Droplet cooling in atomization sprays

Transport between droplets/particles and a gas phase plays an important role in numerous material processing operations. These include rapid solidification operations such as gas atomization and spray forming, as well as chemical systems such as flash furnaces. Chemical reaction rates and solidification are dependent on the rate of gas-particle or gas-droplet transport mechanisms. These gas-based processes are difficult to analyze due to their complexity which include particle and droplet distribution and the flow in a gas field having variations in temperature and velocity both in the jet cross-section and in the axial distance away from the jet source. Thus to study and properly identify the important variables in transport, these gas and droplet variations must be eliminated or controlled. This is done in this work using models based on a single fluid atomization system. Using a heat transport model (referred to as thermal model) validated using single fluid atomization of molten droplets and a microsegregation model, the effect of process variables on heat losses from droplets was examined. In this work, the effect of type of gas, droplet size, gas temperature, gas-droplet relative velocity on the heat transport from AA6061 droplets was examined. It is shown that for a given gas type, the most critical process variable is the gas temperature particularly as affected by two-way thermal coupling and the droplet size. The results are generalized and applied to explain the difference in droplet cooling rate from different atomization processes.     

Sol-solvothermal synthesis and characterization of fine lead zirconate titanate particles

We present a novel low-temperature sol-solvothermal method to synthesize fine lead zirconate titanate (PZT) particles. This sol-solvothermal method combines the advantages of conventional sol–gel process and the solvothermal method, and isopropyl alcohol (IPA) was used as the solvent. The effects of different parameters including KOH concentration, IPA/(IPA + water) ratio and reaction temperature, on the microstructures of the PZT powder were studied. With increasing KOH concentration and reaction temperature, the crystalline structure of as-synthesized PZT transformed from tetragonal to rhombohedral phase. More IPA added in the solvent can effectively reduce agglomeration of the PZT powder and decrease the crystallization temperature, but impurity phase was also detected at high IPA/(IPA + water) ratio. As a result, the synthesis parameters are optimized, and well-crystallized 700 nm PZT particles were successfully synthesized in 2.0 M KOH and 50 % IPA/(IPA + water) ratio at temperatures as low as 120 °C.     

Experimental and Theoretical Studies of Laser Cleaning Mechanisms for Submicrometer Particulates on Si Surfaces

Submicrometer (0.1–1 μm) polystyrene and alumina particles were removed by single-shot nanosecond 248 nm KrF laser radiation from Si wafer surfaces with or without pre-deposited thin liquid layers. Nearly complete (>90%) single-shot laser cleaning has been achieved for combinations of polystyrene and alumina particles, respectively, with 2-propanol and water used as liquid energy transfer media, while for other combinations cleaning was absent or incomplete. Time-resolved optical microscopy studies have for the first time revealed important transient microscopic interactions between these particles and Si substrates in the pre-deposited micron-thick liquid layers, resulting, in some of these cases, in significantly reduced particle-substrate coupling. The visualization results give insight into microscopic particle removal mechanisms relevant to our laser cleaning experimental conditions, which are discussed along with their theoretical analysis. Theoretical modeling has been performed to interpret these experimental results and to provide insight into microscopic particle removal mechanisms.     

Squeezing out ultrafine hydrophobic and poor water-soluble drug particles with water vapour

This short communication describes a scalable new method to produce ultrafine hydrophobic or poorly soluble drug particles. Ultrafine Vitamin D3, Aspirin and Ibuprofen particles in the submicron range were produced. The method is an extension of the antisolvent vapour precipitation technique which exposes a droplet to an antisolvent vapour with reference to the dissolved materials within the droplet. In this work, the drug material was dissolved in ethanol droplets and then exposed to a convective stream of water vapour. Absorption of the water vapour into the droplet resulted in the precipitation of the particles. The precipitated submicron particles showed good dispersion behaviour in water droplets. This work will form the basis for using spray dryers as high-throughput scalable micro-precipitators.     

