Modeling the impact of a molten metal droplet on a solid surface using variable interfacial thermal contact resistance

An analytical model of the true area of contact between molten metal and a rough, solid surface has been used to calculate thermal contact resistance and to predict how it changes with surface roughness, substrate thermal properties and contact pressure. This analytical model was incorporated into a three-dimensional, time-dependent numerical model of free-surface flows and heat transfer. It was used to simulate impact, spreading and solidification of molten metal droplets on a solid surface while calculating contact resistance distributions at the liquid–solid interface. Simulations were done of the impact of 4 mm diameter molten aluminum alloy droplets and 50 μm diameter plasma sprayed nickel particles on steel plates. Predicted splat shapes were compared with photographs taken in experiments and simulated substrate temperature variation during droplet impact was compared with measurements.     

Parameters controlling solidification of molten wax droplets falling on a solid surface

An experimental study was done to identify parameters that determine the shape of splats formed by droplets of paraffin wax impacting and freezing on a polished aluminum surface. Impact velocity was varied from 0.5 to 2.7 m/s and surface temperature from 23 to 73 °C. Droplet impact was photographed, and the splat diameter and liquid-solid contact angle measured from photographs. A simple energy conservation model was used to predict the maximum extent of droplet spread and the rate of droplet solidification. The extent of droplet solidification was found to be too small to affect droplet impact dynamics. Photographs showed liquid recoiling in the droplet center following impact on a cold surface (23 °C); the height of recoil diminished if either substrate temperature or impact velocity was increased. Droplet recoil was attributed to surface tension pulling back the periphery of the splat. Reducing the surface temperature increased surface tension, promoting recoil. At sufficiently large impact velocities droplets fragmented. A model based on the Rayleigh-Taylor instability was used to predict the number of satellite droplets that broke loose after impact.     

Molecular dynamics studies of microscopic wetting phenomena on self-assembled monolayers

Molecular dynamics calculations have been used to investigate the behavior of overlayers of water or n-alkane fluids on solid surfaces formed from “self-assembled” monolayers of long-chain hydrocarbons. A microscopic analog of the wetting contact angle is used to measure the surface wetting characteristics. On a nonpolar surface, formed by close packed chains having -CH₃ tailgroups, the water molecules aggregate to form a compact droplet. The calculated contact angle of the droplet is similar to experimental values for macroscopic water droplets. Contrary to intuition, the overlayers of hexadecane or decane form droplets with smaller contact angles on the same surface. However, the calculated contact angles are again in reasonable accord with experimental values.     

接触角和质量分数对液滴冻结过程的影响

本文通过Fluent软件的凝固/熔化模型,模拟了接触角及质量分数对纯水和氯化钠溶液在冷表面冻结过程的影响,选择铜片为亲水表面,纳米膜表面为疏水表面,对液滴在不同表面特性条件下的冻结过程进行实验研究.结果表明:液滴在冷表面的冻结特性与接触角、质量分数有关.当溶液质量分数一定时,接触角越小,液滴冻结速度越快,完全冻结时间越...     

Jet printing of colloidal solutions – Numerical modeling and experimental verification of the influence of ink and surface parameters on droplet spreading

Ink jet printing of functional materials promises an efficient route for the manufacturing of future low cost and large-area electronics applications. The effect of capillary flow of thin liquid films, the control of droplet spreading by suitably influencing the wetting properties of surfaces, the rheology of the ink and the process design play a relevant role in improvement of ink jet printed patterns. This work presents the experimentally based numerical study of the shape of single ink jetted droplets controlled by homogeneous contact angle distributions. The dynamics of the fluid on the substrate surface is treated in the frame of the lubrication theory using the concept of a precursor film and modeling the equilibrium contact angle by a disjoining pressure. The model describes the spreading of axisymmetric droplets considering different material and process parameter configurations. It is shown that the spreading process can be modeled separately from the drying process within a certain range of contact angles.     

