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Composition-gradient multi-angle static light scattering (CG-MALS) is an emerging technique for the determination of intermolecular interactions via the second virial coefficient B22. With CG-MALS, detailed studies of the second virial coefficient can be carried out more accurately and effectively than with traditional methods. In addition, automated mixing, delivery and measurement enable high speed, continuous, fluctuation-free sample delivery and accurate results. Using CG-MALS we measure the second virial coefficient of bovine serum albumin (BSA) in aqueous solutions at various values of pH and ionic strength of a univalent salt (NaCl). The systematic variation of the second virial coefficient as a function of pH and NaCl strength reveals the net charge change and the isoelectric point of BSA under different solution conditions. The magnitude of the second virial coefficient decreases to 1.13 x 10(-5) ml*mol/g(2) near the isoelectric point of pH 4.6 and 25 mM NaCl. These results illuminate the role of fundamental long-range electrostatic and van der Waals forces in protein-protein interactions, specifically their dependence on pH and ionic strength.

We report a new method for measuring vacuum pressures using Van der Waals (VDW) interactions between reduced graphene oxide (RGO) sheets. For this purpose, we utilized a reaction-based self-assembly process to fabricate various intercalated RGO (i-RGO) films, and monitored their electrical behavior with changing pressure and temperature. Pumping to remove gas from a vacuum chamber produced a decrease in the sheet resistance of i-RGO. With further pumping, distinctly different sheet resistance behaivors were observed depending on the measurement temperature. With increasing vacuum pressure, the resistance increased at 100 °C, whereas it decreased at 30 °C. Two types of VDW interactions are proposed to explain these features: a local VDW interaction between RGO sheets that resulted in V-shaped curves of sheet resistance with pressure changes and broad VDW interactions that occur between RGO sheets when the elastic force required to bend carbon clusters on an RGO sheet exceeds their vibrational energy at low temperatures. On the basis of the results, we propose that the resistance behavior of i-RGO as a function of vacuum pressure can be interpreted as the sum of the two different VDW interactions.

Facile dry decoration of graphene oxide sheets with aerosol Ag nanocrystals synthesized from an arc plasma source has been demonstrated using an electrostatic force directed assembly technique at room temperature. The Ag nanocrystal-graphene oxide hybrid structure was characterized by transmission electron microscopy (TEM) and selected area diffraction. The ripening of Ag nanocrystals on a graphene oxide sheet was studied by consecutive TEM imaging of the same region on a sample after heating in Ar at elevated temperatures of 100 °C, 200 °C, and 300 °C. The average size of Ag nanocrystals increased and the number density decreased after the annealing process. In particular, migration and coalescence of Ag nanocrystals were observed at a temperature as low as 100 °C, suggesting a van der Waals interaction between the Ag nanocrystal and the graphene oxide sheet. The availability of affordable graphene-nanocrystal structures and their fundamental properties will open up new opportunities for nanoscience and nanotechnology and accelerate their applications.

This paper aims to apply the nonlocal Donnell shell theory to study the buckling of double-walled carbon nanotubes (DWCNTs) under axial compression taking into account the effects of internal small length scale and the Van der Waals interactions between layers. The DWCNTs is modeled as nonlocal double circular cylindrical elastic shells. On the basis of nonlocal elasticity theory, governing equations for buckling of Donnell shells are obtained taking into account the Van der Waals force. The nonlocal buckling load of DWCNT is derived without any assumption on radius tubes. However, it is a very difficult task to obtain the analytical solution of nonlocal critical buckling load. In this paper, we develop an approach for prediction of the nonlocal critical buckling load of DWCNTs.

In this paper, we consider an unusual connection between different fluids. Having established a research goal we would like to consider a toy model of the Universe and investigate its behavior, especially for later time evolution for well known facts. The main goal of the article is to consider a toy model of the Universe with generalized ghost dark energy, Van der Waals gas and a phenomenologically modified fluid. The origin of the last component can be understood as a result of interaction between some original fluid and some source of energy or matter in Universe. By unusual connection we mean an assumption that generalized ghost dark energy has its contribution to the model by an interaction term Q and we suppose an interaction of the form Q=3H b(ρ t o t −ρ G D e ). Graphical analysis is performed and the questions of validity of the generalized second law of thermodynamics and stability of the model also approached in this paper.

In the current paper, a coupled two degree of freedom model which considers both bending and torsion of the supporting torsion beams is presented for electrostatically actuated torsional nano/micro-actuators under the effect of van der Waals (vdW) force. Newton’s second law is utilized for finding the normalized equations governing the static behavior of the actuator. The implict function theorem is then utilized for finding the equations governing the pull-in state of the actuator. The related results show that torsion model considerably overestimates the pull-in parameters of the nano/micro-actuator. The concept of the instability mode is introduced, and it is shown that when the ratio of the bending stiffness to the torsion stiffness of the supporting torsion beams is relatively low, the dominant instability mode of the actuator would be the bending mode and otherwise the dominant instability mode would be the torsion mode. It is also observed that the presence of the vdW force can significantly reduce the pull-in angle and pull-in deflection of the nano/micro-actuator. The presented results also show that the vdW force can lead to considerable reduction in the pull-in voltage of the actuator. The equilibrium behavior of the actuator is studied, and it is observed that the vdW force and also bending of the supporting torsion beams greatly reduce the maximum allowable voltage which can be applied to the actuator. Results of this paper can be used for successful design of electrostatically actuated torsional nano/micro-actuators where the size of the actuator is sufficiently small, and as a result, the vdW force plays a major role in the system.

