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In this paper, we perform molecular dynamics simulations to investigate the performance of multilayer graphene slit membranes. Graphene slit membranes at a critical slit size have been found to be promising desalination membranes. In this contribution, it is shown that multilayer slit membranes have the potential to provide significantly better permeability while retaining outstanding salt rejection. Improved permeability of the membrane is achieved by using slits of widths larger than the critical slit size required to reject salt through size exclusion, and desalination of sea water is performed by increased resistance to salt passage through the multilayering. To facilitate the design process of future multilayer membranes, we analyze the flow resistance of the membrane as a combination of electrical resistors in series and show that this analogy works for membranes where the layers possess the same slit size, as well as membranes with layers of different slit sizes. Comparing with single layer graphene membranes, it was shown that it is possible to obtain 55% improvement in permeability without loss in salt rejection capabilities through multilayering. This opens up possibilities for membrane designers to be free from the restrictions of using a single layer graphene slit membrane with a fixed slit width.

It has been shown that the nanoplastic particles present in graphene membranes have a high tendency to cause fouling in them due to the high affinity between graphene and nanoplastic molecules. This poses a significant challenge for the use of graphene membranes for desalination. In this paper, we introduce a double-layer graphene slit membrane as a viable solution to significantly reduce fouling caused by the presence of nanoplastic particles in graphene membranes. The molecular dynamics (MD) simulations performed in this work show that when fouling occurs in a single-layer membrane, the presence of nanoplastics reduces the average permeability by close to 40%, from 1877 LMBH to 1148 LMBH, with a large standard deviation of 26% between runs. With the addition of the secondary membrane, the average permeability increases by 17%, with a significantly reduced standard deviation of 7%. These suggest that the secondary layer acts as a sacrificial shield, attracting the nanoplastic contaminants and preventing them from coming into close proximity with the primary membrane, thus preventing fouling at the primary rejection layer. Furthermore, due to the affinity of the nanoplastic particles with the secondary graphene membrane, this membrane design points toward an effective and efficient way of extracting nanoplastic particles for further analysis or processing.

A key to sustainable management of hypersaline organic-rich wastewaters is to precisely fractionate organic components and inorganic salts (NaCl) as individual resources. Conventional nanofiltration and electrodialysis processes suffer from membrane fouling and compromise the fractionation efficacy. Here, we develop a thin-film composite nanoporous membrane (NPM) via co-deposition of dopamine and polyethyleneimine as a highly anion-conducting membrane (ACM). Experimental results and molecular dynamics simulations show that co-deposition of dopamine and polyethyleneimine effectively tailors the membrane surface properties, intensifying the charge shielding effect and enabling ultrafast anion transfer for highly efficient electrodialysis. The resulting NPM exhibits unprecedented electrodialytic fractionation of organics and NaCl (> 99.3% desalination efficiency; >99.1% organics recovery) with negligible membrane fouling, dramatically outperforming state-of-the-art anion exchange membranes. Our study sheds light on facile design of high-performance ACMs and associated new mass transport mechanisms in electrodialytic separation, paving the way for sustainable management of complex waste streams.

Dynamic characterizations of fractional vibration systems have recently attracted significant research interest. Increasingly, successful applications of fractional derivatives have been found to the modeling of mechanical damping, vibration transmissions, improved fractional vibration controls and nonlinear vibration analyses. To facilitate further development, the eigenvalue problem including its derivatives, which are the central issues of vibration analysis, have to be fully established. This paper examines how eigenvalue and eigenvector derivatives of fractional systems can be derived when system matrices become functions of physical design parameters. First, new important orthonormal constraints are proposed since the modes are no longer orthonormal to the mass matrix, in this case due to its complex and frequency dependent nature. Next, new methods of eigenvector derivatives are developed for distinct eigenvalues for the cases of complete, incomplete and single mode modal data. Realistic and practical FE models incorporating fractional derivatives in the form of viscoelastic supports are employed to demonstrate the numerical accuracy and computational efficiency of the proposed methods. However, when repeated eigenvalues are considered due to structural spatial symmetries, the eigenvector space degenerates and further differentiation of system matrices are required in order to uniquely determine the eigenvector derivatives. Consequently, a new and effective general method is developed which can be applied to compute eigenvector derivatives of repeated eigenvalues with any multiplicity m. A simplified turbine bladed disk vibration model which is known to have repeated eigenvalues due to its cyclic symmetry, is then used to demonstrate the accuracy and salient features of the proposed method.

Possessing exceptional properties, graphene has garnered immense interest in the research community for a wide array of potential applications. The mechanical properties play an important role in the success of the potential applications and thus a thorough understanding is cardinal. Computational modeling and simulation is an important tool in the design process due to the low costs compared to the experimental means. This review aims to consolidate the findings of works of computational modeling and simulation of graphene, with focus on models at the continuum and molecular levels. The review shows intrinsic differences in focus of the applications and types of graphene investigated by the different scale models, thus highlighting the advantages and shortcomings of each type of modeling approach.

