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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.

The expanding capabilities and decreasing costs of additive manufacturing have resulted in the increased adoption of micro-unmanned aerial vehicles (micro-UAVs) among professionals and hobbyists. Due to safety regulatory requirements of UAV operations, weight is generally the overriding design features of interest for micro-drones, but it often comes as a trade-off against the durability, loading constraints, and other subsystem equipment. Nevertheless, ultra-lightweight structures can be realized through the adoption of both 3D-printing and topology optimization without compromising the structural integrity and overall strength and this article explores the use of these two technologies for designing and manufacturing optimized ultralight micro-UAVs. First, material properties of Nylon 12 (PA12) manufactured using selective laser sintering (SLS) were accurately characterized via mechanical testing and ultrasonic means. These properties were verified by comparing the mechanical response of 3-point and 4-point bending tests with corresponding finite element (FE) simulation. Next, topology optimization was performed to produce an optimized structure of a Z-split configured lightweight micro-quadcopter. The optimized design is then 3D-printed and subsequently validated through a load test for verification against the optimized FE simulation-based design. A close correlation was obtained between the numerical and experimental data, suggesting that topology optimization with 3D printing can be safely and reliably adopted for the design and rapid prototyping of micro-UAVs, whilst catering to different specifications and requirements. National Research Foundation (NRF) Submitted/Accepted version This research is jointly supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme, and ST Engineering Aerospace Ltd., under project titled ‘3D Printing of micro UAV’.

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.

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.

Some nonlinear systems possess innate capabilities of enhancing weak signal transmissions through a unique process called Stochastic Resonance (SR). However, existing SR mechanism suffers limited signal enhancement from inappropriate entraining signals. Here we propose a new and effective implementation, resulting in a new type of spectral resonance similar to SR but capable of achieving orders of magnitude higher signal enhancement than previously reported. By employing entraining frequency in the range of the weak signal, strong spectral resonances can be induced to facilitate nonlinear modulations and intermodulations, thereby strengthening the weak signal. The underlying physical mechanism governing the behavior of spectral resonances is examined, revealing the inherent advantages of the proposed spectral resonances over the existing implementation of SR. Wide range of parameters have been found for the optimal enhancement of any given weak signal and an analytical method is established to estimate these required parameters. A reliable algorithm is also developed for the identifications of weak signals using signal processing techniques. The present work can significantly improve existing SR performances and can have profound practical applications where SR is currently employed for its inherent technological advantages.

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.

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.

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.

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.

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.

In this paper, we investigate the potential issues low dimensional carbon material desalination membranes may encounter when nanoplastics are present in the systems. This study is performed via molecular dynamics simulations on desalination systems which comprise the nanoporous graphene membrane. It was found that there is high affinity between nanoplastics and graphene, indicating high tendency of the nanoplastic particles to foul the membranes. Furthermore, it was observed that the presence of nanoplastic molecules reduce the salt rejection rate of the membranes. Through this work, we hope to highlight the potential issues caused by the increasing prevalence of nanoplastics in the environment on carbon based desalination membranes, so as to aid the future design of these high performance membranes. Published version

The underlying mechanisms of surface phenomena are very complex and still not entirely clear. The aim of this work attempts to reveal the important contribution of the nonlocal integral theory of elasticity to surface elasticity, which is of fundamental scientific interest. By considering a uniform and isotropic half-space medium subjected to an arbitrary uniform strain, it is shown that the bulk is homogeneous, however, the whole medium is heterogeneous due to the bond loss near the free surface. The nonlocal effect cannot be observed at all in the homogeneous bulk, but the bond loss characterized by the nonlocal integral theory can show an important contribution to the surface elasticity. This in turn allows to propose a simplified surface model to replace the complex modeling of nonlocal integral theory. The thickness of surface zone can be evaluated from the intrinsic characteristic length used in the nonlocal integral theory. According to the intrinsic correlation and using the molecular dynamics simulations, the thickness of the surface layer of silicon films is 2.6911 A. Furthermore, with application of the simplified surface model to a thin film in tension, it has been noted that the geometric dimension of size-dependence is generally not that of traditional mechanics. For instance, in most situations, the main contribution to size-dependence has actually come from the thickness (or radius) direction of a rod-type structure, rather than its axial direction which is intuitively- and widely-used in the current practice in open literature.

Eigenvalue and eigenvector derivatives with respect to system design variables and their applications have been and continue to be one of the core issues in the design, control and identification of practical engineering systems. Many different numerical methods have been developed to compute accurately and efficiently these required derivatives from which, a wide range of successful applications have been established. This paper reviews and examines these methods of computing eigenderivatives for undamped, viscously damped, nonviscously damped, fractional and nonlinear vibration systems, as well as defective systems, for both distinct and repeated eigenvalues. The underlying mathematical relationships among these methods are discussed, together with new theoretical developments. Major important applications of eigenderivatives to finite element model updating, structural design and modification prediction, performance optimization of structures and systems, optimal control system design, damage detection and fault diagnosis, as well as turbine bladed disk vibrations are examined. Existing difficulties are identified and measures are proposed to rectify them. Various examples are given to demonstrate the key theoretical concepts and major practical applications of concern. Potential further research challenges are identified with the purpose of concentrating future research effort in the most fruitful directions. Ministry of Education (MOE) The first and third authors gratefully acknowledge the financial support from the Singapore Ministry of Education through the award of research project grant AcRF Tier 1 RG183/17.

