Search Results (167)

Hydrogen embrittlement is defined as the phenomenon wherein materials undergo sudden degradation in mechanical properties due to the ingress of hydrogen atoms, and its occurrence is closely linked to hydrogen diffusion behavior. Here, first-principles calculations are employed to systematically investigate the hydrogen diffusion characteristics of both perfect and vacancy-containing α-Fe, γ-Fe, and ε-Fe crystal structures. The dissolution energies of hydrogen atoms in perfect α-Fe, γ-Fe, and ε-Fe crystals were calculated at different interstitial sites and transition states along various pathways. Hydrogen atoms preferentially occupy tetrahedral interstitial sites in α-Fe crystals, with diffusion occurring between two nearest-neighbor tetrahedral interstitial sites. In γ-Fe crystals, hydrogen atoms favor octahedral interstitial sites, diffusing along paths from octahedral sites to tetrahedral sites and then to other octahedral sites. In ε-Fe crystals, hydrogen atoms preferentially occupy octahedral interstitial sites and diffuse along pathways between nearest octahedral interstitial sites. The hydrogen diffusion coefficients calculated based on the Arrhenius equation follow the order α-Fe > γ-Fe > ε-Fe, indicating that hydrogen atoms diffuse most readily in α-Fe crystals. Notably, examination of the relationship between the interatomic distance and interaction energy in α-Fe reveals that hydrogen atoms have difficulty aggregating and forming hydrogen molecules within defect-free α-Fe crystals. However, introducing vacancy defects increases the mutual attraction between hydrogen atoms, thereby facilitating hydrogen bubble nucleation. Furthermore, the introduction of vacancy defects in α-Fe, γ-Fe, and ε-Fe alters the preferential occupancy sites and diffusion pathways of hydrogen because of vacancy trapping effects. Compared with diffusion in perfect crystals, hydrogen atoms must overcome substantially higher energy barriers to escape vacancy trapping and diffuse into defect-free lattice regions. Full article

Bubble nucleation in flowing liquid films is a common interfacial phenomenon affecting the heat and mass transfer at the solid–liquid interfaces in many thermal and functional material production processes, yet realizing its molecular-scale mechanisms under coupled flow, pressure, and heating conditions is important. In this study, molecular dynamics simulations are performed to investigate the bubble nucleation and growth in a liquid argon film on a heated platinum substrate under controlled pressure, with liquid flow driven by an applied body force. Bubble evolution is analyzed by the nucleation time, critical nucleation volume, bubble volume variation, and migration of the bubble’s center of mass. The results show that system pressure and substrate temperature dominantly regulate the nucleation: increasing pressure delays nucleation, whereas increasing substrate temperature accelerates it. Under a fixed system pressure and substrate temperature, liquid flow exhibits a non-monotonic influence. The applied forces from $4.0\times {10}^{-7}\mathrm{e}\mathrm{V}/\mathrm{Å}$ to $1.0\times {10}^{-6}\mathrm{e}\mathrm{V}/\mathrm{Å}$ gradually promote the nucleation and enhance the bubble growth by facilitating near-substrate heat transfer and density fluctuations, while the forces from $1.0\times {10}^{-6}\mathrm{e}\mathrm{V}/\mathrm{Å}$ to $1.4\times {10}^{-6}\mathrm{e}\mathrm{V}/\mathrm{Å}$ suppress nucleation and do not further promote the growth due to the intensified shear and interfacial instability. These findings provide molecular-level insight into the coupled thermodynamic and kinetic effects of pressure, temperature, and flow on bubble nucleation and growth at material interfaces, offering guidance for the design and operation of heat-transfer and functional materials processes. Full article

