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.     

Heterogeneous condensation of a steam on nanoparticles in a laminar diffusion chamber

The heterogeneous condensation of a steam on nanoparticles introduced into a laminar diffusion chamber has been investigated numerically. It has been shown that we can have a growth of droplets with a nanoparticle inside to a radius of the order of a micron even in the case of a moderate temperature difference. The time of droplet growth occurring virtually in a free-molecular regime is nearly 10⁻⁵ sec. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 2, pp. 10–13, March–April, 2006.     

A novel ceramic printing technique based on electrostatic atomization of a suspension

A novel computer-controlled method of depositing ceramic droplets, according to a pre-determined architecture is described. A 21 vol% alumina suspension flowing through a nozzle was subjected to electrostatic atomization in the cone-jet mode at different applied voltages. By using a point-like ground electrode the resulting spray was focused and printed on a substrate placed between the nozzle and the ground electrode. The substrate was moved with the aid of a 2-axis computer controlled stepper motor driven system which enabled the forming of different ceramic architectures. As an example, the word CERAMIC was printed. At an applied voltage of 10 kV, droplet relics in the print were in the size range 30–60 μm. Electronic Publication     

Comparison between the process–structure–property relationships of silica foams prepared through two different processing routes

Cellular silica with improved framework, crosslinking, and stability properties are desirable for applications in thermal insulation. A process for the preparation of cellular silica foam with interconnected cells with tailored porosity and pore size distribution has been attempted. The silica foams have been prepared through two different methods; surfactant- and particle-based stabilization. The silica foams prepared through two different processes namely surfactant-stabilized foams (SSF) and particle-stabilized foams (PSF) have exhibited a wide range of differences in their structure which in turn have shown to affect the final properties of the foam. The cell size distributions in SSF (89 vol% porosity) and PSF (85 vol% porosity) have been found in the range of 50–250 μm (monomodal) and 4–10 μm and 50–100 μm (bimodal), respectively, whereas the cell counts of both have been found in close proximity. The microstructure of both the sintered SSF as well as PSF samples foams have shown an open and interconnected porosity with the permeability of both in the region of ~10⁻⁸ m². The mechanical (compressive) strength and Young’s modulus of the PSF are a third of that in SSF. The structure–property relationship of both the SSF and PSF and their comparison have been discussed.     

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.     

IR Radiative Properties of Solid and Liquid Alumina: Effects of Temperature and Gaseous Environment

An infrared spectrometer (spectral range 2–6 μm), coupled with a 9 μm pyrometer (Christiansen’s wavelength), has been developed to collect time-resolved measurements (32 spectra/s) of the spectral emissivity of alumina droplets (d≈ 3 mm) freely cooling in an aerodynamic levitation system from the liquid-to-solid phases [2800 → 1500 K]. The temperature and nature of the gaseous atmosphere surrounding the droplet (oxidizing/neutral/reducing) are two important parameters affecting the spectral emissivity in the semi-transparent range. Observations are discussed in the framework of the thermal activation and of the chemical interactions of alumina with the environment. Paper presented at the Seventh International Workshop on Subsecond Thermophysics, October 6–8, 2004, Orléans, France.     

Synthesis of 2D micro-assemblies using the interface of immiscible liquids: boundary control of self-assembled structures

We present a novel technique for the fabrication of multi-function microstructures which is a more efficient and economical alternative in the range 10–200 μm to current fabrication methods (e.g., photolithography and standard ink-jet printing). It is a modification of the classical drop-drying technique in which the material content and evaporation of water droplets are controlled by a toluene/water interface. This allows for sequential introduction of materials inside the droplets, with the possibility of in-drop controlled chemical reactions while keeping the form and a pinned contact line for each micro-droplet. It also makes the technique fully compatible with ink-jet printing methods, rendering it amenable to the fabrication of large-scale multi-function micro-arrays. As an illustration of its versatility, the technique is applied to the fabrication of microstructures from colloidal suspensions of nanoparticles, deposition of biological materials, in-drop fabrication of single crystals from water soluble materials, and fabrication of supported and free-standing polymer-based microstructure films.     

