Formation Of Air Pockets Research Articles

Long‐Term Immersion Study for Durability of Interconnected Micropatterned Surfaces with Sustained Water Repellency

AbstractThe sustained water repellency of interconnected micropatterned surfaces is explored over an extended duration, with a focus on their resilience during a 90‐day water‐immersion test. Initially, the microstructure surfaces exhibit high water repellency, a characteristic of the Cassie–Baxter state. However, subsequent detailed temporal analyses reveal varying responses depending on the structural topology. The interconnected micropatterned surfaces exhibit remarkable long‐term resistance to water; this is attributed to the formation of large and stable air pockets enabled by their unique microcavity structures. In comparison, hierarchical microcavity surfaces with micropillars exhibit a notable decrease in water repellency, as evidenced by reduced contact angles, suggesting a transition to a wetting state owing to the emergence of surface hydrophilicity during long‐term water exposure. This study demonstrates the importance of stable air‐pocket effects, particularly in applications where the long‐term stability of liquid repellency is critical, and suggests the role of interconnected structures in maintaining water repellency over time.

Problems of the process of manufacturing precision forgings in multiple systems on a hot hydraulic hammer

The article presents the results of a complex analysis referring to a periodical occurrence of forging defects in the form of twisting of pin-type elements (perpendicular to the main forging axis) as well as lack of total filling of these areas on the forgings in the process of hot precision forging on a hammer. The studies included: an analysis of the forging technology, advanced numerical simulations of the process and dynamic tests of the mutual shifting of the tools with the use of a high-speed measurement camera. The hot precision forging process was realized on a double operation hydraulic hammer with the maximal energy of 16 kJ, as a result of which, a so-called leaf is formed in the dies in two operations, which has 6 relatively small forgings (the mass of one is about 40 g), characterizing in a narrowed scope of dimension-shape tolerance (0.1–0.2 mm and 0.5–1°). The performed preliminary process analysis showed that, for this type of forgings made in a dynamic manner on a hammer, the key role is played by elastic deformations as well as the construction of the dies (application of guide locks) and the impression geometry. Additionally, during the forging process, in the case of pin-type elements (shape of a truncated cone with the convergence of 1°), their twisting was observed as well as small underfills caused by “closed volumes”, connected with the non-vaporized lubricant and the formation of air pockets. The mentioned problems relating to this process significantly affect the quality and precision of the forgings and cause defects. For this reason, based on the results of the conducted tests and analyses, a different approach to the design of dies was applied than in standard hammer forging technologies. It was introduced the necessary changes in the technology as well as modify the construction and geometry of the tools (in the lower die a negative slope was applied) in order to eliminate joggle and twisting of the forging elements, which disqualify such items.

Numerical simulation of wave impact and high pressure characteristics due to violent sloshing in a rectangular tank

ABSTRACT Understanding violent sloshing is of practical concern to a wide range of applications as it may compromise safety in a number of engineering applications. The present study investigates violent sloshing in a 2D tank. In particular, the evolution and influence of an air pocket and the impact pressure on tank walls are numerically studied. Since the problem involves multi-phase flows, standard dam-break problem is used for validations. The liquid depth and amplitude of excitation are systematically varied to assess the impact pressures on walls and roof of the tank. Non-linear effects are captured qualitatively and the distribution of impact pressures on the tank walls is examined. The entrainment of an air pocket by a standing wave at the tank roof is delineated. The violent sloshing wave motion causes the formation of air pockets that arise for a range of forcing amplitudes and a variety of liquid depths. Gradual decay in the impact pressure due to the generation of these air pockets is significant on overall sloshing dynamics. The maximum peak pressure predicted on the tank roof is about 10 times the static pressure, which is also about twice the peak pressure obtained on the side wall. Furthermore, the distribution of average impact-pressure profiles on the tank walls and roof is presented for different loading conditions vis-á-vis global impact characteristics of sloshing wave motion.