Crystallization and microstructure of metastable water quenched nanostructured 8 mol% yttria-stabilized zirconia using the solution precursor plasma spray method

A t′ tetragonally structured metastable 8 mol% yttria-stabilized zirconia (8 mol% YSZ) nanomaterial was synthesized by means of solution thermal plasma spray with water quenching of reacted species. Synthesis of the 8 mol% YSZ powder involved vaporization of a liquid precursor injected into a plasma jet where individual droplets, depending on their trajectory within the plasma, experienced varied thermal histories. Thus, not all the material produced underwent a complete gel → glass → nanocrystalline transformation sequence. Consequently, the collected powder contained a proportion of gel and glass (amorphous) state material. Additionally, the powder contained nano-scale and small micron-scale rapidly solidified 8 mol% YSZ particles. Following thermal treatment, the gel and the amorphous content transformed to produce (i) densely packed nanograin and (ii) chain-like nanograin aggregates. The nanograin aggregates are suggestive of a strong, yet short-range intergranular attraction, as predicted in computer simulation studies presented in the literature. Interestingly, this mixed morphology powder, after compaction and heat treatment at 1400 °C for 2 h, transformed into 98 % dense material with a homogeneous 200–500 nm grain size. For generating 8 mol% YSZ, the solution precursor plasma spray method offers a high synthesis rate using a low-cost precursor to produce powder that can be consolidated into morphologically homogeneous bulk nanomaterial.     

Aerosol Synthesis of Inhalation Particles via a Droplet-to-Particle Method

ABSTRACT

Inhalation powders with consistent particle properties, including particle size, size distribution, and shape were produced with an aerosol synthesis method. Compared to conventional spray drying, the aerosol method provides better control of the thermal history and residence time of each droplet and product particle due to the laminar flow in the heated zone of the reactor where the droplet drying and particle formation take place. A corticosteroid, beclomethasone dipropionate, generally used for asthma treatment was chosen as a representative material to demonstrate the process. Spherical particles were produced with a droplet-to-particle method from an ethanolic precursor solution. The droplets produced with an ultrasonic nebulizer were carried to a heated zone of the reactor at 50–150°C where the solvent was evaporated and dry particles formed. The mass mean diameter of the particles were well within the respirable size range (approximately 2 μm). The geometric standard deviation (GSD) of produced particles was approximately 2. The particle surface structure varied from smooth to rough depending on the degree of particle crystallinity and was affected by the thermal history of the particle. Amorphous particles with smooth surface were most likely obtained due to the rapid evaporation of the solvent from the droplet combined with the slow diffusion of the beclomethasone dipropionate molecule. The amorphous particles were transformed slowly to crystalline particles in the open atmosphere. In addition, the particle surface structure changed from smooth to rough during storage. The process was accelerated by thermal post-annealing. However, additional heating also increased particle sintering. By optimizing the reactor parameters, and thus increasing the molecular diffusion, stable, crystalline particles were produced at 150°C.     

镀件逆流清洗浓度计算公式应用探讨（下）

2问歇换水逆流清洗各级浓度分析 2．1理论公式的推导 设有间歇换水5级逆流清洗系统，当第5级镀液浓度达到某一限定值时，便将第1级的清洗水送去回收，将第2级的清洗水倒换到第1级，将第3级的清洗水倒换到第2级，依此类推，最后倒空的第5级加入不含镀液成分的纯水。各级的浸洗与喷洗都各自独立。     

Encryption of nonlinear optical signals in ZnO:Al thin films by ultrashort laser pulses

Described herein is the effect of optical annealing on the third-order non-linear optical properties exhibited by nanostructured Al-doped ZnO thin films. The samples were synthetized by an ultrasonic spray pyrolysis method. The optical annealing process was carried out by laser pulses at 532, 835 and 1064 nm wavelengths with, ps, fs and ps pulse duration, respectively. The optical non-linearity of the films was measured by the z-scan method with three different irradiations of excitation: 100 fs at 835 nm, 120 ps at 532 nm, and 150 ps at 1064 nm. The as-grown samples showed a saturable optical absorption that evolves into two-photon absorption transitions by a picosecond optical annealing phenomenon induced at 532 nm wavelength. Potential applications for developing optical encryption functions were considered.     