Due to the incorporation of magnetic nanoparticles (MNPs), magnetically tuneable hydrogels have attracted considerable attention recently due to their ability to undergo remotely controlled large deformation. This work investigates the mechanics of the large deformation from the thermodynamics perspective for magneto-thermal sensitive hydrogels. The chemical thermodynamics of a temperature sensitive gel is first recapped before moving on to the thermodynamics of magnetism. Furthermore, an explicit energy form for the magneto-thermal sensitive hydrogel is adopted. The proposed field theory is implemented in a finite element method through the UHYPER subroutine. The finite element simulation results have been validated with analytical solutions at various temperatures and magnetic field strengths for MNPs entrapped PNIPAM hydrogel. We also utilize the numerical models to explain the interesting phenomena, including micro valves, bifurcation, and the opening of gel capsule for drug release delivery. The numerical deformation pattern for bifurcation is consistent with the experimental pattern, thus illustrating our theory and numerical method can provide future perspectives for device design of magneto-thermal sensitive hydrogel.

Fractional derivative is increasingly being deployed to improve existing mathematical models due to its unique capability in describing anomalous behavior and memory effects which are common characteristics of natural phenomena. Improved vibration analysis has been accomplished by introducing fractional derivatives to model viscoelastic damping and vibration propagation through complex media and much research has been carried out to date. However, much of these existing research efforts have been sporadic to the best and there remains a pressing need to develop a consistent and systematic theoretical framework for vibration analysis of fractional systems to synergize for more productive and coordinated efforts in the area. This paper seeks to address some fundamental issues to facilitate further development such as the definition of general form of a fractional vibration system, its eigenvalue problem and methods of solution, definition of frequency response functions and applicability of conventional modal analysis, equivalent eigensystem and its more efficient eigensolution. With these important issues being resolved to clear the myth, vibration studies of fractional systems can be encouraged and expected to grow in a more fruitful direction. New methods developed and concepts discussed in the paper have all been validated through realistic numerical case studies based on a practical GARTEUR structure with viscoelastic supports modeled using fractional derivatives.

There is a pressing need in very small scale three‐dimensional (3D) inkjet printing to control and reduce agglomeration, as agglomeration often leads to nozzle clogging. While agglomeration within ultraviolet ink has been studied, there has been, to our knowledge, no extensive studies conducted for surface agglomeration of the ink on nozzle's wall. This numerical study therefore focuses on investigating if surfactants can effectively control surface agglomeration during nanodroplet formation. Many‐body dissipative particle dynamics is the numerical method of choice here. We found that small amount of surfactant of about 1 wt % is sufficient to effectively reduce ink deposition on the nozzle's wall. However, by using the properties of a commercially available surfactant, sodium dodecyl sulfate, it was found that the maximum reduction achieved by its addition is only 60%. Thus, further physical or chemical deagglomeration techniques are required, and we show that by considering these other techniques, reduction of surface agglomeration to nearly 92% can be achieved. Finally, we found that adding surfactants has the additional benefit of improving total kinetic energy of the ink compositions, lowering possibility of agglomerations within the ink. It also raises the nanodroplet velocity while reducing nanodroplet breakup time, which can help speed up the process of 3D printing process. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1615–1624

We investigate the effect of varying carbon nanotube (CNT) size on the desalination performance through slit confinements formed by horizontally aligned CNTs stacked on top of one another. By increasing the CNT size, the results obtained from this study indicate a corresponding increase in the water flow rate, accompanied by a slight reduction in salt rejection performance. However, due to the increase in the membrane area with CNT size, the permeability performance is observed to reduce as the CNT size increases. Nevertheless, a comparison with nanoporous 2D membranes shows that the permeability of an outer-wall CNT slit membrane remains significantly higher for all CNT sizes considered. This indicates that precise dimensions of the CNTs are not highly crucial for achieving ultra-high permeability performance in such membranes, as long as the critical slit size is maintained. In-depth analytical studies were further conducted to correlate the influence of curvature effects due to increasing CNT size on the flow characteristcis of the outer-wall CNT membrane. These include the analysis of the measured velocity profiles, oxygen density mapping, potential of mean force profile and friction profile. The present numerical results demonstrate the superb desalination performance of the outer-wall CNT slit membrane, regardless of the size of CNTs used. In addition, an extensive analysis conducted provides detailed characterization of how the curvature affects flow across outer-wall CNTs, and can be used to guide future design and fabrication for experimental testing.

Repeated vibration modes often occur in practice in structural vibration systems with general nonproportional viscous damping. However, the topic remains perhaps the least understood in vibration analysis. Some researchers have suggested that the inclusion of nonproportional viscous damping renders a system defective in the case of repeated eigenvalues, while others have assumed that a complete set of linearly independent eigenvectors can always be found regardless of the forms and magnitudes of viscous damping. This paper seeks to first establish that for light non-proportional viscous damping, a system with repeated eigenvalues generally does not become defective based on perturbation theory and realistic numerical examples since rigorous theoretical proof is believed to be difficult. Once non-defectiveness is confirmed, a method for computing the eigen derivatives with repeated eigenvalues in the case of general viscous damping is developed. Effect of mode truncation on numerical accuracy has been discussed. When the magnitude of non-proportional viscous damping becomes considerably high however, it is possible for a damped system to become defective when repeated modes occur. This can have profound practical implications where very high damping is desired as in the cases of vibration suspension and absorber designs, since system defectiveness can lead to major difficulties in the applications of conventional vibration analyses.

