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This paper proposes a hybrid algorithm based on the ant colony and Physarum Polycephalum algorithms. The positive feedback mechanism is used to find the globally optimal path. The crossover and mutation operations of the genetic algorithm are introduced into the path search mechanism for the first time. The Van der Waals force is applied to the pheromone updating mechanism. Simulation results show that the improved algorithm has advantages in quality and speed of solution compared with other mainstream algorithms. This paper provides fast and accurate route methods for solving the Traveling Salesman Problem first and a delivery scheme is also presented for UAVs to realize "contactless delivery" to users in the Changchun Mingzhu District during the COVID-19 epidemic, which confirms the practicability and robustness of the algorithm. [ABSTRACT FROM AUTHOR]

The Lamb shift, one of the most fundamental interactions in atomic physics, arises from the interaction of H atoms with the electromagnetic fluctuations of the quantum vacuum. The energy shift has been computed in a variety of ways. The energy shift, as Feynman, Power, and Milonni demonstrated, equals the change in the vacuum energy in the volume containing the H atoms due to the change in the index of refraction arising from the presence of the H atoms. Using this result and a group theoretical calculation of the contribution to the Lamb shift from each frequency of the vacuum fluctuations, in this paper we obtain an expression for the region of the vacuum energy for each frequency ω around the H atom due to the Lamb shift. This same field plays an essential role in the van der Waals force. We show the ground state atom is surrounded by a region of positive vacuum energy that extends well beyond the atom for low frequencies. This region can be described as a steady state cloud of vacuum fluctuations. For energies E = ℏ ω less than 1 eV, where ℏ is the reduced Planck constant and ω is frequency, the radius of the positive energy region is shown to be approximately 14.4/E Å. For a vacuum fluctuation of wavelength, λ , the radius is (α / 2 π) λ , where α is the fine-structure constant. Thus, for long wavelengths, the region has macroscopic dimensions. The energy–time uncertainty relation predicts a maximum possible radius that is larger than the radius based on the radiative shift calculations by a factor of 1 / 4 α . [ABSTRACT FROM AUTHOR]

Casimir–van der Waals forces are important in the self-assembly processes of nanoparticles. In this paper, using a hybrid approach based on Lifshitz theory of Casimir–van der Waals interactions and corrections due to the shape of the nanoparticles, it is shown that for non-spherical nanoparticles, the usual Hamaker approach overestimates the magnitude of the interaction. In particular, the study considers nanoplates of different thicknesses, nanocubes assembled with their faces parallel to each other, and tilted nanocubes, where the main interaction is between edges. [ABSTRACT FROM AUTHOR]

[Display omitted] • CsPbBr 3 with quantum-limited domain effects have reversible hydrochromic properties. • Different Organic solvents cause polarity-related discoloration of composites. • A detection model was developed for all-organic solvent polarity detection. Hydrochromic materials have received much attention in anticounterfeiting due to their reversible photoemission properties toward water. However, existing hydrochromic materials usually suffer from slow hydrochromic response and limited stability, repeatability, and color tunability. This paper found for the first time that CsPbBr 3 /Cs 4 PbBr 6 composites with quantum-limited domain effects had reversible hydrochromism. CsPbBr 3 /Cs 4 PbBr 6 composites were successfully synthesized in molecular sieve MCM-41 using an improved mechanochemical method. The composites underwent a reversible phase transition from Cs 4 PbBr 6 /CsPbBr 3 to CsPbBr 3 upon treatment with a trace amount of water, the photoluminescence colors underwent a change from blue to green, and reversion occurred due to heating. Mechanistic investigation revealed that quantum dots underwent soft agglomeration due to van der Waals forces in polar solvent environments, affecting the action of quantum domain-limiting effects. Therefore, organic solvents with different polarities can cause different degrees of reversible discoloration of the material. This study provides new ideas for understanding the phase transition and agglomeration of metal halide perovskite quantum dots in polar solvent environments. Additionally, it offered new options for aqueous color-changing materials and organic solvent polar detection materials. [ABSTRACT FROM AUTHOR]

The development of micromanipulation technology has an important impact on the research in the microfield, which mainly focuses on the preparation and assembly of microcomponents, in which the microcomponent picking technology is used. The existing picking methods mainly use manipulation tools to clamp or adsorb components, which may destroy or fail to pick objects in the process of manipulation, and the efficiency is relatively low. In order to solve the problems in the existing manipulation methods, a new metal microcomponent picking method based on electrochemistry is proposed, which mainly uses the electrochemical principle to produce metal deposition, connect the manipulation tool and the object, and indirectly control the object by moving the manipulation tool. In this paper, the simulation software ABAQUS is used to pick up the microcopper wires with the length of 100, 200 and 300 μ m by using a pipette with a nozzle diameter of 15 μ m. The force curve between the tool and the object during the manipulation process is obtained. The feasibility of the method is verified by theoretical analysis and simulation experiments. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we have presented generalized closed form solutions to estimate the permissible dimensional limits for the basic design parameters as well as the accurate pull-in characteristics of electrostatically actuated fixed-fixed nano beam. The coupled electromechanical problem for the nano beam is formulated using Galerkin's approximation while considering the effects of fringe field, mid-plane stretching and intermolecular forces like van der Waals and Casimir. The resulting integrals and higher order polynomials are solved numerically to determine the closed form expressions for allowable detachment length, minimum feasible gap, and critical pull-in parameters. The derived expressions are compared quantitatively with other reported results which show reasonable agreement. The advantage here is that the derived expressions do not involve any complex mathematical operation unlike in existing literature, and thus the expressions can be readily used for quick and accurate estimation of the design parameters. As the applied voltage exceeds beyond pull-in limit, it can lead to severe structural failure during operation; hence the designers must be aware of the accurate pull-in expressions as derived in this paper. It has been observed that the incorporation of the intermolecular forces together with the fringe effect leads to significant reduction in pull-in voltage whereas the mid-plane stretching parameter shows the opposite effect. It is further envisaged that, the proposed generalized closed form expressions will serve as quick guidelines for reliable and safe design and fabrication of various similar electrostatically actuated nanosystems. Highlights • We present general closed form solutions for the threshold for design parameters and pull-in voltage of fixed-fixed nanobeam. • This study ensures reliable and safe design of nanobeam without having structural failure during fabrication or operation. • Coupled electromechanical problem is solved considering the fringe field, mid-plane stretching and intermolecular forces. • The derived expressions are simple and they do not involve complex mathematical operation unlike in reported literature. • The derived closed form expressions will serve as quick guidelines for optimum design and successful fabrication of nanobeam. [ABSTRACT FROM AUTHOR]