How adhesives flow during joining

The highly automated world of manufacturing industry offers new possibilities for the application of adhesives which are not fully utilized yet. In addition to the classical static stress and strength related topics, a more global view on adhesive bonding is needed. This includes the consideration of adhesive flow during application and joining of the substrates with all the information about initial application pattern and the resulting adhesive distribution contained after joining, and thus benefit from the modern manufacturing possibilities. To do so, research which addresses this issue is needed in the first place to elaborate innovative approaches on how to design and develop adhesive application pattern which minimize adhesive spillage and waste while achieving a bond without any air enclosures or other imperfections. This paper systematically investigates different phenomena which can occur during the joining process, such as the formation of air pockets and how they behave once formed. Furthermore, optimized application pattern and the merging behaviour of different flow fronts are considered. In total, four different adhesive application patterns were investigated experimentally and analysed using Particle Image Velocimetry. The investigations were complemented by a numerical simulation of the corresponding squeeze flow experiments — which achieved a high level of agreement. It was found that during the joining process, existing air pockets can reform due to the phenomenon of viscous fingering and subsequently cure into flat cavities, which are more critical in terms of bonding strength than spherical ones.

Air-assisted drag reduction promoted by hydrophobic attraction

Hydrophobic property is of vital significance to entrap air between the solid surface and liquid. In this paper, we observed the formation of air pockets anchored on the wetting solid surface during the lubrication process. It reveals that the fabrication surfaces with various microstructures may achieve different drag reduction effect by using air pockets. The simulation method was presented to visualize the air-solid interface, and obtain the insights of the relationship between the hydrophobic and air-assisted drag reduction. The high hydrophobicity results high drag reduction. And in the paper, it was found that the pit-type micro-texture with the best hydrophobic effect has a significant positive effect on drag reduction, because 90.56% of the solid-liquid interface is replaced by the air-liquid interface.

The use of a computer modeling system for foundry processes in blank production

The article focuses on the analysis of the results of computer simulation of the technological process of casting the part “Stub”. In the work, the elements of the gating-feeding system were calculated. Based on the calculated data, a digital model was built. An important stage of the work was the digital modeling of the technological process of casting the plug part in the ProCast CAE system. The simulation results were viewed and analyzed in the Visual-Viewer postprocessor module. An analysis of the simulation results made it possible to evaluate the efficiency of the gating system, determine the areas of turbulent flow, the nature of filling the zone with a high flow rate, possible places of mold erosion, places of formation of air pockets that can affect the occurrence of gas inclusions in the casting. The nature of the distribution of the temperature gradient in the casting, the time and uniformity of the solidification of the casting were also studied.

Updating of dry shielding of nuclear power plant reactor vessel

Introduction. Dry shielding is a cylindrical structure made of serpentinite concrete in a metal casing with an inner diameter of 5.6 m, an outer diameter of 6.7 m, and a height of 5.3 m, which surrounds the VVER reactor vessel in the vicinity of the core. The purpose of serpentinite concrete, containing an increased amount of chemically bound water, is to soften the spectrum of the neutron flux outside the reactor, increasing the fraction of thermal neutrons in the spectrum, which is necessary for the operation of ionization chambers (IR) of the reactor control and protection system. Dry shielding also performs the functions of radiation and thermal protection, reducing the flux of radiation on ordinary concrete of biological protection. Before the installation of the dry shielding in the reactor shaft, heat treatment (drying) of concrete is carried out at temperatures up to 250 °С to remove unbound water in order to avoid radiolysis. Quality control of concreting and then heat treatment is carried out using a radioisotope device — a neutron moisture meter. These works are very lengthy and costly. Materials and methods. The design of the dry shielding casing was considered in order to perform additional perforation in order to avoid the formation of air pockets during concreting. The possibility of using modern plasticizing additives was considered in order to minimize the consumption of mixing water and, as a result, free water in the body of serpentinite concrete. Results. The possibility of exclusion of the stages of quality control of concreting and heat treatment in their traditional form is shown. Additional perforation of the metal casing, its internal diaphragms in problem areas, the use of a mixture of 20 cm slump or more allows you to completely eliminate the formation of internal voids. According to preliminary estimates, given the intensity of radiation in the NW for a modern reactor with a capacity of 1200 MW, the intensity of the release of hydrogen outside the shell due to radiolysis does not pose any danger. The concentration of hydrogen in the air surrounding the dry shielding is many orders less of magnitude than the dangerous 4 %. Conclusions. The cost of work on the construction of the SZ power unit of a nuclear power plant with a capacity of 1000–1200 MW can be reduced by 70–100 million rubles, the duration of work by 5 months.

