessile drop measurements of the work of adhesion (Wad). The interfacial properties were characterized with impedance spectroscopy to quantify contact resistance and capacitance. It was observed that interfaces with a strong chemical interaction between the metal and the oxygen anion in the ceramic showed large Wad and correspondingly low contact resistances. It is believed that an oxygen depleted region near the surface of the BaTiO3 was formed as a result of the strong metal-oxygen interactions and led to a large Wad and an Ohmic contact.

Measurement of Surface Tension of Liquid Ga Base Alloys by a Sessile Drop Method

Abstract: The surface tensions and the molar volumes of liquid Ga-Bi, Ga-Sn, Ga-In, Sn-Bi, In-Bi and Sn-In binary alloys, and liquid Ga-Bi-Sn and Ga-Bi-In ternary alloys have been measured at 873 K by the sessile drop method. In addition, the calculation of the surface tension of the above binary alloys has been carried out to interpret the composition dependences of the surface tension of Ga-base liquid alloys from the thermodynamic point of view.

Segregation of aluminium at nickel-sapphire interfaces

Abstract: Sessile drop experiments of pure liquid Ni on the basal surface of pure sapphire were conducted under controlled atmosphere and temperature This system has been traditionally considered as non-reactive, based on thermodynamic assessments However, the results of this study demonstrate that a capillary driven interaction exists between the pure liquid Ni and the sapphire, which causes the dissolution of the sapphire substrate mainly at the triple junction Oxygen and Al resulting from the dissolution process diffuse into Ni and segregate at its interfaces with the atmosphere and the sapphire (probably as Al x O y clusters), which reduces the interface energy It is considered that this reduction is beneficial for the adhesion of both liquid and solid Ni on sapphire The amount of Al introduced into the drop, and hence the segregation of Al that affects the interface energy (and adhesion), are related to the size of the sessile drop

Optical measurement of microscale transport processes in dropwise condensation

Abstract: The optimum use of interfacial free-energy gradients to control fluid flow in small regions naturally leads to simpler passive heat transfer systems. In this context, passive refers to the natural pressure field for fluid flow due to changes in the intermolecular force field resulting from an imposed nonisothermal temperature field. Although the particular constrained vapor bubble (CVB) discussed can be viewed as a large version of a wickless heat pipe, it is a much more general heat transfer concept. Herein, it is an ideal system for the optical study of microscale transport processes in droplet condensation due to interfacial phenomena. This article concerns the movement of a single condensed ethanol sessile drop into a concave liquid film. The intermolecular force is found to be much larger than the gravitational force and dominates condensate removal. A dimensionless force balance for viscous shear stress demonstrates the effect of changes in the contact angle and curvature. A dimensionless difference in free energy is identified as the cause of spontaneous condensate removal

Thermodynamic evaluation of surface tension of liquid metal-oxygen systems

Abstract: Thermodynamic equations have been derived to evaluate the surface tension of liquid metal-oxygen systems. On the basis of these equations, the effect of the oxygen on the surface tension of liquid metals has been evaluated by using the fundamental information on the oxygen solubility in the metals, the surface tension and the molar volume of pure liquid metals and oxides. The calculated results from these equations agree with the literature values of the surface tension of liquid Fe-O, Co-O, Ni-O, Cu-O and Si-O systems.

Modified Tilting-Plate Method for Measuring Contact Angles

Abstract: The additivity of the dynamic curvature of a thermocapillary depression and the static curvature of liquid meniscus is experimentally confirmed. A high sensitivity of response shape to the static curvature of liquid surface is used to improve the tilting-plate method. The results of measuring contact angles by this method are well consistent with the data obtained by the sessile drop method. The behavior of meniscus is analyzed in the tilting of a plate with various positioning of its rotation axis in relation to the liquid–gas interface. The applicability limits of the modified method are indicated.

A laser-based technique for the coating of mild steel with a vitreous enamel

Abstract: Marked changes in the wettability characteristics of EN8 mild steel were observed after high power diode laser (HPDL) surface treatment of the metal. The morphological, microstructural and wetting characteristics of the steel were determined using optical microscopy, scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and wetting experiments by the sessile drop technique. Improvements in the wetting action of the mild steel after HPDL surface treatment were identified as being the result of: The HPDL induced melting of the mild steel surface which brought about a reduction of the surface roughness; The small increase in the polar component of the surface energy and the increase in the surface O2 content of the mild steel resulting from HPDL treatment. This work has demonstrated that the use of HPDL radiation to alter the wetting characteristics of mild steel in order to facilitate improved enamelling is a real possibility.

Viscosity and surface tension of liquid Fe-metalloid glass-forming alloys

Abstract: Amorphous and nanocrystalline Fe-based alloys have a wide range of applications as soft magnetic materials. This paper summarises our recent advances in investigation of those thermophysical properties of liquid Fe–metalloid alloys with relevance for both structural evolution of the melt and technological application. Viscosity measurements at high temperatures were performed by oscillating-cup method. Surface tension was determined by the sessile drop method. Composition dependence of the viscosity and surface tension for Fe–B alloys [Phys. Stat. Sol. (a) 31 (1975) K41] in the glass-forming range is discussed. A short review of the results on the temperature variation of viscosity and surface tension for FeBSi alloys is presented. High viscosity values near the liquidus temperature and a specific behaviour of this structure-sensitive property during heating and cooling the molten alloys were found [Jpn. J. Appl. Phys. 17 (1978) 1911]. These results are related to clustering effects in these melts with preferential interaction between unlike atoms.

