Van Der Waals Adhesion Research Articles

Moisture damage mechanism of asphalt mixtures containing reclaimed asphalt pavement binder: A novel molecular dynamics study

Reclaimed asphalt pavement (RAP) stands at the forefront of sustainable practices in road construction and rehabilitation. However, concerns regarding the moisture susceptibility of aged RAP binder have arisen due to its potential hazard to pavement durability. So far, the underlying mechanisms of how moisture infiltrates into the mixture and results in binder stripping remain controversial and scarcely investigated. To address this challenge, a novel molecular dynamics (MD) simulation method has been developed to explore the nanoscale stripping mechanisms, considering the effects of asphalt oxidative aging and rejuvenation. The results indicate that hydrogen bonding interactions between water and the aggregate surface reduce the contact distance compared to the distance between asphalt and aggregate. Under pore water pressure, moisture diffuses into the interface, occupying the van der Waals adhesion sites of asphalt molecules and consequently causing detachment. Asphalt oxidative aging, on one hand, increases the oxygen concentration and hydrophilicity, thereby increasing moisture susceptibility from the energy perspective. On the other hand, it increases the intermolecular friction between aged asphalt molecules, leading to reduced flexibility and age hardening, consequently triggering asphalt premature detachment and diminishing its coating quality on the aggregate surface. Rejuvenators can restore the moisture sensitivity of severely aged asphalt, with a mechanism involving two aspects: firstly, reducing the agglomeration and proportion of polar components in asphalt, thereby weakening its hydrophilicity; secondly, lubricating aged asphalt molecules to restore their flexibility and surface coating properties. Findings from this study contribute to revealing the moisture-induced damage mechanism when recycling RAP binder.

A new discrete element method for small adhesive non-spherical particles

Small adhesive particles with non-spherical shapes are common in natural and industrial processes. However, it is challenging to accurately simulate the dynamic behaviors of non-spherical particulate systems, not only due to the complex local geometry at the contact region but also to the non-linear coupling between the van der Waals adhesion and the elastic deformation. In this paper, we develop a discrete element method (DEM) framework for small non-spherical particles. Particularly, we present a simple algorithm for solving the implicit equations of the dimensionless generalized JKR model, to accurately resolve the local surface geometry at the contact region and the strong coupling between the elastic deformation and the surface adhesion. New explicit expressions are established for the pull-off force, overlap and minor-to-major ratio of the contact ellipse, to naturally capture the pull-off point upon collision. Then the actual contact parameters including the contact radius, curvatures and overlap are used to modify the interparticle interactions, including the normal elastic-adhesive-dissipative force and sliding-rolling-twisting resistances. Finally, the framework is quantitatively validated by four random packing problems, including adhesive spheres, ellipsoids, spherocylinders and dimers, and the representative non-adhesive ellipsoids. Both the microscopic and macroscopic properties of packings agree well with a large number of classic theories, experiments and simulations, showing the satisfactory accuracy and effectiveness. The newly developed adhesive DEM framework has the full capability to dynamically simulate the collision-related behaviors of complex shaped particles in the presence of van der Waals adhesion and frictional forces.

Sonochemical synthesis of bioinspired graphene oxide–zinc oxide hydrogel for antibacterial painting on biodegradable polylactide film

