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.     

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.     

The study of the kinetics of water spreading on solid phases of carbon allotropic materials

Kinetics of water spreading on the surface of solid phases of various carbon materials has been first studied with the use of high-speed video filming (up to 1200 frames per second). It has been found that rates of low-temperature liquid and metal melts spreading and wetting the surface of solid phases are close in time (the process length is 10⁻²–10⁻³s) and in both the cases the spreading occurs in an inert mode. It has been shown that at the final stages the spreading of low-temperature liquids occurs at a viscous mode (10–30 min) caused by the presence of an adsorbed layer (coat) on the solid phase surface due to the environment.     

Adjunctions on the lattices of partitions and of partial partitions

The complete lattice Π(E) of partitions of a space E has been extended into Π*(E), the one of partial partitions of E (where the space covering axiom is removed). We recall the main properties of Π*(E), and exhibit two adjunctions (residuations) between Π(E) and Π*(E). Given two spaces E ₁ and E ₂ (distinct or equal), we analyse adjunctions between Π*(E ₁) and Π*(E ₂), in particular those where the lower adjoint applies a set operator to each block of the partial partition; we also show how to build such adjunctions from adjunctions between P(E₂){\mathcal{P}(E_2)} and P(E₂){\mathcal{P}(E_2)} (the complete lattices of subsets of E ₁ and E ₂). They are then extended to adjunctions between Π(E ₁) and Π(E ₂). We obtain as particular case the adjunction on Π(E) that was defined by Serra (for the upper adjoint) and Ronse (for the lower adjoint). We also study dilations from Π*(E ₁) to an arbitrary complete lattice L; a particular case is given, for L í [0,+￥]{L \subseteq [0,+\infty]} , by ultrametrics; then the adjoint erosion provides the corresponding hierarchy. We briefly discuss possible applications in image processing and in data clustering.     

The effect of the wetting droplets size on power consumption during drum granulation

Changes of torque during fine material (foundry dolomite) granulation in a horizontal drum granulator at changing wetting parameters were studied. The variable parameters were droplet diameter and wetting of granular material. The bed of loose material was sprayed during feeding, at a constant liquid flow rate V_l = 0.012 m³/h. The size of wetting liquid droplets was changed using different rates of air flow through pneumatic spraying nozzles in the range from V_p = 1.0 to 3.0 m³/h and applying a sprinkler which supplied (drop-wise) the liquid uniformly along the entire drum length. In each test, instantaneous values of torque on the granulator shaft were measured in 1 s time intervals. The effect of droplet size and moisture content of the bed of granular material on torque in the whole granulation cycle was estimated. It was found that bed wetting conditions had a significant influence on the nucleation and growth of agglomerates. A minimum (boundary) size of liquid droplets, at which the bed is not transformed into granulated material, depends on the total amount of liquid supplied at the wetting stage. Changes of torque during the drum granulation are a good representation of the phenomena related to the transformation of wetted feed into granulated product and can be an easy indicator of the process.The work was carried out within the project no. 4 T09C 023 22. financed by the State Committee for Scientific Research in the years 2002–2005.     

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.     

Characteristic time for the evolution of instability of a droplet charged to the Rayleigh limit

The nonlinear problem of the capillary oscillations of a charged droplet is solved to estimate the characteristic time for the evolution of instability of a droplet carrying critical charge. It is observed that breakup is delayed because of the hydrodynamic inertia of the droplet. Pis’ma Zh. Tekh. Fiz. 25, 41–45 (August 12, 1999)     

Thermodynamic analysis of growth of iron oxide films by MOCVD using tris(<Emphasis Type="Italic">t</Emphasis>-butyl-3-oxo-butanoato)iron(III) as precursor

Thermodynamic calculations, using the criterion of minimization of total Gibbs free energy of the system, have been carried out for the metalorganic chemical vapour deposition (MOCVD) process involving the β-ketoesterate complex of iron [tris(t-butyl-3-oxo-butanoato)iron(III) or Fe(tbob)₃] and molecular oxygen. The calculations predict, under different CVD conditions such as temperature and pressure, the deposition of carbon-free pure Fe₃O₄, mixtures of different proportions of Fe₃O₄ and Fe₂O₃, and pure Fe₂O₃. The regimes of these thermodynamic CVD parameters required for the deposition of these pure and mixed phases have been depicted in a ‘CVD phase stability diagram’. In attempts at verification of the thermodynamic calculations, it has been found by XRD and SEM analysis that, under different conditions, MOCVD results in the deposition of films comprising pure and mixed phases of iron oxide, with no carbonaceous impurities. This is consistent with the calculations.     

Wetting and interfacial interactions in the CaO–Al2O3–SiO2/silicon carbide system

In this article, the wetting of 23 wt% CaO–15 wt% Al₂O₃–62 wt% SiO₂ molten glass on polycrystalline silicon carbide is studied under air at temperatures between 1,100 and 1,590 °C. Wetting experiments are performed by the sessile drop technique. Good wetting (final contact angle lower than 50°) is observed regardless of the experimental temperature when it is higher than 1,300 °C. Moreover, some specific experiments of wetting of glass on platinum, silica and monocrystalline SiC substrates are also performed. The character of molten glass spreading on silicon carbide (reactive or non-reactive) as well as the role of the atmosphere on interfacial interactions with SiC are identified and discussed.     