This research investigates wettability-induced, preferential, pressure-driven bubble nucleation of gases from a multi-gas dissolved liquid system in hydrophilic and hydrophobic glass vials. The hydrophobic glass surfaces were prepared using (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane (HT). Degassed deionized water in a vial, placed inside a pressure cell, was saturated with a precisely controlled mixture of CO₂ and CH₄ gases at either 6000 mbar or 3000 mbar for 24 h. To initiate the pressure-driven bubble nucleation process, a 500 mbar step-down pressure was applied to the pressure cell every 15 min until bubble nucleation was observed. CH₄ and CO₂ volume fractions were measured using micro-gas chromatography (Micro-GC), while a digital microscope was employed to observe the bubble nucleation process. No bubble nucleation was observed in the case of the hydrophilic vial even when the system pressure was brought to atmospheric pressure. In the case of the hydrophobic vial, the average onset bubble nucleation pressures were 4800 mbar and 2000 mbar for 6000 mbar and 3000 mbar saturation pressures, respectively. The average feed gas concentrations during saturation were 84.44 ± 0.14% and 15.44 ± 0.2% of CH₄ and CO₂, respectively, while at the onset pressure for bubble nucleation, the concentrations shifted to 85.24 ± 0.48% and 13.12 ± 0.52% of CH₄ and CO₂, respectively, when the saturation pressure was 6000 mbar. The average feed gas concentrations during saturation were 85.12 ± 0.28% and 14.67 ± 0.1% of CH₄ and CO₂, respectively, and the average concentrations of CH₄ and CO₂ gases at onset pressure for bubble nucleation were 86.06 ± 1.21% and 12.03 ± 1.03%, respectively, when the saturation pressure was 3000 mbar. The increase in CH₄ concentration is attributed to its preferential separation during the bubble nucleation process. Full article

Bio-inspired engineering draws on principles refined by natural evolution to tackle persistent challenges in fluid mechanics, structural dynamics, and thermal transport. This article presents a critical, mechanism-driven narrative review that integrates recent advances across three complementary domains that are often treated independently, namely: flow-control strategies such as leading-edge tubercles, alula-like devices, riblets, superhydrophobic skins, and hybrid low-Reynolds-number fliers; fluid-structure interactions inspired by aquatic and aerial organisms that leverage compliant foils, flexible filaments, ciliary arrays, and piezoelectric fluttering plates for propulsion, wake regulation, mixing, and energy harvesting; and phase-change heat-transfer surfaces modeled after stomata, porous biological networks, and textured cuticles that enhance nucleation control, liquid replenishment, and droplet or bubble removal. Rather than providing an exhaustive catalog of biological analogues, this review emphasizes the underlying physical mechanisms that link these domains and enable multifunctional performance. These developments reveal shared physical principles, including multiscale geometry, capillary- and vortex-mediated transport, and compliance-enabled flow tuning, which motivate the integrated treatment of aerodynamic, hydrodynamic, and thermal systems in applications spanning aerospace, energy conversion, and microscale thermal management. The review assesses persistent challenges associated with scaling biological architectures, ensuring long-term durability, and modeling tightly coupled fluid-thermal-structural interactions. By synthesizing insights across flow control, fluid-structure interaction, and phase-change heat transfer, this review provides a unifying conceptual framework that distinguishes it from prior domain-specific reviews. Emerging opportunities in hybrid multi-mechanism designs, data-driven optimization, multiscale modeling, and advanced fabrication are identified as promising pathways to accelerate the translation of biological strategies into robust, multifunctional thermal–fluid systems. Full article

Traditional flexible polyurethane (PU) foams frequently exhibit limited mechanical support and suboptimal moisture–heat regulation, which can compromise the microenvironmental comfort required for high-quality sleep. In this study, natural luffa fibers (LF) were incorporated as a microstructural modifier to simultaneously enhance the mechanical and moisture–heat regulation performance of PU foams. PU/LF composite foams with varying LF loadings were prepared via in situ polymerization, and their foaming kinetics, cellular morphology evolution, and physicochemical characteristics were systematically investigated. The results indicate that LF functions both as a reinforcing skeleton and as a heterogeneous nucleation site, thereby promoting more uniform bubble formation and controlled open-cell development. At an optimal loading of 4 wt%, the composite foam developed a highly interconnected porous architecture, leading to a 7.9% increase in tensile strength and improvements of 19.4% and 22.6% in moisture absorption and moisture dissipation rates, respectively, effectively alleviating the heat–moisture accumulation typically observed in unmodified PU foams. Ergonomic pillow prototypes fabricated from the optimized composite further exhibited enhanced pressure-relief performance, as evidenced by reduced peak cervical pressure and improved uniformity of contact-area distribution in human–pillow pressure mapping, together with an increased SAG factor, indicating improved load-bearing adaptability under physiological sleep postures. Collectively, these findings elucidate the microstructural regulatory role of biomass-derived luffa fibers within porous polymer matrices and provide a robust material basis for developing high-performance, sustainable, and ergonomically optimized sleep products. Full article