Roughness and microstructures at the micro and the nano scales have significant influence on the van der Waals forces between the adhering particles and the surface. Previous theoretical models may treat the particle-asperity interactions in an unprecise manner, which may result in inaccurate estimations of the van der Waals force especially when the sphere is much larger than the asperities. In this paper, we proposed a numerical model to calculate the van der Waals force between a spherical particle and a rough surface with periodic nanostructures. The 3D sinusoidal rough surfaces are simplified with their asperities modelled as hemispheres, and the contact is within the Derjaguin-Muller-Toporov (DMT) regime. The proposed model calculates the van der Waals forces between the sphere and all the asperities in the contact surface, which will provide more accurate estimations of the van der Waals force. Moreover, the effect of the planar distribution of asperities on the van der Waals forces is considered, which includes the cases when asperities have close-packed hexagonal lattice, square, and hollow-centered hexagonal packing distributions. The theoretical results calculated using the proposed model are verified by comparing the results to previous theoretical and experimental results and the comparison shows that the proposed numerical model predicts the van der Waals force accurately. • The proposed model in this study considers the contributions to the van der Waals forces from all the asperities under the spherical particle. • This predicts the van der Waals force more precisely than previous models. [Display omitted] • A model has been proposed to calculate the particle-surface van der Waals forces. • The proposed model considered the effect of planar distribution of asperities. • This model can be extended to surfaces with different periodic microstructures. [ABSTRACT FROM AUTHOR]

In this paper, a numerical approach is suggested to find a semi-analytical solution for the common nonlinear boundary value problems (BVPs) of cantilever-type micro-electromechanical system (MEMS) and nano-electromechanical system (NEMS) with a set of model parameters. The nonlinear BVPs that are studied involve the states of the single and double cantilever-shaped beams under the influence of Casimir and Van der Waals force for proper distances of separation. The method is based upon an integral operator that is formed considering Green's function connected with the execution of Picard's or Mann's fixed point schemes. The numerical results for different cases of beam are presented and compared with those obtained by previous works to show the applicability, efficiency, and high accuracy of the suggested method. [ABSTRACT FROM AUTHOR]

• The SPH method is developed to model the van der Waals force. • We utilize SPH to simulate the rupture of thin liquid film without resort to lubrication theory. • We introduce the coordinate transformation in SPH to handle the rupture of thin liquid films. • Both van der Waals force and surface tension are imposed on the outmost layer of SPH particles. The rupture of thin liquid films driven by the van der Waals force is of significance in many engineering processes, and most previous studies have relied on the lubrication approximation. In this paper, we develop a smoothed particle hydrodynamics (SPH) representation for the van der Waals force and simulate the rupture of thin liquid films without resort to lubrication theory. The van der Waals force in SPH is only imposed on one layer, i.e., the outermost layer of fluid particles, where a weighting function is deployed to evaluate the contributions of particles on or near the interface. However, to obtain an accurate hydrostatic pressure in reaction to the van der Waals force, a smaller smoothing length is used for the calculation of the weighting function than that used for SPH discretizations of the bulk fluid. The same surface particles are also used to model the surface tension. To deal with the rupture of a thin liquid film with a very small aspect ratio ε (ε = thickness/length), a coordinate transformation is introduced to shrink the length of the liquid film to achieve accurate numerical resolution with a manageable number of particles. As verifications of our physical model and numerical algorithm, we simulate the hydrostatic pressure in a stationary film and the relaxation of an initially square droplet and compare the SPH results with the analytical solutions. The method is then applied to simulate the rupture of thin liquid films with moderate and small aspect ratios (ε = 0.5 and 0.005). The convergence of the method is verified by refining particle spacing to four different levels. The effect of the capillary number on the rupture process is analyzed. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) is a rapid and flexible technique for the production of metal parts and prototypes from metal powders. The quality of parts manufactured using AM is a strong function of powder physical and mechanical properties. Previous work has shown that powders with a large fraction of fine particles produce better parts in terms of smooth finished surface and high mass density. However, an excessive fine fraction in the source powder causes serious flowability issues, leading to unexpected voids or discontinuities in the finished product. This effect of the fine fraction on the flowability of metal powder is widely encountered but poorly understood in the AM industry. This study presents a three-dimensional (3D) discrete element method (DEM) model to simulate the microscale mechanisms of powder flow considering the effects of van der Waals force. This microscale force has a negligible influence on the flowability of coarse grains, but the effect becomes the dominant factor governing the behavior of fine fractions (micrometer scale). The results show that the numerical model presented in this paper is capable of reproducing the experimental dependency of the powder flowability on the fine fraction. Moreover, it also successfully captures the characteristics of particle flow under the influence of microscale van der Waals force. Image 1 • Powder flowability can be hindered by the van der Waals force effect. • Force ratio and potential energy ratio can be used to quantify powder flowability. • Spreadability increases and then decreases with adding fines. • The flowability decreases with increasing fine fraction. [ABSTRACT FROM AUTHOR]

A novel ionization/sampling method termed triboionization was developed. Triboionization is an ionization method that only uses cohesive substances, such as food wrap or sticky tape, and does not require an electrode, electric power supply, heat source, light source, radiation, or gas, unlike most other conventional ambient ionization methods. In this study, the sample compound attached to adhesive tape or plastic wrap was quickly peeled off at a distance of approximately 2 cm from the atmospheric interface of a mass spectrometer. All of the five types of food wrap and 13 types of adhesive tape tested successfully ionized caffeine. Nine out of ten model compounds were detected as the corresponding molecular ions in the positive or negative mode by this ionizing contrivance using an oriented polypropylene adhesive tape. The detected molecular ions were typically protonated molecules or sodium adducts in the positive mode or deprotonated molecules in the negative mode. The elemental compositions of the observed ions were confirmed within 5 ppm by high-resolution mass spectrometry. The triboionization phenomenon was considered to depend on physical and electronic events caused by peeling off a cohesive substance. Triboionization is able to provide a compact ion source using only mechanical mechanisms. Additionally, triboionization allows sticky tape to be used as a convenient sampling device for surface analysis. [ABSTRACT FROM AUTHOR]

