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A series of azobenzene–cholesterol organogel compounds (M0–M12) with different spacers were designed and synthesized. The molecular structures were confirmed by 1H NMR and 13C NMR spectroscopy. The rapid and reversible photoresponsive properties of the compounds were investigated by UV–vis spectroscopy. Their thermal phase behaviors were studied by DSC. The length of the spacer plays a crucial role in the gelation. Compound M6 is the only one that can gelate in ethanol, isopropanol and 1-butanol and the reversible gel–sol transitions are also investigated. To obtain visual insight into the microstructure of the gels, the typical structures of the xerogels were studied by SEM. Morphologies of the aggregates change from flower-like, network and rod with different sizes. By using IR and XRD characterization, it is found that intermolecular H-bonding, the solvents and van der Waals interaction are the main contributions to the specific superstructure.

A micro cantilever (MC) and a probe are two main components of the atomic force microscope (AFM). The dimensions of these components are in micro scales while their oscillation amplitude is on a nanometer scale. The present study intended to not only increase the accuracy of the simulation with regard to geometric discontinuities based on the Timoshenko beam model using the modified couple stress (MCS) theory but also increase the accuracy of the prediction of a system behavior by considering the hysteresis effect into the system vibration equations based on Bouc–Wen model. Since, the classical theory disregarded the molecular structure of matter and neglected the microstructure size dependency, which raise its physical breakdowns in micro and nanoscale applications. Therefore, some micro continuum theories are developed to account for the microstructure effects by introducing additional material constants to the conventional ones. Due to the lack of the experimental results for this kind of MCs, this paper is focused on the both experimental and simulation results by utilizing the DMASP MC. The roughness of the surfaces affects the MC vibration behavior in the air medium. Therefore, surface roughness affecting van der Waals force has been considered in the air medium. Furthermore, the simulation results are compared with experimental results in the air medium with different moisture content during the surface topography. Due to the results, the best surface topography is achieved in the third vibration mode. Since the piezoelectric MC can tolerate the limited voltage, for this AFM MC type, the best surface topography is achieved in the third mode. Hence, this paper highly recommends using the third vibration mode during the surface topography. Finally, based on the EFAST method the most effective parameters on the AFM MC performance are the piezoelectric layer length, thickness and the probe length.

The present paper is dealing with the reinforcement of high density polyethylene (HDPE) matrix using pre-dried graphene nanoplatelets (GNPs). Nanocomposites of three different wt GNPs contents, i.e. 1%, 2% and 3%, were produced. The so-obtained extruded GNPs/HDPE nanocomposites underwent both tensile and three-point bending tests. Ultimate tensile strength was not affected by the GNPs content. However, the response on plastic deformation under tensile loading differed in each case, depending on the microstructure of the produced nanocomposites. Due to the weak Van der Waals forces that bond the graphene layers into a GNP, these layers are allowed to slip in the in-plane level and they were observed dislocated after tensile loading. Pores with high GNPs concentration were detected for GNPs contents greater than 1%. The flexural response was enhanced and flexural strength values were also correlated with the dimensions of the pores formed.

The barrier coating is the most important and effective method for corrosion protection. However, inevitable defects of various scales from micro to macro on coatings due to mechanical damage, aging, corrosive medium penetration, etc. will lead to coating failure. Usually, the damaged coating can only be removed and then recoated. At present, most of coatings are disposable coating and can not be recycled or healing repeatedly. In this paper, a supramolecular gel composite coating was prepared with ethyl cellulose, castor oil, mineral oil, and Fe3O4 nanoparticle, which are bonded together by hydrogen bonds and Van Der Waals forces. This coating could be multiple healed in situ by an alternating magnetic field based on the electromagnetic heating effect of Fe3O4 particles. The SEM/EDS results indicating that the best content of Fe3O4 in coating is 10% for good dispersion. Under an alternating magnetic field, the composite coating can be uniformly heated through hysteresis loss, and starts to heal when the temperature of the composite coating reaches 80 °C. After self-healing process, the mechanical properties and corrosion resistance of the composite coating have almost no difference with the original intact coating. Since the supramolecular gel is bonded by hydrogen bonding and Van Der Waals forces, these bonds are opened when heated and recombined after cooling without any change of properties. Therefore, the microstructure of supramolecular gel composite coating has no significant change and the corrosion protection performance has not decreased after 10 times of healing.