Removing 20 nm ceramic particles using a supersonic particle beam from a contoured Laval nozzle

Cryogenic particle beam is an effective means of removing nano-sized contaminant particles from a substrate. To overcome the current cleaning limit of 50 nm, a particle beam with novel properties - smaller bullet size moving at a higher velocity - was used. Argon or Ar/He mixture was expanded through contoured Laval nozzles of various expansion angles to generate extremely small particles through genuine nucleation and growth. Argon particles smaller than 100 nm - smaller by a factor of 10 or more than the conventional Argon aerosols - were successfully generated, and could perfectly remove various ceramic particles down to 20 nm.     

Droplet size and morphology analyses of dry liquid

Dry water is a free-flowing powder consisting of numerous solid particle-stabilized water droplets with typical sizes and volumes of 10^?6–10^?4 m and 10^?3–10³ pL, respectively. We describe the first characterization of dry water stabilized with hydrophobic silica nanoparticles, by using laser diffraction droplet size distribution analysis. The water droplet dimensions were measured to be a few tens of micrometers in air, by using the laser diffraction method. These dimensions correspond well with measurements by both laser diffraction and optical microscopy methods for a Pickering-type water-in-n-dodecane emulsion prepared by dispersing dry water in n-dodecane. Optical microscopy confirmed that the dry water consisted of flocs of non-spherical water droplets, and the flocs ranged in size from a few tens of micrometers to a few millimeters in air. On the basis of these results, the flocs of water droplets were proposed to dissociate into individual water droplets under the air blast during droplet size measurement by the laser diffraction method. It was also confirmed that pressurizing the dry water between two glass slides led to encapsulated water leaking from the silica nanoparticle shells. This on-demand pressure-sensitive water leak phenomenon shows a possible usage of the dry waters as a material delivery carrier.     

Experimental investigation on the effect of liquid injection by multiple orifices in the formation of droplets in a Venturi scrubber

Venturi scrubbers are widely utilized in gas cleaning. The cleansing elements in these scrubbers are droplets formed from the atomization of a liquid into a dust-laden gas. In industrial scrubbers, this liquid is injected through several orifices so that the cloud of droplets can be evenly distributed throughout the duct. The interaction between droplets when injected through many orifices, where opposite clouds of atomized liquid can reach each other, is to be expected. This work presents experimental measurements of droplet size measured in situ and the evidence of cloud interaction within a Venturi scrubber operating with multi-orifice jet injection. The influence of gas velocity, liquid flow rate and droplet size variation in the axial position after the point of the injection of the liquid were also evaluated for the different injection configurations. The experimental results showed that an increase in the liquid flow rate generated greater interaction between jets. The number of orifices had a significant influence on droplet size. In general, the increase in the velocity of the liquid jet and in the gas velocity favored the atomization process by reducing the size of the droplets.     

Imaging of droplets and vapor distributions in a diesel fuel spray by means of a laser absorption-scattering technique

The droplets and vapor distributions in a fuel spray were imaged by a dual-wavelength laser absorption-scattering technique. 1,3-dimethylnaphthalene, which has physical properties similar to those of Diesel fuel, strongly absorbs the ultraviolet light near the fourth harmonic (266 nm) of a Nd:YAG laser but is nearly transparent to the visible light near the second harmonic (532 nm) of a Nd:YAG laser. Therefore, droplets and vapor distributions in a Diesel spray can be visualized by an imaging system that uses a Nd:YAG laser as the incident light and 1,3-dimethylnaphthalene as the test fuel. For a quantitative application consideration, the absorption coefficients of dimethylnapthalene vapor at different temperatures and pressures were examined with an optical spectrometer. The findings of this study suggest that this imaging technique has great promise for simultaneously obtaining quantitative information of droplet density and vapor concentration in Diesel fuel spray.     