Photocatalytic Superamphiphobic Coatings and the Effect of Surface Microstructures on Superamphiphobicity.

In recent years, superamphiphobic coatings have been widely used in industrial transportation and environmental treatments because of their unique liquid repellency. In this study, WO3-TiO2 nanorods/SiO2 were used as the constructor of surface microstructures, and 1H,1H,2H,2H-perfluorodecyltriethoxysilane was used as the provider of low surface energy, and a photocatalytic superamphiphobic coating (FTS coating) was prepared. The microstructure and chemical composition of the coating was characterized by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The coating exhibited excellent photocatalytic activity toward degradation methyl red and nitric oxide (NO), and the degradation efficiency to NO reached 47.8%. Also, the advanced contact angle and the hysteresis angle of water, glycol, glycerol, and olive oil was used to evaluate the superamphiphobicity. After 7 days of ultraviolet (UV) aging, five cycles of airbrush flushing and 48 h of immersion in acid, salt, and alkali solutions, the FTS coating still exhibits excellent amphiphobicity, which lays a foundation for its large-scale applications in the concrete exterior wall. The surface microstructure and the formation of air pockets are a prerequisite for superamphiphobicity, which promotes the liquid on the coating surface into the Cassie-Baxter state. Furthermore, the formation of air pockets is closely related to the gas adsorption capacity and the specific surface area (SBET) of the surface microstructure on the coating surface. The coatings with different SBET constructed and the advanced contact angle were measured. The influence of air pockets on the superamphiphobicity of coatings was studied in combination with the optical microscope. The understanding that SBET further influences superamphiphobicity by affecting the surface air pockets is proposed.

Improved dynamic stability of superomniphobic surfaces and droplet transport on slippery surfaces by dual-scale re-entrant structures

There has been substantial recent interest with respect to the broad potential applications of superomniphobic surfaces, but it remains enormously challenging to enhance the dynamic stability of such surfaces impacted by high-pressured droplets. Herein, a robust superomniphobic surface with dual-scale re-entrant structures (DRSs) has been designed and fabricated through a facile combination of magnetic particle-assisted self-assembly and a dip-coating technique. Compared to superomniphobic surfaces with single-scale re-entrant structures, this superomniphobic surface with DRSs shows highly improved repellency against the impact from different high-pressured droplets, a property which is furnished by the extension of three-phase contact lines, the increase in local geometric angles, and the formation of additional air pockets. Furthermore, the superomniphobic surface can serve as a platform to well maintain lubricating fluid so as to realize a photoresponsive slippery surface wherein near-infrared light can be used to manipulate various liquid droplets with a large range of volumes. Overall, this study provides a strategy for obtaining the robust superomniphobic and photoresponsive slippery surfaces through the DRS design, which provides a new insight into enhancing the dynamic stability of superomniphobic surfaces and also demonstrates the application of slippery surfaces for droplet manipulation.

Effect of surface adsorption on icing behaviour of metallic coating

Icephobicity of materials has received intensive attention in recent years due to the increasing requirement of ice protection in aerospace, wind energy and power lines. However, the influencing factors of material icephobicity have not been well identified. In this work, the effect of surface gaseous adsorption on icing behaviour of materials was investigated for the first time. Ni-Cu-P coatings with different surface morphologies were fabricated and used as the objects of the study. Environmental scanning electron microscopy (ESEM) was utilized to observe the water condensation and ice formation on the coatings. X-ray photoelectron spectroscopy (XPS) was employed to analyse the variations of surface adsorption. Droplets icing time and ice adhesion strength of the coatings were also studied. The results showed that the icing time of water droplets on the Ni-Cu-P coatings increased significantly, and the ice adhesion strength decreased sharply with the spontaneous surface adsorption of gaseous species (mainly hydrocarbon groups) in air. The adsorbed hydrocarbon species would promote the formation of air pockets between the ice-coating interface, which could effectively reduce the interfacial contact of the formed ice with the coating. When the adsorbed hydrocarbon species were removed by plasma cleaning, water droplets tended to have more direct contacts with the coatings prior to icing, leading to the formation of interlocked ice and significantly increased the ice adhesion on the surface. The variation of surface icephobicity can also be attributed to the changes of surface energy due to the surface adsorption. The results indicated that the surface gaseous adsorption in air played an important role in determining the surface icing behaviour and the icephobicity of the materials.

