Droplet Splashing Research Articles

Large arc voltage fluctuations and droplet formation in electric arc wire spraying

The diameter of droplets in electric arc wire spraying is critical in determining the microstructure, porosity and oxide content of the resulting sprayed coatings. Droplet diameter controls dynamic and thermal behaviour in the spray, and the splashing and spreading behaviour of droplets at deposition. In electric arc wire spraying, the droplet diameter is determined by a combination of the melting behaviour of the feedstock wires in a direct current arc, combined with primary and secondary atomisation processes under the action of a high velocity atomising gas. The high frequency arc voltage variations during electric arc spraying of Fe-0·8C have been investigated and arc voltage fluctuations related to atomisation events occurring at the wire tips during spraying. A simple analytical model has been proposed which allows the diameter of primary droplets produced by atomisation to be calculated from the wire feedrate and the average time period for material removal, which was obtained from the voltage traces. Primary droplets were then assumed to undergo secondary atomisation according to correlations from the literature, and resulting droplet diameters were compared with diameters measured by experiment. Despite uncertainties in some of the thermophysical data and dynamic conditions during atomisation, predicted droplet diameters were in good agreement with experimental mass mean droplet diameters obtained under a range of atomising gas pressures, arc voltages and wire feedrates.

Epitaxial lithium fluoride films grown by pulsed laser deposition

Lithium fluoride (LiF) films have been prepared on LaAlO3 (LAO), MgO, Si and TiN buffered Si substrates using a pulsed laser deposition (PLD) technique. Their surface morphology and structural qualities were studied by using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Those films deposited on (100)MgO and (100)TiN buffered Si exhibited (200)LiF||(200)MgOand(200)LiF||(200)TiN||(200)Si epitaxial relationships, respectively. These results suggest that LiF films can be epitaxially grown on lattice match substrates and, for the first time, on (100)Si via a buffer layer. LiF grown on bare (100)Si and (100)LAO substrates, however, yielded films of (100) preferred orientation only. Lattice mismatch and thermal effects are invoked to explain these observations. The surfaces of the LiF films are very rough and covered with large globules due to splashing of molten droplets from the target. In order to remedy such deficiencies we used a shadow mask to reduce both the size and the number of these globules while maintaining the heteroepitaxial properties. As a result optically smooth LiF films of excellent structural quality are produced.

Mechanism for electric charge separation by ejection of charged particles from an ice particle growing by riming

Experimental evidence is presented that shows that substantial electric charge separation can occur by the ejection of charged particles during the riming process. Measurements were performed at temperatures between −15 and −30 °C, velocities between 10 and 30 m s −1 and cloud water contents between 0.2 and 1 g m −3. Results show that positively or negatively charged particles were detached from an artificial hailstone, with individual charges ranging between −0.3 and 0.5 pC. This mechanism could separate approximately 5 pC min −1 cm −2 of hail surface area. The charge of the ejected particle depends on the environmental temperature. Charge separation was detected at air velocities above 17 m s −1, which indicates that the mechanism could operate in the presence of hailstones larger than 1 cm in diameter. This mechanism is distinct from the usual noninductive mechanism and does not require ice particle collisions to separate electric charge. The physical mechanism has not been elucidated yet, but the results suggest that the ejected particles could be produced by splashing of droplets impinging on the artificial hailstone or by detachment of rime fragments accreted on it.

Prediction of the size distribution of secondary ejected droplets by crown splashing of droplets impinging on a solid wall

Crown splashing is frequently observed during the impact of a droplet on a rough or wet wall. The size distribution of secondary ejected droplets has been experimentally observed to satisfy the log–normal distribution which contains two free parameters: the most probable diameter d s and the width of the distribution γ. The most probable diameter will be determined through a simplified physical analysis. The width of distribution will be determined by using the principle of maximum rate of entropy production. The theory, containing no free parameters, predicts known experimental results with an acceptable engineering accuracy, provided the Froude number is high enough.

Droplet splashing on a thin liquid film

We propose a theory predicting the transition between splashing and deposition for impacting drops. This theory agrees with current experimental observations and is supported by numerical simulations. It assumes that the width of the ejected liquid sheet during impact is precisely controlled by a viscous length lν. Numerous predictions follow this theory and they compare well with recent experiments reported by Thoroddsen [J. Fluid Mech. 451, 373 (2002)].

Macroscopic erosion of divertor and first wall armour in future tokamaks

Sputtering, evaporation and macroscopic erosion determine the lifetime of the ‘in vessel’ armour materials CFC, tungsten and beryllium presently under discussion for future tokamaks. For CFC armour macroscopic erosion means brittle destruction and dust formation whereas for metallic armour melt layer erosion by melt motion and droplet splashing. Available results on macroscopic erosion from hot plasma and e-beam simulation experiments and from tokamaks are critically evaluated and a comprehensive discussion of experimental and numerical macroscopic erosion and its extrapolation to future tokamaks is given. Shielding of divertor armour materials by their own vapor exists during plasma disruptions. The evolving plasma shield protects the armour from high heat loads, absorbs the incoming energy and reradiates it volumetrically thus reducing drastically the deposited energy. As a result, vertical target erosion by vaporization turns out to be of the order of a few microns per disruption event and macroscopic erosion becomes the dominant erosion source.