Preparation and characterization of spray-dried submicron particles for pharmaceutical application

The versatile use of submicron-sized particles (0.1–1?μm) requires new manufacturing methods. One possibility for the preparation of submicron-sized particles is spray drying. However, the generation of small droplets at a high production rate and the precipitation of submicron particles are quite challenging. In order to produce a sufficient amount of fine and uniform droplets, a two-fluid nozzle with internal mixing was combined with a cyclone droplet separator. The precipitation of particles was realized with an electrostatic precipitator. Considering the difficulty of electrostatic precipitation concerning explosion risks and to make it capable using organic solvents, the spray dryer was integrated in a pressure resistant vessel. Based on previous experiments, the now presented design is compact and the electrostatic precipitator is shortened. In addition, enhanced drying conditions ensured a controlled and reproducible preparation of submicron-sized particles. Thus, high separation efficiencies were shown. Spray-drying experiments were conducted with the model substance mannitol. With the cyclone droplet separator, a fine and uniform spray with a droplet size smaller 2?μm was produced. This robust atomizing technique is capable for high concentrations. For a 10?wt% mannitol solution, particles in the submicron range d_50,3?=?0.7?μm were produced.     

Removal of Nano-sized Particles Using Carbon Dioxide (CO2) Gas Cluster Cleaning without Pattern Damage

As pattern size of semiconductor device becomes less than 20 nm, the removal of particles smaller than 10 nm without pattern damages requires new physical dry cleaning technology. CO₂ gas cluster cleaning is an alternative dry cleaning process to meet these cleaning requirements. To demonstrate gas cluster cleaning performance, particle removal efficiency (PRE) and gate structure pattern damages were evaluated. When pressurized and low temperature CO₂ gas was passed through a convergence–divergence (C–D) nozzle, high energy CO₂ gas clusters were generated at high speed in a vacuum atmosphere. The cleaning force of the CO₂ gas cluster is related to the flow rate of the CO₂ gas. The optimum CO₂ gas flow rate for the particle removal without pattern damage was found to be 6 L/min (LPM). Removal efficiency for 50 nm silica particles was greater than 90%, and no pattern damage was observed on 60 nm poly-Si and a-Si gate line patterns. It was confirmed that the CO₂ gas cluster cleaning force could be controlled by the CO₂ gas flow rate supplied to nozzle.     

Gas phase temperature measurements in the liquid and particle regime of a flame spray pyrolysis process using O₂-based pure rotational coherent anti-Stokes Raman scattering

For the production of oxide nanoparticles at a commercial scale, flame spray processes are frequently used where mostly oxygen is fed to the flame if high combustion temperatures and thus small primary particle sizes are desired. To improve the understanding of these complex processes in situ, noninvasive optical measurement techniques were applied to characterize the extremely turbulent and unsteady combustion field at those positions where the particles are formed from precursor containing organic solvent droplets. This particle-forming regime was identified by laser-induced breakdown detection. The gas phase temperatures in the surrounding of droplets and particles were measured with O₂-based pure rotational coherent anti-Stokes Raman scattering (CARS). Pure rotational CARS measurements benefit from a polarization filtering technique that is essential in particle and droplet environments for acquiring CARS spectra suitable for temperature fitting. Due to different signal disturbing processes only the minority of the collected signals could be used for temperature evaluation. The selection of these suitable signals is one of the major problems to be solved for a reliable evaluation process. Applying these filtering and signal selection steps temperature measurements have successfully been conducted. Time-resolved, single-pulse measurements exhibit temperatures between near-room and combustion temperatures due to the strongly fluctuating and flickering behavior of the particle-generating flame. The mean flame temperatures determined from the single-pulse data are decreasing with increasing particle concentrations. They indicate the dissipation of large amounts of energy from the surrounding gas phase in the presence of particles.     