Graphene oxide nanosheet (GO) is a multifunctional platform for binding with nanoparticles and stacking with two dimensional substrates. In this study, GO nanosheets were sonochemically decorated with zinc oxide nanoparticles (ZnO) and self-assembled into a hydrogel of GO–ZnO nanocomposite. The GO–ZnO hydrogel structure is a bioinspired approach for preserving graphene-based nanosheets from van der Waals stacking. X-ray diffraction analysis (XRD) showed that the sonochemical synthesis led to the formation of ZnO crystals on GO platforms. High water content (97.2%) of GO–ZnO hydrogel provided good property of ultrasonic dispersibility in water. Ultraviolet-visible spectroscopic analysis (UV–vis) revealed that optical band gap energy of ZnO nanoparticles (∼3.2 eV) GO–ZnO nanosheets (∼2.83 eV). Agar well diffusion tests presented effective antibacterial activities of GO–ZnO hydrogel against gram-negative bacteria (E. coli) and gram-positive bacteria (S. aureus). Especially, GO–ZnO hydrogel was directly used for brush painting on biodegradable polylactide (PLA) thin films. Graphene-based nanosheets with large surface area are key to van der Waals stacking and adhesion of GO–ZnO coating to the PLA substrate. The GO–ZnO/PLA films were characterized using photography, light transmittance spectroscopy, coating stability, scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopic mapping (EDS), antibacterial test and mechanical tensile measurement. Specifically, GO–ZnO coating on PLA substrate exhibited stability in aqueous food simulants for packaging application. GO–ZnO coating inhibited the infectious growth of E. coli biofilm. GO–ZnO/PLA films had strong tensile strength and elastic modulus. As a result, the investigation of antibacterial GO–ZnO hydrogel and GO–ZnO coating on PLA film is fundamental for sustainable development of packaging and biomedical applications.

Transfer of III-nitride epitaxial layers onto pre-patterned silicon substrates for the simple fabrication of free-standing MEMS

In recent years, the remarkable properties and potential applications of III-nitride (III-N) semiconductors have sparked a significant interest in the field of microelectromechanical systems (MEMS). Traditionally, III-N MEMS are fabricated through a process involving the epitaxial growth of III-N epilayers on a silicon substrate followed by etching the handle wafer to generate free-standing structures. In this study, we explore the potential of a relatively simple approach based on the two-dimensional (2D) material-based liftoff and transfer to fabricate III-N mechanical resonators. The methodology involves van der Waals epitaxy of III-N layers on 2D hexagonal-boron nitride (h-BN), which leverages the weak van der Waals adhesion between h-BN layers to lift off and transfer these layers from their original growth substrate to an alternative host substrate. The employed method is demonstrated by fabricating 600 nm thick GaN/AlGaN and 2.5 μm thick h-BN micro-resonators onto pre-patterned cavities etched in silicon substrates. These devices are characterized using laser Doppler vibrometry, enabling the observation of well-defined modes of vibration and resonant frequencies. Furthermore, finite element method simulations are performed to gain insights into the experimental observations and the mechanical properties of the transferred layers. This approach could be extended to transfer high-quality III-N MEMS devices onto various host substrates, including flexible substrates, and could be used to assess the mechanical properties of emerging III-N semiconductor materials.

Wetting and strain engineering of 2D materials on nanopatterned substrates.

The fascinating realm of strain engineering and wetting transitions in two-dimensional (2D) materials takes place when placed on a two-dimensional array of nanopillars or one-dimensional rectangular grated substrates. Our investigation encompasses a diverse set of atomically thin 2D materials, including transition metal dichalcogenides, hexagonal boron nitride, and graphene, with a keen focus on the impact of van der Waals adhesion energies to the substrate on the wetting/dewetting behavior on nanopatterned substrates. We find a critical aspect ratio of the nanopillar or grating heights to the period of the pattern when the wetting/dewetting transition occurs. Furthermore, energy hysteresis analysis reveals dynamic detachment and re-engagement events during height adjustments, shedding light on energy barriers of 2D monolayer transferred on patterned substrates. Our findings offer avenues for strain engineering in 2D materials, leading to promising prospects for future technological applications.

Self‐Adhesive, Detach‐on‐Demand, and Waterproof Hydrophobic Electronic Skins with Customized Functionality and Wearability

AbstractThe interfacial design of the electronic skins (E‐skins), which are increasingly crucial in areas like exercise monitoring, healthcare, etc., has a great impact on wearability and functions. The adhesion between E‐skin and human skin serves as the foundation for various functionalities. In addition to robust adhesion strength, detaching‐on‐demand, and waterproof abilities are also important for long‐time wearing and comfort detachment after use. Here, self‐adhesive, detach‐on‐demand, and waterproof hydrophobic E‐skins (PBIA) by copolymerization of acrylates with conductive ionic liquid and adhesive components as the strain sensor and triboelectric nanogenerator (TENG) is developed. The strong van der Waals self‐adhesion (shear adhesion of ≈574 kPa, peeling adhesion of ≈110 N m−1), and waterproof abilities under immersing, flushing, and penetrating situations enable the E‐skin to realize stable and long‐time monitoring for body motions in both the resistance sensing and TENG modes. Meanwhile, the easy detach‐on‐demand ability of van der Waals adhesion (574 to 35 kPa and 110 to 7 N m−1) guarantees the easy and comfortable detachment of PBIA after use, avoiding pain or injury to the skin. The adhesion strategy of PBIA can be expanded to other kinds of E‐skins and accelerate the pace of practical applications of E‐skins.