Melt layer erosion of metallic armour targets during off-normal events in tokamaks

Melt layer erosion by melt motion is the dominating erosion mechanism for metallic armours under high heat loads. A 1-D fluid dynamics simulation model for calculation of melt motion was developed and validated against experimental results for tungsten from the e-beam facility JEBIS and beryllium from the e-beam facility JUDITH. The driving force in each case is the gradient of the surface tension. Due to the high velocity which develops in the Be melt considerable droplet splashing occurs.

Splashing of molten tin droplets on a rough steel surface

We photographed the impact of molten metal droplets on a flat plate. From these images we measured droplet dimensions during spreading and counted the number of fingers around a splashing drop. Experiments were done using stainless steel substrates with average roughness of 0.06, 0.07, 0.56, and 3.45 μm respectively. The temperature of the substrate was kept at either 25 or 240 °C. Droplet diameter (2.2 mm) and impact velocity (4 m/s) were kept constant, giving a Reynolds number ( Re) of 31 135 and Weber number ( We) of 463. Raising substrate roughness from 0.06 to 0.56 μm enhanced the tendency of droplet to splash, whereas increasing roughness even further to 3.45 μm suppressed splashing. This behaviour was attributed to changes in droplet solidification rate with surface roughness. A simple model of droplet spreading was used to estimate thermal contact resistance between the droplet and surface. Increasing surface roughness was found to raise thermal contact resistance and reduce heat transfer from the droplet to the substrate, delaying the onset of solidification and reducing splashing. The number of fingers formed around a droplet splashing on a smooth surface could be predicted reasonably well by a model based on Rayleigh–Taylor instability theory. Increasing surface roughness reduced the number of fingers while enlarging their size.

Complex polymeric binder-lubricant blends

Gas and Fe–0.8 wt.% C droplet dynamics during electric arc spraying have been characterised using particle image velocimetry (PIV). This study has shown that the flow of N2 atomising gas was well-collimated, with an exit velocity of 255 m s−1, decaying to 75 m s−1 at an axial distance of 150 mm, where the jet diameter was ∼50 mm. The presence of atomised steel droplets increased spray divergence. Droplet mode velocity measurements equalled the gas-only velocity measurement of ∼120 m s−1 at an axial distance of 95 mm, and then further increased to 135 m s−1 at 150 mm. Close to the substrate, droplet splashing dominated the flow field with upward and then lateral flow of splashed droplets. In the presence of a vertical step feature, the flow field became complex, with some splash droplets having trajectories that caused secondary deposition on the vertical step wall.

Vertical target and FW erosion during off-normal events and impurity production and transport during ELMs typical for ITER-FEAT

During off-normal events besides evaporation macroscopic erosion might occur. In carbon-based materials (CBMs) macroscopic erosion means brittle destruction and dust formation and in metals melt layer motion and droplet splashing. Dust, melt flow, droplet splashing and redeposited evaporated material produce complex layers with considerable surface roughness and drastically reduced heat conductivity. Moreover flaking and easy levitation of particles might occur. Subsequent ELMs when depositing their energy into such layers might enhance impurity production because of increased heating of the hot spots. For vertical targets and for first walls (FWs) macroscopic erosion is analyzed. For a simplified hot spot scenario a first estimation on maximum tolerable ELM energy is given.

Two-dimensional simulation of liquid metal spraydeposition onto a complex surface: II. Splashing and redeposition

A two-dimensional model (Djuric Z, Newbery P and Grant P 1999 Modelling Simul. Mater. Sci. Eng. 7 553) of liquid metal spray deposition on an arbitrary surface has been extended to include the important processes of splashing of spray droplets during collision with the surface, and the subsequent redeposition of scattered material. Every point on the surface has been treated as both a receiver and an emitter of sprayed material. The action of these micro sources has been modelled by several assumed micro source functions of the spatial distribution of splashed material. The moving surface has been traced by following the trajectories of its points as spraying proceeds. These trajectories were calculated in a way similar to that used previously (Djuric Z, Newbery P and Grant P 1999 Modelling Simul. Mater. Sci. Eng. 7 553), except that they now can, in principle, intersect each other. To achieve a physically reasonable solution, a new algorithm has been developed to resolve this problem.The model has been applied in several cases and the simulations compared with experimental results. By including or excluding droplet splashing effects, it has been possible to analyse the importance of redeposition both in simulations and experiments. In the case of splashing, different assumed micro source functions resulted in slightly different but geometrically similar predicted shapes. The modelling of these functions was based on the concept of the continuity of splashing and re-emission processes, a simplicity of the point source, a qualitative analysis of splashing events observed in experiments by high-speed imaging, the total mass preservation constraint, and some theoretical and experimental results of a single droplet impact onto a hard surface. Using these results, it was possible to predict where direct spraying dominated deposit growth, and where the subsequent redeposition had a major influence on deposit growth and the final shape. While in some cases model predictions are in reasonable agreement with available experimental data, the limitation of the model in accurately describing three-dimensional effects during droplet splashing has been revealed.