Prediction of Powder Stickiness along Spray Drying Process in Relation to Agglomeration

The spray drying process consists of a fast convective drying of liquid droplets by hot air. Initially, the water activity (a_w) of a drop is close to 1. During drying, the drop surface a_w decreases while viscosity increases until reaching a sticky rubbery state before further drying. This can be observed for products such as carbohydrates, leading to particles sticking on walls (product losses) or to adhesion between particles leading to agglomeration. In this study, particle stickiness was investigated in a cocurrent pilot spray dryer by measuring drying air properties (temperature and relative humidity) at different positions. This allowed describing the evolution of temperature and mean water content of the drying drops. Two model products (maltodextrin DE12 and DE21) were spray dried varying process parameters liquid flow rate (1.8, 3.6, and 5.4 kg/h), air temperature (144°, 174°, and 200°C), airflow rate (80–110 kg/h), and rotary atomizer speed (22,500–30,000 rpm). The two products exhibit different drying behaviors in relation to their affinity towards water (sorption isotherms) and glass transition temperature evolution with a_w (stickiness). Depending on drying conditions and product, the drop stickiness was observed very rapidly, close to the atomizer, or later, along the chamber. This approach can be used to identify conditions and positions corresponding to sticky particles.     

Removal mechanisms of dew via self-propulsion off the gecko skin

Condensation resulting in the formation of water films or droplets is an unavoidable process on the cuticle or skin of many organisms. This process generally occurs under humid conditions when the temperature drops below the dew point. In this study, we have investigated dew conditions on the skin of the gecko Lucasium steindachneri. When condensation occurs, we show that small dew drops, as opposed to a thin film, form on the lizard''s scales. As the droplets grow in size and merge, they can undergo self-propulsion off the skin and in the process can be carried away a sufficient distance to freely engage with external forces. We show that factors such as gravity, wind and fog provide mechanisms to remove these small droplets off the gecko skin surface. The formation of small droplets and subsequent removal from the skin may aid in reducing microbial contact (e.g. bacteria, fungi) and limit conducive growth conditions under humid environments. As well as providing an inhospitable microclimate for microorganisms, the formation and removal of small droplets may also potentially aid in other areas such as reduction and cleaning of some surface contaminants consisting of single or multiple aggregates of particles.     

Microstructure of YSZ Coatings Deposited by PS-PVD Using 45 kW Shrouded Plasma Torch

In this study, a conventional 80 kW class plasma spraying system was used to produce yttria-stabilized-zirconia (YSZ) coatings by PS-PVD at a pressure of 100 Pa. A shroud was attached in the front of the plasma nozzle to restrain expansion of plasma jet. The torch was operated at an arc power of 45 kW and YSZ coatings were deposited at a powder feed rate of 0.2 g/min. Optical emission spectroscopy was used to diagnose the particle state in plasma jet. The surface morphology and cross-sectional morphology of coatings was characterized by field emission scanning electron microscope. It is found that the amount of YSZ evaporation is significantly enhanced through using a shroud. The coatings with a hybrid microstructure of splats and nanoclusters were deposited perpendicular to the coatings. The nanostructured clusters deposited out of the vapor are presented at splat interfaces. It is evident that using powders specially designed for PS-PVD and controlling heating of plasma jet to spray particles, PS-PVD deposition for a hybrid microstructure consisting of vapor phase deposit can be realized through conventional plasma spray system. Columnar grain structured YSZ can also be deposited by pure vapor phase at the side surface of the substrate.     

Angular diagram of broadband emission of millimeter-sized water droplets exposed to gigawatt femtosecond laser pulses

We report on the experiments on the interaction of gigawatt femtosecond laser pulses with suspended millimeter-sized water droplets. The transparent droplets experienced laser-induced breakdown and explosive boiling up and emitted a broadband radiation. This radiation covers the spectral range from 450 to 1100?nm and consists of the spectrum of laser pulse scattered and transformed by the droplet due to self-phase modulation and plasma emission produced in water during photoionization. The droplet emission spectrum showed remarkable broadening at all viewing angles and is maximal in the direction of the laser exit from the droplet. The enlargement of the droplet results in additional spectral spreading of the emitted radiation. The depth and amount of laser pulse spectral self-transformations upon propagation through the water droplet are simulated by means of numerical calculations.     

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.