Transport phenomena during horizontal single belt casting process using an optimized inclined feeding system

ABSTRACTHorizontal Single Belt Casting is by far the simplest of all near net shape casting machines and is ideally suited to replace current slab caster operations, given its matching productivity, ‘green’ characteristics, and lower capital and operating costs. In the present paper, numerical simulations based on the Volume of Fluid method were utilized to test the performance of an optimized inclined feeding system that does not rely on the use of magnetohydrodynamics to slow the entry velocity of the liquid metal, so as to allow for a thicker film of liquid metal to be formed (10–15 mm), nor on the use of any moving side-dams so as to constrain the slab being formed. Rather, special attention was paid to the meniscus behaviour, including meniscus formation, meniscus renewal, meniscus turbulence, and the formation of tiny air pockets on the bottom surface of the forming strip.

A Numerical Simulation of Transport Phenomena During the Horizontal Single Belt Casting Process Using an Inclined Feeding System

Horizontal single belt casting (HSBC) has great potential to replace current conventional continuous casting (CCC) processes for sheet metal production, by directly casting 3 to 1 mm sheet for the automobile industry. In the present paper, two-dimensional mathematical models were developed to study transport phenomena, for the case of an inclined wall feeding system for a liquid aluminum wrought alloy (AA6111). Based on the commercial software ANSYS FLUENT 14.5 and user-defined functions, a two-layer turbulence model was used to examine the fluid flow emanating from a slot nozzle set above a water-cooled, high-speed, steel belt. The Volume of Fluid (VOF) method was used to predict the shape of the melt-air interface. A transformed coordinate system (x′, y′) was established in order to analyze the fluid flow on the inclined wall of the feeding system. The total pressure gauge gradient (∂ptotal/∂x) was used to describe the behavior of the melt film inside the slot nozzle of the head box. The modeling results show that during the melt film falling process, the total gauge pressure varies within the slot nozzle, which can decrease the stability of the falling film. The first impingement between the falling film and the inclined refractory wall of the feeding system gives rise to a local oscillation, and this influences the stability of the melt film moving downwards. At the rear meniscus position between the inclined wall and the moving belt, there is a clear vibration of the air-melt interface, together with a recirculation zone. The weak vibration of the air-melt interface could be induced by the periodic variation of the melt-air interface. Moreover, the formation of tiny air pockets is predicted. Finally, on the inclined wall of the feeding system, a suitable length of the transition area is needed to avoid over-acceleration of the melt film due to the force of gravity.

Fluidized bed classification of particles of different size and density

The main objectives of this study were to quantify the classification efficiency of a binary mixture of two different particle types and to demonstrate that CPFD can be used to simulate the main features of the classification process. A lab-scale cylindrical fluidized bed (8.4 cm inner diameter, 150 cm height), equipped with pressure sensors and a video camera for recordings, was applied in the experiments. The particles used in the study were ceramic beads (median diameter 70 µm, skeletal density 3830 kg/m3) and steel shot (290 µm, 7790 kg/m3). Ambient air was used as the fluidization medium.The minimum fluidization velocities of pure ceramic beads and steel shot were found to be 0.015 m/s and 0.240 m/s, respectively. In the experiments with a binary mixture of the two materials, the fluidization column was filled by alternating layers of ceramic beads and steel shot. In principle, segregation of the two particle types can be obtained by applying a suitable gas velocity within the interval defined by the two minimum fluidization velocities, resulting in a top layer of mainly lighter and smaller particles (flotsam) and a bottom layer of mainly heavier and larger particles (jetsam). In the experiments, the air velocity was gradually increased until the entire bed was fluidized. A gradual rearrangement of the multi-layer structure into a two-layer structure was observed. The rearrangement started at the top and then progressed downwards until most of the steel particles were collected at the bottom of the bed and practically all the ceramic beads were gathered at the top. The velocity at which the flotsam and jetsam layers were clearly segregated was found to be 0.180 m/s. The jetsam layer contained less than 0.3 wt% ceramic beads indicating that an almost pure steel shot fraction could be produced through fluidized bed classification. The flotsam layer was, however, less pure, with a steel shot content up to 24 wt%, suggesting that this layer may need a second classification stage for improved purity.Computational particle-fluid dynamics (CPFD) simulations of the same setup were performed using the commercial software Barracuda. The simulated minimum fluidization velocity of steel shot, applying the Ergun drag model, perfectly matched the experimental value. For the ceramic beads, however, the simulations, applying the Wen-Yu drag model, gave a value lower than the experimental value. Still, the simulations were able to capture the general behavior of the particles in the bed observed in the classification experiments, i.e. the rearrangement of the layers, even if a higher gas velocity was required for complete classification of the particles. The formation of air pockets was also observed in the simulations, as in some of the experiments. The results suggest that Barracuda CPFD simulations can be a useful tool in design and evaluation of fluidized bed classifiers.