The problem of molecular docking is defined as that of finding a minimum energy configuration of a pair of molecular structures (usually consisting of proteins, DNA or RNA fragments). It is often assumed that the two interacting structures can be considered as rigid bodies and that it is of interest to researchers to develop methods which enable to discover the potential binding sites. Many different models have been proposed in the literature for the definition of the potential energy between two molecular structures, most of which contain at least a term (known as Van Der Waals interaction) which accounts for pairwise attraction between atoms, a repulsion term and a term which takes into account electrostatic forces (Coulomb interaction). Some well known models, and in particular those used in rigid docking, are based on the assumption that the only terms which are relevant in the process of docking are pairwise interactions between atoms belonging to the two different parts of the structure. In this paper the problem of finding the lowest energy configuration of a pair of biomolecular structures, considered as rigid bodies, is defined and formulated as a global optimization problem. In terms of dimension of the search space this formulation is not 'high-dimensional', as there are only six degrees of freedom: 3 translation and 3 rotation parameters. However the energy surface of the docking problem is characterized by a huge number of local minima; moreover each function evaluation is quite expensive (interesting structures usually possess a few thousand atoms each). So there is a strong need both of local and of global optimization procedures. In this paper a local optimization technique, based upon standard non linear programming software and a penalized objective function, is introduced and its potential usefulness in the context of global optimization is outlined.

Most recently, the whole world is struggling against the virulent pandemic COVID-19. Due to the unbounded global spread of the disease, having biosensors with high performance such as high sensitivity and accuracy is of utmost importance. In this paper, the effects of various parameters on the behaviors of micro-biosensors are investigated in order to enhance their performance. These parameters are related to the geometry and material, and they are assumed to be gradually changing in the longitudinal direction of the biosensor according to a power law. Therefore, they are called functionally graded geometrical and material parameters. Another aspect is when considering microcantilever-based biosensors, the main behavior parameter is the deflection at the free end. In the analyses, the influences of the surface stress and van der Waals intermolecular forces are taken into account. Also, the total energy of the beam, which is the combination of the van der Waals energy and the elastic strain energy, is accomplished. In addition, the equivalent force causing the deflection is also evaluated using Castigliano method for two cases. These cases account for a concentrated force at the free end and a distributed load along the biosensor, respectively. Since the governing equations account for the size dependency and the considered parameters are functions of the position, the solution is too complex to be achieved analytically, and therefore, numerical methods are applied. For uniform biosensors made of homogeneous materials, or in other words when all parameters are not varying with the position, the obtained results are compared with those in the literature, and good agreement is obtained. On the other hand, the performance, which include sensitivity and limit of detection, of functionally graded biosensors can be enhanced by proper choices of the considered parameters and the corresponding exponent of the gradation function.

In this paper, a micro electromagnetic harmonic drive system is proposed. For the drive system, the equation of the displacements for the flexible ring under a magnetic field is deduced. Here, both electromagnetic and Van der Waals forces are considered. The equation of the relationship between flexible ring displacements and coil current is given, and based on this, an equation of the output torque for the drive system is provided. Using these equations, the molecule forces between the flexible ring and stator are investigated, and the relationship between flexible ring displacements and coil current and its changes along with system parameters are analyzed. The output torque of the drive system and its changes along with system parameters are also studied. Findings show that the effects of the initial clearance and the coil current on the Van der Waals force between flexible ring and stator are coupled to each other. For a small initial clearance, the effect of the coil current on the van der Waals force becomes large. When the initial clearance is quite small, the effects of the molecule force on the output torque should be considered.

Phonon polaritons in van der Waals materials show the strong electric confinement and low energy loss. Combining with the materials hexagonal boron nitride, hyperbolic phonon polaritons can be excited in two RS bands, which changes a lot in electric field distribution and transmission characteristics. In order to increase the tunability of hyperbolic phonon polaritons, a structure based on graphene/h-BN heterostructure has been formed in previous studies. In this paper, the electric field distribution and transmissivity characteristics are calculated and discussed based on grapheme–h-BN–graphene heterostructure. The results illustrate the excellent transmissivity tunability in 780 cm−1 incident wavenumber of type-I band and poor transmissivity tunability in 1368 cm−1 of type-II band. In addition, superior field-enhanced characteristic (115.76% transmissivity in 6 V external voltage) is shown in the structure. These excellent properties make such structure a promising component in future optoelectronic devices design.

Abstract The present paper investigated the effects of the electrostatic and non-electrostatic interactions on the water wettability of a surface. Based on this aim, examinations are performed on the molybdenum disulfide (MoS2) as possible nanostructure surface. The results obtained through calculating the water contact angle on the MoS2 surface indicate which this surface is a weak hydrophobic substrate. The present simulations illustrate that the electrostatic interactions have little impact on the wettability amount of the MoS2 substrate. However, the molybdenum disulfide is composed of two charged parts with significant values. In addition, it is observed that the relation between density, van der Waals and electrostatic interactions is different from each other. It have been observed that the van der Waals interactions have direct relationship with water density while there is not anything between the electrostatic interactions and water density peak. The Obtained results via simulation demonstrate that the effectiveness of electrostatic interactions on the wettability depends on the sigma in the Lennard–Jones equation. In the other words, the role of electrostatic and van der Waals interactions on the wettability are not indissociable from each other and this method is sufficient in nanostructure systems. Graphic abstract

In a previous paper (http://www.phys.uri.edu/people/nightingale/publications.html, chem-ph/9406003) we developed a form of variational trial wave function and applied it to van der Waals clusters: five or less atoms of Ar and Ne modeled by the Lennard-Jones potential. In addition, we tested the trial functions for a hypothetical, light atom resembling Ne but with only half its mass. We did not study atoms such as He with larger de Boer parameters, i.e., systems in which the zero point energy plays a more important role relative to the potential energy. This is the main purpose of the present paper. In fact, we study clusters to the very limit where the zero-point energy destroys the ground state as a bound state. A simple picture of this un-binding transition predicts the power law with which the energy vanishes as the de Boer parameter approaches its critical value and the power of the divergence of the the size of the clusters in this limit. Our numerical results are in agreement with these predictions., Comment: text and figures TeX form, 4 pages