Using molecular dynamics (MD) simulations, we investigate the strength and fracture behavior of biphenylene based graphene (BG) monolayer subjected to uniaxial tensile deformation along the x- and y-directions. AIREBO potential is used for modeling the C-C atom interactions, and the simulations are carried out under isothermal conditions at a temperature of 300 K and strain rate of 10 10 s −1 using NVT ensemble. Defects such as cracks and voids are introduced in the monolayer to study their influence on the strength and mechanism of crack propagation. From the results, we conclude that monolayer strength and fracture behavior are dependent on the loading direction. Fracture in the monolayer is observed only when loaded along the x-direction and has comparatively lower strength. Further, the crack propagates by bond breaking along the four-membered rings in the biphenylene unit. As anticipated, the strength of the monolayer decreases in the presence of defects and the crack speed is estimated to be 7 × 10 3 m/s.

The complete mechanical properties (tensile, compressive, and shear properties) of the additively manufactured (AM) continuous carbon fiber-reinforced thermoplastic (CFRTP) fabricated using extrusion-based AM technique were investigated and reported. The fracture modes of the AM CFRTP in various mechanical tests were studied and reported. Anisotropic mechanical properties were observed in all mechanical tests, with the fiber direction having the highest strengths and stiffnesses and the across-the-layer direction having the lowest strengths and stiffnesses. A proof of concept topology optimized unmanned aerial vehicle (UAV) landing gear was designed and fabricated using the mechanical properties obtained experimentally. Finite element analysis and compressive tests conducted show that the UAV landing gear structure fabricated using the AM CFRTP was able to survive the most extreme condition during operation. National Research Foundation (NRF) Accepted version The authors would like to express their appreciation to Nanyang Technological University (NTU) High Performance Computing Centre (HPCC) for providing all the necessary supercomputing resources and support. This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme.

Ultraviolet (UV) ink is a major ink type used in additive manufacturing via 3D inkjet printing. A major challenge in nanoinkjet printing is ink agglomeration. Among the UV ink components, oligomers have the highest tendency to agglomerate which can agitate the stability and quality of the printing fluid and possibly lead to nanoscale nozzle clogging. In this work, the first numerical study on the UV ink fluid, UV ink is modeled by using dissipative particle dynamics to study mesoscale agglomeration. The constituents of the ink model are composed of polystyrene and polyethylene glycol as photopolymers, BZP as a photoinitiator, and SDS as a surfactant. Styrene is a prevalent and established commercial photopolymer in present 3D inkjet applications, while ethylene glycol is a photopolymer known to improve ink viscosity. The morphological characteristics of the UV ink are studied here, where the results for different models from four cases considered here show how the kind of photopolymers and their constituent ratios affect the agglomeration morphology of the fluidic system. The existence of both oligomers and monomers results in mutual morphological benefits against agglomeration, while the photoinitiator occurs between photopolymers. In addition, we find that the surfactant can reduce the average size of agglomeration and improve the dispersion uniformity by increasing the number of agglomerates. These results highlight the important role additives can play to prevent, reduce, and control various forms of agglomeration to achieve enhanced nanoinkjet printing quality. Copyright © 2017 John Wiley & Sons, Ltd.

In this work, the dynamic response of a propane-burnt (C3H8) jet diffusion flame in a longitudinal tube to acoustic waves produced from a loudspeaker are studied. For this, 2-D numerical simulations are conducted by using FLUENT to investigate the interaction of acoustics-flow-flame. Unsteady RANS simulations with one-step Eddy-Dissipation (ED) combustion model are used in the model. And acoustic fluctuations are generated by using User Defined Functions (UDF). The numerical model is validated first by comparing the numerical results with the experimental measurements in the absence of a flame. Further validation is performed by comparing with flame-involved experimental results available in the literature. It is shown that the jet and the flame characteristics are highly sensitive to its axial location, especially when standing waves are present in the tube. The jet experiences large velocity fluctuations. Flow reversal is observed, when the jet is placed at acoustic velocity antinodes. However, the jet and flame are much stable in the velocity node region. Due to the large-amplitude acoustic disturbances, an interesting unsteady mushroom-shaped flame is observed. The numerical model is then used to determine the flame transfer function (FTF), which is important to characterize the flame-acoustic coupling behaviors. It is shown that the flame transfer function is nonlinear. Furthermore, it depends strongly on not only the amplitude of the acoustic disturbances but also the frequency. Such strong responses of the flame to acoustic waves are due to the ‘resonance’ effect of the tube. The present work opens up new applicable way to model and characterize the flame-acoustics-flow interaction via flame transfer function.

This study considers two novel ideas to further explore and enhance the graphene membrane for desalination. Firstly, while earlier molecular dynamics (MD) simulations studies have used frozen membranes, free-standing membrane is considered here. Since 2D membranes are usually embedded on porous support in the experimental reverse osmosis (RO) process, the free-standing membrane can more accurately model the behavior expected during operation. This study showed, using MD simulations, that a free-standing nanoporous graphene membrane can provide a higher salt rejection, but lower water permeability as compared to frozen membrane. Secondly, the performance of a slit membrane as compared to a membrane with circular pore is studied. At a pressure of 268 MPa, the critical diameter of circular pore that can maintain perfect salt rejection is found to be 5 A and the critical size for a slit is determined to be 2.28 A. It is shown that a slit membrane at its critical size can achieve water flux 3.5 times higher than a membrane with circular pore at its critical radius. This finding highlights the importance of slits over circular pores which can potentially widen the options for the design and fabrication of 2D graphene membranes for experimental verification of RO.