Purpose This paper aims to investigate the effect of changing speed of the entraining motion on the formation of ultra-thin lubricating films under different elliptical ratios. The ellipticity parameter (K) varied from 1 (a ball-on-plate configuration) to 6 (a configuration approaching line contact). The influence of the ellipticity parameters, the dimensionless speed and the effects of surface forces on the formation of the minimum film thickness has been demonstrated. The demarcation boundary between region dominated by elastohydrodynamic lubrication (EHL) and that by the surface force action has been demonstrated for different elliptical ratios.Design/methodology/approach The numerical solution has been carried out, using the Newton–Raphson iteration technique, applied for the convergence of the hydrodynamic pressure. The film thickness and pressure distribution are obtained by simultaneous solution of the Reynolds’ equation, the elastic deformation (caused by hydrodynamic pressure, surface force of solvation and Van der Waals force) and the load balance equation. The operating conditions, load and speed of entraining motion, promote formation of ultra-thin films that are formed under the combined action of EHL, surface contact force of solvation and molecular interactions due to presence of Van der Waals force.Findings The paper provides insights about the transition between region dominated by EHL and that by the surface force action for changing ellipticity ratio (K) from 1 (a ball-on-plate configuration) to 6 (a configuration approaching line contact).Originality/value This paper fulfils an identified need to study the effect of changing ellipticity ratio on the formation of ultra-thin films that are formed under the combined action of EHL, surface contact force of solvation and molecular interactions due to presence of Van der Waals force. [ABSTRACT FROM AUTHOR]

In this paper, the vibration of an atomic force microscope (AFM) cantilever in tapping mode with two whole piezoelectric layers submerged in liquid medium is investigated. In the performed modeling, the sample surface has been considered as rough, and to show these surface asperities, two models of Rumpf and Rabinovich have been employed for analyzing the attractive Van der Waals force. This paper has been organized in two sections. The first section deals with the functioning of cantilever over rough surfaces, which accompanies the changes of the attractive Van der Waals force, and the second section involves the changes in the Van der Waals force which lead to a change in the liquid medium. The cantilever is totally submerged in the liquid. To show the effect of liquid on cantilever, first, only the cantilever tip is immersed in the liquid and it is dynamically analyzed. Then, the cantilever is totally submerged and then taken out of the liquid, so that the additional mass and damping of the cantilever could be calculated. In these two manners of cantilever immersion in liquid, the effects of the added mass and damping on the cantilever can be measured. When a cantilever vibrates totally in liquid, since the mass and damping of the liquid that is present on the cantilever cannot be determined, first, the cantilever's natural frequency in liquid is estimated in the laboratory and then by using this frequency and the cantilever stiffness (which is not medium-dependent and is always considered as constant), the additional mass and damping of the cantilever are determined. [ABSTRACT FROM AUTHOR]

In this paper, the Van der Waals force acting between a spherical nanoparticle and a surface with random distribution of asperities has been simulated using the Johnson-Kendall-Roberts (JKR) contact mechanics model. To predict the adhesion force in different mediums and under different contact conditions, it is necessary to calculate the adhesion force in micro/nano systems. For this purpose, four different cases of surface roughness have been studied. The geometries of these rough surfaces have been defined taking into account the two key parameters of asperity radius and asperity height, and the necessary equations to determine the contact area and the magnitude of the adhesion force have been derived. Based on the geometries of the surface cap/trough, the interaction force between a spherical nanoparticle and a rough substrate has been obtained by applying Van der Waals theory. Then, the adhesion force for spherical nanoparticle has been modeled with respect to its height above the rough surface. Finally, the behavior of nanoparticles with different radii on four different rough substrates have been simulated and analyzed. The simulation results show that more precise adhesion forces can be obtained by modelling the surface roughness at the nanoscale. For small distances between spherical nanoparticle and the substrate, the adhesion force determined with the new roughness models is larger for rough substrates than for smooth ones. [ABSTRACT FROM AUTHOR]

This paper investigates the mechanical properties of CNT-reinforced composites via meshless method, and material nonlinearity of its constituents is considered. An equivalent continuum simulation is employed to evaluate the microstructure of CNTs, based on that, both two-dimensional simplified and three-dimensional models are introduced separately. A plasticity model is adopted in the interface between reinforcement and matrix. The effect of van der Waals forces between CNTs is also considered by uniformly aligned transverse springs, which are continuously connected to nanotubes along their length. Parametric studies of the mechanical properties with varying spring stiffness are carried out to prove the suitability of Winkler’s multi-spring model. The governing equations of motion are obtained by establishing energy functional, and displacement fields in the spatial domain are discretized by the moving least-squares (MLS) approximation. The effects of length of reinforcement, node distribution and influence domain are also investigated, and the stability and precision of the presented meshless method are validated by convergence and comparison studies. [ABSTRACT FROM AUTHOR]

Purpose The purpose of this is to study the mechanism of an oil film thickness formation in the nanoscale. A polar lubricant of propylene carbonate is used as the intervening liquid between contiguous bodies in concentrated contacts. A pressure caused by the hydrodynamic viscous action in addition to the double-layer electrostatic force, van der Waals inter-molecular forces and solvation pressure owing to inter-surface forces is considered when calculating the ultrathin lubricating films.Design/methodology/approach Using the Newton–Raphson iteration technique applied for the convergence of the hydrodynamic pressure, a numerical solution has been ascertained.Findings The results show that, at separations beyond about five molecular diameters of the intervening liquid, the formation of a lubricant film thickness is governed by the combined effects of viscous action and surface force of an attractive van der Waals force and a repulsive double-layer force. At smaller separations below five molecular diameters of the intervening liquid, the effect of the solvation force is dominant in determining the oil film thickness.Originality/value This paper fulfils an identified need to study the behavior of polar lubricants in concentrated contacts in ultrathin conjunctions. The effect of the hydrodynamic action, electrostatic force and surface action of van der Waals and solvation forces is considered when calculating the lubricant oil film thickness. [ABSTRACT FROM AUTHOR]

