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.     

Solidification contact angles of molten droplets deposited on solid surfaces

Droplet impact and equilibrium contact angle have been extensively studied. However, solidification contact angle, which is the final contact angle formed by molten droplets impacting on cold surfaces, has never been a study focus. The formation of this type of contact angle was investigated by experimentally studying the deposition of micro-size droplets (∼39 μm in diameter) of molten wax ink on cold solid surfaces. Scanning Electron Microscope (SEM) was used to visualize dots formed by droplets impacted under various impact conditions, and parameters varied included droplet initial temperature, substrate temperature, flight distance of droplet, and type of substrate surface. It was found that the solidification contact angle was not single-valued for given droplet and substrate materials and substrate temperature, but was strongly dependent on the impact history of droplet. The angle decreased with increasing substrate and droplet temperatures. Smaller angles were formed on the surface with high wettability, and this wetting effect increased with increasing substrate temperature. Applying oil lubricant to solid surfaces could change solidification contact angle by affecting the local fluid dynamics near the contact line of spreading droplets. Assuming final shape as hemispheres did not give correct data of contact angles, since the final shape of deposited droplets significantly differs from a hemispherical shape.     

Wettability of NiAl by a liquid Ni–Si–B alloy

An investigation of the wettability of the intermetallic compound NiAl by a liquid Ni–4.5 wt% Si–3.2 wt% B filler metal is presented in this paper. Dynamic observations of spreading of Ni–Si–B droplets, conducted using hot-stage light microscopy, are correlated with post-cooling microscopy and analysis. The paper examines the influence of the oxide layer on the NiAl substrates, on the progression of spreading of the Ni–Si–B liquid. Termination of spreading of the Ni–Si–B droplets by the onset of isothermal solidification at the spreading front is considered. Spreading of the Ni–Si–B droplets was found to be rapid until the onset of isothermal solidification at the spreading front. However, once isothermal solidification commenced, negligible further spreading was observed. The Ni–Si–B filler metal was observed to spread by undermining of the substrate oxide. However, a marked reaction occurred between the substrate oxide and the Ni–Si–B filler metal. This reaction served to remove the substrate oxide layer. The paper contrasts the mechanisms of substrate oxide undermining and isothermal solidification of liquid Ni–Si–B droplets on NiAl with those occurring during the spreading of the same liquid on pure nickel and Ni–Cr alloys. This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

Impact of substrate materials on the processing and properties of melt-spun nickel-metalloid ribbons

The dynamic phenomenology of melt spinning with four different nickel-metalloid alloys on copper, molybdenum, aluminium, and iron wheels as well as nickel- and chromium-plated copper substrates has been evaluated. Puddle dimensions are self-adjusting so that the integrated heat flux through the puddle/substrate contact area is sufficient to convert the melt into foil at the same mass flow rate. The dynamic wetting phenomena are complex because mechanical forces dominate during the initial liquid spreading stage, and the solidification process is highly influenced by the melt/substrate interface conditions. Melt spinning does not automatically produce thin section foils with uniform through thickness microstructure. While structural uniformity frequently occurs with easy glass-forming alloys, lower metalloid content alloys often develop graded through-thickness microstructures. When this occurs, the degree of crystallinity in the contact surface layer is frequently greater than in the opposite free surface layer.     

纳米铝液的转移方式对图案自动成型效果的模拟

目的 研究利用选择性吸附原理实现纳米图案自动成型的可行性,以及转移方式对自动成型效果的影响。方法 通过分子动力学方法分别研究块体铝和两纳米铝团簇的加热升温过程,得到纳米铝液势能–温度变化曲线,并确定纳米铝液图案自动成型的模拟温度;利用LAMMPS软件建立图案自动成型所需的印版模型;分别以液膜转印和液滴喷印的方式,研究纳米铝液在石墨烯–铜基印版表面上润湿铺展情况。结果 纳米铝液能够在石墨烯–铜基印版表面实现图案自动成型;液膜模型和液滴模型的图案自动成型效果不同,液膜较薄时（双层铝原子）模拟时间为1 200 ps时,整个体系达到动态平衡状态,图案区的铝液厚度约为1.8 nm,图案自动成型效果较好;当液膜较厚时（三层铝原子）,模拟时间为1 200 ps时,整个体系达到动态平衡状态,但图案区的两线条间存在较多铝原子残留,自动成型效果较差;而2种液滴模型均实现了较好的图案成型效果,液滴数量为4滴时,模拟时间为3 000 ps时可形成厚度均匀的图案,当液滴数量为6滴时,模拟运行600 ps时图案区即可形成的稳定均匀的线条。结论 利用选择性吸附原理可实现纳米图案自动成型,成型效果受铝液转移方式的影响...     

