Wetting of rare-earth element oxides by metallic melts

The results of studies on the wettability and contact interaction of rare-earth (REE) oxides and aluminium oxide (for comparison) with metallic melts are presented. Proceeding from analysis of the data obtained, a mechanism of interaction of metallic melts with rare-earth element oxides is proposed, based on the interaction of the metallic melt components with the cationic and the anionic sublattice of the oxides. The necessity to take into account the interaction of the melt metal with the cationic sublattice of oxide is determined by a high chemical affinity of the REE for quite a number of metals and by specific features of the structure of the surface of the oxides.     

超声作用下Mg-Zn-Al钎料的润湿与铺展

在大气环境中,对Mg-Zn-Al钎料进行超声振动作用下的铺展,超声时间分别为1,2,3,4s,并与钎剂作用下的钎料铺展行为进行对比。采用体视显微镜观察钎料在超声及钎剂作用下的铺展形貌。采用光学显微镜观察钎料铺展初始端、铺展末端及铺展后钎料的微观组织形貌。结果表明:超声振动作用下的Mg-Zn-Al钎料沿基体表面作受迫铺展,声空化作用于液态钎料产生的冲击波可以破碎基体表面的氧化膜,使液态钎料与母材发生润湿,母材溶解的深度仅有0.12mm。超声时间为2s时,钎料铺展面积最大。超声声空化作用破碎液态钎料在凝固期间产生的α-Mg固溶相及MgZn共晶相,使铺展后的钎料显微组织得到细化。     

Spreading of small liquid drops along a flat surface

We discuss two limiting spreading laws for small drops of a viscous liquid which are supported by the experimental data.Translated from Inzhenerno-Fizicheskii Zhurnal, No. 2, pp. 210–213, August, 1989.     

Wetting ability of biological liquids in presence of metallic nanoparticles

The wetting ability of water and of some biological liquids was measured on different biocompatible surfaces with and without different colloidal metals. Insoluble nanoparticles disperse in biological tissues enhance some properties, such as the interface adhesion between two surfaces, the X-ray contrast of medical images and the absorbed dose during radiotherapy treatments. The introduction of nanoparticles in the liquids generally improves the wetting ability and changes other properties of the solution, due to the different distribution of the adhesion forces, to the nature, morphology and concentration of the added nanoparticles. An investigation on the contact angle of the liquid drops, physiological liquids, including the human blood, placed on different substrates (polymers, ceramics and metals) with and without the use of metallic nanoparticles is presented, evaluated and discussed.     

Wetting pattern characterization of rapidly solidified centrifuge melt-spun metallic ribbons

The wetting pattern characterization of rapidly solidified centrifuge melt-spun metallic Al-Ge ribbons has been enabled by measuring the roughness of the rim-contact side of the ribbons. Roughness parameter values were obtained using the SEM ability of amplitude modulation and line scanning of the smooth ribbon surfaces, and micro-computerized digitizing of the line profiles. In such a manner, quantification of the wetting patterns is possible. As a result, the respective contribution of the ejection pressure and the extraction velocity on the wetting patterns of the solidifying ribbons is evaluated. In centrifuge melt spinning, the roughness of the rim-contact surface of the ribbons increases when extraction velocities are increased. Despite the additional roughness and the bad thermal contact conditions for effective heat transfer, higher cooling rates (as measured by secondary dendritic arm spacings), are achieved. It appears that a kind of dragging occurs, reducing the ribbon thickness by a shear drawing mechanism. This behaviour is characteristic of centrifuge melt spinning because of the specific hydraulic and heat transfer features involved with the counter-rotating of the crucible and the substrate.     

Spreading dynamics: a succinct account of some basic questions

A few basic (theoretical and experimental) questions and some universal results about spreading dynamics are discussed.     

Spreading dynamics of a functionalized polymer latex

Functionalized polymer nanoparticles are used as binders for inorganic materials in everyday technologies such as paper and coatings. However, the functionalization can give rise to two opposing effects: It can promote adhesion via specific interactions to the substrate, but a high degree of functionalization can also hamper spreading on substrates. Here, we studied the spreading kinetics of individual functionalized vinyl acetate-co-ethylene polymer nanoparticles on inorganic substrates by atomic force microscopy (AFM) imaging. We found that the kinetics underwent a transition from a fast initial regime to a slower regime. The transition was independent of functionalization of the particles but depended on the wettability of the substrate. Furthermore, the transition from the fast regime to the slow regime occurred at a size-dependent contact angle, leading to a h ～ a(3/2) scaling dependence between the height (h) and the width (a) of the spreading particles. Thereafter, spreading continued on a slower time scale. In the slow regime, the kinetics was blocked by a high degree of functionalization. We interpret the observations in terms of a nanoscale stick-slip transition occurring at interface stress around 6 kPa. We develop models that describe the scaling relations between the particle height and width on different substrates.     