Microwave plasma enhanced chemical vapor deposition diamond synthesis from high carbon content source

To synthesize diamond films by microwave plasma enhanced chemical vapor deposition (MPECVD), the methane concentration (CH₄/H₂)plays a crucial role. It is well-known that there always exists a critical methane concentration (≤0.6%) only below which a good quality diamond film can be obtained. In this study, however, the phenomena of diamond synthesis resulting from high carbon concentration conditions were observed. The molten metals, e.g., Ag, Cu, were used as the deposition substrates on which crystalline diamonds can be achieved from a methane content of CH₄/H₂≥6% or even from solid carbon sources. These results suggest that there may exist a low methane content boundary layer (<0.6%) in the proximity of molten metal surface on which suitable species, CH, CH⁺, H_α and H_β are composed for the diamond nucleation and growth similar to the condition as in the conventional low methane contents. The molten metal inclines to dissolve other forms of carbonaceous materials other than diamond, and thus keeps a much higher steady supply of carbon atoms that enhances the quality as well as the growth rate of the forming diamonds. Received: 23 June 2001 / Accepted: 23 July 2001     

Rapid Deposition of Buffer Layers for YBCO Coated Conductors on Biaxially-Textured Ni Tapes

Epitaxial growth of CeO₂ buffer layers on biaxially textured (001)Ni tapes was studied using pulsed laser deposition. Relationships between microstructure and deposition parameters were systematically studied in order to develop reliable long tape coating processes. It was found that orientation and texture of CeO₂ buffer layers were sensitive to deposition parameters. X-ray diffraction analyses showed that CeO₂ buffer layers had pure (001) orientation at 860 °C. Under optimized deposition conditions, highly (001) oriented CeO₂ buffer layers have been achieved at a high deposition rate of 1.5 nm/s in-plane texture and out-plane texture of CeO₂ buffer layers were 4.25 degrees and 5.85 degrees, respectively.     

Electron microscopic studies of vacuum-evaporated (Pb1 −x Sn x )1 −y Te y thin films

Thin films of (Pb_{1 −x} Sn _x )_{1 −y} Te _y have been deposited by vacuum evaporation onto glass, mica, CaF₂ and NaCl substrates heated to various temperatures at low deposition rates. Transmission HEED, TEM and SEM studies have been carried out for as-grown films. HEED photographs show that (i) the degree of crystallinity of the films improves at higher substrate temperatures and (ii) the crystallites have smaller dimensions on CaF₂ than on NaCl, mica and even glass, under identical conditions of deposition. SEM studies reveal that with increasingT _sub, grains tend to form clusters through mobility coalescence. Needleshaped grains are formed when the deposition rate exceeds a certain critical value. Formation of needle-like grains has been attributed to the excess Te atoms possessing trigonal structure so that such grains can be observed only inp-type PbSnTe films.     

Complex impedance analysis of layered perovskite structure electroceramics—NaDyTiO<Subscript>4</Subscript>

A ceramic oxide (NaDyTiO₄), having layered perovskite structure, has been prepared by a standard high-temperature solid-state reaction technique. X-ray diffraction (XRD) studies have confirmed material formation under reported condition along with the presence of impurity (Na₂Dy₂Ti₃O₁₀) as the minor phase. Complex impedance spectroscopy (CIS) analysis has been carried out to investigate its microstructure and electrical properties as a function of frequency and temperature. CIS analysis has indicated that the electrical behavior of the material sample shows negative temperature coefficient of resistance (NTCR) typical to a semiconductor. Impedance studies have also indicated the presence of temperature dependent relaxation process in the material with a spread of relaxation time. The d.c. conductivity of the material as evaluated from the impedance spectrum has been observed to be ∼10⁻⁹ Scm⁻¹ at room temperature (RT). It has been observed to increase as a function of temperature with a maximum of ∼10⁻⁵ Scm⁻¹ at 550∘C. The conductivity variation shows a cross over from Mott-type hopping phenomena at lower temperatures to a thermally activated Arrhenius type behavior at high temperature.     

Broadband dielectric response of amorphous arsenic triselenide layers grown by different methods

The polarization processes in arsenic triselenide (As₂Se₃) films grown by thermal vacuum deposition and high-frequency ion plasma sputter methods have been studied in alternating electric field at frequencies in a range of 10⁻² to 10⁷ Hz and temperatures of 253–343 K. It has been established that the method of preparation significantly influences the dielectric response of samples, which is related to specific features of the localized energy states.     