Fabrication of ZnO tetrapods on silicon substrate by thermal evaporation

Large-scale ZnO tetrapods have been fabricated on silicon substrate by a simple thermal evaporation method at 700 °C without vapor transportation and characterized by FESEM, XRD, Micro-Raman, and PL, respectively. FESEM images indicate that the length of tetrapod arm is about 3–4 μm, and the diameter of the tip is about 50 nm. XRD and Raman spectrum reveal that ZnO tetrapods are highly pure hexagonal wurtzite structure. The PL spectrum indicates that the ZnO tetrapods have strong green emission at 510 nm.     

Experimental and Theoretical Investigation on Rapid Evaporation of Ethanol Droplets and Kerosene Droplets During Depressurization

This paper reports an experimental and theoretical study of rapid evaporation of ethanol droplets and kerosene droplets during depressurization. For experimental method, an ethanol droplet or a kerosene droplet was suspended on a thermocouple, which was also used to measure the droplet center temperature transition. And the droplet shape variation was recorded by a high speed camera. A theoretical analysis was developed based on the heat balance to estimate the droplet center temperature transition, and the evaporation model proposed by Abramzon and Sirignano was used to describe the droplet vaporization. According to the experimental data and theoretical analysis, both of the environmental pressure and the initial droplet diameter have a prominent influence on the droplet temperature transition. Comparing the evaporation processes of ethanol droplets and kerosene droplets with water droplets, the ethanol droplets have the fastest evaporation rate, followed by water, and the evaporation rates of kerosene droplets are the slowest. Also it was found that a bubble can easily emerge within kerosene droplet, and its lifetime is more than 1 s.     

Preparation of Ag–Fe-decorated single-walled carbon nanotubes by arc discharge and their antibacterial effect

A simple one-step approach for the preparation of Ag–Fe-decorated single-walled carbon nanotubes (Ag–Fe/SWCNTs) by DC hydrogen arc discharge is presented in this article. The growth of SWCNTs and the attachment of Ag and Fe nanoparticles to the SWCNTs occur simultaneously during the arc discharge evaporation process. It has been confirmed that the Ag and Fe nanoparticles in the diameter range of 1–10 nm are well dispersed and tightly attached to the outer surfaces of SWCNTs. The as-grown Ag–Fe/SWCNTs have been purified by high-temperature hydrogen treatment to remove amorphous carbon and carbon shells. Antibacterial tests show that the antibacterial activity of the purified Ag–Fe/SWCNT hybrid nanoparticles is excellent against Escherichia coli. The percentage of the E. coli killed by 100 μg/ml Ag–Fe/SWCNTs can reach up to 85.1 % at a short residence time of 2 h, suggesting that the purified Ag–Fe/SWCNTs may have potential antibacterial applications.     

Investigation of aluminium thin layer microstructure on BOPP polymer substrate

In this work, 40 μm biaxially oriented polypropylene (BOPP) polymer substrate is coated in a roll coater system. The single- and double-coated aluminium thin layers are analyzed by XRD, SEM, TEM, AFM and optical light microscopy. The size and density of pinholes are investigated by using TEM, the size of the pinholes are in the range of 0.8–8 μm, it is shown that with increasing sample thickness, the dimension of pinholes decreases. SEM and AFM are applied to study the surface morphology. The results show that the surface roughness of double-coated film is better than the single-coated one and the size of pinholes is smaller. The transmittance through the samples is measured on the UV–Vis range. The results show that double-coated film has significantly low transmittance (almost zero transmittance) in UV–Vis region.     

Hysteresis and metastability of the transition from evaporative to droplet flow of a liquid through a capillary

A model is constructed for the transition from evaporative to droplet flow of a liquid through a capillary in a gravitational field allowing for the mutual influence of the droplets. An S-shaped dependence of the flow on the pressure drop at the capillary is obtained which for certain (critical) values of the control parameter gives a monotonic curve. Values of the pressure drop are determined for which the droplet flow regime and the droplet-free regime become unstable. It is shown that in a certain range of pressure drops in the presence of noise transitions may take place from evaporative to droplet flow and back (intermittence). Pis’ma Zh. Tekh. Fiz. 25, 30–36 (December 26, 1999)     

Structural and electrical studies of Cd<Subscript>1−x</Subscript>Zn<Subscript>x</Subscript> Te thin film by vacuum evaporation technique