Optimization of Layer Transfer and Photolithography for Device Integration of 2D-TMDC

Extensive research into two-dimensional transition metal dichalcogenides (2D-TMDCs) over the past decade has paved the way for the development of (opto)electronic devices with enhanced performance and novel capabilities. To realize devices based on 2D-TMDC layers, compatible and optimized technologies such as layer transfer and photolithography are required. Challenges arise due to the ultrathin, surface-only nature of 2D layers with weak van der Waals adhesion to their substrate. This might potentially compromise their integrity during transfer and photolithography processes, in which prolonged exposure at usually high temperature to reactive chemicals and strong solvents are conventionally used. In this paper, we show that employing a dry-transfer technique based on thermal release tape (TRT) as an alternative to wet processes based on KOH solution better preserves layer quality. In the succeeding device fabrication process, an optimized photolithography as a cost-effective and widely available method for device patterning is utilized. The introduced photolithography protocol presents a near-perfect yield and reproducibility. To validate our optimized techniques, we fabricated field-effect transistors (FETs) using 2D-MoS2 layers from metal–organic chemical vapor deposition (MOCVD), wet- and dry-transferred onto SiO2/Si substrates. Our findings mark a significant stride towards the efficient and industry-compatible utilization of 2D van der Waals materials in device fabrication.

Tunable Contacts of Bi2 O2 Se Nanosheets MSM Photodetectors by Metal-Assisted Transfer Approach for Self-Powered Near-Infrared Photodetection.

Owing to the Fermi pinning effect arose in the metal electrodes deposition process, metal-semiconductor contact is always independent on the work function, which challenges the next-generation optoelectronic devices. In this work, a metal-assisted transfer approach is developed to transfer Bi2 O2 Se nanosheets onto the pre-deposited metal electrodes, benefiting to the tunable metal-semiconductor contact. The success in Bi2 O2 Se nanosheets transfer is contributed to the stronger van der Waals adhesion of metal electrodes than that of growth substrates. With the pre-deposited asymmetric electrodes, the self-powered near-infrared photodetectors are realized, demonstrating low dark current of 0.04 pA, high Ilight /Idark ratio of 380, fast rise and decay times of 4 and 6ms, respectively, under the illumination of 1310nm laser. By pre-depositing the metal electrodes on polyimide and glass, high-performance flexible and omnidirectional self-powered near-infrared photodetectors are achieved successfully. This study opens up new opportunities for low-dimensional semiconductors in next-generation high-performance optoelectronic devices.

Analytical disk–cylinder interaction potential laws for the computational modeling of adhesive, deformable (nano)fibers

The analysis of complex fibrous systems or materials on the micro- and nanoscale, which have a high practical relevance for many technical or biological systems, requires accurate analytical descriptions of the adhesive and repulsive forces acting on the fiber surfaces. While such analytical expressions are generally needed both for theoretical studies and for computer-based simulations, the latter motivates us here to derive disk–cylinder interaction potential laws that are valid for arbitrary mutual orientations in the decisive regime of small surface separations. The chosen type of fundamental point-pair interaction follows the simple Lennard-Jones model with inverse power laws for both the adhesive van der Waals part and the steric, repulsive part. We present three different solutions, ranging from highest accuracy to the best trade-off between simplicity of the expression and sufficient accuracy for our intended use. The validity of simplifying approximations and the accuracy of the derived potential laws is thoroughly analyzed, using both numerical and analytical reference solutions for specific interaction cases. Most importantly, the correct asymptotic scaling behavior in the decisive regime of small separations is achieved, and also the theoretically predicted (1/sinα)-angle dependence (for non-parallel cylinders) is obtained by the proposed analytical solutions. As we show in the outlook to our current research, the derived analytical disk–cylinder interaction potential laws may be used to formulate highly efficient computational models for the interaction of arbitrarily curved fibers, such that the disk represents the cross-section of the first and the cylinder a local approximation to the shape of the second fiber.

