Hydrogen Embrittlement as a Conspicuous Material Challenge─Comprehensive Review and Future Directions.

Hydrogen is considered a clean and efficient energy carrier crucial for shaping the net-zero future. Large-scale production, transportation, storage, and use of green hydrogen are expected to be undertaken in the coming decades. As the smallest element in the universe, however, hydrogen can adsorb on, diffuse into, and interact with many metallic materials, degrading their mechanical properties. This multifaceted phenomenon is generically categorized as hydrogen embrittlement (HE). HE is one of the most complex material problems that arises as an outcome of the intricate interplay across specific spatial and temporal scales between the mechanical driving force and the material resistance fingerprinted by the microstructures and subsequently weakened by the presence of hydrogen. Based on recent developments in the field as well as our collective understanding, this Review is devoted to treating HE as a whole and providing a constructive and systematic discussion on hydrogen entry, diffusion, trapping, hydrogen-microstructure interaction mechanisms, and consequences of HE in steels, nickel alloys, and aluminum alloys used for energy transport and storage. HE in emerging material systems, such as high entropy alloys and additively manufactured materials, is also discussed. Priority has been particularly given to these less understood aspects. Combining perspectives of materials chemistry, materials science, mechanics, and artificial intelligence, this Review aspires to present a comprehensive and impartial viewpoint on the existing knowledge and conclude with our forecasts of various paths forward meant to fuel the exploration of future research regarding hydrogen-induced material challenges.

Unraveling Ice-Solid Interface Rupture Dynamics: Insights from Molecular Dynamics Simulations.

Unwanted icing on exposed surfaces poses significant risks, driving the quest for effective anti-icing mechanisms. While fracture mechanics concepts have been developed for designing coatings that weaken the ice-solid interface on soft surfaces, the factors that dictate ice adhesion strength and its counterpart, ice removal force, on hard surfaces remain poorly understood. In this study, we employ molecular dynamics simulations to investigate the interface rupture between ice and a hard solid substrate. The results indicate that the ice adhesion strength is contingent on the length of the ice cube. By examining the shearing behavior, we reveal a nanoscale critical force-bearing length. The shear force required to detach the ice scales proportionally with the length of the ice cube when it is smaller than the critical length. Once the ice cube length exceeds the critical length, the shear force stabilizes at a constant maximum value, revealing the existence of a maximum ice-removal force. The results align with the so-called strength versus toughness-controlled deicing regimes and are in agreement with cohesive zone modeling at the continuum length scale and recent experimental results. Our results extend this understanding to the nanoscale, confirming consistency between macro and micro scales. This consistency suggests that the toughness of the ice-solid interface is intrinsically governed by ice-surface interactions. By unraveling key intrinsic factors and their scale-dependent effects on the interface rupture of ice on surfaces, this study lays a solid theoretical foundation for the design and fabrication of next-generation anti-icing surfaces.

Recombinant irisin enhances the extracellular matrix formation, remodeling potential, and differentiation of human periodontal ligament cells cultured in 3D.

BACKGROUND: Irisin is expressed in human periodontal ligament (hPDL), and its administration enhances growth, migration and matrix deposition in hPDL cells cultured in monolayers in vitro. OBJECTIVES: To identify whether irisin affects the gene expression patterns directing the morphology, mechanical properties, extracellular matrix (ECM) formation, osteogenic activity and angiogenic potential in hPDL cell spheroids cultured in 3D. MATERIALS AND METHODS: Spheroids of primary human hPDL cells were generated in a rotational 3D culture system and treated with or without irisin. The gene expression patterns were evaluated by Affymetrix microarrays. The morphology of the spheroids was characterized using histological staining. Mechanical properties were quantified by nanoindentation. The osteogenic and angiogenic potential of spheroids were assessed through immunofluorescence staining for collagen type I, periostin fibronectin and von Willebrand factor (vWF), and mRNA expression of osteogenic markers. The secretion of multiple myokines was evaluated using Luminex immunoassays. RESULTS: Approximately 1000 genes were differentially expressed between control and irisin-treated groups by Affymetrix. Several genes related to ECM organization were differentially expressed, and multiple deubiquitinating enzymes were upregulated in the irisin-exposed samples analyzed. These represent cellular and molecular mechanisms indicative of a role for irisin in tissue remodeling. Irisin induced a rim-like structure on the outer region of the hPDL spheroids, ECM-related protein expression and the stiffness of the spheroids were enhanced by irisin. The expression of osteogenic and angiogenetic markers was increased by irisin. CONCLUSIONS: Irisin altered the morphology in primary hPDL cell-derived spheroids, enhanced its ECM deposition, mechanical properties, differentiation and remodeling potential.