Wetting dynamics versus interfacial reactivity of AlSi alloys on carbon

The wetting dynamics of silicon carbide forming AlSi alloys on carbon substrates is studied by the dispensed drop technique in high vacuum by varying the parameters: alloy composition, temperature and type of carbon (vitreous carbon, pyrocarbon and pyrographite). The results on wetting are analysed in relation to those on interfacial reactivity and compared with model predictions.     

Effect of evaporation on the dynamics of falling drops

The relationship between the dimensionless intensity of the evaporation of a drop and the Reynolds number of the separating vapor is shown. Conditions are indicated under which the Dukovicz correction to the resistance coefficient of a falling evaporating drop cannot be ignored. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 71, No. 2, pp. 222–224, March–April, 1998.     

Wetting and coalescence prevention of drops in a liquid matrix. Ground and parabolic flight results

An experimental and numerical analysis of the behavior of drops in a liquid matrix, in presence of temperature differences, is carried out in preparation for a MAXUS sounding rocket flight to study wetting and coalescence prevention induced by thermal Marangoni effect. On-ground experimentation has been carried out using micro zone apparatus. Different pairs of liquids (drop and matrix) have been analyzed; wetting prevention has been observed with drops of different diameter and critical temperature differences have been measured for each pair. To avoid buoyancy effects similar experiments have been carried out during the 30^th ESA parabolic flight campaign. The main objective of the experiments is facility tests in a parabolic flight for the MAXUS flight. During the parabolic flight a hanging drop of Silicone oil 10 mm diameter has been injected in a matrix of Fluorinert and in air; wetting prevention has been observed in presence of a temperature difference between the drop and the lower surface. The theoretical-numerical study of the problem has been carried out with a thermofluidynamic model based on the assumption of the existence of a fluid film between the drop and the lower surface. After the campaign, the video images of the experiment have been analyzed and velocity measurements have been obtained analyzing the motion of tracers. Measured and computed velocities are in sufficient agreement, in particular for the Silicone oil/Fluorinert configuration.     

Vibrational dynamics of Zr-based bulk metallic glasses

The vibrational dynamics of some Zr-based bulkmetallic glasses were studied at room temperature in terms of phonon eigen frequencies of longitudinal and transverse modes employing three different approaches proposed by Hubbard-Beeby (HB), Takeno-Goda (TG) and Bhatia-Singh (BS). The well recognized model potential is employed successfully to explain electron-ion interaction in the metallic glass. The present findings of phonon dispersion curve are found to be in fair agreement with available theoretical as well as experimental data. The thermodynamic properties obtained by the HB and TG approaches are found to be much lower than those obtained by the BS approach.     

Solidification dynamics of spherical drops in a free fall environment

Silver drops (99.9%, 4, 5, 7, and 9 mm diameter) were levitated, melted, and released to fall through Marshall Space Flight Center’s 105 meter drop tube in helium −6% hydrogen and pure argon atmospheres. By varying a drop’s initial superheat the extent of solidification prior to impact ranged from complete to none during the ∼4.6 seconds of free fall time. Comparison of the experimental observations is made with numerical solutions to a model of the heat transfer and solidification kinetics associated with cooling of the drop during free fall, particularly with regard to the fraction of liquid transformed. Analysis reveals the relative importance of the initial parameters affecting the cooling and solidification rates within the drop. A discussion of the conditions under which the actual observations deviate from the assumptions used in the model is presented.     

Spreading of liquid lead droplets on metallic iron covered by silicon oxide particles or films

As well known, the spreading of a liquid metal droplet on a solid metal is very sensitive to the presence of chemical heterogeneities on the solid metal. In this study, wetting experiments with liquid lead on heterogeneous surfaces composed of iron and silicon oxide particles or films were performed using the dispensed drop technique. High purity iron and binary iron–silicon substrates with different silicon contents were studied. Before the wetting experiments, the substrates are annealed at 850 °C in a N₂–H₂ atmosphere in order to reduce iron oxides and to form silicon oxide particles or films on the surface. The liquid lead droplet is then released onto the metallic substrate partly or wholly covered by the oxides. The spreading of the liquid metal droplet strongly depends on the surface area fraction covered by the oxides.     