The equilibrium shape of an axisymmetric sessile drop subject to local stresses

Abstract: The exact equation describing the shape of a fluid drop under the action of local surface stresses induced by colloidal interactions is derived without resorting to any of the approximations inherent in the profile equation currently employed in the literature. The exact equation implies, and numerical examples confirm, that repulsive external (i.e. positive) surface energies assist in stabilizing the drop against deformation, while attractive (i.e. negative) energies destabilize the drop, promoting or enhancing deformation. An inherent singularity in the governing differential equation (absent from the approximate equations currently used) when the surface energy (surface tension) is identically matched by an external attractive energy represents an instability limit. Explicit bounds are established for a further instability criterion and for the hydrostatic pressure difference across the interface. An exact equation for the radial extent of the sessile drop and some numerical examples are also presented.

Surface energy, adsorption, and wetting transitions in ternary liquid alloys

Abstract: Surface energy measurements have been performed on liquid Ga-Sn binary and on Ga-Pb-Sn as well as Ga-Pb-Tl ternary alloys, by the sessile drop technique under ultrahigh vacuum conditions. The purpose of these measurements was to investigate the change in adsorption behavior of Pb in Garich alloys as a consequence of ternary additions. In order to aid in the interpretation of the results, a multilayer model of surface segregation was formulated in the regular solution approximation. The results show evidence of site competition effects between the two solutes, which reduce the surface concentration of Pb in relation to binary Ga-Pb alloys. The effects of Sn on previously studied wetting-related adsorption transitions in Ga-Pb were also investigated. It was concluded that Sn additions are likely to raise the wetting temperature of the Ga-Pb alloy.

Wetting of alumina, iron and stainless steel substrates by molten magnesium

Abstract: An approach was developed for observation of the wetting behavior of molten magnesium on solid substrates by pre-filling magnesium vapor in the reaction chamber from a special magnesium evaporation source to control its evaporation from the sessile drop. Based on this approach, the wetting behavior of four kinds of substrates (alumina, iron, SUS316 and SUS430 stainless steel) by molten magnesium was investigated. The results showed that the molten magnesium drop revealed a spreading tendency during the initial wetting process stages on alumina and SUS316 stainless steel solid substrate and gave a good wettability of 89° and 71°, respectively, for the initial contact angle at 973 K. In these wetting processes, there were two distinct behaviors of the advancing wetting because of the good wettability of molten magnesium and the subsequent drop-receding, resulting from the evaporation of magnesium. Furthermore, the interfacial reaction products that induce good wettablity could be observed in other specimens obtained under higher reactive conditions, although they were hardly detected at the interface between molten magnesium and the substrate at the end of the wetting experiment. On the contrary, the molten magnesium drop did not advance on the surface of iron and SUS430 substrates because of the poor wettability at 115° and 116°, respectively, for the initial contact angle at 973 K.

Effects of Evaporation and Thermocapillary Convection on Volatile Liquid Droplets

Abstract: Results of an experimental investigation of evaporating sessile drops on a glass-slide surface for three volatile liquids show that both evaporation and thermocapillary convection in the sessile drop strongly affect the drop spreading and contact angle. The evolution of contact diameter of the drops can be divided into four stages: 1) initial spreading, 2) spreading-evaporation balance, 3) evaporation-dominating contraction, and 4) final rapid contraction. Molecular-kinetic spreading always occurs in the early first stage and is rapidly restrained and then taken over by the effects of evaporation. Thermocapillary convection, induced by the evaporation, promotes the competition of evaporation over the spreading and shortens the spreading-evaporation balance stage to become undetectable. Evaporation may increase or decrease the contact angle of the evaporating sessile drops, depending on the evaporation rate

Wetting characteristics of CaO−SiO2−Al2O3 ternary slag on refractory oxides, Al2O3, SiO2 and TiO2

Abstract: The wettabilities of the metallurgical slag, CaO−55.2 wt.%SiO2−15 wt.%Al2O3 on refractory oxides, Al2O3, SiO2 and TiO2 were investigated at temperatures of 1350, 1400 and 1470°C. Contact angle measurements were performed using a combination of the sessile drop method and the X-ray fluoroscopic technique. The contact angle was obtained from the numerical solution of the Young-Laplace equation. The steady contact angles obtained were 31°, 24°, and 15° for SiO2, Al2O3 and TiO2 at 1470°C, respectively. The spreading rate of the liquid slag was characterized by the formation of cognate interface between the liquid slag and solid oxide, and further enhanced by the formation of a diffuse layer around the slag drop at higher temperatures.

Wettability of non-reactive Cu/Si–Al–O–N systems I. Experimental results

Abstract: The purpose of this work was to study the evolution of ceramic/liquid metal interactions according to the ionocovalent nature of the anion–cation bond in the ceramic. In order to explore a wide ionicity scale, Si–Al–O–N systems were chosen, enabling variation in either the anion or the cation or both at the same time. The experimental work of adhesion, W a , of pure liquid copper on seven compounds (oxides, nitrides and oxynitrides) was measured. Analysis (SEM, EDS and Auger) confirmed the non-reactivity of the systems studied. The values of contact angle, θ , and surface tension, σ lv , have been obtained by the sessile drop method. There is no significant dependence on the nature of the substrate and satisfactory agreement with the literature data was found.