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Over the past decade, four distinct differences have been summarized regarding ripplocation and kink boundaries in layered solids: spontaneous reversibility, atomic sharpness, delamination, and residual stress. However, according to molecular dynamics simulations, these four distinct differences are not completely applicable to the ripplocation and kink boundaries in graphene/copper nanolaminates. Contrary to conventional understanding, ripplocation boundaries can transform into kink boundaries without triggering the collective movement of other ripplocations. In addition, ripplocation boundaries can be irreversible with out-of-plane slip, leading to coherent twins in [110] orientation and stacking faults in [111] orientation. Moreover, delamination only occurs at kink boundaries rather than ripplocation boundaries. Wide-spreading residual stresses are absent for both ripplocation and kink boundaries due to plasticity. These findings broaden the current understanding of ripplocation and kink boundaries in layered solids. [ABSTRACT FROM AUTHOR]

The presence of fractures increases the difficulty of flow mechanisms analysis, and it remains unclear how fractures affect multiphase flow displacement in the layered rock matrix. Herein, a pore-scale imbibition model considering the layered matrix-fracture system is established using the phase-field method, where oil is displaced by a range of fluids with various properties. Two typical flow modes are carefully analyzed, depending on the locations of the fracture and the interfaces between different layers of the matrix: fracture is parallel to the interface (mode I), and it penetrates through the interface (mode II), which are dominated by the co-current imbibition and countercurrent imbibition mechanisms, respectively. Interestingly, the surface tension is found to be negatively correlated with the ultimate oil recovery rate for mode I and plays an opposite effect on that of mode II. For flow mode I, the conditions of lower injection rate, higher viscosity ratio, higher grain diameter ratio, and injection of the invading fluid from the larger pore throat size (positive direction flow) can improve oil recovery. For flow mode II, the fracture bifurcation angle has little effect on the positive direction flow, while it can significantly regulate the phase distribution in the negative direction flow. Based on scaling analysis of relating pore-filling events to displacement modes and the equilibrium relationship between capillary and viscous forces, two theoretical models are derived to predict the imbibition patterns, and the variation of the flow regime under various parameters in the typical layered matrix-fracture models is systematically concluded. [ABSTRACT FROM AUTHOR]

In this work, we explored how the structure of monolayer water confined between two graphene sheets is coupled to its dynamic behavior. Our molecular dynamics simulations show that there is a remarkable interrelation between the friction of confined water with two walls and its structure under extreme confinement. When the water molecules formed a regular quadrilateral structure, the friction coefficient is dramatically reduced. Such a low-friction coefficient can be attributed to the formation of long-range ordered hydrogen bond network, which not only decreases the structure corrugation in the direction perpendicular to the walls but also promotes the collective motion of the confined water. The regular quadrilateral structure can be formed only if the number density of confined water falls within a certain range. Higher number density results in larger structure corrugations, which increases the friction, while smaller number density leads to an irregular hydrogen bond network in which the collective motion cannot play the role. We demonstrated that there are four distinct stages in the diagram of the friction coefficient vs the number density of confined water. This research clearly established the connection between the dynamic characteristics of confined monolayer water and its structure, which is beneficial to further understand the mechanism of the high-speed water flow through graphene nanocapillaries observed in recent experiments. [ABSTRACT FROM AUTHOR]

Spall damage caused by multiple shockwaves is a crucial and significant issue worth exploring. Herein, we perform molecular dynamics simulations to investigate shock damage and the recompression process in double-shock-loaded copper. Compared to the traditional spall signal, two significant velocity peaks with different time intervals between peaks are observed in the free-surface velocities under two shock loadings. The dynamic processes are then revealed via the evolution of voids and the complex propagation of waves. Meanwhile, the states of the recovered samples after shock loading are found to depend on the loading conditions. Both intact samples that undergo recompression and broken samples with separated spall scabs are observed in our simulations. Secondary spallation is observed after the recompression process, and the associated spall strength is significantly lower than that under the first shock loading. Recrystallization is observed in the spall region after secondary shock loading with a remnant of highly localized plastic deformation. The simulation results are confirmed based on the thermodynamic properties of the samples obtained via the temperature–pressure pathway. [ABSTRACT FROM AUTHOR]

