Recent progress of simulations/modeling at the atomic level has led to a better understanding of the mechanical behaviors of graphene, which include the linear elastic modulus E, the nonlinear ...elastic modulus D, the Poisson’s ratio ν, the intrinsic strength σint and the corresponding strain εint as well as the ultimate strain εmax (the fracture strain beyond which the graphene lattice will be unstable). Due to the two-dimensional geometric characteristic, the in-plane tensile response and the free-standing indentation response of graphene are the focal points in this review. The studies are based on multiscale levels: including quantum mechanical and classical molecular dynamics simulations, and parallel continuum models. The numerical studies offer useful links between scientific research with engineering application, which may help to fulfill graphene potential applications such as nano sensors, nanotransistors, and other nanodevices.
Using molecular dynamics (MD) simulations, a new approach based on the behavior of pressurized water out of a nanopore (1.3-2.7 nm) in a flat plate is developed to calculate the relationship between ...the water surface curvature and the pressure difference across water surface. It is found that the water surface curvature is inversely proportional to the pressure difference across surface at nanoscale, and this relationship will be effective for different pore size, temperature, and even for electrolyte solutions. Based on the present results, we cannot only effectively determine the surface tension of water and the effects of temperature or electrolyte ions on the surface tension, but also show that the Young-Laplace (Y-L) equation is valid at nanoscale. In addition, the contact angle of water with the hydrophilic material can be further calculated by the relationship between the critical instable pressure of water surface (burst pressure) and nanopore size. Combining with the infiltration behavior of water into hydrophobic microchannels, the contact angle of water at nanoscale can be more accurately determined by measuring the critical pressure causing the instability of water surface, based on which the uncertainty of measuring the contact angle of water at nanoscale is highly reduced.
The working mechanism of nanoporous energy absorption system (NEAS) under high speed impact loading conditions are explored using molecular dynamics simulations, and the effects of loading rate and ...tube size are also considered. The present NEAS is composed of a single-walled carbon nanotube (CNT) segment and water molecules. The work done by the impact load can be converted into three parts: (I) water molecules potential change due to nanoconfinement, (II) solid–liquid interaction energy, and (III) the heat dissipated by the solid–liquid surface friction. We found that with a small tube size, part III provides the main contribution to the overall energy absorption; with the increase of tube size, part III rapidly decreases, and part I begins to give the main contribution. This result is different with the reported mechanism in which part II is the main contributor. The energy absorption density of NEAS is much higher than that of the conventional energy absorption materials, which decreases with the tube size and slightly increases with the impact loading rate. In addition, water molecules can transport through CNTs very fast under the high loading rate, thus NEAS can meet the requirement of a very low response time to prevent against the high speed impact load. On the basis of our simulations, NEAS can be a very promising candidate to protect against high speed loading.
The transport behavior of water molecules inside a model carbon nanotube is investigated by using nonequilibrium molecular dynamcis (NMED) simulations. The shearing stress between the nanotube wall ...and the water molecules is identified as a key factor in determining the nanofluidic properties. Due to the effect of nanoscale confinement, the effective shearing stress is not only size sensitive but also strongly dependent on the fluid flow rate. Consequently, the nominal viscosity of the confined water decreases rapidly as the tube radius is reduced or when a faster flow rate is maintained. An infiltration experiment on a nanoporous carbon is performed to qualitatively validate these findings.
Nanoindentation has been widely adopted for mechanical properties characterization of two-dimensional (2D) materials, where one typically starts with measuring the indentation load-displacement ...relationship of a selected 2D material, either free-standing or on a substrate, and then fits the result to an analytical model to extract the elastic modulus and strength of the material. However, the existing indentation models were not originally intended to be used for atomically thin materials, which has led to some controversies and confusion in the field. There is now an urgent need to develop new analytical models capable of describing the indentation response of 2D materials for accurate characterization of their mechanical properties. Here, we review recent progress in this field to identify existing issues and opportunities for future studies.
The interaction between mechanical wave initiated by impact load and nanoporous energy absorption system (NEAS) is investigated using molecular dynamics (MD) simulations, which includes the forward ...procedure (stage I) and the reflected procedure (stage II) of mechanical wave. The current NEAS is made of one-end closed single-walled carbon nanotubes (CNTs) and water. The effects of impact loading rate and tube size on the energy absorption of NEAS are also considered. After the bulk water is pushed into CNTs by the impact load, a new water interface will be created. The water molecules on interface have a much higher potential energy than their bulk counterpart, and this energy change can be defined as liquid interfacial energy, which gives the main contribution to the energy absorption of NEAS. As compared to stage I, the liquid interfacial energy density at stage II is significantly increased, especially for smaller tubes with a higher loading rate, and thus stage II can provide the significant contribution to the system energy absorption and must be considered in the energy absorption procedure. In addition, when water transports through CNTs, the solid–liquid friction based on the ven der Waals (VDW) interaction between CNTs and water is very small because the VDW repulse force can be partially canceled by the VDW attraction force, and the net force is very small; thus, the contribution of the solid–liquid friction to the energy absorption of NEAS can be neglected.
Using molecular mechanics simulations, we investigate the deformation mechanism of graphene monolayer under free standing indentation. During indentation, the van der Waals (VDW) interaction between ...the indenter tip and graphene monolayer will cause: (i) a phase lag between the indentation force P and the indentation displacement δ (i.e., δ>0 when P=0); (ii) a different strain energy function than that in in-plane tension; (iii) a larger nonlinear deformation than its counterpart in in-plane tension. Thus, the elastic properties of graphene monolayer (including the second-order elastic stiffness E and the third-order nonlinear elastic constant cmi) determined by free standing indentation are different than those determined by in-plane stretching, especially for cmi. The VDW effect rapidly decreases with the increase of the indentation load. Under a small load (i.e., in-plane strain ε⩽2.5%, typically used in the real tests), the classic indentation analysis cannot give the correct results, especially for cmi. Under a large load (e.g., ε increases to 5%), the error caused by the classic indentation analysis can be effectively reduced. Therefore, the deformation mechanism must be understood in order to accurately determine the elastic properties of graphene from free standing indentation.
Most previous studies on nanofluidic motions were focused on liquid−solid interactions, with the important role of gas phase being ignored. Through a molecular dynamics simulation, we show that the ...gas−liquid interaction can be an indispensable factor in nanoenvironments. Gas molecules in relatively large nanochannels can be dissolved in the liquid during pressure-induced infiltration, leading to the phenomenon of “nonoutflow”. By contrast, gas molecules tend to form clusters in relatively small nanochannels, which triggers liquid defiltration at a reduced pressure. The results qualitatively fit with the observations in a high-pressure-resting experiment on nanoporous silica gels.
Using molecular mechanics simulations we investigate the elastic properties of monolayer graphene determined from free standing indentation and the effects of graphene size and indenter tip size are ...considered. In free standing indentation, the overall system potential energy includes two parts: the strain energy of graphene monolayer and the van der Waals (VDW) interaction energy between indenter tip and graphene. The VDW interaction will strongly affect the elastic properties of graphene monolayer determined from free standing indentation. Without considering the VDW interaction, the classical free standing indentation analysis is not able to accurately estimate the second-order elastic stiffness (E) and the third-order nonlinear elastic constant (cm) (E will be underestimated and cm will be overestimated). It is found that both E and cm determined from free standing indentation are quite close to their counterparts determined from in-plane tension after properly considering the VDW interaction. The results can provide a useful guideline to understand the elastic properties of graphene monolayer determined from free standing indentation tests.
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