Experiments depict that the physico-mechanical response of miniature devices is microstructure-dependent. However, the classic continuum theory cannot correctly predict the microstructure-dependency. In this paper, the strain gradient theory is employed to examine the dynamic behavior and instability characteristics of miniature varactor with trapezoidal geometry. The governing equation of the varactor is obtained incorporating the effects of Coulomb force, van der Waals (vdW) attraction, squeeze film damping and structural damping. The influences of microstructure on the dynamic instability of equilibrium points are studied by plotting the phase portrait and bifurcation diagrams. It is found that increase in the microstructure parameter enhances the torsional stability. In the presence of the applied voltage, the phase portrait shows the saddle-node bifurcation while for free-standing varactor a subcritical pitchfork bifurcation is observed. [ABSTRACT FROM AUTHOR]

This paper aims at investigating free vibrations and stability of double-walled carbon nanotube (DWCNT)-based spinning nanobearings. The so-called nanobearing consists of two coaxial carbon nanotubes (CNTs) where either of the two CNTs can be a rotor while the other takes the role of stator. Euler-Bernoulli beam model along with the Eringen's nonlocal theory of elasticity are employed to obtain governing equations of transverse vibrations for the CNTs. The coupling of the two CNTs originates from the van-der-Waals (vdW) forcing present in the interface of the two CNTs. The coupling is taken into account as distributed spring foundation with an equivalent elastic stiffness. Based on the obtained model, effects of small-scale parameter, vdW interaction between CNTs and the diameter ratio of CNTs on the critical spinning speed are investigated. Finally, the stability margins of the nanobearings are determined and some general conclusions are drawn. [ABSTRACT FROM AUTHOR]

In contrast to terrestrial environment, the harsh lunar environment conditions include lower gravity acceleration, ultra-high vacuum and high (low) temperature in the daytime (night-time). This paper focuses on the effects of those mentioned features on soil cutting tests, a simplified excavation test, to reduce the risk of lunar excavation missions. Soil behavior and blade performance were analyzed under different environmental conditions. The results show that: (1) the cutting resistance and the energy consumption increase linearly with the gravity. The bending moment has a bigger increasing rate in low gravity fields due to a decreasing moment arm; (2) the cutting resistance, energy consumption and bending moment increase significantly because of the raised soil strength on the lunar environment, especially in low gravity fields. Under the lunar environment, the proportions of cutting resistance, bending moment and energy consumption due to the effect of the van der Waals forces are significant. Thus, they should be taken into consideration when planning excavations on the Moon. Therefore, considering that the maximum frictional force between the excavator and the lunar surface is proportional to the gravity acceleration, the same excavator that works efficiently on the Earth may not be able to work properly on the Moon. [ABSTRACT FROM AUTHOR]

In this paper we derive explicit expressions for the Green’s functions in the case of an anisotropic elastic half-space and bimaterial subjected to a line force and a line dislocation. In contrast to previous studies in this area, our analysis includes the contributions of both anisotropic surface elasticity and surface van der Waals interaction forces. By means of the Stroh sextic formalism, analytical continuation and the state-space approach, the corresponding boundary value problem is reduced to a system of six (for a half-space) or 12 (for a bimaterial) coupled first-order differential equations. By employing the orthogonality relations among the corresponding eigenvectors, the coupled system of differential equations is further decoupled to six (for a half-space) or 12 (for a bimaterial) independent first-order differential equations. The latter is solved analytically using exponential integrals. In addition, we identify four and seven non-zero intrinsic material lengths for a half-space and a bimaterial, respectively, due entirely to the incorporation of the surface elasticity and surface van der Waal forces. We prove that these material lengths can be only either real and positive or complex conjugates with positive real parts. [ABSTRACT FROM AUTHOR]

Recently, electrostatic torsional nanomirrors are applied widely in various industries. In previous lectures, coefficient of squeeze film air damping in the gap between main plate and substrate has been constant, while this coefficient is the function of viscosity and air pressure and has nonlinear equation. In this paper, the effect of nonzero initial conditions also, and the effect of nonlinear coefficient of squeeze film air damping on the stability of torsional nanomirrors using higher order continuum theories are investigated. First considering two-degree-of-freedom coupled bending/torsion system, the governing equations of system are derived, then using the fourth order Runge-Kutta method and nonzero initial conditions, the equations of motion are solved. Dynamic pull-in instability voltage is obtained and by plotting of response curves and phase portraits, the size effect is investigated. Study of phase portraits shows that size effect causes increasing in the range of stability of nanomirror and increasing of length scale parameter increases the mentioned surface area. As well as assuming constant squeeze film air damping causes the error in investigating of its behavior. Therefore in equation of motion, for getting more accurate response, this coefficient must be considered nonlinear. [ABSTRACT FROM AUTHOR]

The physico-mechanical behavior of nanoscale devices might be microstructure dependent. However, the classical continuum theory cannot correctly predict the microstructure dependency. In this paper, the strain gradient theory is employed to examine the instability characteristics of a nanoscanner with circular geometry. The governing equation of the scanner is derived incorporating the Coulomb and van der Waals (vdW) forces. The influences of applied voltage, squeeze damping and microstructure parameters on the dynamic instability of equilibrium points are studied by plotting the phase portrait and bifurcation diagrams. In the presence of the applied voltage, the phase portrait shows the saddle-node bifurcation while for freestanding scanner a subcritical pitchfork bifurcation is observed. It is concluded that the microstructure parameter enhances the torsional stability. [ABSTRACT FROM AUTHOR]