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.

This paper presents the development and validation of a numerical simulation method using the Lattice Boltzmann Method (LBM) to predict the permeability of epoxy resin (ER) composites with graphene nanoplatelets (GNPs).Gas permeability tests were carried out for a series of GNP/ER nanocomposites with different loadings and diameters of GNPs. The experimental results confirm that inclusion of GNPs in ER significantly decreased the effective gas permeability, with the highest reduction of 66% when the GNP loading was 3 wt%. The effects of using different diameters of GNPs show that using GNPs of 25 µm in diameter achieved less reduction in gas permeability than using GNPs of smaller diameters of 5 and 15 µm at the same loading of 1 wt%. This unexpected result has now been explained by the developed numerical model.The microstructures of GNPs filled ER composites were numerically reconstructed for the relative gas permeability prediction model using LBM. The 3D X-ray CT scan images clearly show agglomeration of GNPs, in particular when the diameter of GNPs is large (25 µm), due to strong Van der Waals forces. An agglomeration sub-model was thus incorporated when numerically constructing the microstructure of GNPs filled ER composites. Agglomeration of GNPs results in the formation of a small number of super-thick GNPs, leaving large spaces as ER-rich area without any GNP. This led the GNPs filled ER with 25 µm of GNP diameter to obtain a lower reduction in gas permeability than smaller GNPs filled ER. The results of numerical sensitivity studies on surface area, rotation, curling and folding of GNP flakes suggest that it is acceptable to use flat disk shaped flakes to represent amorphous GNPs with small degrees of deformation (less than 20o and 1.5 for folding angle and curling rate respectively). The results also show that the projection area perpendicular to the overall gas flow direction dominates the overall gas barrier effect of GNPs. The feasibility of using 2D models is demonstrated and it is acceptable to assume that the GNPs in the prepared samples are uniformly sized with a diameter equal to the nominal diameter. This numerical simulation model significantly improves the accuracy for prediction of reduction in gas permeability, over that of existing analytical models, when compared against the authors’ experimental results and experimental data from literature.

Annealing is a commonly used fabrication technology of graphene-assembled materials, which serves as an efficient method to control material properties. In graphene-assembled materials, the multilayer folded configuration of graphene has been widely observed due to the two dimensional characteristic of graphene. However, the manipulation on the mechanical properties of graphene-assembled materials by annealing has not been fully understood yet, especially considering the effect of folded microstructures. In this paper, we focus on the effect of annealing temperature on the mechanical properties of multilayer folded graphene. The dependences of elastic modulus, tensile strength, ultimate strain and fracture toughness on the annealing temperature have been systematically studied by molecular dynamics simulations. Moreover, the mechanisms behind the manipulations by annealing temperature have been revealed combining the structural evolutions obtained from the simulations. Our results indicate that the multilayer folded graphene after annealing under higher temperature exhibits significant reinforcement on its elastic modulus and tensile strength, while its ultimate strain drops instead. The fracture toughness is enhanced only within a certain range of annealing temperature. The controllable mechanical properties are attributed to the formation of interlayer covalent bonds between carbon atoms belonging to adjacent layers during the annealing processing. With the annealing temperature increases, more interlayer crosslinks are observed from simulations, which greatly strengthens the interlayer interaction. For the cases with lower annealing temperature, the folded graphene can be unfolded easily then finally flattened under tensile stretch, and the structural failure originates from the interlayer slippage in the folded area. However, for the cases with higher annealing temperature, the unfolding deformation is prevented since the folded graphene is blocked by much denser interlayer crosslinks, and the origins of structural failure transforms to the intralayer fracture in graphene plane. Considering the intralayer covalent bond interaction is far more powerful than the interlayer van der Waals interaction, the higher annealing temperature will bring higher elastic modulus and tensile strength due to the change on the structural failure mode, but it will sacrifice the ductility at the same time due to the blocked unfolding process of folded area. It is confirmed in our study that the annealing is an effective approach for the synthetic modulation on the stiffness, strength, ductility and toughness of multilayer folded graphene.