Cd_1−xZn_xTe (where x = 0.02, 0.04, 0.06, 0.08) thin film have been deposited on glass substrate at room temperature by thermal evaporation technique in a vacuum at 2 × 10⁻⁵ torr. The structural analysis of the films has been investigated using X-ray diffraction technique. The scanning electron microscopy has been employed to know the morphology behaviour of the thin films. The temperature dependence of DC electrical conductivity has been studied. In low temperature range the thermal activation energy corresponding to the grain boundary—limited conduction are found to be in the range of 38–48 μeV, but in the high temperature range the activation energy varies between 86 and 1.01 meV. The built in voltage, the width of the depletion region and the operating conduction mechanism have been determined from dark current voltage (I–V) and capacitor-voltage (C–V) characteristics of Cd_1−xZn_xTe thin films.     

Geometrical characterization of inkjet-printed conductive lines of nanosilver suspensions on a polymer substrate

The effect of droplet/substrate interactions on the geometrical characteristics such as shape and morphology of as-printed conductive lines of nanosilver suspensions was investigated by varying the surface energy, substrate temperature and droplet spacing. With a plasma surface treatment using a mixture of C₄F₈ and O₂ gases, various surface wettability conditions were obtained that could produce desired droplet diameters on the substrate from 30 μm to 70 μm. The substrate temperature varied from room temperature to 75 °C, and ink droplets ejected from a 30 μm nozzle were printed with various overlaps from 10% to 60%. When printed at room temperature, continuous lines are not formed due to line instability issues such as merging of neighboring droplets and line bulges. By heating the substrates, continuous lines without bulges could be obtained on the relatively hydrophobic substrate because the heat flux from the substrate enhances the evaporation rate of the solvent. The coffee ring effect in the droplets and lines is more enhanced as the substrate temperature increases. This effect is weaker in the lines than in the single droplets due to less edge length in the lines. Under appropriate conditions, well-defined continuous lines could be printed without coffee rings.     

Investigation of spontaneous and coherent radiation in the 3–4 μm wavelength range in an InGaAsSb/AlGaSbAs laser heterostructure

Coherent radiation in the 3–4 μm spectral range has been obtained for the first time in an InGaAsSb/AlGaSbAs double heterostructure grown on a GaSb substrate. It has been shown that spontaneous and coherent radiation is generated mainly as a result of interband transitions in the narrow-gap InGaAsSb layer. It has been established that the optical confinement of the electromagnetic wave is sufficient for operation of the laser up to 120 K with a square-wave supply. Pis#x2019;ma Zh. Tekh. Fiz. 23, 66–71 (November 26, 1997)     

Preparation and thermoelectric properties of BP films on SOI and sapphire substrates

The chemical vapor deposited (CVD) BP films on Si(100) (190 nm)/SiO _x (370 nm)/Si(100) (625 μm) (SOI) and sapphire (R-plane) (600 μm) substrates were prepared by the thermal decomposition of the B₂H₆–PH₃–H₂ system in the temperature range of 800–1050 °C for the deposition time of 1.5 h. The BP films were epitaxially grown on the SOI substrate, but a two-step growth method, i.e., a buffer layer at lower temperature and sequent CVD process at 1000 °C for 1.5 h was effective for obtaining a smooth film on the sapphire substrate. The electrical conduction types and electrical properties of these films depended on the growth temperature, gases flow rates and substrates. The thermal conductivity of the film could be replaced by the substrate, so that the calculated thermoelectric figure-of-merit (Z) for the BP films on the SOI substrate was 10⁻⁴–10⁻³/K at 700–1000 K. Those on the sapphire substrate were 10⁻⁶–10⁻⁵/K for the direct growth and 10⁻⁵–10⁻⁴/K for the two-step growth at 700–900 K, indicating that the film on a sapphire by two-step growth would reduce the defect concentrations and promote the electrical conductivity.     