Abstract: Van der Waals forces often dominate interactions and adhesion between fine particles and, in turn, decisively influence the bulk behaviour of powders. However, so far there is no effective means to characterize the adhesive behaviour of such particles. A complication is that most powder particles have rough surfaces, and it is the asperities on the surfaces that touch, confounding the actual surface that is in contact. Conventional approaches using surface energy provide limited information regarding adhesion, and pull-off forces measured through atomic force microscope (AFM) are highly variable and difficult to interpret. In this paper we develop a model which combines the Rumpf–Rabinovich and the JKR–DMT theories to account simultaneously for the effects of surface roughness and deformation on adhesion. This is applied to a ‘characteristic asperity’ which may be easily obtained from AFM measurements. The concept of adhesiveness, a material property reflecting the influences of elastic deformability, surface roughness, and interfacial surface energy, is introduced as an efficient and quantitative measure of the adhering tendency of a powder. Furthermore, a novel concept of specific adhesiveness is proposed as a convenient tool for characterizing and benchmarking solid materials. This paper provides an example to illustrate the use of the proposed theories. [Copyright &y& Elsevier]

Abstract: The present paper examines the instability of a double-walled carbon nanotube embedded in an elastic medium under pure bending. Effect of surrounding elastic medium and van der Waals forces between the inner and outer nanotubes is taken into consideration. From the point of view of continuum modeling, an elastic double-shell model is presented for the pure bending buckling of a double-walled carbon nanotube. Based on this model, a condition is derived in terms of the buckling modes of the shell, from which the critical bending moment can be predicted. The paper emphasizes bifurcation instability. It is shown that buckling may occur. Finally, a simplified analysis is carried out to estimate the moment causing bifurcation instability of the double-walled carbon nanotube. [Copyright &y& Elsevier]

The van der Waals force is significant for the formation of microstructures of wind-blown deposits, such as loess soil. However, it is difficult to observe the effect of the van der Waals force in the natural environment and laboratory. The conventional DDA model, which simplifies contact as compressed elastic springs, has been successfully applied in simulating blocky rock behaviors, but it is incapable of simulating tension forces. In this paper, we modified the conventional DDA model by considering blocks as soil particles and by applying the van der Waals force among the particles to investigate the micro-behavior of fine soil. The modified DDA is validated by simulating two ideal experiments. Then, the falling process of fine grains (<65 μm), similar to wind-blown dust, is simulated by the modified and conventional DDA models separately. The simulated deposits of the falling grains by the two models show significant differences in microstructure. The microstructure generated by the modified DDA model is much closer to the loose structure of a real loess specimens, while that generated by the conventional DDA model is much more compact. This verifies that the van der Waals force is crucial for the formation of the loose microstructure of loess. [ABSTRACT FROM AUTHOR]

Si surfaces were functionalized by anodization to create nanoneedles. Such surfaces allow Velcro-like bonding at room temperature with large bond strength for such functionalized surfaces. In addition, Velcro-like bonding has a capability of multiple bonding and debonding of the same surfaces analogous to the Velcro principle. To employ such surfaces and the bonding technique in wafer or chip bonding or in integrated circuit packaging applications, understanding the fabrication and the bonding mechanism between such surfaces is very crucial. This paper presents a simple silicon-based technology to generate Velcro-like surfaces using the anodization of Si wafer in a hydrofluoric acid-based electrolyte, and describes the bond strength between such surfaces using the van der Waals force approach. The capability of the technique and the validity of the proposed van der Waals force model are shown by comparison with the experimental results. [2015-0308] [ABSTRACT FROM PUBLISHER]

For many reactions performed in fluidized beds, the mechanical stability of particles is of major importance, and yet, despite this, previous studies have disagreed on the elutriated amount and origin of fine particles. This paper therefore investigates the attrition of particles in bubbling fluidized beds in more detail to determine whether these differences are due to electrostatic or other interparticle forces. To achieve this, the forces acting during fluidization are mathematically derived, and two particle types (alumina and dolomite) are experimentally investigated. Based on the results obtained, a simple model is developed to describe the attrition of both particle types. [ABSTRACT FROM AUTHOR]

It is well known that surface effect forces, such as van der Waals force and capillary force, are the major contributions to adhesion when microsized particles are in contact in humid environment. But it is very complex to calculate the adhesion force between two smooth unequal particles. In conventional approaches, the effective particle radius approximation and the constant half-filling angle assumptions are often used for computing the van der Waals forces between two microparticles. However, the approximation and the assumption are actually difficult to accurately model the forces between unequal particle sizes when the surfaces are with different properties. In this paper, we present a theoretical study of the van der Waals force and capillary force between two microparticles with different radii and the surface properties linked by a liquid bridge. The proposed model provides the adhesion force predictions in good agreement with the previous formula and existing experiment data. Considering the solid particles are partially wetted by the liquid bridge, the van der Waals force is calculated by divided the particle surface into a wetted part and a dry portion in our stimulation. Since the wetted surface portion of the particle is determined by the half-filling angle, the relationship between two half-filling angles of the unequal size particles is developed from the geometrical consideration, which is relate to the size ratio of the particles, the contact angle, and the separation distance. Then, the van der Waals force is determined using the surface element integration. Moreover, the influences of humidity, particles size, contact angle, and separation distance toward the adhesion forces are discussed using the proposed method. Simulations indicate that a higher relative humidity leads to bigger liquid bridges, suggesting a higher capillary force, but at the same time, the van der Waals force decreases due to the decrease in surfaces energy. As for the influence of contact angle, results show that a higher contact angle, that is, a more hydrophobic surface, reduces the capillary force but increases the van der Waals force (absolute value). The simulations also show that the both the capillary force and the van der Waals force (absolute value) increase as the particle size increases. When the particles are separated from each other, the capillary force and van der Waals force decreases gradually. These results are helpful to understand and utilize the adhesion interaction between particles with unequal sizes at the ambient condition. [ABSTRACT FROM PUBLISHER]