Combustion of aluminum nanoparticles (AlNPs) has long been investigated experimentally because of their use in various energetic formulations for propellants and explosives. But the limited spatiotemporal resolution in experiments, in particular, makes it challenging to explore the microstructural evolution of AlNP oxidation and associated mechanisms. Here, we perform large-scale reactive molecular dynamics simulations to study the structural evolution of AlNPs with a 2–4 nm thick oxide shell in an oxygen environment. We find the temporal hollowing processes of AlNPs for both symmetrical and asymmetrical oxidations, in which the morphological evolution can be understood by a discrepant electric field and temperature distributions for different systems. In the early time, core aluminum atoms experience a fast reaction with an oxide shell. Environmental oxygen does not react with AlNPs until the surface O/Al ratio decreases to ∼1.2. Moreover, based on our simulation results, previous experimental data agree well with the proposed model, which can well describe the relationship between combustion efficiency and oxide shell thickness, confirming that the oxide shell promotes rather than hinders the combustion of AlNPs. The molecular insights obtained here would be significant for understanding the underlying mechanism and further modeling of AlNP combustion. [ABSTRACT FROM AUTHOR]

The interaction of shock-induced ejecta with gas beyond the free surface is a critical unsolved issue and being investigated broadly. Using models containing micrometer-sized gas environments, we perform molecular dynamics simulations to investigate the coupling interactions of surrounding gases with ejecta from shock-loaded tin surface. Ejected microjets experience progressively aggravated deceleration with increasing gas density, and particle flows ahead of jet tips are suppressed. Despite the drag effect, the primary fragmentation process is yet intrinsically dominated by a velocity gradient. The continuous interaction between ejecta and gas leads to the progressive formation of transmitted shock waves in background gases, which is jointly determined by ejecta velocity and thermophysical properties of gas. Meanwhile, a mixing layer between ejecta and gas is directly observed, leading to discrepant mass distributions of ejecta along shock direction. With increasing gas density, the volume density tends to rise in the mixing zone while the zone thickness decreases. Further, with the presence of gases, the size distribution of ejected particles is altered with an outstanding feature of enhanced formation of atomic particles. It is found that the stripping effect of gas dominates the growth of ejecta clusters in the transport process. The stripped particles strongly couple and flow with compressed gas, accompanied by recombination into subsequent clusters. As the gas density increases, both formation and annihilation of atomic particles are promoted. The revealed peculiarities provide microscopic views of ejecta interaction with ambient gas, which would further the understanding of gas effects on the breakup and transport of ejected particles. [ABSTRACT FROM AUTHOR]

Evaporation-driven liquid flow through nanochannels has attracted extensive attention over recent years due to its applications in mass and heat transfer as well as energy harvesting. A more comprehensive understanding is still expected to reveal the underlying mechanisms and quantitatively elucidate the transport characteristics of this phenomenon. In this study, we investigated evaporation-driven liquid flow through nanochannels using molecular dynamics simulations. The evaporation flux from the solid-liquid interface was higher than that from the middle region of the channel or the liquid-vapor interface. This finding may explain why experimental observations of evaporation flux through nanochannels exceed the limits predicted by the classical Hertz–Knudsen equation. Upon increasing the interaction strength between liquid atoms, the liquid exhibited enhanced solid-liquid interfacial evaporation and higher surface tension, albeit with reduced total flux. We also found that lyophilic channels exhibited higher evaporation fluxes than lyophobic channels, which can be interpreted by a Gibbs free energy analysis. The energy conversion analysis indicated that the effective pressure gradient exerted on a liquid flow by evaporation depends on the channel length. This was consistent with our simulations. Evaporation-driven liquid flow through nanochannels could be modeled quantitatively using this knowledge. [ABSTRACT FROM AUTHOR]

Micro-ejecta, an instability growth process, occurs at metal/vacuum or metal/gas interface when compressed shock wave releases from the free surface that contains surface defects. We present molecular dynamics (MD) simulations to investigate the ejecta production from tin surface shocked by supported and unsupported waves with pressures ranging from 8.5 to 60.8 GPa. It is found that the loading waveforms have little effect on spike velocity while remarkably affect the bubble velocity. The bubble velocity of unsupported shock loading remains nonzero constant value at late time as observed in experiments. Besides, the time evolution of ejected mass in the simulations is compared with the recently developed ejecta source model, indicating the suppressed ejection of unmelted or partial melted materials. Moreover, different reference positions are chosen to characterize the amount of ejecta under different loading waveforms. Compared with supported shock case, the ejected mass of unsupported shock case saturates at lower pressure. Through the analysis on unloading path, we find that the temperature of tin sample increases quickly from tensile stress state to zero pressure state, resulting in the melting of bulk tin under decaying shock. Thus, the unsupported wave loading exhibits a lower threshold pressure causing the solid-liquid phase transition on shock release than the supported shock loading. [ABSTRACT FROM AUTHOR]

The effect of interlayer shear on bending property of multi-layer graphene is investigated using molecular dynamics simulations. Interlayer shear modulus is three orders lower than in-plane Young's modulus, thus the bending behavior is dominated by interlayer shear. Continuum theory and atomistic simulations show that bending rigidity is proportional to layer number when the layer number exceeds 5. The small difference of these two proportional constants results from the stacking pattern, and turbostratic stacking decreases the interlayer shear modulus. For layer number smaller than 4, intrinsic ripples also play an important role and further decrease the interlayer shear modulus. [ABSTRACT FROM AUTHOR]