Interface interaction in the (B4C + TiB2)/Cu system

Interface phenomena in the TiB₂/(Cu–B) and (B₄C + TiB₂)/(Cu–B) systems were investigated in order to determine conditions for cermet preparation by free infiltration. The wetting behavior of the two-phase ceramic substrate may be accounted for as a superposition of behavior patterns characteristic of the two ceramic phases. The relatively low wetting angles of the liquid Cu on the titanium diboride substrate is attributed to a minor departure of the ceramic phase composition from stoichiometry. Copper alloyed with above 8 at.% B yields contact angles less than 20° sufficient for fabrication of cermets based on the two-phase ceramic by free infiltration. The enhanced wetting and the absence of a new phase formation were confirmed by SEM and TEM analysis of the infiltrated cermets.     

Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273–343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using α = α_max + (α_min − α_max)/[1 + exp 2(E − E_g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E_g(T) = E_g(273 K) − (8.8 × 10⁻⁴T²)/(483 + T) + 0.088 (Varshni empirical formula) and E_g(T) = E_g(273 K)−0.231447/[exp(362/T)−1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor ∼0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.     

Epitaxial growth of Sr x Ba1−x Nb2 O6 (SBN) thin films on Pt coated MgO substrates: the determining control of platinum crystallographic orientation

The growth of (001) oriented Sr _x Ba_1−x Nb₂ O₆ (SBN:x) thin films on MgO single crystals substrates has focused attention due to its potential application to integrated signal waveguiding and electro-optic processing. This paper addresses the possible insertion of a platinum bottom electrode to implement the electro-optic effect, and examines the influence of Pt orientation on the subsequent growth of SBN thin films deposited by R.F. magnetron sputtering. We have first investigated the optimal sputtering conditions of a (001) Pt oriented growth. Experimental evidence is provided for an orientation switching temperature sensitive to R.F. power but insensitive to deposition rate which suggests that kinetic phenomena are not involved in the orientation development of Pt films. Seeding the MgO substrate with Fe, Cu or Cr inhibits the competitive (111) Pt growth without modifying the optimal temperature for (001) growth. Annealing at a temperature sufficient to crystallise the top SBN film causes a drastic evolution of the initial (111) and (001) Pt volume fractions, making particularly critical the deposition conditions for a final dominant (001) Pt texture on unseeded substrates. The (001) oriented growth of SBN is shown to occur exclusively on (001) Pt crystallites stable against subsequent annealing steps. As a final result, (001) SBN films have been obtained epitaxial on both Pt coated and not coated (001) MgO substrates. They display two in-plane orientations mirror symmetric (±α) to the axis of (001) MgO, with α shifted from 31° to 18° by (001) Pt coating.     

Dynamics of Drop Coalescence on a Surface: The Role of Initial Conditions and Surface Properties

An investigation of the coalescence of two water drops on a surface is presented and compared with drop spreading. The associated capillary numbers are very low (< 10⁻⁵). The drops relax exponentially towards equilibrium. The typical relaxation time t_c decreases with contact angle. t_c is proportional to the drop size R, thus defining a characteristic velocity U^* = R/t_c. The corresponding U* values are smaller by many orders of magnitude than the bulk hydrodynamic velocity (U = σ /η, with σ the gas–liquid surface tension and η the viscosity). The dynamics of receding (coalescence) and spreading motion is found to be of the same order when coalescence or spreading is induced by a syringe. The dynamics of coalescence induced with the syringe deposition is systematically faster by an order of magnitude than condensation-induced coalescence. This disparity is explained by the coupling of the contact line motion with the oscillation of the drop observed for syringe deposition but absent for condensation-induced coalescence. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

The wetting and spontaneous infiltration of ceramics by molten copper

Infiltration trials have been conducted by filling Cu tubes withceramic powders and melting them under argon. No externalforces were applied; successful infiltration of the ceramicrelied solely upon favourable metal-ceramic wetting conditions.Oxides and covalently bonded compounds could not bespontaneously infiltrated but transition metal compounds such asNbC, Cr₃C₂, WC, NbB₂ and Cr₂N were. It was impossible toinfiltrate any ceramics when oxygen was present in the system.Contact angle data in the literature were found to predict, withfair reliability, infiltration events in Cu-ceramic systems.The good correlation is thought to be due to the ease with whichthe oxide film can be prevented from forming on molten Cu duringboth sessile droplet experiments and infiltration processing.     

Wettability of transition metal boride eutectic alloys to graphite

The wetting of nickel, platinum and palladium boride eutectics (Ni₅₅B₄₅, Ni₆₂B₃₈, Pd₇₂B₂₈, and Pt₇₂B₂₈) to polycrystalline graphite has been studied by sessile drop and surface analytical techniques. The wettability is critically influenced by surface segregation of low-level bulk impurities in the alloys. At the melting temperature, the impurities segregate to the surface of the liquid alloy and inhibit wetting. A poorly-wetted droplet with a large contact angle results. Good wetting results by introducing materials into the contact system which possess high chemical reactivity to the major segregant species, either as a surface coating or by incorporation into the alloy during manufacture. The materials act to suppress segregation by tying up the alloy impurities through compound formation. The results indicate that the use of surface sensitive techniques in the study of wetting is essential.