Self-driven photodetectors that can detect light without any external voltage bias are important for low-power applications, including future internet of things, wearable electronics, and flexible electronics. While two-dimensional (2D) materials exhibit good optoelectronic properties, the extraordinary properties have not been fully exploited to realize high-performance self-driven photodetectors. In this paper, a metal–semiconductor–metal (MSM) photodetector with graphene and Au as the two contacts have been proposed to realize the self-driven photodetector. Van der Waals contacts are formed by dry-transfer methods, which is important in constructing the asymmetrical MSM photodetector to avoid the Fermi-level pinning effect. By choosing graphene and Au as the two contact electrodes, a pronounced photovoltaic effect is obtained. Without any external bias, the self-driven photodetector exhibits a high responsivity of 7.55 A W−1 and an ultrahigh photocurrent-to-dark current ratio of ~108. The photodetector also shows gate-tunable characteristics due to the field-induced Fermi-level shift in the constituent 2D materials. What is more, the high linearity of the photodetector over almost 60 dB suggests the easy integration with processing circuits for practical applications.

Graphene nanosheets prepared by liquid-phase exfoliation tend to aggregate easily and irreversibly in most solvents due to van der Waals forces and high surface energy. This article presents a facile, low-cost and novel approach for the preservation and separation of graphene dispersions by adjusting the temperature and solvent. Using IPA-water mixtures can realize green production of graphene nanosheets, and different physicochemical parameters are achievable by changing the proportion of components. Two valid methods for improving stability were discussed in depth: low-temperature storage as a liquid and as a solid. When graphene dispersions are stored in a liquid phase, agglomeration of nanosheets in mixed solvents can be effectively retarded by strong viscous resistance induced by low temperatures. Frozen powder prepared by liquid nitrogen maintains the nanosheets in a dispersed state and is suitable for long-term preservation in the solid phase in ordinary freezers. Furthermore, rapid separation of graphene nanosheets is a challenging problem that retards industrial production. Flocculation induced by slow freezing can accelerate solid–liquid separation, offering a novel approach to obtain easily dispersed powders. Various patterns could be printed on paper and poly(ethylene terephthalate) by a simple and low-cost screen printing technique using the graphene powder, providing a new platform for scalable, low-cost printing of electronics. Consequently, this scalable and simple strategy can be satisfactorily applied to the preservation and separation of graphene and is expected to extend to other nanomaterials, including MoS2 and h-BN.

This paper presents a size-dependent, nonlinear beam model for bending, buckling and free vibrations of electrically actuated viscoelastic clamped–clamped microbeams. The modified couple stress theory is used to model the size effect. The Euler–Bernoulli beam model including von Karman geometric nonlinearity and intermolecular van der Waals and Casimir forces is adopted. The Boltzmann superposition viscoelastic model is used to simulate the linear behavior of the viscoelastic material. The governing integral–differential equation and corresponding boundary conditions are obtained using Hamilton’s principle. Galerkin’s discretization is used to obtain a reduced-order model of the problem. The quasi-elastic method is used to approximate the viscoelastic behavior. The model has been verified by comparing some results with those available in the literature, and a good agreement is obtained. The influence of the small size, creep modulus, relaxation time and intermolecular forces on the electrostatic bending, buckling and free vibrations are investigated and found significant.

Two-dimensional (2D) van der Waals (vdW) materials have recently attracted considerable attention due to their excellent electrical and mechanical properties. TmPSx (where Tm = a transition metal), which is a new class of 2D vdW materials, is expected to show various physical phenomena depending on the Tm used. In this paper, the unprecedented synaptic behavior of a vertical Ag/CrPS4/Au capacitor structure, where CrPS4 is a single-crystalline 2D vdW layer, is reported. Multi-stable resistive states were obtained using an external voltage of less than 0.3 V. Both short-term plasticity and long-term potentiation were observed by controlling the interval of the external voltage pulse. Simple mechanical exfoliation was used to develop a synaptic device based on a very thin CrPS4 layer with a thickness of ~17 nm. Therefore, it was demonstrated that vertical Ag/CrPS4/Au capacitors could be promising inorganic synaptic devices compatible with next-generation, flexible neuromorphic technologies. A capacitor made of a chromium thiophosphate (CrPS4) layer sandwiched between a silver electrode and a gold electrode behaves like a synapse. A Korea-based team led by Bae Ho Park from Konkuk University, Seoul, peeled very thin, crystalline CrPS4 layers of different thicknesses from the bulk material and fabricated top and bottom electrodes to prepare a series of Ag/CrPS4/Au capacitors. Under electrical pulses of different amplitudes, durations and intervals, these structures mimicked short-term and long-term variations in synaptic strength—neural behaviors involved in learning and memory. The devices’ behavior is attributed to the electrochemically induced migration of silver ions across the CrPS4 layer, leading to the formation and rupture of a conducting filament between the electrodes. The Ag/CrPS4/Au structure is promising for computing technologies that imitate neural pathways in the nervous system. We suggest synaptic devices using cation migration along thickness direction in a new class of 2D layered materials. An electrochemically active metal, such as Ag and Cu, is used for the operation of the synaptic device and chromium thiophosphate (CrPS4) single crystal is used as an electrolyte material. Multi-stable resistive states, short-term plasticity, and long-term potentiation are observed by controlling external voltage pulse with height smaller than 0.3 V. Given that simple mechanical exfoliation can generate very thin CrPS4 layers, the vertical Ag/CrPS4/Au capacitor offers a promising inorganic synaptic device compatible with next-generation flexible neuromorphic technology.