Sessile droplet formation in the laser-induced forward transfer of liquids: A time-resolved imaging study

The formation process of sessile droplets in the laser-induced forward transfer of aqueous solutions was analyzed through time-resolved imaging. At the irradiation conditions which lead to the deposition of well-defined droplets, a cavitation bubble is generated in the laser irradiated area. Such bubble evolves into a high-speed liquid jet which propagates towards the receptor solid substrate. Once the jet impinges on the receptor substrate, liquid gently starts accumulating on the impact position, and the growth of a sessile droplet initiates. In a first stage, which only lasts a few microseconds, the forming droplet suffers a fast spreading process. Then, the jet continues feeding the forming droplet for some hundreds of microseconds, but the droplet diameter remains constant, and thus the contact angle increases. Finally, liquid feeding stops due to jet breakup, and the sessile droplet initiates a slow relaxation process in which its contact angle diminishes and its diameter increases. This deposition process results in the deposition of a single sessile droplet up to donor film-receptor substrate distances of the order of the millimeter. At higher separations, satellite droplets appear, and at even higher separations only randomly distributed small droplets are deposited.     

Influence of substrate surface conditions on the deposition and spreading of molten droplets

This paper reports the finding of the role of substrate surface chemistry on the interaction between the molten droplets (splats) and solid substrates in thermal spray coating. The substrate surfaces were modified by thermal treatments to grow specific types of oxide and hydroxide layers on the surface. It was found that water released from the dehydration of surface hydroxide triggered by the impact of the droplets provoked splat fragmentation and splashing, resulting in the poor splat-substrate bonding. Although this finding is used to address the question of the nature and the influence of surface adsorbates on the splat formation and morphology in thermal spray coating, it can be applied to other technologies and material processes involving the contact between the molten droplets and metal surfaces.     

Numerical analysis of the deformation and solidification of a single droplet impinging onto a flat substrate

An existing model has been modified to explore the deformation and solidification of a single droplet impinging on a substrate. The modification accounts for possible solid fraction of material at impact. Numerical results predict that the kinetic energy dominates the process at impinging velocities greater than about 100 m s⁻¹. In addition, the thermal diffusivity of the solidifying material controls the process, but the temperature of the substrate relative to the melting temperature of the material must be considered when comparing materials. It is believed that droplets solidifying into thinner, wider discs would reduce porosity; therefore, dense materials accelerated to high speed would solidify into masses with the highest bulk density.     

Heat transfer aspects of splat-quench solidification: modelling and experiment

In this paper a combined theoretical and experimental study is reported on the process of solidification of a liquid metal droplet by impaction on a cold substrate (splat-quenching). The study is focused on the heat transfer aspects of this process and on the identification of parameters affecting the heat transfer mechanism. To this end, the effect of the droplet impact velocity and temperature, the effect of the substrate material and its initial temperature, and the effect of the thermal contact resistance between the splat and the substrate are investigated. A two-dimensional conduction model accounting for the freezing process in the splat and for the solidification kinetics has predicted reasonably well the trends observed in the experimental part of the study.     

Effect of temperature and surface roughness on the wettability of boron nitride by molten Al

Reactive wetting of hexagonal BN by molten Al at 1073–1273 K was studied using an improved sessile drop method. The temperature and substrate surface roughness have a remarkable effect on the wetting behavior. Reasons for the large discrepancy in the final contact angles reported in the literature were addressed.     

A lattice Boltzmann model for solidification of water droplet on cold flat plate

Since the solidification of water droplet is the initial and essential process in the whole process of frosting, a model is developed by the lattice Boltzmann method (LBM) that applies the velocity and temperature distribution functions to investigate the solidification process of water droplet on cold flat plate. The thermal transport and liquid–solid phase transition in the present model are both based on the pseudo-potential model combined with the enthalpy formation. By this LB model, the solidification process is simulated in form of temperature and solid phase variations in water droplet on cold flat plate, and the shape of solid phase in freezing can also be predicted. In addition, we apply the present LB model to preliminarily study the frost formation process. Numerical results agree well with our experimental data.     

Spallation of a substrate by thermal shock and thermal expansion differential during splat cooling

The manufacture of metal-matrix composite materials by spray deposition is a very attractive process, but impaired by the spallation that may take place after impact of molten metal droplets on the fibers. In this work, the spallation of a quartz substrate was investigated through video and acoustic measurements and through temperature measurements of the splat surface. The time scales pertaining to the fracture mechanisms are examined from acoustic measurements of the spallation. The spall formation mechanism was quantified by analyzing the geometric configuration of the splats and spalls under varying conditions of droplet superheat, droplet size, and droplet or substrate material. Furthermore, the thermal contact resistance between the splat and the substrate was evaluated by matching the measured temperatures of the top or bottom surface of the splat with numerical results from a heat conduction model with phase change.     

