Graphene oxide (GO) has recently emerged as a promising 2D nanomaterial to make high-performance membranes for important applications. However, the aqueous-phase separation capability of a layer-stacked GO membrane can be significantly limited by its natural tendency to swell, that is, absorb water into the GO channel and form an enlarged interlayer spacing (d-spacing). In this study, the d-spacing of a GO membrane in an aqueous environment was experimentally characterized using an integrated quartz crystal microbalance with dissipation and ellipsometry. This method can accurately quantify a d-spacing in liquid and well beyond the typical measurement limit of ∼2 nm. Molecular simulations were conducted to fundamentally understand the structure and mobility of water in the GO channel, and a theoretical model was developed to predict the d-spacing. It was found that, as a dry GO membrane was soaked in water, it initially maintained a d-spacing of 0.76 nm, and water molecules in the GO channel formed a semiordered network with a density 30% higher than that of bulk water but 20% lower than that of the rhombus-shaped water network formed in a graphene channel. The corresponding mobility of water in the GO channel was much lower than in the graphene channel, where water exhibited almost the same mobility as in the bulk. As the GO membrane remained in water, its d-spacing increased and reached 6 to 7 nm at equilibrium. In comparison, the d-spacing of a GO membrane in NaCl and NaSO solutions decreased as the ionic strength increased and was ∼2 nm at 100 mM.
Recent developments of meshfree and particle methods and their applications in applied mechanics are surveyed. Three major methodologies have been reviewed. First, smoothed particle hydrodynamics �SPH � is discussed as a representative of a non-local kernel, strong form collocation approach. Second, mesh-free Galerkin methods, which have been an active research area in recent years, are reviewed. Third, some applications of molecular dynamics �MD � in applied mechanics are discussed. The emphases of this survey are placed on simulations of finite deformations, fracture, strain localization of solids; incompressible as well as compressible flows; and applications of multiscale methods and nano-scale mechanics. This review article includes 397 references. �DOI: 10.1115/1.1431547�
This paper explores a Reproducing Kernel Particle Method (RKPM) which incorporates several attractive features. The emphasis is away from classical mesh generated elements in favour of a mesh free system which only requires a set of nodes or particles in space. Using a Gaussian function or a cubic spline function, flexible window functions are implemented to provide refinement in the solution process. It also creates the ability to analyse a specific frequency range in dynamic problems reducing the computer time required. This advantage is achieved through an increase in the critical time step when the frequency range is low and a large window is used. The stability of the window function as well as the critical time step formula are investigated to provide insight into RKPMs. The predictions of the theories are confirmed through numerical experiments by performing reconstructions of given functions and solving elastic and elasticplastic one-dimensional (1-D) bar problems for both small and large deformation as well as three 2-D large deformation non-linear elastic problems. Numerical and theoretical results show the proposed reproducing kernel interpolation functions satisfy the consistency conditions and the critical time step prediction; furthermore, the RKPM provides better stability than Smooth Particle Hydrodynamics (SPH) methods. In contrast with what has been reported in SPH literature, we do not find any tensile instability with RKPMs.
Membranes made of layer-stacked two-dimensional molybdenum disulfide (MoS) nanosheets have recently shown great promise for water filtration. At present, the reported water fluxes vary significantly, while the accountable structure and properties of MoS nanochannels are largely unknown. This paper aims to mechanistically relate the performance of MoS membranes to the size of their nanochannels in different hydration states. We discovered that fully hydrated MoS membranes retained a 1.2 nm interlayer spacing (or 0.9 nm free spacing), leading to high water permeability and moderate-to-high ionic and molecular rejection. In comparison, completely dry MoS membranes had a 0.62 nm interlayer spacing (or 0.3 nm free spacing) due to irreversible nanosheet restacking and were almost impermeable to water. Furthermore, we revealed that the interlayer spacing of MoS membranes in aqueous solution is maintained by comparable van der Waals and hydration forces, thereby ensuring the aqueous stability of MoS membranes without the need of cross-linking. In addition, we attributed the high water flux (30-250 L m h bar) of MoS membranes to the low hydraulic resistance of smooth, rigid MoS nanochannels. We also concluded that compaction of MoS membranes with a high pressure helps create a more neatly stacked nanostructure with minimum voids or looseness, leading to stable water flux and separation performance. Besides, this paper systematically compares MoS membranes with the widely studied graphene oxide membranes to highlight the uniqueness and advantages of MoS membranes for water-filtration applications.
Not all integrals of the motion, however, are of equal importance in mechanics. There are some whose constancy is of profound significance, deriving from the fundamental homogeneity and isotropy of space and time." L. D. Landau and E. M. Lifshitz, MechanicsDedicated to Professor Karl. S. Pister on the occasion of his 70th birthday.Abstract. In a previous work, the authors have presented a formalism for deriving systematically invariant, symmetric finite difference algorithms for nonlinear evolution differential equations that admit conserved quantities. This formalism is herein cast in the context of exact finite difference calculus. The algorithms obtained from the proposed formalism are shown to derive exactly from discrete scalar potential functions using finite difference calculus, in the same sense as that of the corresponding differential equation being derivable from its associated energy function (a conserved quantity). A clear ramification of this result is that the derived algorithms preserve certain discrete invariant quantities, which are the consistent counterpart of the invariant quantities in the continuous case. Results on the nonlinear stability of a class of algorithms that are derived using the proposed formalism, and that preserve energy or linear momentum, are discussed in the context of finite difference calculus. Some numerical experiments are presented to illustrate the conservation property of the proposed algorithms.
Two-dimensional (2D) materials have been incorporated into calcium silicate hydrate (C–S–H) gel to enhance its mechanical performance for decades, while the modified C–S–H gel exhibits poor toughness, tensile strength, and ductility. In this work, we report a new design strategy and synthesis route to strengthen C–S–H interface by intercalating a silicene sheet of one atom thickness. The hybrid C–S–H/Silicene gel shows superb mechanical properties, with a remarkable enhancement in strength and other functional properties. By using density functional theory (DFT) and molecular dynamics (MD) simulations, we have demonstrated that Si–O bonds between silicene and C–S–H are stable and covalent, and the interaction energy of this bilayer gel nearly doubles by forming a 3D covalent network with a strong bridging effect. Owing to its better crystallinity enrichment and its induced dislocation dissipation mechanism, the hybrid C–S–H/Silicene gel possesses a higher tensile ductility (∼118% average enhancement and ∼228% in the c direction) and a much smaller elastic stiffness (59.04 GPa for average Young’s modulus). This work offers an ingenuous route in turning brittle C–S–H gel into a soft gel, which provides opportunities for fabricating ultrahigh performance cementitious materials.
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