This paper presents the results on interaction behaviour between spinnable multiwall carbon nanotube bundles. Spinnable carbon nanotube webs were used as test specimens. Mechanical peel tests were performed to investigate the effect of average orientation angles on interactions between nanotube bundles. Thus, in the absence of any binding agents, the effect of orientation directions on Van Der Waals forces between the nanotube bundles was revealed. The experimental results obtained from this study are consistent with theoretical calculations from published literature. It was found that the interaction force of the spinnable nanotube bundles is inversely related to their average orientation angle. It is shown that the force for parallel nanotube bundles at 0° angle is higher than the force for crossed nanotube bundles at 90° angle. [ABSTRACT FROM AUTHOR]

Recently, micro- and nanomachines for microelectromechanical systems and the mechanism of bio-adhesive pads have attracted great interest. Nanoscale structures are affected by forces such as the van der Waals force, even though they are neglected in large-scale structures. In this paper, the van der Waals force is introduced to a boundary element method (BEM) program for analyzing adhesion in arbitrarily shaped bodies. The van der Waals force is described by a nonlinear function of the distance between two surfaces in close proximity, and the adhesion and repulsion forces vary greatly within the atom equilibrium distance. Therefore, it is difficult to obtain a solution for the simultaneous equation in the BEM. We propose a method to compute the body deformation and apply it to the adhesion between a cylinder and a flat, rigid substrate. The solution is compared with previously published theoretical results, and the validity of the algorithm for approaching the solution is demonstrated. Further calculations for the adhesive contact of softer and harder cylinders with a rigid substrate are conducted. [ABSTRACT FROM AUTHOR]

Abstract: The grains of lunar regolith are characterized with rough surfaces, angular shapes and mutual adhesions due to short-range interactions. These features control the macroscopic mechanical behavior of lunar regolith but have not been completely captured by contact models in previous Discrete Element Method (DEM) analyses. In this paper, a simplified two-dimensional microscopic contact model is proposed for high efficiency DEM analyses of lunar regolith. The model consists of three components in the normal, tangential and rolling directions respectively, plus two new parameters. A shape parameter is used to control the rolling resistance ability at the contact area between two particles to capture the features of grain shape and interlocking. The second parameter, micro-separation, which denotes the nominal minimum distance between the molecules of the two contacting particles, is introduced to account for van der Waals force as the major component of the short-range interactions that contribute to the adhesion of regolith grains in lunar environment conditions. The novel model has been implemented in a two-dimensional DEM code for numerical simulations of biaxial compression tests on lunar regolith. The effects of interparticle friction, grain shape, lunar environment conditions and void ratio on the strength of lunar regolith were numerically investigated. The results show that soils in the simulated lunar environment exhibit greater strength and more apparent strain-softening and shear dilatancy than on the Earth. The proposed model can capture the main features of the mechanical behavior of lunar regolith (apparent cohesion and high peak friction angle) and a wide range of strength indices can be obtained by the contact model. [Copyright &y& Elsevier]

Abstract: In this paper we solve the common nonlinear boundary value problems (BVPs) of cantilever-type micro-electromechanical system (MEMS) and nano-electromechanical system (NEMS) using the distributed parameter model by the Duan–Rach modified Adomian decomposition method (ADM). The nonlinear BVPs that are investigated include the cases of the single and double cantilever-type geometries under the influence of the intermolecular van der Waals force and the quantum Casimir force for appropriate distances of separation. The new Duan–Rach modified ADM transforms the nonlinear BVP consisting of a nonlinear differential equation subject to appropriate boundary conditions into an equivalent nonlinear Fredholm–Volterra integral equation before designing an efficient recursion scheme to compute approximate analytic solutions without resort to any undetermined coefficients. The new approach facilitates parametric analyses for such designs and the pull-in parameters can be estimated by combining with the Padé approximant. We also consider the accuracy and the rate of convergence for the solution approximants of the resulting Adomian decomposition series, which demonstrates an approximate exponential rate of convergence. Furthermore we show how to easily achieve an accelerated rate of convergence in the sequence of the Adomian approximate solutions by applying Duan''s parametrized recursion scheme in computing the solution components. Finally we compare the Duan–Rach modified recursion scheme in the ADM with the method of undetermined coefficients in the ADM for solution of nonlinear BVPs to illustrate the advantages of our new approach over prior art. [Copyright &y& Elsevier]

The effect of magnetic fields on water is still a highly controversial topic despite the vast amount of research devoted to this topic in past decades. Enhanced water evaporation in a magnetic field, however, is less disputed. The underlying mechanism for this phenomenon has been investigated in previous studies. In this paper, we present an investigation of the evaporation of water in a large gradient magnetic field. The evaporation of pure water at simulated gravity positions (0 gravity level (ab. g), 1 g, 1.56 g and 1.96 g) in a superconducting magnet was compared with that in the absence of the magnetic field. The results showed that the evaporation of water was indeed faster in the magnetic field than in the absence of the magnetic field. Furthermore, the amount of water evaporation differed depending on the position of the sample within the magnetic field. In particular, the evaporation at 0 g was clearly faster than that at other positions. The results are discussed from the point of view of the evaporation surface area of the water/air interface and the convection induced by the magnetization force due to the difference in the magnetic susceptibility of water vapor and the surrounding air. [ABSTRACT FROM AUTHOR]

A physical mechanism of phononic band gap and resonant nanoacoustic scattering in an aggregate of two elastic nanospheres is presented in this paper. By considering the Van der Waals (VdW) force between two nanospheres illuminated by nanoacoustic wave, phononic band gap and frequency shift at the lower frequency side, and largely enhanced nanoacoustic scattering at the other frequency range have been found through calculating the form function of the acoustic scattering from the nanosystem. This VdW-force-induced band gap is different from the known mechanisms of Bragg scattering and local resonances for periodic media. It is shown that when the separation distance between two nanospheres is decreasing from 20 to 1 nm, due to the increasing VdW force, the nanoacoustic scattering is much heightened by two order of magnitude, and meanwhile the frequency shift and phononic band gap at the low frequencies are both widened. These results could provide potential applications of nanoacoustic devices. [ABSTRACT FROM AUTHOR]