Droplet Splashing during Arc Spraying of Steel and the Effect on Deposit Microstructure

The mechanism by which droplet deposition occurs is important when filling substrate features for the electric arc spray forming of steel tooling. Particle image velocimetry and high-speed video imaging techniques have been used to observe droplet deposition, particularly with regard to the behavior of droplets originating from splashing. Droplet splashing on deposition has been seen to be significant, and splash droplets form a large proportion of the overspray. The splash droplets are smaller and, when first created, move slower than the parent droplet. When spraying into deep features, the lateral and upward movement of splash droplets acts as a mechanism for deposit formation onto surfaces in shadow from the main spray. Microstructural study has shown that oxidation of the splash droplets before redeposition leads to a deposit with a high fraction of oxide. Simultaneous growth of deposit formed directly from the spray, and from splash droplets, results in a banded microstructure containing elongated macropores. A mechanism for such microstructural evolution is proposed.

Impact, recoil and splashing of molten metal droplets

We studied the impact and solidification of molten tin droplets on a stainless steel surface. Droplet impact velocity was varied from 1.0 to 4.0 m/s and substrate temperature from 25 to 240°C (above the melting point of tin, 232°C). We photographed droplet impact and measured splat diameter and liquid–solid contact angle from these photographs. Substrate temperature under an impacting droplet was measured using a fast response thermocouple. Thermal contact resistance at the droplet–substrate interface was calculated by matching measured surface temperature variation with an analytical solution. A simple energy conservation model was used to predict the maximum spread of droplets during impact. Predictions agreed well with measured values. Instabilities were observed on the periphery of the droplet, which led to the formation of fingers. A model based on the Rayleigh–Taylor instability was used to predict the number of fingers around the periphery of the droplet.

Droplet interaction with shear-driven liquid films: analysis of deposition and secondary droplet characteristics

Abstract The paper describes the results of a detailed investigation of droplet interaction with shear-driven liquid films. Particularly the mechanism of droplet splashing and the characteristics of the deposited mass fraction had been studied experimentally. A new integral method based on conductivity and volume flux measurements was used. The deposited mass fraction and the composition ratio of the secondary droplets are given as a function of the impact angle and the film height. Comprehensive measurements of the primary and secondary droplets had been performed by means of a Phase-Doppler-Anemometer (PDA). The resulting distributions for droplet diameter, velocity and angle had been used to evaluate correlations for the interaction between single droplets and a shear-driven wall film. Together with the results on droplet deposition they can serve as basis for a complete 2D model simulation in CFD codes.

Formation of splat morphology during thermal spraying

Mechanisms of splashing of droplets impinging onto the substrate surface during thermal spraying are considered. Supercooling formed in the flattening droplet is shown to consist of thermal supercooling, which occurred due to the high pressure developed upon droplet impact. Solidification starts when the supercooling exceeds some critical value. For a `cold' substrate, i.e., a substrate whose temperature is less than a transition temperature, the marked contribution to supercooling is due to the high pressure term. In this case a regular disc-shaped splat will be formed in the central part of the flattening droplet and splashing will occur in the periphery. For a `hot' substrate, i.e., a substrate whose temperature exceeds the transition temperature, thermal supercooling is sufficiently high, no splashing occurs and a regular disc-shaped splat is formed. Theoretical results agree with the experimental observations.

Thermomechanical Simulation of the Splashing of Ceramic Droplets on a Rigid Substrate

Finite element simulation techniques have been applied to the spreading process of single ceramic liquid droplets impacting on a flat cold surface under plasma-spraying conditions. The goal of the present investigation is to predict the geometrical form of the splat as a function of technological process parameters, such as initial temperature and velocity, and to follow the thermal field developing in the droplet up to solidification. A non-linear finite element programming system has been utilized in order to model the complex physical phenomena involved in the present impact process. The Lagrangean description of the motion of the viscous melt in the drops, as constrained by surface tension and the developing contact with the target, has been coupled to an analysis of transient thermal phenomena accounting also for the solidification of the material. The present study refers to a parameter spectrum as from experimental data of technological relevance. The significance of process parameters for the most pronounced physical phenomena is discussed as are also the consequences of modelling. We consider the issue of solidification as well and touch on the effect of partially unmelted material.