Atomistic Insights into the Droplet Size Evolution during Self-Microemulsification.

Microemulsions have been attracting great attention for their importance in various fields, including nanomaterial fabrication, food industry, drug delivery, and enhanced oil recovery. Atomistic insights into the self-microemulsifying process and the underlying mechanisms are crucial for the design and tuning of the size of microemulsion droplets toward applications. In this work, coarse-grained models were used to investigate the role that droplet sizes played in the preliminary self-microemulsifying process. Time evolution of liquid mixtures consisting of several hundreds of water/surfactant/oil droplets was resolved in large-scale simulations. By monitoring the size variation of the microemulsion droplets in the self-microemulsifying process, the dynamics of diameter distribution of water/surfactant/oil droplets were studied. The underlying mass transport mechanisms responsible for droplet size evolution and stability were elucidated. Specifically, temperature effects on the droplet size were clarified. This work provides the knowledge of the self-microemulsification of water-in-oil microemulsions at the nanoscale. The results are expected to serve as guidelines for practical strategies for preparing a microemulsion system with desirable droplet sizes and properties.

MFGM components promote gut Bifidobacterium growth in infant and in vitro.

PURPOSE: Infant gut microbiota which plays an important role in long-term health is mainly shaped by early life nutrition. However, the effect of nutrients on infants gut microbiota is less researched. Here, we present a study aiming to investigate in vitro a modified formula that is supplemented with milk fat globule membrane (MFGM) that were missing in common formulas when compared with human milk and to assess the impact of feeding scheme on microbiota and metabolism. METHODS: A total of 44 infants including 16 from breast milk feeding, 13 from common formula feeding and 15 from modified formula feeding were analyzed, and A cross-sectional sampling of fecal and urine was done at 1 month-of-age. Stool microbiota composition was characterized using high-throughput DNA sequencing, and urinary metabolome was profiled by nuclear magnetic resonance (NMR). In vitro growth experiment of Bifidobacterium with key components from MFGM was performed and analyzed by both DNA and RNA. RESULTS: Stool samples from the infants who were breastfed had a higher relative abundance of Bifidobacterium and a lower relative abundance of Escherichia than the formula-fed infants. The stool microbiome shifts were associated with urine metabolites changes. Three substances including lactadherin, sialic acid and phospholipid, key components of MFGM were significantly positively correlated to Bifidobacterium of stool samples from infants, and stimulated the growth rate of Bifidobacterium significantly by provided energy in vitro growth experiment with RNA analysis. CONCLUSIONS: These findings suggest that the key components from MFGM could improve infants' health by modulating the gut microbiome, and possibly supporting the growth of Bifidobacterium. REGISTRATION: Clinicaltrials.gov NCT02658500 (registered on January 20, 2016).

Unraveling Adhesion Strength between Gas Hydrate and Solid Surfaces.

Natural gas hydrate is a promising future energy source, but it also poses a huge threat to oil and gas production due to its ability to deposit within and block pipelines. Understanding the atomistic mechanisms of adhesion between the hydrate and solid surfaces and elucidating its underlying key determining factors can shed light on the fundamentals of novel antihydrate materials design. In this study, large-scale molecular simulations are employed to investigate the hydrate adhesion on solid surfaces, especially with focuses on the atomistic structures of intermediate layer and their influences on the adhesion. The results show that the structure of the intermediate layer formed between hydrate and solid surface is a competitive equilibrium of induced growth from both sides, and is regulated by the content of guest molecules. By comparing the fracture behaviors of the hydrate-solid surface system with different intermediate structures, it is found that both the lattice areal density of water structure and the adsorption of guest molecules on the interface together determine the adhesion strength. Based on the analysis of the adhesion strength distribution, we have also revealed the origins of the drastic difference in adhesion among different water structures such as ice and hydrate. Our simulation indicates that ice-adhesion strength is approximately five times that of lowest hydrate adhesion strength. This finding is surprisingly consistent with the available experimental results.