To tackle challenges including excessive initial boiling superheat and low heat transfer coefficients inherent in conventional working fluids, hydrophobic-modified TiO₂@carbon nanotube (MWCNT) composite nanofluids were fabricated. Subsequently, the boiling heat transfer mechanisms were systematically investigated and visually verified. Hydrophobic TiO₂ nanofluids exhibit enhanced stability, whereas hydrophobic TiO₂@MWCNTs composite nanofluids demonstrate improved thermal conductivity. At a mass ratio of hydrophobic-modified TiO₂ to MWCNTs of 2:1, the optimal heat transfer performance was attained, with a 31.6% increase in heat transfer coefficient (HTC) and a 46.5% increase in critical heat flux (CHF) density relative to hydrophobic-modified TiO₂ nanofluids. Composite nanofluids exert effective regulation over bubble kinetic parameters: hydrophobic nanoparticles increase vaporization core density, reduce bubble nucleation energy barriers, and mitigate initial boiling superheat. Benefiting from the superior thermal conductivity and mechanical properties, MWCNTs remarkably promote heat transfer efficiency. The synergistic effect between the two components enables the concurrent enhancement of HTC and CHF, thus highlighting the promising application potential of hydrophobic-modified TiO₂@MWCNTs composite nanofluids in intensifying pool boiling heat transfer. Full article

This experimental study comparatively investigates flow boiling performance and mechanisms in open and closed rectangular microchannels (ORMs/CRMs) with a high aspect ratio of 4. Fabricated on a copper substrate and sealed with a transparent window for visualization, the systems were tested using refrigerant R245fa. Experiments spanned mass fluxes from 89 to 545 kg/m²·s and heat fluxes from 6.3 to 218.5 W/cm² at an inlet temperature of 14 °C. Flow visualization reveals that the ORM configuration accelerates the transition from bubbly to slug and churn flow regimes and facilitates a unique stratified flow pattern absent in the CRM. Quantitatively, the ORM enhances the heat transfer coefficient by 4.2–14.1% while reducing the system pressure drop by 11.5–58.6% within the low mass flux range (89–269 kg/m²·s). Conversely, at a high mass flux of 545 kg/m²·s, the ORM’s pressure drop increases substantially by 29.9–246.8%, attributed to significant two-phase losses in the top-gap region. As heat flux increases, inertial forces dominate over gravitational effects, shifting the primary heat transfer contribution from nucleate to flow boiling. The figure of merit (FOM) confirms the overall performance superiority of the ORM at low mass fluxes. This work provides valuable insights and design guidelines for high-performance, high-aspect-ratio microchannel heat sinks in advanced thermal management systems. Full article

Laboratory-scale cryogenic X-ray microtomography was employed for the first time to investigate the early structural evolution of polyurethane (PU) foams. This method enables ex situ studying the internal morphology of the frozen reactive mixture at various times before cell impingement. In this work, the precision of the method was evaluated by studying the early bubble formation and growth under different blowing agents and catalyst contents. It was detected that tripling the catalyst weight content doubled cell nucleation density, from 8.9 × 10⁵ to 1.8 × 10⁶ cells cm⁻³. Yet, doubling the water content has lesser impact on nucleation but leads to fast speeds of cell growth and, in turn, lower relative density at equal reaction times. Overall, it is demonstrated that laboratory cryogenic microtomography can be used to democratise the 3D investigation of the internal structure of foams which was until now only possible in synchrotron facilities. In addition, this method can help elucidate the mechanisms of nucleation and degeneration via directly measuring the density of bubbles and distance between them in the reactive mixture. Finally, this methodology could be extended to recent laboratory nanotomography systems utilizing X-ray tubes with nanometric spot sizes, thereby enabling the confident identification of nucleation events. Full article