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.

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.

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

This paper investigates the orthotropic properties of Fused Deposition Modeling (FDM)-printed Acrylonitrile Styrene Acrylate (ASA) material with different raster configurations. The elastic properties were determined using a non-destructive ultrasonic technique. This technique allows us to deduce the orthotropic elastic constants from the material density and the velocities of the longitudinal and shear waves propagating through the material along different directions. Tensile tests were performed in addition to ultrasonic tests to obtain the yield properties of the ASA material and to validate the elastic constants determined by the ultrasonic tests, which have shown very close correspondence. Finally, numerical verification was performed by comparing the experimental results of the three-point and four-point bending tests with the finite element simulation results which have as input the material properties from the ultrasonic testing. The simulation results have shown excellent agreement with the experimental results, implying that the material properties obtained from the ultrasonic testing were highly accurate comparing to the actual orthotropic elastic properties of the 3D-printed ASA material. NRF (Natl Research Foundation, S’pore) Accepted version

Although single layer transverse flow carbon nanotube (CNT) membrane (TFCM) has been shown to be ultra-permeable with high salt rejection, its physical fabrication with sub-nanometre slits remains a significant challenge to its development. This work presents the multilayer TFCM, which resembles vertically aligned CNT arrays, as an alternative candidate for efficient desalination. Using molecular dynamics, this work shows that multilayer TFCM can provide permeability and salt rejection on par with its single layer counterpart. By multilayering, the slit size between neighbouring CNTs can be increased to nanometre range while still maintaining high salt rejection. The increase in slit size counteracts the reduction in permeability due to multilayering, thereby allowing multilayer TFCM to achieve permeability performance comparable to the single layer TFCM. The effects of the number of layers n and other design parameters (interlayer distance d , CNT diameter D C N T , offset h ) on the desalination performance of multilayer TFCM are investigated thoroughly using results from non-equilibrium and equilibrium molecular dynamics. It was found that the desalination performance is not sensitive to variations in d , D C N T or h . Finally, this work provides computational evidence that the multilayer TFCM, which could be fabricated using techniques for current dense vertically aligned CNT arrays, can make an efficient design for future low dimensional materials membrane.

In parallel with recent developments in carbon nanomaterials, there is growing interest in using these nanomaterials for desalination. To date, many studies have affirmed the potential of using such nanomaterials for constant pressure desalination operation. In this work, the performance of such membrane when subjected to oscillatory pressure at sub-nanosecond is investigated in detail. Using the transverse flow CNT membrane operating at periods ranging from 0.02 to 0.1 ns, we find that oscillatory pressure operation can increase the permeability of the membrane by 16% with a salt rejection close to 100%. Detailed studies on the salt concentration profile, water orientation and water permeance behavior revealed that this increase in permeability is due to the development of resistance to reverse flow at higher periods of oscillation. Further extension of the analysis to periods on the order of 0.1 ns and beyond do not show a positive influence on water permeability. Thus, this work shows that periods on the order of 10−2 ns are required for improved performance of low dimensional nanomaterials membrane. The results from this work shows that nanomaterials membrane is suitable for oscillatory operation, such as electrodialysis reversal. Due to the nanoscale sized of the membrane channels, sub-nanoseconds pulsations are more effective in introducing instabilities to the system to positively influence the water permeance behavior of the membrane.

Many different methods have been developed since the pioneering works of Mills-Curran (1988) and Dailey (1989) on eigenvector derivatives with repeated eigenvalues. In spite of the increasing mathematical complexities witnessed in many of the newly emerged methods, some underlying fundamental theories governing the eigenvector derivatives have neither been much discussed, nor fully established to date. The present approach seeks to fill such an outstanding theoretical gap and to lay down the necessary theoretical foundation on which existing methods can be mathematically unified and further improved in numerical accuracy and computational efficiency. The particular solutions of eigenvector derivatives generally required have been derived in terms of modal properties, thereby avoiding the computationally expensive and potentially erroneous procedure of solving a set of algebraic equations of system dimension. The contributions of higher unavailable modes have been theoretically derived, enhancing the practical applicability of the proposed method to the general case where only partial eigensolutions are made. To avoid degeneration of eigenvector space in the case of repeated eigenvalues, a concept of global design variable is developed in which all intended multivariate design modifications are grouped into a single global variable to which eigenvector derivatives are derived, rendering real major applications of the proposed method to the predictions of structural design modifications. A discrete parameter model of a turbine bladed disk assembly, which is known to have many pairs of repeated eigenvalues due to its cyclic symmetry, as well as a finite element model of a cantilevered beam with large DOFs have been employed. Numerical results have demonstrated the accuracy and the practical applicability of the proposed new theoretical developments, as well as the proposed new method.