CO2 wetting on pillar-nanostructured substrates.

CO2 capture by dropwise CO2 condensation on cold solid surfaces is a promising technology. Understanding the role of the nanoscale surface and topographical features of CO2 droplet wetting characteristics is of importance for CO2 capture by this technology, but this remains unexplored as of yet. Here, using large-scale molecular dynamics (MD) simulations, the contact angle and wetting behaviors of CO2 droplets on pillar-structured Cu-like surfaces are investigated for the first time. Dynamic wetting simulations show that, by changing the strength of the solid-liquid attraction [Formula: see text] a smooth Cu-like surface offers a transition from CO2-philic to CO2-phobic. By periodically pillared roughening of the Cu-like surfaces, however, a higher contact angle and a smaller spreading exponent of a liquid CO2 droplet are realized. Particularly, a critical crossover of CO2-philic to CO2-phobic can appear. The wetting of the pillared surfaces by a liquid CO2 droplet proceeds non-uniformly. A liquid CO2 droplet is capable of exhibiting a transition from the Cassie state to the Wenzel state with increasing [Formula: see text] increasing inter-pillar distance, and increasing pillar width. The wetting morphologies of the metastable Wenzel state of a CO2 droplet are very different from each other. The findings will inform the ongoing design of CO2-phobic solid surfaces for practical dropwise condensation-based CO2 capture applications.

Stability, deformation and rupture of Janus oligomer enabled self-emulsifying water-in-oil microemulsion droplets.

Microemulsions exist widely in nature, daily life and industrial manufacturing processes, including petroleum production, food processing, drug delivery, new material fabrication, sewage treatment, etc. The mechanical properties of microemulsion droplets and a correlation to their molecular structures are of vital importance to those applications. Despite studies on their physicochemical determinants, there are lots of challenges of exploring the mechanical properties of microemulsions by experimental studies. Herein, atomistic modelling was utilized to study the stability, deformation, and rupture of Janus oligomer enabled water-in-oil microemulsion droplets, aiming at revealing their intrinsic relationship with Janus oligomer based surfactants and oil structures. The self-emulsifying process from a water, oil and surfactant mixture to a single microemulsion droplet was modulated by the amphiphilicity and structure of the surfactants. Four microemulsion systems with an interfacial thickness in the range of 7.4-17.3 Å were self-assembled to explore the effect of the surfactant on the droplet morphology. By applying counter forces on the water core and the surfactant shell, the mechanical stability of the microemulsion droplets was probed at different ambient temperatures. A strengthening response and a softening regime before and after a temperature-dependent peak force were identified followed by the final rupture. This work demonstrates a practical strategy to precisely tune the mechanical properties of a single microemulsion droplet, which can be applied in the formation, de-emulsification, and design of microemulsions in oil recovery and production, drug delivery and many other applications.

Breathable Nanowood Biofilms as Guiding Layer for Green On-Skin Electronics.

Thin-film electronics are urged to be directly laminated onto human skin for reliable, sensitive biosensing together with feedback transdermal therapy, their self-power supply using the thermoelectric and moisture-induced-electric effects also has gained great attention (skin and on-skin electronics (On-skinE) themselves are energy storehouses). However, "thin-film" On-skinE 1) cannot install "bulky" heatsinks or sweat transport channels, but the output power of thermoelectric generator and moisture-induced-electric generator relies on the temperature difference (∆T ) across generator and the ambient humidity (AH), respectively; 2) lack a routing and accumulation of sweat for biosensing, lack targeted delivery of drugs for precise transdermal therapy; and 3) need insulation between the heat-generating unit and heat-sensitive unit. Here, two breathable nanowood biofilms are demonstrated, which can help insulate between units and guide the heat and sweat to another in-plane direction. The transparent biofilms achieve record-high transport// /transport⊥ (//: along cellulose nanofiber alignment direction, ⊥: perpendicular direction) of heat (925%) and sweat (338%), winning applications emphasizing on ∆T/AH-dependent output power and "reliable" biosensing. The porous biofilms are competent in applications where "sensitive" biosensing (transporting// sweat up to 11.25 mm s-1 at the 1st second), "insulating" between units, and "targeted" delivery of saline-soluble drugs are of uppermost priority.