Numerical investigations of acoustic agglomeration of liquid droplet using a coupled CFD-DEM model

Acoustic agglomeration is widely considered a potentially effective technology for application in artificial defogging and precipitation. A coupled three-dimensional Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) model was constructed to investigate the agglomeration performance of liquid droplets in the acoustic field. The acoustic field is calculated by solving the Linearized Navier-Stokes Equations (LNSEs) in the time domain, and the background flow is initially obtained using the Reynolds-averaged Navier-Stokes (RANS) equations with a k–ε turbulence model. The motion of the droplet aerosol follows Newton’s second law with fluid-particle and particle-particle interactions, including collision, agglomeration, and fragmentation. The agglomeration performance of liquid droplets under high-intensity acoustic waves was numerically investigated in terms of the effects of the acoustic properties as well as the droplet characteristics.The numerical results show that it is necessary to consider droplet fragmentation in the process of acoustic agglomeration under the action of high-speed jet. The sprayed droplets are more likely to collide and condense than those without a breakup model, which has rarely been reported in previous studies. Acoustic frequency has a significant effect on agglomeration behavior, with optimal frequencies of about 225 Hz, 150 Hz, and 125 Hz corresponding to droplets with mode diameters of 15.97 μm, 25.85 μm, and 42.88 μm, respectively. However, despite the fact that most studies favoured large acoustic intensity for agglomeration performance, the agglomeration performance of aerosol particles is not always positively correlated with acoustic intensity, especially for large droplets. The optimal intensity of droplet with d_p = 42.88 μm is in the range of 120-130 dB, which is smaller than the maximum operation pressure of 150 dB used in this study. In addition, an effective approach to increase the agglomerate size is to extend the residence time that liquid droplets are exposed in the acoustic and flow field, especially because the typical acoustic intensity of actual operation is usually not that high.     

Investigation of the structure of thin layers of hexadecane on a metal substrate by infrared spectroscopy

Infrared spectroscopy was used to analyze the structure of 0.5–10 μm thick layers of hexadecane on a metal substrate in the range 3000–2800 cm⁻¹. The results suggest that density fluctuations occur in layers between 3 and 10 μm thick, whereas the hexadecane crystallizes in thinner layers. Zh. Tekh. Fiz. 24, 24–28 (May 26, 1998)     

Development of a Multiband near-to-far Infrared Radiance COmparator (MIRCO)

In recent years, there has been a growing demand to calibrate industrial blackbodies both at short wavelengths for lower temperatures and at long wavelengths for higher temperatures. User requests cover a very wide temperature range, from  −20°C to 1,500°C in the infrared bands used by thermal cameras or as defined by specific applications (especially the 1–3 μm, 3–5 μm, and 8–12 μm bands). Therefore, LNE (Laboratoire National de Métrologie et d’Essais) has developed a radiance comparator with a mirror-based optical system, an associated set of interference filter wheels, a modular holder for several infrared detectors, and a lock-in amplifier. This setup is designed to be very versatile in terms of wavelength and temperature. Targeted performances have a thermal resolution better than 0.05°C, and a known and controlled size-of-source effect (SSE). A silicon detector and a visible-to-near infrared integrating sphere were used to assess the stray light inside the housing, and supplementary baffles and stops were used to reduce it to an acceptable level. The investigation included measurement of the SSE for this comparator layout. Then, the performance in the 3–5 μm and 8–12 μm bands, using, respectively, indium antimonide (InSb) and mercury cadmium telluride (MCT) detectors, was evaluated using a water heat-pipe blackbody. This paper describes the modeling and the technical solutions implemented to optimize the optical system. Preliminary results are presented for the short-term stability, the thermal resolution between  −20°C and 960°C, and also the SSE up to 60 mm in these bands.     

Gas Pressure Dependence of the Heat Transport in Porous Solids with Pores Smaller than 10 μm

To elucidate the gaseous heat transfer in open porous materials with pore sizes below 10 μm, an experimental setup for hot-wire measurements at high gas pressures was designed and tested. The samples investigated were organic, resorcinol–formaldehyde-based aerogels with average pore sizes of about 600 nm and 7μm. The range in gas pressure covered was 10 Pa to 10 MPa. To avoid effects due to mass transport along the inner surface of the porous backbone of the samples, He and Ar, i.e., gases with very low interaction with the sample surface at ambient temperature, were chosen. The study reveals a significant contribution of coupling effects to the thermal transport in nanoporous media. A model has been developed that qualitatively describes the observed gas pressure dependence of the heat transport.