Patterned hydrophobic-hydrophilic templates made from microwave-plasma enhanced chemical vapor deposited thin films

In the present research, nano-structured materials exhibiting super-hydrophobic behavior obtained by microwave-plasma enhanced chemical vapor deposition (MPECVD) had their surface chemical status altered through vacuum ultraviolet (VUV) light irradiation. Falling water droplets rolled and bounced without wetting or spreading over the initially super-hydrophobic surfaces. We demonstrate a surface preparation technique to create a patterned super-hydrophobic/super-hydrophilic substrate in which micropatterns with super-hydrophobic and super-hydrophilic regions were prepared through irradiation with VUV light. To confirm the method, growth of water droplets is observed in situ on such super-hydrophobic/super-hydrophilic micropatterns. We discuss the applicability of the super-hydrophobic/super-hydrophilic pattern to the bottom-up assembling of materials, like site-selective electroless Cu plating on patterned substrates made of paper and selective cell culture experiments.     

激光熔覆玻璃涂层

本文研究在Ａ３钢基材上激光熔覆玻璃涂层的可行性，激光熔覆玻璃涂层的过程是玻璃熔体在基材表面上湿润、铺展，然后冷凝的过程。玻璃熔体在基材表面上铺展，不仅与它们之间的浸润有关，而且与玻璃熔体的流动性有关。玻璃熔体的流动性在熔覆过程中随时间变化；激光处理参数的不同会改变熔体的随时间变化过程。合理选择激光参数能熔覆形成均匀的、光滑的、粘附的、无裂纹的玻璃涂层。在激光熔覆过程中，会在玻璃熔体中出现结晶现象，     

Spreading characteristics of nanofluid droplets impacting onto a solid surface

This paper reports an experimental investigation on the spreading characteristics of nanofluid droplets impinging on aluminum substrate under the influence of several key factors such as nanoparticle volume fraction, substrate temperature, and the Weber number. Sample nanofluid used is prepared by dispersing several volumetric concentrations (1 to 5%) of titanium dioxide nanoparticles in ethylene glycol. The entire dynamic process of each droplet collision with the substrate surface and the spreading phenomena is captured by using a high speed camera and then the transient spreading diameter and height of droplet are determined. It is found that the higher the concentration of nanoparticles the larger the spreading diameter of nanofluid droplet. As the surface temperature increases, the overall spreading diameter and height of nanofluid droplet significantly decreases and increases, respectively. At larger Weber number, the final spreading of the nanofluid droplet is also found to be larger than that of lower Weber number. Present results demonstrate that spreading characteristics of nanofluid droplets impacting onto solid surface are greatly influenced by each of the aforementioned factors.     

Application of push-off shear test for evaluation of wetting-interface structure-bonding relationship of solder joints

The relationship between wetting behavior, interface structure and mechanical properties of solder/substrate couples has been studied on example of Sn-alloys and Cu substrates. The sessile drop method was used to investigate the solder wetting and spreading on polished Cu substrates in vacuum at a temperature of 503 K. The sessile drop samples after solidification were bisected perpendicularly to the substrate at the mid-plane of the contact circle. The first half of each sample was used for structural characterization of interfaces and evaluation of their mechanical properties by improved push-off shear test. The second half was used for investigation of the effect of thermocycling on structural stability and corresponding mechanical behavior of model solder/Cu joints. A comparison with the results obtained on standard solder joints has shown the usefulness of the improved push-off shear test performed directly on solidified sessile drop samples as an express test for evaluation of technological and mechanical compatibility of solder/substrate couples, particularly at the first stage of solder candidate selection.     

Anisotropic wetting of microstructured surfaces as a function of surface chemistry

In order to study the influence of surface chemistry on the wetting of structured surfaces, microstructures consisting of grooves or squares were produced via hot embossing of poly(ethylene-alt-tetrafluoroethylene) ETFE substrates. The structured substrates were modified with polymer brushes, thereby changing their surface functionality and wettability. Water droplets were most strongly pinned to the structure when the surface was moderately hydrophilic, as in the case of poly(4-vinylpyridine) (P4VP) or poly(vinyl(N-methyl-2-pyridone) (PVMP) brush-modified substrates. As a result, the droplet shape was determined by the features of the microstructure. The water contact angles (CA) were considerably higher than on flat surfaces and differed, in the most extreme case, by 37° when measured on grooved substrates, parallel and perpendicular to the grooves. On hydrophobic substrates (pristine ETFE), the same effects were observed but were much less pronounced. On very hydrophilic sampes (those modified with poly(N-methyl-vinylpyridinium) (QP4VP)), the microstructure had no influence on the drop shape. These findings are explained by significant differences in apparent and real contact angles at the relatively smooth edges of the embossed structures. Finally, the highly anisotropic grooved microstructure was combined with a gradient in polymer brush composition and wettability. In the case of a parallel alignment of the gradient direction to the grooves, the directed spreading of water droplets could be observed.     