HighlightsA novel re-formulation of quadrotor dynamics leads to a cascaded form that allows the use of a backstepping controller and results in a more disturbance tolerant control as the use of algebraic inversion is avoided.Partial contraction is used to derive the backstepping control that guarantees stability and convergence.It is shown conclusively that the full state backstepping controller is more superior than the inverse kinematics-based backstepping controller in terms of unmodeled effects rejection.The backstepping controller formulated by the contraction theory in this paper is generic and can be applied to any dynamical systems satisfying the cascaded structure.Extensive simulation studies have been carried out to include aerodynamic effects and wind disturbances, and the proposed controller is verified to perform better than the benchmark controller. In this paper, a tracking controller is formulated for a quadrotor to track a moving ground target. The quadrotor exhibits distinct hierarchical dynamics that allows its position to be controlled by its attitude. This motivates the use of backstepping control on the underactuated quadrotor. Most backstepping architecture controls the quadrotor position and attitude independently, and couples them with inverse kinematics. Inverse kinematics computes the attitude angles required to achieve a desired acceleration. However unmodeled effects are shown to cause inexact inversion resulting in tracking error. The approach proposed in this paper uses a re-formulated full state cascaded dynamics to eliminate the need for inverse kinematics in a full state backstepping control architecture. It is shown that zero steady state error is achieved in the presence of unmodeled aerodynamics effect and wind disturbance despite no integral action. In addition, a backstepping formulation is derived using contraction theory that guarantees the boundedness of state response under bounded disturbances such as wind. This improves the system performance. Numerical simulations are performed using the proposed controller to track a target moving along predefined paths and the results are compared with a benchmark controller derived using inverse kinematics. The results show that the proposed controller is able to achieve better tracking performance under unmodeled aerodynamic effects and wind disturbance as compared with the benchmark controller.

The present work investigates the deformation characteristics of a temperature sensitive hydrogel. The state of mechanical and chemical equilibrium is first formulated via a variational approach, upon which a finite element model is developed and implemented through user-defined material subroutine UMAT in the commercial software ABAQUS. We will show that this UMAT implementation allows for more versatility in the imposition of initial conditions over existing models developed using UHYPER subroutine. Furthermore, we propose an approach to simulate the transient swelling process of a temperature sensitive hydrogel. This is achieved through the simultaneous application of three user-defined subroutines, which model the constitutive properties of the gel, as well as the diffusion of solvent molecules within the gel. Several numerical case studies are presented to verify the present model developed with experimental data, as well as to illustrate its capabilities in simulating a wide array of complex gel phenomena, including surface creasing, bifurcation and buckling of gels. Through these numerical examples, we are able to gain deeper insights, and explain some of the new interesting physical phenomena observed in reported experiments.

In this paper, comparisons of the adhesive strengths of two commercially available adhesives, epoxy and cyanoacrylate, on 3D printed plastic materials, Acrylonitrile Styrene Acrylate (ASA) and Nylon 12 Carbon Fiber (NCF) were carried out. The single lap shear test is used to determine the adhesive properties of the specimens with and without post-curing at elevated temperature. A comparison is made with fully printed, non-bonded specimens to give a relative gauge of the performance of the adhesives. It was found that for ASA and NCF, the adhesive strength for cyanoacrylate (CA) is much higher than that of epoxy. ASA and NCF bonded with CA had average failure load of 1810 kN and 2310 kN, respectively, as compared to those bonded with epoxy which had significantly lower failure load of 470 kN and 860 kN, respectively. It was observed that although heat treatment and surface treatment improve the adhesive strength of epoxy with both adherend materials, the improved adhesive strength of epoxy is still observed to be significantly weaker than that of CA. Accepted version

Hydrogels and shape memory polymers (SMPs) possess excellent and interesting properties that may be harnessed for future applications. However, this is not achievable if their mechanical behaviors are not well understood. This paper aims to discuss recent advances of the constitutive models of hydrogels and SMPs, in particular the theories associated with their deformations. On the one hand, constitutive models of six main types of hydrogels are introduced, the categorization of which is defined by the type of stimulus. On the other hand, constitutive models of thermal-induced SMPs are discussed and classified into three main categories, namely, rheological models; phase transition models; and models combining viscoelasticity and phase transition, respectively. Another feature in this paper is a summary of the common hyperelastic models, which can be potentially developed into the constitutive models of hydrogels and SMPs. In addition, the main advantages and disadvantages of these constitutive modes are discussed. In order to provide a compass for researchers involved in the study of mechanics of soft materials, some research gaps and new research directions for hydrogels and SMPs constitutive modes are presented. We hope that this paper can serve as a reference for future hydrogel and SMP studies.

The widespread use of low dimensional carbon membrane for desalination and gas separation is limited by the difficulty to physically realise such membrane designs on a meaningful scale. This review aims to bring together results achieved in this field, hoping to inspire new designs or developments that could bridge this technical challenge. The focus of this paper is on sub-nanometer separation operations such as desalination or gas separation. This is because such operations consume the most energy, and there is thus much interest to reduce this cost. Three groups of low dimensional carbon materials are considered: graphene, carbon nanotubes (CNT) and graphene oxide (GO). Graphene and CNT membranes have the advantage of high permeability but are difficult to manipulate to form membranes that separate efficiently. GO, on the other hand, has the advantage of ease of fabrication but suffers in terms of separation performance. This review dives deep into the innovative ideas proposed for these low dimensional carbon membrane design, deliberating their strengths and weaknesses, in a consolidated effort to generate new ideas for further advancements.