The nanorods of natural palygorskite exist as aggregates or bundles. High-pressure homogenization technology can disaggregate effectively the bundles or aggregates of palygorskite in water. In order to further explore the effect of the dispersion medium on the dispersion efficiency, in this paper, palygorskite was dispersed in ethanol–water mixture with the aid of high-pressure homogenization. The effects of various ethanol/water ratios on the microstructure and colloidal properties of palygorskite were investigated. It was found that the dispersion efficiency of aggregates during the homogenization process was affected as increasing the ethanol/water ratio from 0:10 to 4:6. The introduction of ethanol into the aqueous suspension of palygorskite could weaken the van der Waals forces and hydrogen bonding interaction among rods, and a stable suspension with higher shear stress and shear modulus was obtained at the ethanol/water ratio of 6:4 (v/v).

Recent years have witnessed an increased demand for a method for precise measurement of the microstructures of mechanical microparts, microelectromechanical systems, micromolds, optical devices, microholes, etc. This paper presents a measurement system for three-dimensional (3D) microstructures that use an optical fiber probe. This probe consists of a stylus shaft with a diameter of 2.5 µm and a glass ball with a diameter of 5 µm attached to the stylus tip. In this study, the measurement system, placed in a vacuum vessel, is constructed suitably to prevent adhesion of the stylus tip to the measured surface caused by the surface force resulting from the van der Waals force, electrostatic force, and liquid bridge force. First, these surface forces are analyzed with the aim of investigating the causes of adhesion. Subsequently, the effects of pressure inside the vacuum vessel on surface forces are evaluated. As a result, it is found that the surface force is 0.13 µN when the pressure inside the vacuum vessel is 350 Pa. This effect is equivalent to a 60% reduction in the surface force in the atmosphere.

A novel hydrophilic polyurethane (abbreviated as W-OH) was developed and applied as a sustainable sand-fixing material. This paper on the chemical sand fixation mechanism of W-OH discusses the adhesive force between the W-OH solid and sand particles. The solidification mechanism was investigated and the solidifying time was tested. And then the thickness and porosity of the W-OH sand-fixing layer were investigated. By scanning electron microscopy (SEM), the microstructure of the W-OH sand-fixing layer was examined. The hardness and compressive stress of the sand-fixing specimens were studied at W-OH different concentrations. Finally, the resistance to wind erosion of the W-OH sand-fixing layer was investigated by a wind tunnel test. The results demonstrated that the W-OH aqueous solution had an excellent affinity for water on the surface of the sand particles, and the adhesive force between the W-OH solid and sand was primarily hydrogen bonding, covalent bonds and physical absorption, such as Van Der Waals forces. W-OH is a prepolymer of hydrophilic polyurethane containing groups of -NCO that can quickly react with water and other groups containing active H. The W-OH aqueous solution solidified in the range of 2 min to 15 min. The solidifying time decreased with increasing temperature and concentration. Before solidifying it had a good permeability of sand and the formed sand-fixing layer had a thickness of 8 - 35 mm and a porosity of 25% - 8% at a spraying concentration of 2 - 10 L/m2. The hardness index of the sand-fixing layer was in the range of 21 mm to 28 mm and compressive stress was in the range from 0.21 MPa mm to 1.27 MPa, both of which increased linearly with W-OH concentration. Sand treated by over 3% W-OH concentrations showed excellent resistance to wind/sand erosion of more than 25 m/s.