Reinitiation of substrate oxide undermining by Ni-P filler metals during wetting of nickel-base alloys

The process of wetting of nickel and nickel-chromium substrates by nickel-phosphorus high-temperature brazing filler metals has been examined. The filler was found to wet the substrate by undermining the oxide layer on the substrate. Evidence was found that the undermining process is halted temporarily at discontinuities in the substrate oxide which typically occur above substrate grain boundaries. It is suggested that it is initially more favourable energetically for the filler to spread along the oxide defects than to continue undermining. Mechanisms for the reinitiation of undermining have been considered. It is proposed that undermining recommences after the onset of isothermal solidification of the filler spreading along oxide defects.     

Reaction enhanced wetting of quartz by silicon droplets and its instabilities

A miniature reaction cell was developed in order to study the influence of gas phase components on reactive wetting and dewetting processes. Small silicon droplets on fused silica can be observed in situ at 4000 frames per second under reaction conditions by a microscope equipped with a high speed camera. Additionally, ex-situ investigations of etch profiles originating from the reactive wetting process are conducted by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Under certain reactive conditions large dynamic spreading modes exhibiting a new type of droplet instability are observed. Some spread-out droplets suddenly disrupt and decay into a ring of smaller droplets. Each of these then spreads again and disrupts into a circular array of even smaller droplets forming a ring-shaped structure. Whole cascades of decay can be traced by means of the etch profiles found on the substrate. The results are discussed within a simple thermodynamic model that relates the changes in the oxygen chemical potential to the changes of solid–liquid interface tension.     

Analysis of Solidiflcation in Spray Atomized and Codeposited Metal-matrix Composites Part Ⅱ: Reinforcement Injection and Deposition

The influence of the injection of reinforcing particles (for the production of metal matrix composites and of the droplets-to-substrate heat transfer on the resulting microstructural uniformity of spray atomized and codeposited composite material is analyzed. The reinforcement particles injection velocity has to be limited between an upper and a lower critical values. in order to ensure entrapment into the matrix droplets in flight. The thermal history of the injected droplets during the deposition stage is calculated with the assumption that the in-flight solidifying droplets reach the substrate while containing still at least 20% liquid volume fraction, in order to avoid porosity of the deposited material. The substrate to pouring-tube orifice distance where that condition is achieved depends strongly on the atomization pressure and the convective heat transfer coefficient of the substrate. It is demonstrated that "tailoring" the microstructures and the reinforcement volume percent in the deposited material is feasible. The critical process parameters : the atomization pressure, the melt flow rate. the substrate to pouring-tube orifice distance, the reinforcement particles injection location and rate can all be adequately chosen in order to obtain any desired microstructure, grain size, reinforcement volume percent, with the additional benefit, if wanted, of rapid solidification processing     

Atomistic simulations of reactive wetting in metallic systems

Atomistic simulations were performed to investigate high temperature wetting phenomena for metals. A sessile drop configuration was modeled for two systems: Ag(l) on Cu and Pb(l) on Cu. The former case is an eutectic binary and the wetting kinetics were greatly enhanced by the presence of aggressive interdiffusion between Ag and Cu. Wetting kinetics were directly dependent upon dissolution kinetics. The dissolution rate was nearly identical for Ag(l) on Cu(100) compared to Cu(111); as such, the spreading rate was very similar on both surfaces. Pb and Cu are bulk immiscible so spreading of Pb(l) on Cu occurred in the absence of significant substrate dissolution. For Pb(l) on Cu(111) a precursor wetting film of atomic thickness emerged from the partially wetting liquid drop and rapidly covered the surface. For Pb(l) on Cu(100), a foot was also observed to emerge from a partially wetting drop; however, spreading kinetics were dramatically slower for Pb(l) on Cu(100) than on Cu(111). For the former, a surface alloying reaction was observed to occur as the liquid wet the surface. The alloying reaction was associated with dramatically decreased wetting kinetics on Cu(100) versus Cu(111), where no alloying was observed. These two cases demonstrate markedly different atomistic mechanisms of wetting where, for Ag(l) on Cu, the dissolution reaction is associated with increased wetting kinetics while, for Pb(l) on Cu, the surface alloying reaction is associated with decreased wetting kinetics.     