Transmission of natural gas with methane as the main constituent has been a subject of interest to industrial companies. Predicting hydrate formation conditions is important to prevent formation of methane hydrate in gas pipeline. Also, attention has been taken to account for capture and storage of pure methane. In this paper, a comprehensive comparison is performed between empirical correlations and different equation of state in van der Waals Platteeuw (VdW-P) thermodynamic model to determine the most accurate method of hydrate formation condition of methane. In addition, a novel, simple and accurate correlation is developed to predict methane hydrate formation temperature using genetic programming. Error analysis on a wide range of experimental data indicates that the new proposed correlation is superior over existing correlations and all VdW-P models with R 2 = 0.999.

This paper provides the static and dynamic pull-in behavior of nano-beams resting on the elastic foundation based on the nonlocal theory which is able to capture the size effects for structures in micron and sub-micron scales. For this purpose, the governing equation of motion and the boundary conditions are driven using a variational approach. This formulation includes the influences of fringing field and intermolecular forces such as Casimir and van der Waals forces. The differential quadrature (DQ) method is employed as a high-order approximation to discretize the governing nonlinear differential equation, yielding more accurate results with a considerably smaller number of grid points. In addition, a powerful analytical method called parameter expansion method (PEM) is utilized to compute the dynamic solution and frequency-amplitude relationship. It is illustrated that the first two terms in series expansions are sufficient to produce an acceptable solution of the mentioned structure. Finally, the effects of basic parameters on static and dynamic pull-in instability and natural frequency are studied.

International audience; The fourth-gradient model for fluids-associated with an extended molecular mean-field theory of capillarity-is considered. By producing fluctuations of density near the critical point like in computational molecular dynamics, the model is more realistic and richer than van der Waals' one and other models associated with a second order expansion. The aim of the paper is to prove-with a fourth-gradient internal energy already obtained by the mean field theory-that the quasi-linear system of conservation laws can be written in an Hermitian symmetric form implying the stability of constant solutions. The result extends the symmetric hyperbolicity property of governing-equations' systems when an equation of energy associated with high order deformation of a continuum medium is taken into account.

The present work deals with the axial and torsional buckling analysis and design of carbon nanotubes (CNTs) by finite element. Computer-aided design, computer-aided engineering simulation, and finite element modeling are applied for the calculation of axial and torsional displacement and von Mises stresses. The smallest diameter of single-walled, double-walled, and multi-walled carbon nanotubes have been selected to survey armchair and zigzag types of CNTs. Nanotube diameters, covalent and van der Waals bonding, interlayer spacing, and chirality is considered in nanotubes construction based on actual dimensions. This paper illustrates that armchair-type nanotube is more resistant than zigzag-type against axial buckling for single-walled carbon nanotubes, whereas, in the case of torsional loading, zigzag-type show superior performance against buckling. By increasing the diameter and number of layers, due to van der Waals interaction, elastic buckling properties of nanotubes can be improved significantly. Data analysis for double-walled carbon nanotubes has shown that armchair-type is a better choice under combined loading, whereas, in the meantime, zigzag-type for triple-walled carbon nanotubes is preferred in the comparative analysis under combined loading. The results demonstrate the vital role of covalent intermolecular forces for single-walled nanotubes and van der Waals interlayer interactions for multi-walled nanotubes under buckling loading. The present study investigated the effect of covalent and van der Waals bonding for small diameter nanotubes.

Theoretical description of molecular interactions remains a challenge for computational chemistry. In particular, systems dominated by static correlation, in which bonds are stretched or twisted, are often beyond capabilities of methods based on a single-electron approximation, being usually a method of choice. For interacting multireference systems, it is necessary to apply either high-level coupled-cluster methods (which however include multireference effects only partially) or base the theoretical description on a multireference wavefunction. Popular multireference methods like CASPT2 (complete active space perturbation theory) do not provide satisfactory results since they may suffer from problems with size consistency and poor accuracy. Recently we have shown that combining a simple multireference wavefunction, perfect-pairing generalized valence bond (GVB) with extended random phase approximation in embedding framework (EERPA) leads to a method EERPA-GVB providing accurate results for challenging multireference systems. In this paper, good performance of EERPA-GVB is confirmed by its application to van der Waals and hydrogen-bonded complexes. In addition, we show that the decomposition of the EERPA-GVB correlation energy into contributions from pairs of geminals can provide useful insight into the investigated interactions.

Pore solution salinity has important bearing on engineering behavior of marine sediments as they influence electrochemical stress (A-R) and differential osmotic stress (Delta pi) of the salt-enriched clays. The electrochemical stress (A-R) is contributed by van der Waals (A) attraction and diffuse ion layer repulsion (R) , while the differential osmotic stress (Delta pi) is governed by the differences in dissolved salt concentrations in solutions separated by osmotic membrane. The paper examines the relative influence of differential osmotic stress (Delta pi) and electrochemical stress (A-R) on the consolidation behavior of slurry consolidated kaolinite specimens, which are known to be encountered in recent alluvial marine sediments. Methods are described to evaluate the magnitudes of these physico-chemical components and their incorporation in true effective stress. Results of the study demonstrate that differential osmotic stress finitely contributes to true effective stress. The contribution from differential osmotic stress enables kaolinite specimens to sustain larger void ratio during consolidation.

Since surface roughness greatly influences the force of adhesion between two bodies, the dynamic behavior of different biological micro/nanoparticles on rough surfaces during 3D manipulation operations has been studied in this paper by employing the atomic force microscope (AFM). In this regard, the force of adhesion between two surfaces has been simultaneously investigated on the basis of two important approaches. First, the interactions between two surfaces have been evaluated based on the available contact mechanics theories, and depending on the type of biological nanoparticle studied in this research, an appropriate model has been presented. Then, the common adhesion models of Rumpf and Rabinovich have been modified based on the Van der Waals force and with regard to the cylindrical geometry of the nanoparticles. Finally, based on the Schwartz method, by developing the adhesion model based on the two approaches mentioned above, the adhesion force between cylindrical biological nanoparticles and rough surfaces has been extracted for the first time. The dynamic behavior of nanoparticles in 3D manipulation on a rough substrate has been modeled and then simulated and analyzed. The simulation results indicate that, by increasing the asperity radius, the critical force needed to manipulate a nanoparticle increases. Also, in contrast to the sliding mode, the critical force of movement in the rolling mode diminishes with the increase of nanoparticle radius. The experimental results obtained from the topography of glass and silica substrates indicate that the Rumpf model underestimates the adhesion force relative to the Rabinovich model.