Design of artificial lotus leaves using nonwoven fabric

The apparent contact angles of a droplet deposited on the surfaces of thermal-bonded nonwoven fabrics were presented, and the characteristics required for a superhydrophobic surface were described. For a nonwoven superhydrophobic surface, the Cassie–Baxter model describes the wetting of rough surfaces. Using topological and chemical surface modifications of nylon 6,6 nonwoven fabric, artificial Lotus leaves having water contact angles >150° were prepared. Good agreement between the predictions based on the original Cassie–Baxter model and experiments was obtained. The angle at which a water droplet rolls off the surface has also been used to define a superhydrophobic surface. Superhydrophobic surfaces were prepared by two criteria: a low-surface energy and a properly designed surface roughness.     

Sessile Drop Wettability in Normal and Reduced Gravity

This paper presents initial work performed to develop a database of contact angles of sessile drops in reduced gravity. Currently, there is no database of wettability of sessile drops in reduced gravity. The creation of such a database is imperative for continued investigations of heat and/or mass transfer in reduced gravity and future engineering designs. In this research, liquid drops of water and ethanol were created on aluminum and PTFE substrates. The formed drops were characterized by their dimensions including contact angle, wetted perimeter and droplet shape in both normal gravity and reduced gravity. The droplets were recorded during testing with high definition video and the images obtained digitally analyzed, post-test, to determine their characteristics as a function of the experimental parameters. The Queensland University of Technology (QUT) Drop Tower Facility was utilized for the reduced gravity experimentation. For droplets with diameters above their capillary length, the changes in drop dimensions and/or wettability was observed. The Young-Laplace equation was validated to accurately predict the contact angle in reduced gravity for small droplets, however it was not adequate to describe the contact angle for larger drops (above the drops associated capillary length).     

Influence of temperature on the bonding mechanism of plasma-sprayed coatings

Although considerable work has been carried out on the bonding mechanism of sprayed coatings, the relationship between the temperatures of substrate and particle and the adhesion of the coatings was not clearly pointed out. The purpose of this paper is to analyse the influence of temperature on the adhesive mechanism of thermally sprayed coatings. In this paper the importance of the temperature of both the substrate and the particle in the adhesive mechanism is emphasized. In order to clarify the adhesive mechanism at the boundary between the coating and the spreads on it is made. The model of a uniform layer of liquid (the particle) at initial temperature T_p which is suddenly brought into contact at time zero with a substrate at initial temperature T_s is considered. The diameter of the splat particle is assumed to be much larger than its thickness; edge effects are neglected and the problem is then considered as one-dimensional heat flow. Two phase changes are under consideration: the solidification of the spray droplet and the melting of the substrate.By analogy with the method of Weiner who obtained the solution for transient heat conduction in a multiphase media, the exact solution for this solidification- melting problem is found. The solution gives the pertinent information on the thermal behaviour of both the particle and the substrate. Moreover it gives the relationship between the initial molten droplet temperature and the initial solid substrate temperature for the onset of melting in the solid.The results of the analysis indicate the importance of physical properties of both the spray droplet and the substrate in the incipient melting of the substrate and emphasize the importance of the substrate because the temperature of the particle is usually difficult to control.Obviously, a particle-substrate system where the solidification-melting conditions are easily realized has greater bond strength than a system where these conditions are difficult to meet. Thus the reason why some materials stick generally to all surfaces is clarified, particularly if the solidification-melting conditions are favourable to the creation of a metallurgical bond.     