Effect of temperature and substrate surface texture on wettability and morphology of IMCs between Sn–0.7Cu solder alloy and copper substrate

In the present work, the effect of soldering temperature (270 and 298?°C) and substrate surface texture (0.02 and 1.12?μm) on wetting characteristics and morphology of intermetallic compounds (IMCs) between Sn–0.7Cu lead-free solder on copper substrates was investigated. It was found that increase in temperature and substrate surface roughness improved the wettability of solder alloy. However, the effect of surface roughness on wettability was significant as compared to that of temperature. The spreading of solder alloy was uniform on smooth substrate, whereas spreading of the alloy on rough substrate resulted in an oval shape. The morphology of IMCs transformed from long needle shaped to short and thick protrusions of IMCs with increase in surface roughness of the substrate. Needle shaped and thick protruded intermetallics formed at the solder/Cu interface were identified as Cu₆Sn₅ compounds. The formation of Cu₃Sn IMC was observed only for the spreading of solder alloy at 298?°C which contributed to improvement in the wettability of solder alloy on both smooth and rough substrate surfaces.     

The wettability of titanium diboride by molten aluminum drops

Titanium diboride is widely accepted to be completely wet by liquid aluminum, yet few published wetting studies demonstrate this behavior, and reported contact angles vary widely. Sessile drop substrates from four different sources were selected and their microstructures and chemistries characterized. The results of sessile drop experiments using four techniques to modify oxide film behavior were compared. The Al-TiB₂ interfaces were examined in metallographic sections or after chemical removal of the Al drop. Al wets a material containing 5.5 wt% Ni in vacuum experiments before the hold temperature of 1025^° C is reached. The other TiB₂ substrates are completely wet by Al at 1025^° C, but only after prolonged holds under vacuum. Elimination of boron oxide from the TiB₂ surface leads to a spreading condition. The role of the substrate microstructure (porosity, grain size, roughness, and carbon content) in altering the wetting kinetics is discussed.     

Micro/nano engineering on stainless steel substrates to produce superhydrophobic surfaces

Creating micro-/nano-scale topography on material surfaces to change their wetting properties has been a subject of much interest in recent years. Wenzel in 1936 and Cassie and Baxter in 1944 proposed that by microscopically increasing the surface roughness of a substrate, it is possible to increase its hydrophobicity. This paper reports the fabrication of micro-textured surfaces and nano-textured surfaces, and the combination of both on stainless steel substrates by sandblasting, thermal evaporation of aluminum, and aluminum-induced crystallization (AIC) of amorphous silicon (a-Si). Meanwhile, fluorinated carbon films were used to change the chemical composition of the surfaces to render the surfaces more hydrophobic. These surface modifications were investigated to create superhydrophobic surfaces on stainless steel substrates. The topography resulting from these surface modifications was analyzed by scanning electron microscopy and surface profilometry. The wetting properties of these surfaces were characterized by water contact angle measurement. The results of this study show that superhydrophobic surfaces can be produced by either micro-scale surface texturing or nano-scale surface texturing, or the combination of both, after fluorinated carbon film deposition.     

Experimental Study on Distribution Characteristics of Condensate Droplets Under Ultrasonic Vibration

This paper studied the effect of ultrasound on distribution characteristics of condensate droplets on a vertical metal surface. The surface was made of aluminum and coated with PVC film to obtain durable condensate droplets. Visualization of the condensation process was carried out under the action of ultrasonic vibration with a constant frequency of 20 kHz. The effects of ultrasonic power on surface coverage of condensate droplets, first shedding time of condensate droplets, total number of shedding, heat flux and condensation heat transfer coefficient were analyzed. Furthermore, the mechanism of ultrasonic vibration on accelerating the shedding of condensate droplets was discussed. The results indicated that the shedding of condensate droplets was accelerated by ultrasound compared with those without ultrasound. In addition, the shedding period of condensate droplets was decreased with the increase of ultrasonic power. Contrarily, the heat flux and the condensation heat transfer coefficient were increased with the increase of ultrasonic power. The maximum enhancement ratio of heat transfer coefficient reached 2.67 compared with that without applying ultrasound. This study shows that ultrasound has a good application prospect in strengthening condensation heat transfer, particularly for space applications in microgravity environment.     

Physicochemical aspects of quantum dot array formation in the InAs/GaAs system by droplet epitaxy under MOVPE conditions

Conditions of the deposition of indium droplets on GaAs(100) substrates during low-temperature (100°C) decomposition of trimethylindium have been studied. It is established that, in order to eliminate the partial coalescence of the indium droplets, it is possible to use subsequent heat treatment for evaporating excess indium. The heat treatment at a temperature of 350–500°C, only slightly modifies the composition of indium drops as a result of the substrate solution and, hence, does not significantly change the composition of quantum dots grown in this system.