Lidocaine HCl and procaine HCl are local anesthetics widely used in minor chirurgic procedures; nevertheless, spectral information about their behavior is not complete at present. In this context, in this paper, absorbance and fluorescence studies of both the drugs with α- and β-cyclodextrins (CDs) have been measured at 298.15 K. The thermodynamic parameters: binding constant (K) and standard state Gibbs energy (∆G°) were calculated for the systems in which complex formation was observed. The α- and β-CD study indicates that the lidocaine HCl and procaine HCl drugs form 1:1 inclusion complex with α- and β-CD. Both experimental and theoretical studies revealed that the phenyl ring with the amide group of the drug is encapsulated in the hydrophobic CD cavity. van der Waals interactions are mainly responsible for complex formation of lidocaine hydrochloride and procaine hydrochloride with CDs.

The research field of oxide heteroepitaxy suffers from the characteristics of misfit strain and substrate clamping, hampering the optimization of performance and the gain of fundamental understanding of oxide systems. Recently, there are demonstrations on functional oxides epitaxially fabricated on layered muscovite substrate. In these heterostructures, due to the weak interaction between substrate and film, they show the lattice of films close to bulk with excellent strictive properties, suggesting that these critical problems can be potentially solved by van der Waals oxide heteroepitaxy. In addition, by exploiting the transparent and flexible features of muscovite, such a heteroepitaxy can deliver new material solutions to transparent soft technology. In this paper, the history, development, and current status of van der Waals oxide heteroepitaxy are addressed and discussed. In the end, new research directions in terms of fundamental study and practical application are proposed to highlight the importance of this research field.

This paper considers the problem of stiction in electrostatic–elastic deflections whereby elastic surfaces adhere to one another after coming into physical contact under attracting Coulomb interactions. This phenomenon is studied in a family of recently derived models which account for forces which become important when the elastic surfaces are in close proximity. The presence of bistability in these models results in hysteresis, or nonreversibility, which accounts for the difficulty in achieving separation after an initial contact event. We use singular perturbation techniques to derive explicit formula for the critical parameters over which bistability occurs and discuss new operational modes which arise when bistability is present.

Along the c axis of the crystal lattice, Bi2Te3 has periodic quintuple layers “ Te1 Bi Te2 Bi Te1–” which are connected by Van der Waals bonding. The weak bonding between Te1–Te1 layers substantially affects the mechanical properties of Bi2Te3. In the work discussed in this paper, the molecular dynamics method was used to study the mechanical properties of cuboid single-crystal of bulk Bi2Te3 under compressive loads. The emphasis was on the effects of the Van der Waals bonding on the deformation and failure mechanism. The molecular dynamics simulation results revealed that Van der Waals bonding has a dominant effect on the mechanism of deformation, and fundamentally determines the ultimate stress and fracture strain. Furthermore, the compressive load along the a and c axes lead to quite different failure behavior, which can be distinguished by their specific effects on the deformation of the Van der Waals bonding. However, only models with the load along the a axis dramatically demonstrate the effect of strain rate on the stress–strain curves, in accordance with the poor structural stability.

In this paper, evaporation of sessile water droplets containing fluorescent polystyrene (PS) microparticles on polydimethylsiloxane (PDMS) surfaces with different curing ratios was studied experimentally using laser confocal microscopy. At the beginning, there were some microparticles located at the contact line and some microparticles moved towards the line. Due to contact angle hysteresis, at first both the contact line and the microparticles were pinned. With the depinning contact line, the microparticles moved together spontaneously. Using the software ImageJ, the location of contact lines at different time were acquired and the circle centers and radii of the contact lines were obtained via the least square method. Then the average distance of two neighbor contact lines at a certain time interval was obtained to characterize the motion of the contact line. Fitting the distance-time curve at the depinning contact line stage with polynomials and differentiating the polynomials with time, we obtained the velocity and acceleration of both the contact line and the microparticles located at the line. The velocity and the maximum acceleration were, respectively, of the orders of 1 µm/s and 20-200 nm/s2, indicating that the motion of the microparticles located at the depinning contact line was quasi-static. Finally, we presented a theoretical model to describe the quasi-static process, which may help in understanding both self-pinning and depinning of microparticles.

In this paper we report the growth of high quality SnS thin films with good crystallinity deposited on two-dimensional (2D) mica substrates. It is believed that the 2D nature of SnS, with strong intra-layer covalent bonds and weak inter-layer van der Waals interactions, is responsible for its relative insensitivity to lattice mismatch. We also investigated the reduction of Sn vacancies in the material using Sn-compensation technique during the material growth process. The experimental results clearly demonstrated substantial enhancements in the electrical and structural properties for films deposited using Sn-compensation technique. A mobility of 51 cm2 V−1 s−1 and an XRD rocking curve full width at half maximum of 0.07° were obtained. Sn-compensated SnS/GaN:Si heterojunctions were fabricated and significant improvement in both the I-V characteristics and the spectral responsivities of the devices were characterized.