SYNOPSIS. Many organisms have evolved a fibrillated interface for contact and adhesion as shown by several of the papers in this issue. For example, in the Gecko, this structure appears to give them the ability to adhere and separate from a variety of surfaces by relying only on weak van der Waals forces. Despite the low intrinsic energy of separating surfaces held together by van der Waals forces, these organisms are able to achieve remarkably strong adhesion. To help understand adhesion in such a case, we consider a simple model of a fibrillar interface. For it, we examine the mechanics of contact and adhesion to a substrate. It appears that this structure allows the organism, at the same time, to achieve good, ‘universal’ contact and adhesion. Due to buckling, a carpet of fibrils behaves like a plastic solid under compressive loading, allowing intimate contact in the presence of some roughness. As an adhesive, we conjecture that energy in the fibrils is lost upon decohesion and unloading. This mechanism can add considerably to the intrinsic work of fracture, resulting in good adhesion even with only van der Waals forces. Analysis of the mechanics of adhesion through such a fibrillar interface provides rules for the design of the microstructure for desired performance as an adhesive.

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.

The structure and mechanical properties of a simple two-dimensional model of a cohesive powder are investigated by molecular dynamics simulations. Micromechanical ingredients involve elasticity, friction, a short range attraction and, possibly, rolling resistance (RR) in contacts. The microstructure of the cohesive packing varies according to the assembling procedure, from rather densely packed if isolated particles are directly compressed to much looser if the formation of large aggregates is allowed prior to compression. A crucial parameter is the ratio P*= Pa/F0 of applied pressure P, acting on grains of diameter $a$, to maximum tensile contact force F0. At low P* the final structure depends on the level of velocity fluctuations at the early stages of cluster aggregation. With RR the coordination number approaches 2 in the limit of low initial velocities or large rolling friction. The force network generally comprises small hyperstatic clusters with forces of the order of F0, joined by nearly unstressed, barely rigid arms. As P* grows, it quickly rearranges into force chain-like patterns. Density correlations witness a fractal structure, with dimension Df, up to some density-dependent blob size. WIth RR Df coincides with the ballistic aggregation result, despite a possibly different connectivity. Possible effects of some parameters on material strength are evoked., Comment: 31 pages Only 26 pages in the published PRE paper, but figures look better in THIS version !

In an electrostatic powder coating process, several forces effect the deposition of charged polymer paint particles and the adhesion of the deposited particles on the grounded substrate being painted. In this paper, the authors discuss the roles and relative magnitudes of these forces. They present a model on the deposition of a charged particle approaching the surface of the substrate. Once the particle deposits on the surface and when the high voltage is turned off, the particle experiences (1) the force due to the image charge of the underlying powder layer, (2) the force due to the image charge of the particle, (3) the repulsive force between the charged particle and the charge on the powder layer, and (4) the van der Waals force between the particle and the adjacent particles deposited on the substrate. The model shows criteria on whether the particles will deposit on the substrate and, if deposited, whether it will remain on the surface of the powder layer once the corona field is turned off. These relative forces influence the microstructure of the powder layer and may affect the ultimate appearance or the texture of paint film after curing. >

This paper reports on particle pushing and engulfment by solidification fronts and on respective microgravity experiments. The absense of thermal convection and of sedimentation under microgravity conditions renders possible a direct observation of the relevant forces and the corresponding critical growth rates.

The high mechanical strength and excellent flexibility of 2D materials such as graphene are some of their most important properties [1]. Good flexibility is key for exploiting 2D materials in many emerging technologies, such as wearable electronics, bioelectronics, protective coatings and composites [1] and recently bending has been suggested as a route to tune electronic transport behaviour [2]. For virtually all crystalline materials macroscopic deformation is accommodated by the movement of dislocations and through the formation of twinning defects [3]; it is the geometry of the resulting microstructure that largely determines the mechanical and electronic properties. Despite this, the atomic microstructure of 2D materials after mechanical deformation has not been widely investigated: only by understanding these deformed microstructures can the resulting properties be accurately predicted and controlled. In this paper we describe the different structural features that can form as a result of bending in van der Waals (vdW) crystals of 2D materials. We show that twin boundaries, an important class of crystal defect, are delocalised by several nm and not atomically sharp as has been assumed for over half a century [4]. In addition, we demonstrate that different classes of microstructure are present in the deformed material and can be predicted from just the atomic structure, bend angle, and flake thickness. We anticipate that this new knowledge of the deformation structure for 2D materials will provide foundations for tailoring transport behaviour[2], mechanical properties, liquid-phase [5,6] and scotch-tape exfoliation [7,8], and crystal growth.