Enhanced surface hydrophobicity of poly(lactic acid) by Co$^{60}$ gamma ray irradiation

Contact angle measurements were performed on Co$^{60}$ gamma-ray irradiated poly(lactic acid) (PLA) samples at several doses, using the sessile drop method. It was found that irradiation alters the wettability of PLA. In particular, PLA wetting behavior changes from moderately hydrophilic at low dose ($< 100$ kGy), to hydrophobic after the samples were exposed above a threshold dose ($\approx 200$ kGy). At low doses, wettability follows the Wenzel relation but beyond a threshold dose the Cassie-Baxter regime takes place. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) show that surface roughness of the samples increases as the applied dose increases. PLA wettability changes are mainly a consequence of surface modifications as roughness promotes the formation of air pockets under a drop, which reinforces the hydrophobic nature of is surface. Co$^{60}$ gamma ray irradiation can be used to tailor wetting properties of PLA. The method might have also application to produce biphilic PLA. Finally it is very important to remark that reported contact angles values for PLA (by different authors) of untreated samples vary from 60 to 85 degrees. Reasons for this behavior are given in this article.

Air pockets and water lenses in the ice cover of the Slave River

Communities along the Slave River are concerned about changes in the river ice cover, such as the formation of air pockets and double layers of ice in the ice cover. These anomalies make it challenging to find suitable locations for river ice crossings in winter, necessary for the transportation of goods to and from remote regions and to access hunting, trapping and fishing areas. This study utilized satellite imagery and field observations to document these anomalies of the river ice during the winters from November 2013 to March 2016. Backscatter analysis of the satellite imagery identified low and high density areas of air pockets along the river. Satellite images also revealed cracks and flooding of the river ice, particularly along the downstream reach of the river.

Laser welding of vertically aligned carbon nanotube arrays on polymer workpieces

Here we demonstrate laser transmission welding of vertically aligned carbon nanotube (VACNT) arrays for joining polymer sheets. The unique characteristics of VACNTs make them suitable for use in laser welding. First, the excellent light absorption of the VACNTs induces selective heating at the contact plane with a polymer sheet, minimizing thermal damage to the polymer. Second, the porous and compliant structure of the VACNTs prevents the formation of air pockets inside the contact space. Successful welding is obtained when the laser irradiation power is at an optimal level, below which the adhesion is too weak and above which the excessive heat causes periodic damage along the scanning path. The optimized laser welding technique is expected to become a new method for implementing carbon nanotubes as mechanical linkers for various thermoplastic polymers.

Thermal Performance of Liquid Metal Alloy with Graphene Addition as Thermal Interface Material

A high thermal conductivity thermal paste can be developed by mixing the oxidized liquid metal alloy (OLMA) with graphene. Four kinds of graphene-OLMA pastes were synthesized at graphene concentrations of 0.25 wt%, 0.75 wt%, 1.5 wt%, and 2.0 wt%, respectively. The paste structures were characterized by MicroXCT-400, which can be used to readily measure the air pocket size, and their thermal conductivities measured by a laser flash analysis method. It is found that the OLMA structure is very different from the liquid metal alloy (LMA), and a small amount of air pockets were formed in the OLMA. The air pocket size significantly affected the thermal conductivity of the graphene-OLMA paste. When the graphene concentration increased, as shown in Fig. 1(c)-(e), the paste's thermal conductivity increased. However, more air pockets were formed around the graphene. In particular, when the graphene concentration increased to 2.0 wt%, clusters of graphene, as shown in Fig. 1(f), were formed resulting in the formation of big air pockets in the thermal paste, which directly affected the thermal conductivity as shown in Fig. 1(g). We thought that when the graphene concentration increases, the thermal conductivity should increases. But the results show that it was not and then we used MicroCT to see the internal structure of the thermal paste and found that the air pockets were formed and significantly affects the thermal performance.