This paper studies the dynamic shell buckling behavior of multi-walled carbon nanotubes (MWNTs) embedded in an elastic medium under step axial load based on continuum mechanics model. It is shown that, for occurrence of dynamic shell buckling of MWNTs or MWNTs embedded in an elastic medium, the buckling stress is higher than the critical buckling stress of the corresponding static shell buckling under otherwise identical conditions. Detailed results are demonstrated for dynamic shell buckling of individual double-walled carbon nanotubes (DWNTs) or DWNTs embedded in an elastic medium. A phenomenon is shown that DWNTs or embedded DWNTs in dynamic shell buckling are prone to axisymmetric buckling rather than non-axisymmetric buckling. Numerical results also indicate that the axial buckling form shifts from the lower buckling mode to the higher buckling mode with increasing buckling stress, but the buckling mode is invariable for a certain domain of buckling stress. Further, an approximate analytic formula is presented for the buckling stress and the associated buckling wavelength for dynamic axisymmetric buckling of embedded DWNTs. The effect of radii is also examined.

In this paper, an approach is identified using concepts in molecular dynamics (MD) and discrete element method (DEM) to generate the microstructure of polycrystalline materials. Using the proposed methods, different types of particles with different grain size and volume fraction in the real material, can be easily generated. It is assumed that the particles can be randomly packed together into a simulation region, by defining artificial interaction forces among them. Such forces may be either adopted from Van der Waals potential energy, or Hooke pair and gravity forces. The proposed method has proved to be fast due to the fact that the algorithm has been implemented on graphical processing units (GPU). Utilizing the Voronoi tessellation method, the set of the generated discrete grains have been altered to space-filling, adjoining polyhedrons with respect to the real geometry. Moreover, as an advantage, the boundary and the interface region of the microstructures were modeled.

Few studies in shear band formation have considered the environmental conditions on the Moon, which however are significant for lunar regolith failure in future lunar exploration activities. This paper presents a numerical investigation into the mechanical behavior and strain localization of lunar regolith by means of the discrete element method (DEM). A micromechanical contact model for lunar regolith accounting for van der Waals forces and rolling resistance has been developed, then implemented into a DEM code, PFC2D, and finally applied to analyze the strain localization of lunar regolith through biaxial tests. Biaxial tests without considering van der Waals force effect were also performed as reference to compare with. The distributions inside the sample of grid deformation, void ratio, velocity, averaged pure rotation rate (APR), force chains and local stress during shear banding are analyzed. The simulations show that persistent bands are differently formed under Moon and Earth conditions. Van der Waals forces and rolling resistance play crucial roles in choosing persistent bands from various transient micro-bands before the peak state. Van der Waals forces lead to increased dilation and particle rotation, and enlarged “meso-voids” in force chain distributions within the persistent shear bands. The thickness (inclination to the horizontal) of shear band for the regolith under Moon condition is smaller (larger) than that for regolith under Earth condition.The fields of velocity and APR can reveal the finest heterogeneity in particle displacement (translation and rotation) in the form of transient micro-bands even at the very beginning of shear.

The paper presents how to study the thermal vibration of a double-walled carbon nanotube (DWCNT) by using a model of double-Euler beams, together with the law of energy equipartition, with the energy of van der Waals interaction between layers taken into consideration. The basic finding of the study is the relation, derived via the model of double-Euler beams and the law of energy equipartition, between the temperature and the root-of-mean-squared (RMS) amplitude of the thermal vibration at any cross section of the DWCNT. The molecular dynamics simulations of thermal vibration of the DWCNT in argon atmosphere show that the model of double-Euler beams can predict the RMS amplitude of the thermal vibration of the DWCNT reasonably well.

In our previous paper, we extended the Tao and Mason equation of state (TM EOS) to pure ionic liquids. Here we apply TM EOS based on statistical–mechanical perturbation theory to binary mixtures of ionic liquids. Three temperature-dependent quantities are needed to use the equation of state: the second virial coefficient, B2, effective van der Waals co-volume, b, and a scaling factor, α. The second virial coefficients are calculated from a correlation that uses the normal boiling temperature and normal boiling density. α and b can also be calculated from the second virial coefficient by scaling. In this procedure, the number of input parameters, for calculation of B2, α, and b reduced from 5 (i.e., critical temperature, critical pressure, acetric factor, Boyle temperature TB, and the Boyle volume υB) to 2 (i.e., Tbp and ρbp). At close inspection of the deviations given in this work, the TM EOS predicts the densities with a mean AAD of 1.69%. The density of selected system obtained from the TM EOS has been compared with those calculated from perturbed-hard-sphere equation of state. Our results are in favor of the preference of the TM EOS over another equation of state. The overall average absolute deviation for 428 data points that calculated by perturbed-hard-sphere equation of state is 2.60%.

In our previous paper, we extended the Tao and Mason equation of state (TM EOS) to refrigerant fluids, using the speed of sound data. Here, we predict the equation of state for ionic liquids (ILs). The considered ILs are [Bmim][PF6], [C2mim][NtF2], [C3mim][NtF2], [C6mim][NtF2], [C7mim][NtF2], [C2mim][EtOSO3], [Bmim][MeSO4], [Bmim][OcSO4], and [C4mim][dca]. The equation of state consists of three temperature-dependent parameters: the second virial coefficient, a constant for scaling the softness of repulsive force, and an effective hard-sphere diameter equivalent to the van der Waals co-volume. The second virial coefficients of ILs are scare and there is no accurate potential energy function to allow their theoretical calculation. In this work, the second virial coefficient have been calculated using corresponding states correlation based on temperature and density at normal boiling point. The other two parameters of the equation of state can be calculated using a scaling rule. Analysis of our predicted results shows that the Tao–Mason equation of state is capable of accurately predicting the density of ILs at any temperature and pressure. The overall average absolute deviation densities for 1,633 data points are 2.05%. Also, the density of ILs obtained from the TM EOS has been compared with those calculated from vdW–CS–β and Peng–Robinson (PR) equation of state. Our results are in favor of the preference of the TM EOS over the two other equations of state. The overall average absolute deviation for 1,633 data points calculated by vdW–CS–β and PR equation of state are 6.63% and 12.19%, respectively.