The use of two-dimensional (2D) materials as ultra-thin desalination membrane has garnered much interest in the scientific community. Various 2D materials membrane designs have been tested both numerically and experimentally. However, a detailed direct comparison between different 2D materials membrane is lacking. This study uses molecular dynamics (MD) to characterize and compare the desalination performance of graphene, borophene, molybdenum disulphide (MoS2) and MXene slit membranes. It was found that monoelemental graphene and borophene have the best desalination performance. They achieve 100% monovalent salt rejection with permeability 25% higher than MoS2 membranes and 87% higher than MXene membranes. Three factors are found to influence the desalination characteristics. Firstly, the hydrophobicity of the material which is found to increase salt rejection but reduce permeability. Second the material's inherent interaction parameters, and third the shape of the slits formed by atomic arrangement of the 2D material. Furthermore, although desalination performance varies across 2D materials membrane, all the membranes tested have permeability two orders of magnitude higher than current desalination membrane. All in all, this study highlights the varying characteristics of membrane made of different 2D materials and relate the material's properties with the resulting desalination performance.

Undergoing large volumetric changes upon incremental environmental stimulation, hydrogels are interesting materials which hold immense potentials for utilization in a wide array of applications in diverse industries. Owing to the large magnitudes of deformation it undergoes, swelling induced instability is a commonly observed sight in all types of gels. In this work, we investigate the instability of photo-thermal sensitive hydrogels, produced by impregnating light absorbing nano-particles into the polymer network of a temperature sensitive hydrogel, such as PNIPAM. Earlier works have shown that by using lights of different intensities, these hydrogels follow different swelling trends. We investigate the possibility of utilizing this fact for remote switching applications. The analysis is built on a thermodynamic framework of inhomogeneous large deformation of hydrogels and implemented via commercial finite element software, ABAQUS. Various examples of swelling induced instabilities, and its corresponding dependence on light intensity, will be investigated. We show that the instabilities that arise have their morphologies dependent on the light intensity.

Graphene oxide quantum dots (GO-QDs) have distinct optoelectronic properties for their application in bio-imaging, drug delivery and photovoltaics. Herein, the effect of OH functionalisation on the optical properties of GO-QDs is studied based on state-of-the-art theoretical simulations. Our calculations predict the effect of OH groups on ionisation potentials, light absorption and emission properties. The mechanism of fluorescence is analysed considering the role of geometry distortion and charge transfer. Moreover, selective functionalisation of positions with large electron–hole separation offers a strategy to tune the optical gap and photoluminescence properties. These results open up new opportunities for the design of GO-QDs for a wide range of applications.

The study of crystal structures in shape memory alloys is of fundamental importance for understanding the shape memory effect. In order to investigate the mechanism of how Cu content affects martensite crystal structures of TiNiCu alloys, the present research examines the atomic displacement of Ti50Ni50−xCux (x = 0, 5, 12.5, 15, 18.75, 20, 25) shape memory alloys using density functional theory (DFT). By the introduction of Cu atoms into TiNi martensite crystal to replace Ni, the displacements of Ti and Ni/Cu atoms along the x-axis are obvious, but they are minimal along the y- and z-axes. It is found that along the x-axis, the two Ti atoms in the unit cell move in opposite directions, and the same occurred with the two Ni/Cu atoms. With increasing Cu content, the distance between the two Ni/Cu atoms increases while the Ti atoms draw closer along the x-axis, leading to a rotation of the (100) plane, which is responsible for the decrease in the monoclinic angle. It is also found that the displacements of both Ti atoms and Ni/Cu atoms along the x-axis are progressive, which results in a gradual change of monoclinic angle and a transition to B19 martensite crystal structure. Published version

The inability of membranes to handle a wide spectrum of pollutants is an important unsolved problem for water treatment. Here we demonstrate water desalination via a membrane distillation process using a graphene membrane where water permeation is enabled by nanochannels of multilayer, mismatched, partially overlapping graphene grains. Graphene films derived from renewable oil exhibit significantly superior retention of water vapour flux and salt rejection rates, and a superior antifouling capability under a mixture of saline water containing contaminants such as oils and surfactants, compared to commercial distillation membranes. Moreover, real-world applicability of our membrane is demonstrated by processing sea water from Sydney Harbour over 72 h with macroscale membrane size of 4 cm2, processing ~0.5 L per day. Numerical simulations show that the channels between the mismatched grains serve as an effective water permeation route. Our research will pave the way for large-scale graphene-based antifouling membranes for diverse water treatment applications., Intrinsic limitations of nanoporous graphene limit its applications in water treatment. Here the authors produce post-treatment-free, low-cost graphene-based membranes from renewable biomass and demonstrate their high water permeance and antifouling properties using real seawater.

In this paper, we perform molecular dynamics simulations to propose a novel bio-inspired nanopumping mechanism that is achieved through the rotation of graphene nanoribbons. Due to the rotation and interaction with water, the graphene nanoribbons undergo morphological transformation. It is shown that with appropriate geometrical and spatial parameters, the resulting morphology is twisted ribbon, which is efficient in pumping of water through a channel. This mimics the propulsive behavior of bacterial flagella through continual rotation at the base and causing morphology of the geometry into twisted ribbons, thus driving flow. It was observed that the maximum flux rate decreases upon reaching the optimal configuration even with increasing rotational speed and graphene width. This is due to the development of cavitation near the region of the nanoribbon with tip velocities approaching the speed of sound in water. The simulation shows promising results where the flux rate of the driven flow outperforms various nanopump configurations that have been reported in recent literature by more than one order.