Understanding the role of hollow sub-surface structures in reducing ice adhesion strength.

The accretion of ice on exposed surfaces results in detrimental effects in many aspects of life and technology. Passive icephobic coatings, designed by strategies towards lowering ice adhesion to mitigate icing problems, have recently received great attention. In our previous studies, incorporation of hollow sub-surface structures which act as macro-scale crack initiators has been shown to drastically lower the ice adhesion on PDMS surfaces. In this study, the effects of hollow sub-surface structure geometry, such as the heights, shapes, and distributions, as well as the directions of the applied shear force, are experimentally investigated. Our results show that the number of potential macro-scale crack initiation sites dictates ice adhesion strength. The directions of the applied shear force also influence the ice adhesion strength when the potential crack length is dependent on the applied shear force direction. The inter-locking effect between ice and the coating, caused by the pre-deformation, needs to be considered if one of the dimensions of the hollow sub-surface structures approaches the millimeter scale. These results improve the understanding of the role of hollow sub-surface structures in reducing ice adhesion, providing new insights into the design principles for multi-scale crack initiator-promoted icephobic surfaces.

An ultra-durable icephobic coating by a molecular pulley.

Slide-ring crosslinked polydimethylsiloxane (PDMS) is designed and prepared for anti-icing/deicing applications. Compared with the covalent crosslinks, the slidable crosslinks enhance the mobility of polymer networks and endow the materials with low elastic modulus. The PDMS matrix guarantees the hydrophobicity of as-prepared coatings. These properties synergistically lead to ultra-low ice adhesion strength (13.0 ± 1.3 kPa) and excellent mechanical durability. The ice adhesion strength on the coating maintains a value of ∼12 kPa during 20 icing/deicing cycles, and increases gradually to a value of ∼22 kPa after 800 cycles of abrasions. The novel design strategy provides one-step forward to anti-icing/deicing solutions for targeted applications.

Grain-Size-Controlled Mechanical Properties of Polycrystalline Monolayer MoS2.

Pristine monocrystalline molybdenum disulfide (MoS2) possesses high mechanical strength comparable to that of stainless steel. Large-area chemical-vapor-deposited monolayer MoS2 tends to be polycrystalline with intrinsic grain boundaries (GBs). Topological defects and grain size skillfully alter its physical properties in a variety of materials; however, the polycrystallinity and its role played in the mechanical performance of the emerging single-layer MoS2 remain largely unknown. Here, using large-scale atomistic simulations, GB structures and mechanical characteristics of realistic single-layered polycrystalline MoS2 of varying grain size prepared by confinement-quenched method are investigated. Depending on misorientation angle, structural energetics of polar-GBs in polycrystals favor diverse dislocation cores, consistent with experimental observations. Polycrystals exhibit grain-size-dependent thermally induced global out-of-plane deformation, although defective GBs in MoS2 show planar structures that are in contrast to the graphene. Tensile tests show that presence of cohesive GBs pronouncedly deteriorates the in-plane mechanical properties of MoS2. Both stiffness and strength follow an inverse pseudo Hall-Petch relation to grain size, which is shown to be governed by the weakest link mechanism. Under uniaxial tension, transgranular crack propagates with small deflection, whereas upon biaxial stretching, the crack grows in a kinked manner with large deflection. These findings shed new light in GB-based engineering and control of mechanical properties of MoS2 crystals toward real-world applications in flexible electronics and nanoelectromechanical systems.

Design and preparation of sandwich-like polydimethylsiloxane (PDMS) sponges with super-low ice adhesion.