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.

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.

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.

It is well known that encryption algorithms developed based on Logistic map suffer from limited key space due to the narrow regions of system parameters which can be used, potential risk of security in the presence of numerous periodic windows within the key space, and weakness in known-plain-text attack due to the inherent correlation among the chaotic sequence used for encryption. To overcome these existing problems, this paper presents a secure image encryption algorithm based on a new highly nonlinear discrete dynamical system with ideal chaotic characteristics. Transcendental functions are introduced together with modulo operations which physically represent discontinuous time-varying nonlinearities, leading to extremely complex chaotic behavior that is highly sensitive to system parameters and initial conditions, both of which are considered as the key for the cryptosystem. Extensive numerical experiment results have revealed that the proposed image encryption algorithm offers advantages of unlimited key space and high-level security, since those problematic periodic windows are no longer present within the key space, and it is extremely robust against known-plain-text attack, since the chaotic sequence generated bears no correlation whatsoever due to the folding effect of modulo operation. The algorithm makes truly efficient yet highly secure image encryption based on chaotic systems a reality. Published version

Many modern structural systems are found to contain localized areas, often around structural joints and boundaries, where the actual dynamic behavior is far from linear. Such nonlinearities need to be properly identified so that they can be incorporated into the improved mathematical model used for design and operation. One approach to this task is to use higher-order frequency response functions (FRFs). Identification of structural nonlinearities using higher-order FRFs is currently an active and growing emerging research area and to date, much research has been conducted. However, most existing research seems to be confined to very simple nonlinear system models such as SDOF mass-spring-damper models or to the most, MDOF mass-spring chain models. For more general continuous nonlinear structures with discrete localized nonlinearities however, analytical derivation of higher-order FRFs remains unknown and as a result, identification of nonlinearities of such systems using measured higher-order FRFs becomes impossible to achieve. This missing link between higher-order FRFs and physical parameters of nonlinearities of continuous structures has to be established before any real progress can be possibly made. In this paper, a new novel method is developed which can be used to derive analytical higher-order FRFs of continuous structural systems with discrete localized nonlinearities. The method is generally applicable and theoretically exact, and it serves exactly as that missing link. Having established analytical higher-order FRFs which serve as the theoretical foundation for any subsequent identification, method of parameter identification of nonlinearity is then further developed. Important characteristics of higher-order FRFs are discussed, some of which are revealed the first time since there has never been higher-order FRFs derived from nonlinear continuous structures. Various numerical aspects on how to improve accuracy of identified nonlinear system parameters are discussed.

Two types of cracks are often encountered in engineering structural systems and these are the open cracks and the breathing cracks. Existence of open cracks often leads to the loss of physical stiffness, resulting in a mostly linear structure with reduced load bearing capacity and vibration frequencies. The development of breathing cracks not only reduces structural stiffness, but tends to render the otherwise linear structure to become nonlinear, due to their bilinear stiffness characteristics associated with open and closed states. The nonlinear structural vibration responses can then be investigated and potentially employed to detect and identify breathing cracks within a structural system. In the present study, breathing cracks are modeled based on fracture mechanics from which bilinear stiffness values are obtained. These values are then incorporated into finite element models to compute the first- and second-order frequency response functions (FRFs) based on a proposed correlation technique which is both very accurate and resilient against measurement uncertainties. The existence of well-defined second-order FRFs has been firmly established for the bilinear oscillator, as well as a cantilevered beam with breathing cracks. By expressing the bilinear restoring force as a polynomial series, analytical derivation of second-order FRFs of general structural systems such as the GARTEUR AG11 structure with breathing cracks has been established for the first time. Further, a method of identification has been developed to identify the physical parameters of breathing cracks using second-order FRFs. With these new developments that are presented in this paper, a solid foundation has been laid for the potential applications of higher-order FRFs to damage identification and assessment of general real structural systems. MOE (Min. of Education, S’pore)

It is well known that elastic instabilities induce pattern transformations when a soft cellular structure is compressed beyond critical limits. The nonlinear phenomena of pattern transformations make them a prime candidate for controlling macroscopic or microscopic deformation and auxetic properties of the material. In this present work, the novel mechanical properties of soft cellular structures and related hydrogel–elastomer composites are examined through experimental investigation and numerical simulations. We provide two reliable approaches for fabricating hydrogel–elastomer composites with rationally designed properties and transformed patterns, and demonstrate that different geometries of the repeat unit voids of the periodic pattern can be used to influence the global characteristics of the soft composite material. The experimental and numerical results indicate that the transformation event is dependent on the boundary conditions and material properties of matrix material for soft cellular structures; meanwhile, the deformation-triggered pattern of matrix material affects the pattern switching and mechanical properties of the hydrogel–elastomer material, thus providing future perspectives for optimal design, or serving as a fabrication suggestion of the new hydrogel–elastomer composite material. 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.

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.