Many biological attachment devices of insects, spiders and geckos consist of arrays of hairs (setae), which are terminated by contact elements of different shapes. However, the most frequently observed shape is a thin plate-like spatula. In spite of a rather wide range of sizes, most spatulae of different animals are not uniform, but rather possess a gradient in thickness and width. Here we show that the spatulae of insects and geckos become gradually thinner and wider approaching the end. This geometrical effect is explained in the present paper, by using a numerical approach for the modelling of the van der Waals adhesion and friction between the contact elements and the substrate. The approach suggests that the observed negative thickness gradient contributes to the improvement of the adhesion resistance, whereas the positive width gradient increases the stability of the detachment, probably a key factor in controlling the animal walking.

This paper reports the results of an investigation into the effect of transverse magnetic fields on dynamic characteristics of multi-walled carbon nanotubes (MWNTs). Couple dynamic equations of MWNTs subjected to a transverse magnetic field are derived and solved by considering the Lorentz magnetic forces induced by a transverse magnetic field exerted on MWCNTs. Results show that the transverse magnetic field exerted on MWNTs makes the lowest frequency of the MWNTs nonlinearly decrease and the highest frequency, changeless. When the strength of applied transverse magnetic fields is larger than a given value the two walls of MWNTs appear in the radial and axial coaxial vibration phenomena.

A lively philosophical debate has lately arisen over the nature of elementhood in chemistry. Two different senses in which the technical term ELEMENT is currently in use by chemists have been identified, leaving chemistry open to the logical fallacy of equivocation. This paper introduces a third, more elemental candidate: the high-enthalpy short-lived unbonded atom. An enthalpy index based on free-atoms-as-elements is established, whereby one can monitor the degree to which an atom’s spin-based attractive force is implemented exo-enthalpically when the atom binds chemically to others. Enthalpy indexing shows that the strength of an atom’s attractive force is proportional to its spin angular momentum. Vibrational spectroscopy shows that the force varies inversely as the fourth power of the inter-atom distance. Both features differentiate the chemical force from the stronger electromagnetic force and from the weaker Van der Waals force.

Axisymmetric vibrations of multi-walled carbon nanotubes (MWCNTs) with finite length are investigated in this paper. A multi-walled carbon nanotube is modeled with multiple elastic isotropic shells. Based on a continuum model and considering van der Waals forces between tubes, a two-dimensional finite element model is developed to obtain axisymmetric natural frequencies and mode shapes of MWCNTs. First, the axisymmetric vibrational characteristics of single-walled carbon nanotubes are investigated, and then, they are compared with those of MWCNTs. The effects of van der Waals forces on radial vibration of MWCNTs are also explained. Moreover, influences of end conditions on radial and axial natural frequencies of carbon nanotubes (CNTs) with wide range of length-to-thickness ratio are studied by considering the free-free (F-F) and simply supported (S-S) end conditions. Besides, it is focused on dependence of axisymmetric mode frequencies on dimensional parameters such as length, diameter, as well as number of layers of MWCNTs. Through this, explicit expressions are found for calculating the radial breathing mode and longitudinal mode frequencies of MWCNTs.

The capabilities of the proposed nonlocal beam models in the companion paper in capturing the critical velocity of a moving nanoparticle as well as the dynamic response of double-walled carbon nano- tubes (DWCNTs) under a moving nanoparticle are scrutinized in some detail. The role of the small-scale effect parameter, slenderness of DWCNTs and velocity of the moving nanoparticle on dynamic deflections and nonlocal bending moments of the innermost and outermost tubes as well as their maximum values are then investigated. The results reveal that the critical velocity increases with the slenderness of DWCNTs and the magnitude of the van der Waals interaction force. Nevertheless, the critical velocity generally decreases with the small-scale effect as well as the ratio of the mean diameter to the thickness of the innermost tube. Additionally, the predicted maximum dynamic deflections and nonlocal bending moments of the innermost and outermost tubes by using the nonlocal Euler-Bernoulli and Timoshenko beam theories are generally the lower and upper bounds of those obtained by the nonlocal higher-order beam theory (NHOBT). In the case of λ1 < 20, the use of the NHOBT is highly recommended for more realistic prediction of dynamic response of DWCNTs under a moving nanoparticle.

Harmonic dynamical behaviour of thallous halides (TlCl and TlBr) have been studied using the new van der Waals three-body force shell model (VTSM), which incorporates the effects of the van der Waals interaction along with long-range Coulomb interactions, three-body interactions and short-range second neighbour interactions in the framework of rigid shell model (RSM). Phonon dispersion curves (PDC), variations of Debye temperature with absolute temperature and phonon density of state (PDS) curves have been reported for thallous halides using VTSM. Comparison of experimental values with those of VTSM and TSM are also reported in the paper and a good agreement between experimental and VTSM values has been found, from which it may be inferred that the incorporation of van der Waals interactions is essential for the complete harmonic dynamical behaviour of thallous halides.

This paper investigates surface instability of a functionally graded and layered elastic film interacting with another flat rigid body [or interacting with another functionally graded and layered elastic film (or simply-supported elastic plate)] through surface van der Waals forces under plane strain conditions. The shear modulus in each functionally graded layer is assumed to be exponentially varied in the thickness direction. A homogeneous elastic layer, which is the focus of this research, can be considered as a special case of the functionally graded layer by taking the magnitude of the gradient parameter to be very small. The solution for any functionally graded layer is obtained in terms of the pseudo-Stroh formalism; then the solution for the multilayered system is derived based on the transfer-matrix method. As a result the displacement and traction vectors at the top surface of the layered film (or plate) can be expressed in terms of those at the bottom surface of the layered film (or plate). We can thus obtain simple relationships between the surface normal traction and surface deflection. Expressions for the interaction coefficient as a function of the wave number of the instability mode are therefore obtained. The critical value of the interaction coefficient for surface instability and the associated instability mode can be determined easily by identifying the minimum of the interaction coefficient. An advantage of the present method lies in that it is very convenient to address a film (or a plate) with an arbitrary number of layers. The correctness of the present approach is verified by comparison with known results. The results show that it is possible to find N distinct surface instability modes for an N-layered elastic film interacting with another flat rigid body; and that it is also possible to find that there are at most N1+N2 distinct surface instability modes for an N1-layered elastic film interacting with another N2-layered elastic film. When a multilayered elastic film interacts with a simply-supported multilayered elastic plate, the film-plate system will exhibit the instability mode of the film or that of the plate depending on the stability strength of the plate versus that of the film.