In this paper the effects of temperature on the radial breathing modes (RBMs) and radial wave propagation in multiwall carbon nanotubes (MWCNTs) are investigated using a continuum model of multiple elastic isotropic shells. The van der Waals forces between tubes are simulated as a nonlinear function of interlayer spacing of MWCNTs. The governing equations are solved using a finite element method. A wide range of innermost radius-to-thickness ratio of MWCNTs is considered to enhance the investigation. The presented solution is verified by comparing the results with those reported in the literature. The effects of temperature on the van der Waals interaction coefficient between layers of MWCNTs are examined. It is found that the variation of the van der Waals interaction coefficient at high temperature is sensible. Subsequently, variations of RBM frequencies and radial wave propagation in MWCNTs with temperatures up to 1 600 K are illustrated. It is shown that the thick MWCNTs are more sensible to temperature than the thin ones. [ABSTRACT FROM AUTHOR]

Casimir and van der Waals dispersive forces between real material surfaces can be strongly influenced by surface roughness and the frequency dependent dielectric functions of the interacting materials. The Lifshitz theory allows calculations of these forces between two flat plates if the frequency dependent dielectric function is known. Even in this case in order to compare different measurements of the Casimir force the accuracy must be higher than the force uncertainty arising as a result of the variation in the measured dielectric functions for a given material. Therefore, when we are dealing with dispersive forces between real materials, precise characterization of the particular interacting samples is critical. For this reason the effects of optical properties and surface roughness on dispersive forces will be first discussed in this review paper. Finally, we will compare research results among various research groups, at relatively large surface separations where roughness plays insignificant role, in order to show the level of consensus that has developed in this field in the recent years. [ABSTRACT FROM AUTHOR]

The dynamics of the myosin molecular motor as it binds to actin filaments during muscle contraction are still not clearly understood. In this paper, we focus on the coupling mechanism of multi-force interactions in the myosin molecule during its interaction with actin. These forces include the electrostatic force, the van der Waals force and the Casimir force in molecular dynamic simulations of the molecules in solvent with thermal fluctuations. Based on the Hamaker approach, van der Waals and Casimir potentials and forces are calculated between myosin and actin. We have developed a Monte Carlo method to simulate the dynamic activity of the molecular motor. We have shown that because of the retardation effect, the van der Waals force falls into the Casimir force when the distance between the surfaces is larger than 3 nm. When the distance is smaller than 3 nm, the electrostatic force and the van der Waals force increase until the myosin becomes attached to the actin. Over the distances studied in the present work, the electrostatic force dominates the attractive interactions. Our calculations are in good agreement with recently reported experimental results. [ABSTRACT FROM AUTHOR]

A new inverse approach is proposed in this paper, which combines elements of nonlocal theory and molecular mechanics, based on the experimental results available in the nanoindentation literature. The effect of the inlayer van der Waals atomistic interactions for carbon nanotubes with multiple walls (MWCNT) is included by means of the Brenner -Tersoff potential and experimental results. The neighboring walls of MWCNT are coupled through van der Waals interactions, and the shell buckling would initiate in the outermost shell, when nanotubes are short. The nanoindentation technique is simulated for the axially compressed of individual nanotubes, in order to evaluate the loadunloaded- displacement, the curve critical buckling and the appropriate values for local Lame constants. [ABSTRACT FROM AUTHOR]

Abstract: The variations of critical axial forces for the inner and outer tubes of the double-walled carbon nanotube (DWCNT) in different buckling modes are studied in this paper. The effects of growth in the radius, length, and aspect ratio (length/radius) on the stability of a DWCNT and its inner and outer tubes are also considered, while the effects of the surrounding elastic medium on the outer tube and the van der Waals forces between two adjacent tubes are taken into account. Comparison between the DWCNT as a whole, its outer tube, inner tube, and single-walled carbon nanotube (SWCNT) indicates that the critical axial force of the inner tube of a DWCNT is almost two times more than the critical axial force of a SWCNT, showing the positive effect of the van der Waals interaction on the inner tube, DWCNT surrounded by an elastic medium has greater axial stability than DWCNT, without the surrounding elastic medium. This study demonstrates that the critical axial force of DWCNT is larger than for SWCNT, and it rises as the radius and aspect ratio increase. [Copyright &y& Elsevier]

Abstract: In this paper, a distributed parameter model is used to study the pull-in instability of cantilever type nanomechanical switches subjected to intermolecular and electrostatic forces. In modeling of the electrostatic force, the fringing field effect is taken into account. The model is nonlinear due to the inherent nonlinearity of the intermolecular and electrostatic forces. The nonlinear differential equation of the model is transformed into the integral form by using the Green’s function of the cantilever beam. Closed-form solutions are obtained by assuming an appropriate shape function for the beam deflection to evaluate the integrals. The pull-in parameters of the switch are computed under the combined effects of electrostatic and intermolecular forces. Electrostatic microactuators and freestanding nanoactuators are considered as special cases of our study. The detachment length and the minimum initial gap of freestanding nano-cantilevers, which are the basic design parameters for NEMS switches, are determined. The results of the distributed parameter model are compared with the lumped parameter model. [Copyright &y& Elsevier]

The interface behavior may significantly influence the mechanical properties of carbon nanotube (CNT)-reinforced composites due to the large interface area per unit volume at the composite. The modeling of CNT/polymer interfaces has been a challenge in the continuum modeling of CNT-reinforced composites. This paper presents a review of recent progress to model the CNT/matrix interfaces via a cohesive law established from the van der Waals force. A simple, analytical cohesive law is obtained from the inter-atomic potential, and is used to study the effect of CNT/matrix interfaces on the macroscopic properties of CNT-reinforced composites. [ABSTRACT FROM AUTHOR]

It has been confirmed experimentally that the adhesion force measured between an atomic force microscope (AFM) tip and the self-assembled monolayers (SAMs) has a direct correlation with the chain length of SAMs, and that the adhesion force decreases with the increase of the chain length. In this paper, a theoretical model is put forward to calculate the adhesion force between the AFM tip and the SAMs by integrating the Lennard–Jones potential. The theoretical results are in good agreement with the existing experimental results. [ABSTRACT FROM AUTHOR]