Detailed evaluation of drug powder deposition in swirl-type dry powder inhalers

Based on numerical simulations by the Euler/Lagrange approach in combination with the RNG-k-ε turbulence model flow field and particle transport through inhaler devices are studied. Since one major problem in inhaler performance and drug powder release is their deposition on the inhaler walls this issue is analysed in more detail. For computing fine particle motion through the stationary flow field all relevant particle forces and turbulent dispersion are considered. A potential deposition of fine particles is governed by two elementary processes, namely the transport of the fine drug particles towards the walls and their possible wall adhesion. Wall impingement of particles is induced by fluid transport and particle inertia as well as turbulence and for very fine particles also Brownian random motion. The possibility of particle sticking on the inhaler walls is modelled by considering the van der Waals adhesion force. Based on an energy balance for an oblique elastic-plastic wall impingement, a critical impact velocity is derived below which the particles are depositing. This approach accounts for the mechanical properties of wall and particle materials. First, the performance of different deposition models was analysed and the results were compared with literature experiments. The local and global deposition of particles in the considered swirl-type inhalers is determined in dependence of particle size, stationary flow rates (between15 l/min and 90 l/min) and particle release location. The range of particle sizes considered is between 0.05 μm and 10 μm, representing a typical spectrum for fine drug powders in use. Additionally, the spatial distribution of the deposits within the two inhaler types is evaluated for allowing inhaler optimisation and the predicted emitted fraction of fine particles is compared with experimental observations.

Chemical Vapor-Deposited Graphene on Ultraflat Copper Foils for van der Waals Hetero-Assembly.

The purity and morphology of the copper surface is important for the synthesis of high-quality, large-grained graphene by chemical vapor deposition. We find that atomically smooth copper foils—fabricated by physical vapor deposition and subsequent electroplating of copper on silicon wafer templates—exhibit strongly reduced surface roughness after the annealing of the copper catalyst, and correspondingly lower nucleation and defect density of the graphene film, when compared to commercial cold-rolled copper foils. The “ultrafoils”—ultraflat foils—facilitate easier dry pickup and encapsulation of graphene by hexagonal boron nitride, which we believe is due to the lower roughness of the catalyst surface promoting a conformal interface and subsequent stronger van der Waals adhesion between graphene and hexagonal boron nitride.

Adhesion effect analysis of ultra-fine lunar dust particles on the aluminum-based rough surface based on the fractal theory

Investigation on the adhesion effect of lunar dust particles on contacting interface improves in lunar dust protection and prolongs the equipment service life. In this work, the contact mathematical adhesion model between an ultra-fine lunar dust particle and the aluminum-based rough surface is established based on the fractal theory. The research shows that the van der Waals force dominates adhesion force for ultra-fine lunar dust particles in space environment and is directly related to the rough surface fractal parameters, namely the fractal dimension parameter D , fractal roughness G as well as root-mean-square roughness(rms). Based on van der Waals adhesion model, the optimum rough surface fractal parameters are obtained through model optimization when the van der Waals force is minimal. The van der Waals forces reaches the minimum for lunar dust particle with diameters of 500 nm and 2.5 μm when the rms is 2.689 nm and 4.7 nm respectively. To validate the effectiveness of using fractal theory to the surface topography in van der Waals force calculation, the dust residual rates have been compared with the aluminum surfaces prepared using chemical etching method, electrochemical etching method, and combined chemical method. Based on the AFM topography of the surfaces, the van der Waals forces have been calculated between the lunar dust particles and the surfaces prepared using different etching methods. The centrifugal experiments suggest that van der Waals force is positively related to the dust residual rates. In summary, the fractal theory is effective in describing the surface roughness in lunar dust adhesion problems and it can be used in future lunar dust protection surface design and applications.