Transverse wave propagation in single-layer graphene sheet (SLGS) is studied via molecular dynamics simulation, continuum, and non-continuum analysis. We found that the propagation of transverse waves with frequency over 3 THz is remarkably chirality-dependent. Furthermore, the wave propagation in zigzag direction remains undistorted only when the frequency is below 16 THz, while this threshold is 10 THz in the armchair direction. The minimum permissible wavelength is proposed to explain the frequency limitation due to non-continuity. Our findings lead to an improved fundamental understanding on the vibration of graphene-based nanodevices and have potential applications in design and fabrication of nanoelectromechanical systems. [ABSTRACT FROM AUTHOR]

When water is constrained between two sheets of graphene, it becomes an intriguing monolayer solid with a square pattern due to the ultrahigh van der Waals pressure. However, the square ice phase has become a matter of debate due to the insufficient experimental interpretation and the slightly rhomboidal feature in simulated monolayer square-like structures. Here, we performed classical molecular dynamics simulations to reveal monolayer square ice in graphene nanocapillaries from the perspective of structure and dynamic characteristics. Monolayer square-like ice (instantaneous snapshot), assembled square-rhombic units with stacking faults, is a long-range ordered structure, in which the square and rhombic units are assembled in an order of alternative distribution, and the other rhombic unit forms stacking faults (polarized water chains). Spontaneous flipping of water molecules in monolayer square-like ice is intrinsic and induces transformations among different elementary units, resulting in the structural evolution of monolayer square ice in dynamics. The existence of stacking faults should be attributed to the spontaneous flipping behavior of water molecules under ambient temperature. Statistical averaging results (thermal average positions) demonstrate the inherent square characteristic of monolayer square ice. The simulated data and insight obtained here might be significant for understanding the topological structure and dynamic behavior of monolayer square ice.

The freezing and melting of low-dimensional materials, either via a first-order phase transition or without any discontinuity in thermodynamic, still remain a matter of debate. Melting (superheating) in two-dimensional (2D) ice is fundamentally different from that in bulk counterpart. Here, we perform comprehensive molecular dynamics simulations of the superheating of monolayer ice in graphene nanocapillaries to understand the nature of melting transition in 2D water/ice. We find four different superheating (melting) scenarios can happen in the superheating of monolayer square-like ice, which are closely related to the lateral pressure and the channel width. The anomalous two-stage melting transition with arisen coexistence phase is found, which reveals the unknown extraordinary characteristics of melting in 2D water/ice. Under ultrahigh lateral pressure, the intermediate monolayer triangular amorphous ice will be formed during the superheating of monolayer square-like ice with both continuous-like and first-order phase transitions. Whereas, under low lateral pressure, the melting in monolayer square-like ice manifests typical discontinuity with notable hysteresis-loop in potential energy during the heating/cooling process. Moreover, we also find that highly puckered monolayer square-like ice can transform into bilayer AB-stacked amorphous ice with square pattern in the superheating process. The superheating behavior under high lateral pressure can be partly regarded as the compression limit of superheated monolayer water. The intrinsic phenomena in our simulated superheating of monolayer ice may be significant for understanding the melting behavior in 2D water/ice.

Graphene confinement provides a new physical and mechanical environment with ultrahigh van der Waals pressure, resulting in new quasi-two-dimensional phases of few-layer ice. Polymorphic transition can occur in bilayer constrained water/ice system. Here, we perform a comprehensive study of the phase transition of AA-stacked bilayer water constrained within a graphene nanocapillary. The compression-limit and superheating-limit (phase) diagrams are obtained, based on the extensive molecular-dynamics simulations at numerous thermodynamic states. Liquid-to-solid, solid-to-solid, and solid-to-liquid-to-solid phase transitions are observed in the compression and superheating of bilayer water. Interestingly, there is a temperature threshold (∼275 K) in the compression-limit diagram, which indicates that the first-order and continuous-like phase transitions of bilayer water depend on the temperature. Two obviously different physical processes, compression and superheating, display similar structural evolution; that is, square ice-nanotube arrays (BL-VHDI) will bend first and then transform into bilayer triangular AA stacking ice (BL-AAI). The superheating limit of BL-VHDI exhibits local maxima, while that of BL-AAI increases monotonically. More importantly, from a mechanics point of view, we propose a novel mechanism of the transformation from BL-VHDI to BL-AAI, both for the compression and superheating limits. This structural transformation can be regarded as the "buckling failure" of the square-ice-nanotube columns, which is dominated by the lateral pressure.