Abstract: The influence of van der Waals (vdW) force on the stability of electrostatic torsional nano-electro-mechanical systems (NEMS) actuators is analyzed in the paper. The dependence of the critical tilting angle and voltage is investigated on the sizes of structure with the consideration of vdW effects. The pull-in phenomenon without the electrostatic torque is studied, and a critical pull-in gap is derived. A dimensionless equation of motion is presented, and the qualitative analysis of it shows that the equilibrium points of the corresponding autonomous system include center points, stable focus points, and unstable saddle points. The Hopf bifurcation points and fork bifurcation points also exist in the system. The phase portraits connecting these equilibrium points exhibit periodic orbits, heteroclinic orbits, as well as homoclinic orbits. [Copyright &y& Elsevier]

Abstract: In this paper, the deformation of single- and multi-walled carbon nanotubes under radial pressure has been studied by using an analytical molecular structural mechanics model. The elastic in-plane properties (in-plane stiffness in the hoop direction, Y θ) and Poisson''s ratio () of single-walled carbon nanotubes (SWCNTs) has been derived and shown to be equal to the axial direction in-plane properties (Y z, ) for all tube diameters. In-plane properties are sensitive to tube diameter at small diameter ranges and insensitive to tube size at large diameters. Using the obtained hoop stiffness, the radial deformation of SWCNTs under radial pressure is formulated. The effect of van der Waals forces between layers in multi-walled carbon nanotubes (MWCNTs) has been incorporated within the analytical molecular structural mechanics model to analyze the radial deformation of MWCNTs. Closed form solution for the radial deformation of MWCNTs is obtained by using the universal graphite potential function for the interlayer van der Waals interactions. Load transfer between layers is considered. The behavior of MWCNTs under large radial deformation has also been studied by incorporating nonlinear van der Waals interactions. The present theoretical investigation provides a very simple approach to predict the mechanical properties of carbon nanotubes. [Copyright &y& Elsevier]

Abstract: Drop dynamics plays a central role in defining the interfacial morphology in two-phase complex fluids such as emulsions and polymer blends. In such materials, the components are often microstructured complex fluids themselves. To model and simulate drop behavior in such systems, one has to deal with the dual complexity of non-Newtonian rheology and evolving interfaces. Recently, we developed a diffuse-interface formulation which incorporates complex rheology and interfacial dynamics in a unified framework. This paper uses a two-dimensional implementation of the method to simulate drop coalescence after head-on collision and drop retraction from an elongated initial shape in a quiescent matrix. One of the two phases is a viscoelastic fluid modeled by an Oldroyd-B equation and the other is Newtonian. For the parameter values examined here, numerical results show that after drop collision, film drainage is enhanced when either phase is viscoelastic and drop coalescence happens more readily than in a comparable Newtonian system. The last stage of coalescence is dominated by a short-range molecular force in the model that is comparable to van der Waals force. The retraction of drops from an initial state of zero-velocity and zero-stress is hastened at first, but later resisted by viscoelasticity in either component. When retracting from an initial state with pre-existing stress, produced by cessation of steady shearing, viscoelasticity in the matrix hinders retraction from the beginning while that in the drop initially enhances retraction but later resists it. These results and the physical mechanisms that they reveal are consistent with prior experimental observations. [Copyright &y& Elsevier]

This paper describes a numerical method to estimate adhesion forces that are present in different handling operations in the micro- and nanoworld. Emphasis is on the calculation of the van der Waals force, although the results can be generalized in a straightforward manner to also cover the electrostatic force. The presented method enables force calculation between objects that have arbitrary shapes and material properties together with arbitrary alignments relative to each other. The estimation accuracy is enhanced and the computational complexity is reduced by using surface formulation of the force instead of conventional volume formulation. The surface formulation also enables division of the surfaces into separate regions. This guarantees better accuracy in the computations and makes it possible to use only partial surfaces in the force evaluation. [ABSTRACT FROM AUTHOR]

Abstract: The interaction between electrically neutral surfaces at sub-micron separation is dominated by the force arising from quantum fluctuations of the electromagnetic field, known as the Casimir force. This effect has been witnessing a renewed interest because of its potential impact in micro- and nanotechnology. Most recent literature has focused on the study of the attraction between bulk-like metallic surfaces in vacuum. Because electromagnetic fluctuations depend on the dielectric function of the surfaces, the use of different materials might reveal new aspects of the Casimir force and suggest novel solutions for the design of micro- and nanofabricated devices. Following this approach, we have measured the Casimir force using Hydrogen Switchable Mirrors—a metallic mirror that switches from highly reflective to transparent when exposed to hydrogen. The comparison of the results obtained in air and in hydrogen sheds light on the relative contribution of visible and infrared wavelengths to the Casimir interaction. We have also studied the dependence of the Casimir force on the metallic film thickness and have shown the effect of the skin-depth. The final section of the paper discusses the torque induced by quantum fluctuations on two birefringent plates and describes an experiment that should allow us to observe this phenomenon. [Copyright &y& Elsevier]

This paper employs continuum principles combined with van der Waals theory to estimate the thermal contact resistance between nanowires and planar substrates. This resistance is modeled using elastic deformation theory and thermal resistance relations. The contact force between a nanowire and substrate is obtained through a calculation of the van der Waals interaction energy between the two. The model estimates numerical values of constriction and gap resistances for several nanowire-substrate combinations with water and air as the surrounding media. The total interface resistance is almost equal to the gap resistance when the surrounding medium has a high thermal conductivity. For a low-conductivity medium, the interface resistance is dominated by the constriction resistance, which itself depends significantly on nanowire and substrate conductivities. A trend observed in all calculations is that the interface resistance increases with smaller nanowires, showing that interface resistance will be a significant parameter in the design and performance of nanoelectronic devices. [ABSTRACT FROM AUTHOR]

This paper presents the generalized multipole, local, and translation operators for three-dimensional static potentials of the form r-λ where λ is any real number. Addition theorems are developed using Gegenbauer polynomials. Multipole expansions and error bounds are presented in a manner similar to those for truncated classical multipole expansions. Numerical results showing error behavior versus number of terms, distance, and λ are presented. [ABSTRACT FROM AUTHOR]