Both elastic and fracture behavior of hydrogel are affected by its water content. As shown by extensive experimental data, currently prevalent models, which are primarily based on the Flory–Rehner theory (F–R theory), are unable to correctly capture the effect of water content (or conversely polymer fraction) on the elastic modulus of hydrogels. Lake–Thomas theory cannot provide correct predictions on fracture toughness with different water content conditions as well. In this work, we carry out experiments on polyacrylamide (PAAm) gel and discover scaling-laws that differ significantly in the swollen and dehydrated state in addition to contradicting F–R model. We also derive scaling laws that are consistent with our experiments. Intriguingly, we find that the application of the scaling theory to fracture problems of the hydrogel can also provide a better theoretical prediction. An intriguing implication of this result is that the study of the fracture threshold of soft matter may be replaced to some extent by merely the studying of their elastic modulus.

The present research has investigated the martensite crystal structures and electronic structures of Ti50Ni50−xCux (x = 0, 5, 12.5, 15, 18.75, 20, 25) shape memory alloys using density functional theory (DFT). The computational results are compared with the reported data and it is found that the equilibrium lattice constants are in good agreement with reported values. It is also found that with Cu addition to NiTi, the lattice parameters (a and c) and the monoclinic angle decrease, whereas the lattice parameter b increases. With increasing Cu content, fewer electrons were transferred from Ti to Ni in comparison with that in binary NiTi alloys, and the NiTi monoclinic structure becomes unstable. When Cu content is increased to around 20 at%, an orthorhombic crystal structure is formed which agrees well with reported experimental observations.

Polymeric gels can undergo large deformation when subjected to external solutions of varying pH. It is imperative to understand the deformation process of pH-sensitive hydrogels for the effective application of these attractive materials in the biomedical and microfluidic fields. In the modeling of these multi-phase materials, finite element (FE) modeling is a useful tool for the development of future applications, and it allows developers to test a wide variety of material responses in a cost-effective and efficient manner, reducing the need to conduct extensive laboratory experiments. Although a FE user-defined material model is available for the equilibrium state, the transient response of pH-sensitive gels has not been effectively modeled. Based on our recent work using the heat transfer analogy to tap into the readily available coupled temperature–displacement elements available in the commercial FE software ABAQUS for simulation of the transient swelling process of neutral hydrogels, the transient swelling process of a pH-sensitive hydrogel is studied and a FE model is further developed to simulate the transient phenomena. Some benchmark examples are investigated to demonstrate the model's capabilities in the simulation of nonlinear deformation kinetics relevant to several applications of pH-sensitive hydrogels. © 2013 Society of Chemical Industry

Classical non-equilibrium molecular dynamics is employed to model short-strips of single-layered materials consisting of either carbon (graphene) or silicon (silicene) atoms. Both materials are modeled using their respective parameterizations of the Tersoff potential, and their thermal conductivities are then determined through non-equilibrium molecular dynamics. The present results indicate that both materials experienced increasing thermal conductivities as length increased, and graphene had far more rapid increases than silicene. Both armchair and zigzag chiralities in silicene has significant differences in thermal conductivities but not in graphene. Graphene possesses significantly higher thermal conductivities than silicene at every length scale and chirality, and this is found to be attributed to the fewer excitable phonon frequencies, as shown through the vibrational density of states.

The transient swelling process of polymeric gels is studied and a finite element model is introduced to simulate the phenomena based on existing hyperelastic theory for inhomogeneous swelling of gels and analogies made to transient heat conduction analysis. Examples of free swelling of a cube and a fixed cylinder were investigated, showing the highly non-linear deformation present during transition from the initial state to equilibrium.

In this study, classical molecular dynamics with the well-known van Beest, Kramer and van Santen potential are used for the first time to investigate the solid thermal conductivity of silica aerogel. Aerogel samples at various densities are obtained through negative pressure rupturing of dense silica samples, and reverse non-equilibrium molecular dynamics is employed to determine the thermal conductivity at each density. Results indicate that a power-law fit of the thermal conductivity obtained varies almost linearly with density, where decreasing density and increasing porosity led to an almost linear decrease in thermal conductivity. This is reflective of the trend observed in experimental bulk sintered silica aerogel. The results also showed that the thermal conductivity is of the same order of magnitude as bulk sintered aerogel. The power-law fit of the results also accurately reflected the variation found in bulk sintered aerogel.

In the present work, we present a systematic analysis of the chemisorption process pathway of hydrogen atoms onto the exterior wall of (5,5) carbon nanotubes using the ONIOM2 (B3LYP(6–31+G(d,p):UFF)) scheme, and we avoid the gross assumption of fixing any of the carbon atoms during the simulation. It is shown that the adsorption of hydrogen atoms onto the sidewall of CNTs are energetically favorable and the most stable state is to form two H–C σ-bonds while the original σ-bond between the carbon atoms is totally severed. In particular, we examined the molecular thermodynamics properties for the reaction at a range of temperatures from 77 K to 1000 K, and the results suggests that the reaction is possible at ambient temperature, but it is less favorable than that at lower temperatures.