This study aims to demonstrate the presence of a pronounced coseismic increase in atmospheric methane concentrations during the 2024 Noto Peninsula Earthquake and to examine whether this increase may have originated from underground natural gas release. By analyzing hourly CH₄ data from the Ministry of the Environment’s monitoring network, this study shows that significant methane increases occurred only in areas with seismic intensity of 6– or greater, and that an exceptional anomaly—reaching 29 times the standard deviation of the past year—was recorded at the Nanao station. The validity of this anomaly was confirmed through consultation with local atmospheric officer, and high-time-resolution data (6 min values) were provided, verifying continuous instrument operation. Detailed analysis further shows that two major methane peaks occurred, each rising not immediately after the main shock but synchronously with two large aftershocks approximately 8 and 44 min later. Geological and hydrogeological information indicates the presence of water-soluble gas and unsaturated hydrocarbons beneath the Nanao region, suggesting that seismic shaking may have ruptured clay layers and released accumulated gas. Analyses of public reports and interviews with local officials show that alternative explanations—such as fire smoke, pipeline rupture, instrument malfunction, and gas-cylinder damage—were unlikely. These findings indicate that the observed methane anomaly was most likely caused by earthquake-synchronous underground gas release, suggesting that methane-release risk should be considered in post-earthquake fire-hazard assessments. Full article

Experimental and theoretical techniques have been developed to quantify foamy oil behaviour of solvent-heavy oil systems at bubble level during a gas exsolution process. During constant composition expansion (CCE) tests, we artificially induced foamy oil dynamics for solvent-heavy oil systems by gradually reducing pressure and recorded the changed pressures and volumes in an isolated PVT setup at a given temperature. By discretizing gas bubbles on the basis of the classical nucleation theory, we theoretically integrated the population balance equation (PBE), Fick’s law, and the Peng–Robinson equation of state (PR EOS) to reproduce the experimental measurements. Pseudo-bubblepoint pressure for a given solvent-heavy oil system can be increased with either a lower pressure depletion rate or a higher temperature, during which gas bubble growth is facilitated with a reduction in viscosity and/or an increase in solvent concentration, but gas bubble nucleation and mitigation is hindered with an increase in solvent concentration. Compared to CO₂, CH₄ is found to yield stronger and more stable foamy oil, indicating that foamy oil is more stable with a larger amount of dispersed gas bubbles at lower temperatures. Using the PR EOS together with the modified alpha functions at T_r = 0.7 and T_r = 0.6, the absolute average relative deviation (AARD) is reduced from 4.58% to 2.24% with respect to the predicted pseudo-bubblepoint pressures. Full article

This study experimentally investigated boiling phenomena and heat transfer enhancement on sintered Cu micro/nanoporous surfaces under saturated pool boiling conditions. To evaluate the effects of the combined micro/nanostructures, microporous Cu layers and pillar-integrated surfaces were fabricated using micro-sized (diameter <75 mm) metal powder sintering, while nanostructures were formed through thermal oxidation. Boiling experiments revealed that the boiling heat transfer coefficient (BHTC) and critical heat flux (CHF) of the microporous Cu surfaces surpassed those of the reference surface SiO₂. The microporous pillar surface exhibited the best performance, demonstrating enhancements of approximately 2.7-fold and 7.3-fold in CHF and BHTC, respectively. High-speed imaging attributed this improvement to increased nucleation site density, rapid detachment and generation of small bubbles, efficient surface rewetting by capillary wicking, and liquid–vapor pathway separation enabled by the pillar geometry. Distinct transient temperature peaks and recoveries were observed on the oxidized pillar surfaces. Despite temporary overheating, strong capillary wicking from the superhydrophilic nanostructures recovered to the nucleate-boiling regime, which suppressed irreversible dryout and extended the boiling performance beyond the smooth surface CHF by 2.1 times. The results revealed that increasing the nucleation site density, enhancing the capillary-driven liquid supply, and ensuring effective separation of the vapor and liquid pathways improved the boiling heat transfer in multiscale porous structures. The sintered Cu micro/nanoporous surfaces demonstrated stable and efficient heat transfer across a wide range of heat fluxes, highlighting their potential for advanced thermal management applications and realizing optimally designed high-performance boiling surfaces. Full article