Effects of pores on shear bands in metallic glasses: A molecular dynamics study

The effects of nanoscale pores on the strength and ductility of porous Cu₄₆Zr₅₄ metallic glasses during nanoindentation and uniaxial compression tests are modelled and investigated using molecular dynamics (MD) simulations. In the MD simulations, atomistic amorphous samples were digitally prepared through fast quenching from the liquid states of copper and zirconium alloy. In both of the nanoindentation and uniaxial compression simulations, shear transformation zones and shear bands are observed through the local deviatoric shear strains in the samples. The results show that the existence of pores causes strain concentrations and greatly promotes the initialization and propagation of shear bands. Importantly, only pores reaching critical size can effectively facilitate the formation of multiple shear bands. It is also observed that hardening occurs through pore annihilation and the shear band stops in porous metallic glasses.     

Modeling the pullout test of nanoreinforced metallic matrices using molecular dynamics

Molecular dynamics (MD) simulations of pullout tests are developed to determine the effect of the different parameters influencing the interfacial shear strength (ISS) of carbon nanotube-reinforced metallic matrices. Unlike earlier works, the current study focuses on the effect of the cell size, cell geometry, and the potential functions adopted in the MD simulations. The basic MD cell was created in two steps. The metal atoms were initially created using the built-in tools in the molecular dynamics code “LAMMPS” guided by the specific metal lattice parameters with a pre-defined constraint of a central hole to accommodate the CNT at a later stage. The cell was equilibrated with Brownian dynamics prior to the placement of the CNT reinforcement. The CNT was then placed in the central hole. This was then followed by equilibrating the entire system prior to pulling out the CNT, to release spurious stresses arising during the build up of the cell, initially with Brownian dynamics and later with the nvt ensemble. Our ISS predictions agreed very well with earlier research work. Additionally, our results show that box-shaped MD cells are more suitable for the pullout test simulations with nvt or nve ensembles, while npt scheme produces additional forces to the system. The MD cell length was found to have insignificant effect on the pullout force.     

Contact formation dynamics: Mapping chemical bond formation between a molecule and a metallic probe

We present a study that maps out chemical bond formation between a Pt-inked probe and a single 1,3-cyclohexadiene (1,3-CHD) molecule on Si(100). By separating the mechanical and electronic contributions to the current during the approach to contact, we show that there are significant forces between the probe and the C=C of the molecule and we track the relaxation of the molecule, the emergence of a chemical bond feature in the LDOS, and the quenching of specific molecular vibrations during bond formation.     

Wetting transparency of graphene

We report that graphene coatings do not significantly disrupt the intrinsic wetting behaviour of surfaces for which surface-water interactions are dominated by van?der?Waals forces. Our contact angle measurements indicate that a graphene monolayer is wetting-transparent to copper, gold or silicon, but not glass, for which the wettability is dominated by short-range chemical bonding. With increasing number of graphene layers, the contact angle of water on copper gradually transitions towards the bulk graphite value, which is reached for ~6 graphene layers. Molecular dynamics simulations and theoretical predictions confirm our measurements and indicate that graphene's wetting transparency is related to its extreme thinness. We also show a 30-40% increase in condensation heat transfer on copper, as a result of the ability of the graphene coating to suppress copper oxidation without disrupting the intrinsic wettability of the surface. Such an ability to independently tune the properties of surfaces without disrupting their wetting response could have important implications in the design of conducting, conformal and impermeable surface coatings.     

Wetting of Rough Walls

Quenched geometric disorder of a wall delimiting a spectator phase can have dramatic effects on the nature of critical wetting transitions. We consider self-affine walls in 2D with roughness exponent _W. Transfer matrix results for directed interfacial models with short-range interactions suggest that wetting turns first-order as soon as _W exceeds ₀, the anisotropy index of interface fluctuations in the bulk. Discontinuous interface depinning is best identified by a peculiar two-peak structure in the statistical distributions of wall–interface contacts obtained by sampling over disorder. On the other hand, for _W<₀ wetting remains continuous, most plausibly in the same universality class as with flat walls. This occurs both with ordered (₀ = 1/2) and with bond-disordered (₀ = 2/3) bulk. A precise location of the thresholds at _W = ₀ can be argued on the basis of an analysis of different terms in the interfacial free energy. This analysis elucidates the peculiar role played by the intrinsic interfacial roughness and suggests extensions of the results to 3D and to long-range substrate forces.