This paper investigated the formation of chain aggregates from fine particles suspended in gas stream onto material surfaces under the action of electric field. The results showed that the shape of aggregate formed on material surface was greatly influenced by the field intensity and the surface condition of materials. In a weak electric field without corona discharge, particles tended to form clustered aggregates on a metal plate with smooth surface, but on a metal mesh and a porous alumina substrate, to form chain aggregate. On the other hand, in a corona discharge field, these surfaces were coated uniformly. Consequently, for forming chain aggregates on material surface, an electric field without corona discharge and a rough surface are necessary conditions. On rough surface, chain aggregates of dielectric particles or conductive particles grew from the protrusions of the surface and could form a rough and porous layer. When the external electric field was removed, the chain aggregates remained long time due to the Van der Waals forces. After sintered at proper temperature, the chain aggregates became fiber-like. The results indicate that the formation of chain aggregate can be controlled by electrostatic force, and sintering can be used as a method for increasing their mechanical strength.

The dynamical stability of carbon nanotubes embedded in an elastic matrix under time-dependent axial loading is studied in this paper. The effects of van der Waals interaction forces between the inner and outer walls of nanotubes are taken into account. Using continuum mechanics, we apply an elastic layered shell model to solve the transverse parametric vibrations of a carbon nanotube. Both the Gaussian wide-band axial temperature changes and physically realizable temperature changes with known probability distributions are assumed as the tube axial loading. The energy-like functionals are used in the stability analysis. The emphasis is placed on a qualitative analysis of dynamic stability problem. Stability domains in the space of geometric, material and loading parameters are presented in analytical forms.

Debonding of a single CNT (carbon nanotube) from the midway or one end of a CNT rope is studied in this paper. The analysis of buckling-driven debonding of an individual CNT is based on an elastic beam model, while the local debonding growth, stability and arrest are explained by the energy release rate criterion. The debonding from a preloaded CNT rope is driven by excess energy released from the unbuckled to the buckled state of an individual CNT. Some interesting debonding behaviors are displayed for varying dimensions, compression strain and van der Waals (vdW) adhesive energy. The growth of the debonding may be stable, unstable or unstable growth followed by a stable growth when an initial debonding CNT rope is loaded.

This paper investigates the intertubular van der Waals interactions that produce the initial cross sectional distortion of single-walled carbon nanotubes during a bundle formation. By combining the analysis of molecular dynamics with the continuum mechanics, the distributions of the van der Waals forces were determined. The dependence of the load parameters, deformation variables and the lattice constant on the nanotube radius, was also investigated. It was found that the van der Waals forces are attractive and vary circumferentially in a harmonic manner. For the considered zigzag nanotubes of radius 7–14 A, the intensity of the van der Waals forces is radius-dependent and can be as large as 6–11 GPa in the channels of the bundle and 1–5 GPa at the closest points between the single-walled nanotubes.

In this paper we employ the hydrodynamic formulation of time-dependent density functional theory to obtain the van der Waals coefficientsC 6 andC 8 of alkali metal clusters of various sizes including very large clusters. Such calculations become computationally very demanding in the orbital-based Kohn-Sham formalism, but are quite simple in the hydrodynamic approach. We show that for interactions between the clusters of the same sizes,C 6 andC 8 scale as the sixth and the eighth power of the cluster radius, respectively, and approach their classically predicted values for the large size clusters.

In this paper, we use complex variable techniques to analyze the surface instability of a harmonic solid attracted by a rigid body through the influence of van der Waals forces. Our results indicate that the surface of the solid is always unstable, and the parameter α, characterizing the solid, plays an important role in its surface instability.

In this paper, vibrations, instability, and control of nanotube- and nanowire-based nanoelectromechanical systems (NEMS) are investigated. A dynamic analysis is presented, which includes the role of the electric field, van der Waals forces, temperature, and the uncertainty principle. Furthermore, the system instability, arising at the so-called pull-in voltage and corresponding to the on-off transition of the device, is also quantified. Active current and voltage controls based on the quantum tunneling effect are discussed. It is shown that the tunneling current between the cantilever nanotube/nanowire tip and the electrode substrate would correspond to the realization of an analogic device (e.g., nanotweezers) if the current is in control or to a digital device (e.g., nanoswitch) if the voltage is controlled. Finally, the characteristic electromechanical curves voltage/current-displacement for the NEMS are deduced.

In a previous publication, we reported a new cell model for the equation of state of polymer melts. The equation contains external degrees of freedom explicitly as a function of molecular weight of repeating unit. In this paper, we apply the theory to 20 polymer melts and a blend melt. The linear lengths of the coarse-grained particle (CGP) of several polymers coincide with those obtained by scattering experiments. In addition, we estimated the characteristic quantity T* of the equation of state per repeating unit using van der Waals constants and the critical temperature of the repeating unit or monomer. The values, T*, agreed with those from our result per repeating unit. This result suggests the quantitative validity of our theory. We strongly suggest that our theory must be related to the conformer model by Matsuoka and the helical wormlike chain model by Yamakawa, because T* must be dependent on the chemical structure and stiffness of the polymers.