A novel method to control hydrolytic degradation of nanocomposite biocompatible materials via imparting superhydrophobicity

Acceleration of hydrolytic degradation of biomedical materials is not always desirable. For instance, terpolymers based on l-lactide, glycolide and trimethylene carbonate exhibit very fast hydrolytic degradation due to their amorphous structure, hydrophilicity, and high water absorption capability. Therefore, an attempt was made in the current study to impede the hydrolytic degradation for these materials through imparting superhydrophobicity to their surfaces. The used terpolymer has been shown to have promising potential applications as bio-absorbable surgical sutures and other biomedical materials, and thus, its applicability could be further extended upon impeding its hydrolytic degradation. Moreover, a novel method including combined use of non-solvent and nanoparticles was utilized to achieve superhydrophobicity. Very diverse wettability results were obtained which were attributed to the obtained various morphologies according to scanning electron microscopy results. More importantly, a unique hierarchical morphology was found to be responsible for the observed water repellent behavior. X-ray photoelectron spectroscopy results revealed co-existence of nanosilica particles and terpolymer chains on the surface's top layer. Finally, it was found that the superhydrophobic sample exhibited a significantly impeded hydrolytic degradation as compared with the hydrophilic pure terpolymer which was attributed to the formation of air pockets on the surface's top layer.

Assessment of Graft Compatibility of Some Olive Cultivars on Gemlik Rootstock by Florescence Microscopy = تقييم التوافق التطعيمي في الزيتون بواسطة المجهر الفلوريسيني

A florescence microscopy was used to assess the graft compatibility of five scions of olives and one rootstock. The cultivars inspected were ‘Gemlik’, ‘Memecik’, ‘Sari Ulak’, ‘Nizip Yaglik’ and ‘Domat’ where the rootstock was ‘Gemlik’. Anomalies have been observed in some grafts, such as differences in the growth of scions and rootstock trunk diameters. The study reports the extent of callus growth, cambial differentiation, cambial continuity and formation of vascular tissues of these olive cultivars in applied grafts by florescence microscopy. A good callus growth was observed in the cambial development of vascular tissues in these samples. The formation of air pockets in side grafts was also found. Successful compatible grafts of ‘Gemlik’, ‘Memecik’, ‘Nizip Yaglik’, and ‘Sari Ulak’ cultivars were identified using ‘Gemlik’ rootstock. The scions of ‘Domat’ grafted onto ‘Gemlik’ induced limited callus and cambial development, resulting only in partial success of the grafts, and with the formation of air pockets. Weak grafts; dark brown necrotic regions along the lines of the graft in the callus tissue borders were also observed. Phenolic compounds causing necrosis in the graft zone were also identified by histochemical investigation.

Effects of Manufacturing Parameters in Planar Flow Casting Process on Ribbon Formation and Puddle Evolution of Fe–Si–B Alloy

The effects of manufacturing parameters in the planar flow casting process on the ribbon formation and the puddle stability of Fe78Si9B13 alloy are investigated experimentally. The ribbon morphology, surface quality, and puddle geometry are examined at different conditions and the transient evolution processes of puddle for molten metal passing through a rectangular nozzle are observed. The successful operability window for the production of Fe78Si9B13 ribbon is established and it is found the scope is different from that of Al-based alloy. The trend lines of the alloys on the plane of Reynolds number versus Weber number corresponding to obtain a successful ribbon are established. The ribbon thickness is found to vary with the applied pressure across the crucible and the wheel speed to the power of 0.45 and −0.9, respectively. The formation of small air pockets could be enhanced by increasing the applied pressure difference and wheel speed, or by decreasing the nozzle-wheel gap and the jetting temperature.