Effects of temperature,size of water droplets,and surface roughness on nanowetting properties investigated using molecular dynamics simulation

The wetting properties of water nanodroplets on a gold substrate are studied using molecular dynamics (MD) simulations. The effects of temperature, droplet size, and surface roughness are evaluated in terms of molecular trajectories, internal energy, dynamic contact angle, and the radial distribution function. The simulation results show that the wetting ability and spreading speed of water greatly increases with increasing temperature. The dynamic contact angle of water on the gold substrate decreases with increasing temperature and decreasing droplet size and surface roughness, which leads to an increase in wetting ability. The compactness of a water droplet increases with decreasing temperature and droplet size, and slightly increases with degree of roughness. The internal energy of a water droplet decreases with increasing surface roughness, indicating that droplets form more stably on a rough surface.     

Droplet velocity and thermal state from hot gas atomization of steel melt: Impact on the quality of the spray-formed tubular deposit

The velocity and thermal behavior (temperature, enthalpy, solid fraction) of atomized droplets in a metal spray play the most important role in the spray forming process. These properties mainly determine the materials yield and the final product quality (e.g., porosity, microstructure) of the as-sprayed materials. Changing the gas temperature in the atomization process directly influences these droplet properties in the spray. To understand the droplet behavior in the spray at various atomization gas temperatures (i.e., room temperature RT 293 K, 573 K, 873 K), numerical simulations using computational fluid dynamics (CFD) techniques have been performed and validated by experiments. A series of atomization runs (powder production and spray-forming with AISI 52100 steel) has been conducted at different atomization gas temperatures and pressures with a close-coupled atomizer (CCA). The in-situ temperature detection of the deposit surface (pyrometer) and in the substrate (thermocouples) has been performed to observe the effect of particle properties on the deposit. The result shows that hot gas atomization provides smaller droplets with faster velocity in the spray, affecting the droplet impact and deformation time in the deposition zone. A higher solid fraction of the smaller droplets by hot gas atomization also reduces the deposit surface temperature. Increasing the substrate diameter further decreases the deposit surface temperature without compromising the deposit quality (i.e., porosity) and also refines the grain size. Pre-heating of the substrate up to 573 K results in lower porosity in the vicinity of the substrate.     

Superhydrophobic surface directly created by electrospinning based on hydrophilic material

We describe a method to form hydrophobic surfaces using PHBV (Poly (hydroxybutyrate-co-hydroxyvalerate))—a kind of intrinsically hydrophilic material. The concentration of polymer solutions was varied to control the surface morphology and resultant wetting property. The as-prepared films were characterized by micro-scale valley-and-hill structure, which was formed by aggregating of electrospun beads. The bead morphology changed from smooth to porous and popcorn-like with decreased concentrations. The shape of water droplet on these surfaces had contact angles ranging from 110.7 to 158.1°, with a maximum standard deviation of 2.5°. It was found that both the micro and nanostructure were important to create a superhydrophobic surface.     

Nanobubbles at Water-Solid Interfaces: Calculation of the Contact Angle Based on a Simple Model

Nanobubbles have been found to form at the interface of water and solid surfaces. We examine the conditions for such bubbles to form and estimate the pressure inside the bubble based on thermodynamic considerations. Using a simple model we calculate the contact angle for a wide range of temperatures and hypothetical substrates possessing a continuous range of strengths. We show that as the temperature increases the shape of a bubble changes continuously from a spherical cap with low curvature to a complete sphere. An equivalent effect results from either increasing the strength of the solid or decreasing the surface tension. A model of a substrate formed by layers of materials is proposed to obtain a nanobubble with a particular contact angle.     

Wetting and reaction between Si droplet and SiO<Subscript>2</Subscript> substrate

The wetting and reaction between Si melt and SiO₂ substrate were investigated as a function of the atmosphere, temperature, and degree of vacuum. The results revealed that below 2 Torr with an Ar flow, the wetting angle is finally 90°. The Si droplet was stationary at a wetting angle of 90°. Videos indicated that the droplets moved and vibrated; Above 20 Torr, the Si droplet vibrated up and down with a frequency of approximately 2 Hz, thereby changing the wetting angle. Further, the droplet remained stationary on a substrate on which grooves with a width of 100 μm and depth of 100 μm were etched with a pitch of 1 mm. The presence of grooves or dimples on the substrates facilitated the leakage of SiO gas; as a result, the Si droplet did not vibrate. A vibration model was proposed in which the SiO gas produced at the interface between the Si droplet and the substrate according to the reaction Si + SiO = 2SiO expands and leaks continuously.