The mitigation of ice on exposed surfaces is of great importance to many aspects of life. Ice accretion, however, is unavoidable as time elapses and temperature lowers sufficiently. One practical solution is to reduce the ice adhesion strength on a surface to as low as possible, by either decreasing the substrate elastic modulus, lowering surface energy or increasing the length of cracks at the ice-solid interface. Herein, we present a facile preparation of polydimethylsiloxane (PDMS) based sandwich-like sponges with super-low ice adhesion. The weight ratio of the PDMS prepolymer to the curing agent is tuned to a lower surface energy and elastic modulus. The introduction of PDMS sponge structures combined the advantages of both a reduced apparent elastic modulus and most importantly, the macroscopic crack initiators at the ice-solid interface, resulting in dramatic reduction of the ice adhesion strength. Our design of sandwich-like sponges achieved a low ice adhesion strength as low as 0.9 kPa for pure PDMS materials without any additives. The super-low ice adhesion strength remains constant after 25 icing and deicing cycles. We thus provide a new and low-cost approach to realize durable super-low ice adhesion surfaces.

Atomistic dewetting mechanics of Wenzel and monostable Cassie-Baxter states.

Water adhesion underlies wettabilities, and thus hydrophobicities, and defines surface properties like self-cleaning, icephobicity and many others. The nanomechanics of water adhesion, especially in the dynamic dewetting processes, has not been fully investigated. Here in this article, atomistic modeling and molecular dynamics simulations were utilized to probe the adhesion mechanics of water droplets on nanopillars and flat surfaces, covering dewetting in the Wenzel and the newly discovered monostable Cassie-Baxter states. The simulations were able to identify intermediate dewetting states on rough surfaces, and resolve the transition between wetting states under force. The results revealed characteristic features of dynamic water adhering stress underpinning dewetting on the nanoscale, which provided deeper knowledge on surface dewetting mechanics. This work complements nanoscale dewetting experiments for new fundamental insights in studies including nanoroughness design, enhanced oil recovery, anti-icing and others.

Atomistic insights into the nanofluid transport through an ultra-confined capillary.

Nanofluid or nanoparticle (NP) transport in confined channels is of great importance for many biological and industrial processes. In this study, molecular dynamics simulation has been employed to investigate the spontaneous two-phase displacement process in an ultra-confined capillary controlled by the surface wettability of NPs. The results clearly show that the presence of NPs modulates the fluid-fluid meniscus and hinders the displacement process compared with the NP-free case. From the perspective of motion behavior, hydrophilic NPs disperse in the water phase or adsorb on the capillary, while hydrophobic and mixed-wet NPs are mainly distributed in the fluid phase. The NPs dispersed into fluids tend to increase the viscosity of the fluids, while the adsorbed NPs contribute to the wettability alteration of the solid capillary. Via capillary number calculations, it is uncovered that the viscosity increase of fluids is responsible for the hindered spontaneous displacement process by hydrophobic and mixed NPs. The wettability alteration of the capillary induced by adsorbed NPs dominates the enhanced displacement in the case of hydrophilic NPs. Our findings provide guidance for modifying the rate of capillary filling and reveal the microscopic mechanism transporting NPs into porous media, which is significant to the design of NPs for target applications.

Multiscale crack initiator promoted super-low ice adhesion surfaces.

Preventing icing on exposed surfaces is important for life and technology. While suppressing ice nucleation by surface structuring and local confinement is highly desirable and yet to be achieved, a realistic roadmap of icephobicity is to live with ice, but with lowest possible ice adhesion. According to fracture mechanics, the key to lower ice adhesion is to maximize crack driving forces at the ice-substrate interface. Herein, we present a novel integrated macro-crack initiator mechanism combining nano-crack and micro-crack initiators, and demonstrate a new approach to designing super-low ice adhesion surfaces by introducing sub-structures into smooth polydimethylsiloxane coatings. Our design promotes the initiation of macro-cracks and enables the reduction of ice adhesion by at least ∼50% regardless of the curing temperature, weight ratio and size of internal holes, reaching a lowest ice adhesion of 5.7 kPa. The multiscale crack initiator mechanisms provide an unprecedented and versatile strategy towards designing super-low ice adhesion surfaces.

Size and shape effects on the thermodynamic properties of nanoscale volumes of water.