Development of substrate-coating adhesion in thermal spraying

Different mechanisms for the development of substrate-coating adhesion during thermal spraying are considered. The most important is mechanical interlocking formed chiefly due to roughness of the substrate surface, high pressures developed on the droplet impact, and solidification of the lower part of the splat. Possible deformation of the substrate surface, rebounding of the impinging droplets, and influence of spraying at off-normal angles on the development of adhesion are also considered. Thermal mechanisms involving partial or complete melting and subsequent solidification in the substrate interfacial region are shown to be effective in creation of adhesive bonds. The roles of the diffusion processes and the influence of the splat morphology on adhesion are also discussed. The theoretical results agree well with the observed tendencies of behaviour of the thermal spray coatings and in the case of the thermal mechanisms of adhesion these results are in good agreement with the experimental data. The splashing of droplets impinging on to the substrate surface during thermal spraying is considered. Supercooling formed in the flattening droplet is shown to consist of the thermal supercooling and that which arises due to the high pressure developed on the droplet impact. Solidification starts when the supercooling exceeds some critical value. With a ‘cold’ substrate, when its temperature is less than a critical value, a marked contribution to the supercooling can be attributed to the high pressure effect. In this case a regular disc shaped splat will be formed in the central part of the flattening droplet and splashing will occur at the periphery. With a ‘hot’ substrate, when the pressure effect on the supercooling is reduced and the thermal effect is increased, no splashing occurs and a regular disc shaped splat is formed. The influence of the residual stresses on substrate-cooling adhesion is discussed.

Development of substrate-coating adhesion in thermal spraying

Different mechanisms for the development of substrate-coating adhesion during thermal spraying are considered. The most important is mechanical interlocking formed chiefly due to roughness of the substrate surface, high pressures developed on the droplet impact, and solidification of the lower part of the splat. Possible deformation of the substrate surface, rebounding of the impinging droplets, and influence of spraying at off-normal angles on the development of adhesion are also considered. Thermal mechanisms involving partial or complete melting and subsequent solidification in the substrate interfacial region are shown to be effective in creation of adhesive bonds. The roles of the diffusion processes and the influence of the splat morphology on adhesion are also discussed. The theoretical results agree well with the observed tendencies of behaviour of the thermal spray coatings and in the case of the thermal mechanisms of adhesion these results are in good agreement with the experimental data. The splashing of droplets impinging on to the substrate surface during thermal spraying is considered. Supercooling formed in the flattening droplet is shown to consist of the thermal supercooling and that which arises due to the high pressure developed on the droplet impact. Solidification starts when the supercooling exceeds some critical value. With a ‘cold’ substrate, when its temperature is less than a critical value, a marked contribution to the supercooling can be attributed to the high pressure effect. In this case a regular disc shaped splat will be formed in the central part of the flattening droplet and splashing will occur at the periphery. With a ‘hot’ substrate, when the pressure effect on the supercooling is reduced and the thermal effect is increased, no splashing occurs and a regular disc shaped splat is formed. The influence of the residual stresses on substrate-cooling adhesion is discussed.

Fusarium oxysporum f. sp. gladioli . [Descriptions of Fungi and Bacteria

Abstract A description is provided for Fusarium oxysporum f. sp. gladioli . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Gladiolus sp. Also known to infect Babania, Crocosmia (= Montbretia), Crocus, Freesia , bulbous Iris, Ixia, Sparaxis, Streptanthera, Tritonia and Washingtonia (Iridaceae ), causing corm rot or yellows symptoms (McClellan, 1945; 73, 3171; 74, 1746). DISEASE: Corm rot, basal dry rot, yellows. GEOGRAPHICAL DISTRIBUTION: Widespread. TRANSMISSION: By infected corms used as planting material. Via movement of contaminated soil. Local dispersal is by water flow and droplet splash containing macro- or microconidia.

Pulsed laser deposition of some II–VI compounds and alloys

AbstractPulsed laser deposition (PLD) of materials is a growing field with applications to many systems. With many materials, however, a major problem is particulate formation due to splashing of molten droplets or ablation of solid clusters from the target. For the case of II–VI semiconductors we have found that careful selection of the laser power density and appropriate mastering of the beam could reduce particulate formation to a negligible level even with the use of pressed targets. In this work we have deposited a number of II–VI semiconductors and their alloys, using XeCl and Nd:YAG lasers, from pressed and bulk targets onto various substrates in high vacuum. The ternaries of ZnTe and ZnSe as well as of ZnTe and CdTe were studied in detail. We determined the optimum growth temperatures and deposition rates for the growth of optical quality films. X‐ray diffraction, optical absorption, energy‐dispersive X‐ray analysis and Raman scattering were used to characterise these films or grain size and orientation, optical band gap and alloy effects on photon bands.