Computer simulations and in particular mesoscopic simulation techniques such as the dissipative particle dynamics (DPD) technique, enable researchers to study the complexities of soft material and polymeric systems by performing in silico experimentations alongside in vivo experiments. In addition, these mesoscopic simulations allow scientists and engineers to characterize and optimize the actual experiments in a more efficient manner. The DPD is one the most reliable mesoscopic simulation techniques for phenomenological investigation of soft matter and polymeric systems. In this review, which is complimentary to an earlier review also by the present authors on DPD methodology and complex fluid application (Moeendarbary et al., 2009), we categorize and review the notable published works, and document efforts that applied the DPD simulation technique to various important soft matter and polymeric applications, over the last decade.

A chemo-electro-mechanical formulation, referred to as the multi-effect-coupling pH-stimulus (MECpH) model, is presented in this paper for the analysis of the effects of the initial geometrical size on the responsive behavior of pH-sensitive hydrogels subject to the coupled stimuli of environmental solution pH and externally applied electric voltage. The model is composed of coupled nonlinear partial differential equations, and it accounts for the diffusion of ionic species, distributive electric potential and large mechanical deformation. In addition, the correlation between the diffusive hydrogen ion within the hydrogel and charge groups fixed to the polymeric network chains is incorporated quantitatively into this MECpH model. For the simulation of the response characteristics of the smart hydrogel, we solve the one-dimensional steady-state problem using the Hermite–Cloud meshless technique. For the MECpH model, the present numerical simulations were compared with experimental data available from literature to validate the accuracy and robustness of the model, and good agreement was observed. Several parameter studies were then carried out in the analysis of the hydrogel swelling when immersed in solution, and it was observed that the initial geometrical size has significant influence on the volume variations of these pH-responsive hydrogels.

The dissipative particle dynamic (DPD) method is applied to simulate the DNA separation process based on a mechanism developed earlier by other authors, who demonstrated a micro-device for separating DNA molecules (>2 kbp) through an entropic trapping mechanism. The DPD is used to model the process because it is intrinsic to modeling hydrodynamic interactions, which is lacking in some earlier works. We use the worm-like chain model to represent DNA molecule and Lennard–Jones potential at mesoscopic level to avoid phantom collision between the chain beads. Our simulations show that longer DNA strands do move faster than shorter ones, as observed in available experimental data. We also confirm that the delayed entrance is the cause of entropic trapping. Contrary to some earlier reported data, we found that the corner trapping is not a contributor to DNA separation.

The dissipative particle dynamics (DPD) technique is a relatively new mesoscale technique which was initially developed to simulate hydrodynamic behavior in mesoscopic complex fluids. It is essentially a particle technique in which molecules are clustered into the said particles, and this coarse graining is a very important aspect of the DPD as it allows significant computational speed-up. This increased computational efficiency, coupled with the recent advent of high performance computing, has subsequently enabled researchers to numerically study a host of complex fluid applications at a refined level. In this review, we trace the developments of various important aspects of the DPD methodology since it was first proposed in the in the early 1990's. In addition, we review notable published works which employed DPD simulation for complex fluid applications.

A comprehensive three-dimensional nonlinear finite element analyses were carried out to examine the effects of the geometry and the incidence angle of canonical 4-lb bird striking a stationary fan blade using LS-DYNA. Three aspects of the work were accordingly examined. The first was concerned with the modeling framework used. The second with the constitutive law that governs the behaviour of the bird. The third was concerned with the influence of the geometry and the angle of incidence of the bird on the integrity of the blade. This article combines and extends the earlier works on both the Lagrangian modeling methodology (Mao et al. AIAA J. Aircr. 44(2), 583–596, 2007) and the bird geometry studies (Meguid et al. 2008) in systematic and integrated manner. Both the bird and the blade were simulated in a Lagrangian framework. Furthermore, the homogenized fluidic constitutive equation of the bird follows the Brockman hydrodynamic model, while the blade is modeled as a viscoplastic material of the Perzyna type. The study focused on the three most-frequently used configurations in the literature; namely, hemispherical-ended cylinder, straight-ended cylinder, and ellipsoid, at various length-to-diameter aspect ratios. The results show that the initial contact area between the bird and target in the early phase of the impact event would have a significant effect on the peak impact force. Normal incidence results in maximum impact forces and plastic strains leading to severe deformation. For the case where the incidence angle is equal to or larger than 60°, the impact forces are significantly reduced and the blade deformation remains within the elastic range. The maximum impact force and the resulting plastic strain are not merely governed by the size of the bird but also its density.

Aerogels are ultralight solids with nanoporous structure and are one of the world’s lightest materials available in the market. It is a dry gel, principally made up of 99.8 % of air and weighing just around three times that of air. The first aerogels were realized in 1931, when Kistler (J Phys Chem 36:52–64, 1932) attempted to remove liquid from a wet gel. It started out with the testing of the hypothesis that the liquid in a jelly can be replaced by a gas so as to avoid the collapse of the wet gel. He postulated that it was possible to slowly expand the supercritical fluid within a gel and obtain an air-filled non-collapsed gel structure. He subsequently succeeded in producing silica aerogels with densities in the range 20–100 kg/m3, as well as aerogels of alumina, tungsten, ferric, and stannic oxides. Today, silica aerogel is frequently used in nanocomposites for their light weight and excellent thermal insulating properties. In this chapter, we document some of the silica aerogel-filled carbon composite sandwich structures we have recently developed and also numerically examine the underlying mechanisms which enable silica aerogels to possess extreme insulation properties and especially how pore size plays a major role.