Small systems are known to deviate from the classical thermodynamic description, among other things due to their large surface area to volume ratio compared to corresponding big systems. As a consequence, extensive thermodynamic properties are no longer proportional to the volume, but are instead higher order functions of size and shape. We investigate such functions for second moments of probability distributions of fluctuating properties in the grand-canonical ensemble, focusing specifically on the volume and surface terms of Hadwiger's theorem, explained in Klain, Mathematika, 1995, 42, 329-339. We resolve the shape dependence of the surface term and show, using Hill's nanothermodynamics [Hill, J. Chem. Phys., 1962, 36, 3182], that the surface satisfies the thermodynamics of a flat surface as described by Gibbs [Gibbs, The Scientific Papers of J. Willard Gibbs, Volume 1, Thermodynamics, Ox Bow Press, Woodbridge, Connecticut, 1993]. The Small System Method (SSM), first derived by Schnell et al. [Schnell et al., J. Phys. Chem. B, 2011, 115, 10911], is extended and used to analyze simulation data on small systems of water. We simulate water as an example to illustrate the method, using TIP4P/2005 and other models, and compute the isothermal compressibility and thermodynamic factor. We are able to retrieve the experimental value of the bulk phase compressibility within 2%, and show that the compressibility of nanosized volumes increases by up to a factor of two as the number of molecules in the volume decreases. The value for a tetrahedron, cube, sphere, polygon, etc. can be predicted from the same scaling law, as long as second order effects (nook and corner effects) are negligible. Lastly, we propose a general formula for finite reservoir correction to fluctuations in subvolumes.

Molecular dynamics study of di-CF4 based reverse micelles in supercritical CO2.

Reverse micelles (RMs) in supercritical CO2 (scCO2) are promising alternatives for organic solvents, especially when both polar and non-polar components are involved. Fluorinated surfactants, particularly double-chain fluorocarbon surfactants, are able to form well-structured RMs in scCO2. The inherent self-assembly mechanisms of surfactants in scCO2 are still subject to discussion. In this study, molecular dynamics simulations are performed to investigate the self-aggregation behavior of di-CF4 based RMs in scCO2, and stable and spherical RMs are formed. The dynamics process and the self-assembly structure in the RMs reveal a three-step mechanism to form the RMs, that is, small RMs, rod-like RMs and fusion of the rod-like RMs. Hydrogen-bonds between headgroups and water molecules, and salt bridges linking Na+ ions, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The results show that di-CF4 molecules have a high surfactant coverage at the RM interface, implying a high CO2-philicity. This mainly results from bending of the short chain (C-COO-CH2-(CF2)3-CF3) due to the flexible carboxyl group. The microscopic insight provided in this study is helpful in understanding surfactant self-assembly phenomena and designing new CO2-philic surfactants.

A SERS Study on the Assembly Behavior of Gold Nanoparticles at the Oil/Water Interface.

Herein, the assembly behavior of gold nanoparticles (AuNPs) at the oil/water interface is studied by surface-enhanced Raman scattering (SERS) spectroscopy. Two selected chemicals [1-dodecanethiol (DDT) and tetramethylammonium ion (TMA(+))] are applied to tune the surface properties of AuNPs and the corresponding assembly behaviors at the oil/water interface are thoroughly investigated. Various AuNPs films, namely sparse 2D film, perfect monolayer, and multilayers are obtained. The SERS spectra analyses show that the surface composition of AuNPs is strongly dependent on the chemical environment around AuNPs and results in different morphologies of AuNPs film at the oil/water interface. Accordingly, we propose a rational relationship between AuNPs assembly behavior at the oil/water interface and their surrounding chemical environment, and thus reveals the physical mechanism underlying the nanoparticle assembly.

Extraordinary deformation capacity of smallest carbohelicene springs.

The extraordinary deformation and loading capacity of nine different [∞]carbohelicene springs under uniaxial tension up to their fracture were computed using the density functional theory. The simulations comprised either the experimentally synthetized springs of hexagonal rings or the hypothetical ones that contained irregularities (defects) as, for example, pentagons replacing the hexagons. The results revealed that the presence of such defects can significantly improve mechanical properties. The maximum reversible strain varied from 78% to 222%, the maximum tensile force varied in the range of 5 nN to 7 nN and, moreover, the replacement of hexagonal rings by pentagons or heptagons significantly changed the location of double bonds in the helicenes. The fracture analysis revealed two different fracture mechanisms that could be related to the configurations of double and single bonds located at the internal atomic chain. Simulations performed with and without van der Waals interactions between intramolecular atoms showed that these interactions played an important role only in the first deformation stage.