An experimental technique was developed for two-dimensional Particle Image Velocimetry (PIV) measurement of wall shear flow around a bubble growing under dissolved gas supersaturation. Carbonated water was used as dissolved-gas-supersaturated liquid, and its flow was created in a small container with a tube pump. An isolated CO₂ bubble nucleated from an intentionally created scratch on the glass surface was placed in the flow. The mass-diffusion-driven growth of the bubble (from nucleation to detachment from the surface) was recorded using a video camera with backlighting; the radius of the detached bubble was below 1 mm in the present experimental conditions. The velocity field without and with the wall-attached bubble was obtained through PIV with the water (seeded with fluorescent particles) and with a planar laser sheet, which enables one to obtain local shear flow. With the measured liquid velocity at the bubble center, the particle Reynolds number was found to be below 1. The proposed PIV measurement technique allows for careful examination of bubble detachment dynamics and convective mass transfer around attached bubbles. Full article

The present investigation conducts a comparative analysis of the nucleate pool boiling heat transfer performance of two dielectric fluids, a low-GWP hydrofluoroolefin-based fluid (commercially known as Opteon™ SF33, referred to hereafter as SF33) and a perfluoropolyether-based fluid with a high GWP (commercially known as Galden^R HT55, referred to hereafter as HT55) under atmospheric pressure conditions. Pool boiling experiments and visual observations were performed to assess essential performance parameters, such as critical heat flux, heat transfer coefficient, and boiling dynamics. SF33 exhibits enhanced heat transfer performance, achieving markedly higher heat transfer coefficient values at all the heat flux levels and attaining superior critical heat flux relative to HT55. The results show that SF33 provides a consistently higher heat transfer coefficient, reaching approximately 12 W/m²·K at maximum heat flux, compared to only 6 W/m²·K for HT55, representing nearly a 100% improvement. The visual observations indicated that reduced surface tension and increased latent heat of vaporization of SF33 facilitate more frequent bubble nucleation and smaller bubble departure, thereby enhancing its boiling performance. Properties of SF33 render it a superior candidate for high-performance cooling systems in data centers and power electronics. The study concludes that SF33 is a more efficient and adaptable fluid for next-generation cooling systems, providing superior heat dissipation and energy efficiency relative to HT55. Full article

This study numerically investigates the growth and dynamics of a single vapor bubble in a rectangular microchannel under subcooled and saturated inlet conditions using the phase-field method coupled with the Lee phase-change model. Results demonstrate that subcooled flow induces early bubble nucleation, pronounced lateral expansion along the heated wall, and prolonged bubble-wall contact due to stronger condensation at the interface and thinner microlayer formation. Enhanced recirculating vortices and steeper thermal gradients promote vigorous evaporation and increased local heat flux, resulting in faster downstream bubble propagation driven by significant axial pressure gradients. Analysis of temperature gradient and heat flux profiles confirms that subcooled conditions produce higher wall heat flux and more frequent peaks in evaporative flux compared to the saturated case, indicating intensified phase-change activity and thermal transport. Conversely, saturated conditions produce more spherical bubbles with dominant vertical growth, weaker condensation, and symmetrical thermal and pressure fields, leading to slower growth and delayed detachment near the nucleation site. These findings highlight the critical influence of inlet subcooling on bubble morphology, flow structures, heat transfer, and pressure distribution, underscoring the thermal management advantages of subcooled boiling in microchannel applications. Full article

Nanomaterials play a beneficial role in regulating the function of cement-based materials. The effects and mechanism of graphene oxide (GO) on foam behavior in solutions and air-entraining behavior of cement mortar were studied, and its effect on the microstructure of cement mortar was also investigated. The results show that a synergy between GO’s hydrophobicity and the air-entraining agent’s hydrophobic chains drove more agent molecules to adsorb onto the GO surface, subsequently spreading and aggregating across the bubbles. GO effectively assisted the air entraining agent to refine the bubble size, improved the bubble stability of aqueous solutions, and had excellent air entraining performance in the fresh cement mortar, as well as the optimum air-void adjustment performance of hardened cement mortars. With the addition of 0.4‰ GO, the loss rate of gas content in the GO mixed mortar was 10.3%, which was 55.8% lower than that when only using AEA. The addition of 0.4‰ of GO effectively increased the volume fraction of the cement mortar system. GO reduced the pore volume in the mortar through the filling effect and nucleation effect to reduce the total porosity and refine the microstructure of the mortar. Full article