In this paper, the dynamic stability analysis of thin, laminated cylindrical panels under static and periodic axial forces is presented by using the mesh-free kp-Ritz method. The mesh-free kernel particle estimate is employed to approximate the 2D transverse displacement field. A system of Mathieu–Hill equations is obtained through the application of the Ritz minimization procedure to the energy expressions. The principal instability regions are then analyzed via Bolotin's first approximation. Effects of lamination schemes such as number of layers, ply-angle and boundary conditions on the instability regions are examined in detail.

The effect of Young's modulus on the behavior of pH-sensitive hydrogels is studied when subjected to coupled stimuli of solution pH and externally applied electric field. The study is the first instance for coupled stimuli numerical analysis of the hydrogels, in which a multi-effect-coupling pH-stimulus (MECpH) model is presented. The model considers ionic diffusion, electric potential, and large mechanical deformation. A correlation between diffusive hydrogen ion and charge groups fixed onto the hydrogel polymeric chains by the Langmuir absorption isotherm is incorporated into the model. To validate the model, the computed results by the meshless Hermite-Cloud technique are compared well with experimental data available from the literature. Then several case studies are conducted for the swelling hydrogel immersed in buffered solution subject to applied electric voltage, especially for the Young modulus effects on the volume variations of the hydrogels.

When exposed to an external solvent, a dry polymeric network imbibes the solvent and undergoes large deformation. The resulting aggregate is known as a hydrogel. This swelling process is diffusion driven and thus results in differential swelling during transient swelling. When subjected to external geometrical constraints, such as being rigidly fixed or attachment to a compliant substrate, wrinkles have been shown to appear due to mechanical instabilities. In the case of free swelling, there are no external constraints to induce the instabilities accounting for wrinkling patterns. However, during the transient swelling process, the swelling differential between the gel on the exterior and the interior causes compressive stresses and gives rise to mechanical instabilities. It is also observed that the time dependence of the swelling profile causes the wrinkles to evolve with time. In this work, we investigate this interesting phenomenon of transient wrinkle mode evolution using the finite element and state-space methods. From our simulations and prediction, we find that there is an inverse relation between critical wave number and time, which has earlier been observed in experiments.

In this paper, a novel numerical technique is developed for the dynamic stability analysis of composite laminated cylindrical shells under static and periodic axial forces. The mesh-free kernel particle (kp) estimate is employed in hybridized form with harmonic functions, to approximate the 2-D transverse displacement field. A system of Mathieu–Hill equations is obtained through the application of the Ritz minimization procedure to the energy expressions. The principal instability regions are then analyzed via Bolotin’s first approximation. The mesh-free kp-Ritz method is validated through comparison with existing available numerical data taken from open literature. Effects of boundary conditions and lamination schemes on the instability regions are also examined in detail.

In this paper, the free vibration of conical panels is analyzed by the mesh-free kp-Ritz method. Both 1-D and 2-D versions of the kp-Ritz approach are formulated for conical panels. For conical panels with two simply supported straight edges, the 1-D kp-Ritz version is used, where the kernel particle estimation is employed in hybridized form with harmonic functions to approximate the 2-D displacement field. For conical panels having arbitrary boundary conditions, the displacement field is approximated by the 2-D kp-Ritz version, with 2-D form of kernel particle functions employed. The classical thin shell theory based on Love's hypothesis is employed in the present analyses, and based on the kernel particle concept and Ritz technique, the eigenequations of the frequencies of the conical panels are obtained. To validate the accuracy of stability of the present method, convergence studies were carried out based on the influences of the support size and the number of nodes. Comparisons were also made with existing results available in the open literature. This study also examines in detail the effects of variation in the semi-vertex angle and boundary conditions, on the frequency characteristics of conical panels.

In this work, a novel true meshless numerical technique is proposed. It is termed the Hermite–Cloud method and is based on the classical reproducing kernel particle method except that a fixed reproducing kernel approximation is used instead. Another distinction is that the point collocation technique is used for the discretization of the governing partial differential equations. In this method, the Hermite theorem is employed for the construction of the interpolation functions. Through the constructed Hermite-type interpolation functions, we are able to generate the expressions of approximate solutions of both the unknown functions and the first-order derivatives, in a direct manner. A set of auxiliary conditions have also been developed so as to construct a complete set of PDEs with mixed Dirichlet and Neumann boundary conditions. Through several structural analysis examples, it is shown that the numerical results at the scattered discrete points generated by the Hermite–Cloud method are distinctly improved, for both the approximate solutions as well as the first-order derivatives.

The combination of PM3 semi-empirical method for geometry optimization, and ab initio DFT (density functional theory) for energy calculation, is used to study the configurations of hydrogen molecules at 0 K within the vacuum of C60 fullerene and carbon nanocapsules. The obtained structural information including the molecular arrangement and structural state of the clusters of H2 are mainly determined by two kinds of repulsive energies, namely that between the H2 molecules and the wall of the spheroidal or capsule like carbon structure, and that between interacting H2 molecules. It is further established that the repulsive energy among the H2 molecules is not purely a function of the number of encapsulated H2, and there is a tradeoff between the two kinds of repulsive energies.