Tearing behavior induced by van der Waals force at heterogeneous interface during two-dimensional MoS<sub>2</sub> nanoindentation

Combining with <i>in situ</i> nanomechanical testing system and video module of scanning electron microscope, the nanoindentation testing is performed to study the peeling-tearing behavior of two-dimensional material van der Waals heterostructures. After two-dimensional MoS<sub>2</sub> nanosheets prepared by chemical vapor deposition are assembled into MoS<sub>2</sub>/SiO<sub>2</sub> heterostructures by wet transfer, the nanoindentation is carried out by manipulating the tungsten probe in the<i> in situ</i> nanomechanical testing system. When the tungsten probe is tightly indenting into MoS<sub>2</sub> nanosheets, a new W/MoS<sub>2</sub>/SiO<sub>2</sub> heterostructure is assembled. With the tungsten probe retracting, the adhesive effect makes the two-dimensional MoS<sub>2</sub> nanosheet peel off from SiO<sub>2</sub>/Si substrate to form a bulge. After reaching a certain height, under the van der Waals adhesion interaction, an incomplete penetration fracture occurs along the arc line contacting the needle. Then cleavage appears and produces two strip cracks and MoS<sub>2</sub>/SiO<sub>2</sub> interface separation takes place simultaneously, before a large area of MoS<sub>2</sub> nanosheet is teared. Based on the density functional theory calculation of interface binding energy density of van der Waals heterogeneous interface, the interface binding energy density of MoS<sub>2</sub>/W is verified to be larger than that of MoS<sub>2</sub>/SiO<sub>2</sub>, which explains the adhesion peeling behavior of MoS<sub>2</sub> induced by van der Waals force between heterogeneous interfaces, perfectly. By using the peeling height and tearing length of MoS<sub>2</sub> recorded by video module, the fracture strength of MoS<sub>2</sub> is obtained to be 27.055 GPa and stress-strain relation can be achieved according to the film tearing model. The density functional theory simulation results show that the fracture strength of MoS<sub>2</sub> is in a range of 21.7–32.5 GPa, and the stress-strain relation is consistent with the experimental result measured based on film tearing model. The present work is expected to play an important role in measuring the fracture strengths of two-dimensional materials, the assembly, disassembly manipulation and reliability design of two-dimensional materials and van der Waals heterostructures devices.

Exceptional Elasticity of Microscale Constrained MoS2 Domes.

The outstanding mechanical performances of two-dimensional (2D) materials make them appealing for the emerging fields of flextronics and straintronics. However, their manufacturing and integration in 2D crystal-based devices rely on a thorough knowledge of their hardness, elasticity, and interface mechanics. Here, we investigate the elasticity of highly strained monolayer-thick MoS2 membranes, in the shape of micrometer-sized domes, by atomic force microscopy (AFM)-based nanoindentation experiments. A dome’s crushing procedure is performed to induce a local re-adhesion of the dome’s membrane to the bulk substrate under the AFM tip’s load. It is worth noting that no breakage, damage, or variation in size and shape are recorded in 95% of the crushed domes upon unloading. Furthermore, such a procedure paves the way to address quantitatively the extent of the van der Waals interlayer interaction and adhesion of MoS2 by studying pull-in instabilities and hysteresis of the loading–unloading cycles. The fundamental role and advantage of using a superimposed dome’s constraint are also discussed.

Tiny yet tough: Maximizing the toughness of fiber-reinforced soft composites in the absence of a fiber-fracture mechanism

Tiny yet tough: Maximizing the toughness of fiber-reinforced soft composites in the absence of a fiber-fracture mechanism

The effects of surface and particle properties on van der Waals (vdW) adhesion quantified by the enhanced centrifuge method

The topographical features on the individual particles in a powder and the surfaces with which they interact affect the powder adhesion. An experimental and modelling framework called the Enhanced Centrifuge Method (ECM) maps particle and surface properties onto 'effective' Hamaker constant distributions that describe the van der Waals (vdW) adhesion between a powder and a surface. The ECM was used to study the adhesion between polystyrene powder and silica surfaces to exemplify the 'effective' Hamaker constants quantify the effects of particle and surface properties on adhesion. Silica surfaces were polished to alter their surface roughness profiles to reveal levels of roughness in which the adhesion was controlled by either the powder or surface properties. This study revealed surface roughness height descriptions are inadequate descriptors of adhesion, and instead the frequency of roughness peaks to the particle size must be considered. This result was verified using an existing particle adhesion simulator.

Mechanism for clogging of microchannels by small particles with liquid cohesion

AbstractWe numerically investigate the effect of liquid cohesion on the clogging of microchannels induced by small wet particles. The computer simulation is performed by the discrete element method (DEM) with cohesive contact models in presence of pendular liquid bridges, which is embedded into the computational fluid dynamics (CFD). We find that liquid cohesion significantly promotes particle deposition and agglomerate growth. A clogging phase diagram, in the form of Weber number and Stokes number, is constructed to quantify the clogging‐nonclogging transition. The competition between particle–particle and particle–fluid interactions is quantitatively discussed in terms of particle velocity and slip velocity. Strong cohesion can address a greater slip velocity or drag between particles and fluid, which depresses the resuspension of deposited particles and results in clogging. Finally, we compare our results with clogging induced by van der Waals adhesion of small dry particles and find that the competence of liquid cohesion is more prominent.

Charge Density Fluctuations on a Dielectric Surface Exposed to Plasma or UV Radiation

Dust particles on a nonconductive surface are known to acquire electric charge and detach from the surface under plasma conditions and/or when affected by ultraviolet radiation. Similar phenomena occur as a result of electrostatic surface cleaning (shedding) as well as in nature, e.g., when observing levitation of dust particles above the lunar surface. A detachment of dust particles from the surface should occur when the electrostatic forces of their repulsion Fc exceed the sum of the gravitation Fg forces and the adhesive van der Waals FvdW forces acting on the particle on a nonconducting surface. However, a paradoxical situation usually arises: the three primary forces of different nature Fc, Fg, and FvdW, acting on a speck of dust with a characteristic size of the order of hundreds or thousands of nanometers, are completely incomparable in magnitude, herewith Fc << Fg << FvdW. In the last decade, numerous attempts have been made to explain how a particle on a nonconducting surface can acquire a charge sufficient for the electrostatic forces that arise to approach the adhesive forces’ values. However, despite some successes, many questions remain unanswered. This article presents a brief analysis of the charge appearance process on a solitary dust speck and a speck lying on the surface. To explain the detachment of dust particles from the surface caused by electrostatic forces and the accumulation of a charge on those particles sufficient for levitation, one should take into account the charge density fluctuations on the surface.

Numerical modeling of particle motion in traveling wave solar panels cleaning device

One of the promising solutions of dust accumulation on solar panels consists of using electric potential waves to prevent dust adhesion and to remove the dust layer. This study aims to understand the particle motion mechanisms on an electrostatic traveling wave conveyor. Understanding the effect of the operating parameters on particle trajectories and displacement distance is an essential step to improve the efficiency of this device and to understand the factors that can limit its performance. A numerical model that takes into account Coulomb force, dielectrophoretic, and image forces, but also gravity, drag force, and van der Waals adhesion force is carried out. The effect of rotating electric field, frequency, and electric potential harmonics on the particle trajectories and their characteristics are analyzed and discussed. The results reveal that the particles move according to four main movement modes. The frequency is a key parameter to control particle velocity and displacement. A hyper-synchronous movement during which the particles reach velocities much higher than the propagation velocity of the traveling wave has been obtained under certain conditions.

Modification of the van der Waals interaction at the Bi2Te3 and Ge(111) interface

We present a structural and density-functional theory study of the interface of the quasi-twin-free grown three-dimensional topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ on Ge(111). Aberration-corrected scanning transmission electron microscopy and electron energy-loss spectroscopy in combination with first-principles calculations show that the weak van der Waals adhesion between the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ quintuple layer and Ge can be overcome by forming an additional Te layer at their interface. The first-principles calculations of the formation energy of the additional Te layer show it to be energetically favorable as a result of the strong hybridization between the Te and Ge.