Van Der Waals Adhesion Forces Research Articles

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.

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.

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.

Determination of Hamaker constant for CNT and AFM probe configuration inside SEM

Mounting the atomic force microscope (AFM) probe on the gripper of the robotic nanoscale manipulators system enables the combination of the AFM and scanning electron microscope (SEM). The AFM–SEM-based method of determining Hamaker constant for carbon nanotube (CNT) and AFM cantilever in the vacuum condition of SEM, which is completed through a robotic nanoscale manipulators system, is presented for the first time. During Hamaker constant determining experiment, the elastic force is measured utilising integrated AFM–SEM imaging technique through detecting the deflection of AFM cantilever that actually equals van der Waals adhesion force. The van der Waals attraction force between CNT and AFM cantilever is calculated through the modelling of a cylindrical nanotube and a flat gold-coated surface. The experiment results indicate that Hamaker's constant measured are comparable to typical values published by previous researchers, being of the order of 100 zJ, which indicates that the present approach is valid and reliable.

Adhesion force of W dust on tokamak W plasma-facing surfaces: The importance of the impact velocity

The adhesion of tungsten (W) dust particles on W tokamak plasma-facing surfaces is investigated. When spherical dust hits the tokamak walls, its shape is altered as a contact area is created by the adhesion work and irreversible plastic deformation. The van der Waals adhesion force between such a deformed grain and a flat surface is estimated, and the link between the force and the impact velocity is determined. We show that adhesion can be increased by several orders of magnitude when the impact velocity approaches the adhesion velocity. Finally, the effects of surface roughness are incorporated.

The number of measurements of a surface’s topography and the expected variation in adhesion to the surface

Variations in roughness on a surface spawn variations in adhesion force between the surface and any particles that contact the surface. To fully characterize the adhesion that will be exhibited when a particle contacts any location on the surface, it is desirable to map the surface with nanoscale detail. Since it is impractical to make nanoscale roughness measurements over the entirety of a surface with a characteristic dimension on the order of centimeters, a relationship between the number of surface measurements and the likely variation in the expected adhesion force is similarly desirable. In this work, the predicted van der Waals force was used to describe the particle adhesion force. The bootstrap statistical method was employed to estimate the error associated with the predicted mean adhesion force between a smooth spherical particle and a rough surface as a function of the number of locations on the surface where the roughness was measured. Specifically, 40 atomic force microscope (AFM) topographical scans (5 × 5 μm) were taken of three different surfaces and used as model surface inputs to an existing van der Waals adhesion force simulator. The simulator described the expected adhesion force resulting from 1200 contacts between the smooth, spherical particle (10 μm diameter) and random locations on each scanned area. After analyzing the results using the bootstrap method, it was determined that the adhesion between the particle and 10–15 scanned areas (out of 40) optimizes the accuracy of the predicted adhesion with respect to the researcher’s labor.

Reflections on and Extensions of the Fuller and Tabor Theory of Rough Surface Adhesion

The classical Fuller and Tabor theory of rough surface adhesion is reviewed, and its limited applicability noted. New results using an extension of the JKR adhesion theory to include the effect of van der Waals adhesion forces are described. These show the influence of the Tabor parameter μ: moderate values of µ (µ<100) lead to increased forces during loading, and so are found to predict much lower hysteresis losses. A further extension is to replace the GW surface roughness model used by Fuller and Tabor by a model based on the Nayak model of a surface as a random field, as in the Bush, Gibson and Thomas (BGT) theory of non-adhesive contact. The results are qualitatively the same, and the differences may well be largely due to the necessary redefinition of the adhesion index using the rms profile curvature in place of the asperity radius of curvature used in the GW model. `

Simulation of Micro/Nanopowder Mixing Characteristics for Dry Spray Additive Manufacturing of Li-Ion Battery Electrodes

A new dry spraying additive manufacturing method for Li-ion batteries has been developed to replace the conventional slurry-casting technique for manufacturing Li-ion battery electrodes. A dry spray manufacturing process can allow for the elimination of the time- and energy-intensive slurry drying process needed due to the use solvents to make the electrodes. Previous studies into the new manufacturing method have shown successful fabrication of electrodes which have strong electrochemical and mechanical performance. Li-ion battery electrodes typically consist of three basic materials: active material (AM), binder particle additives (BPA), and conductive particle additives (CPA). In this paper, a discrete element method (DEM) simulation was developed and used to study the mixing characteristics of dry electrode powder materials. Due to the size of the particles being in the submicron to micron size range, the mixing characteristics are heavily dependent on van der Waals adhesive forces between the particles. Therefore, the effect the Li-ion battery electrode material surface energy has on the mixing characteristics was studied. Contour plots based on the DEM simulation results where the surface energy components of selected material types are changed were used to predict the mixing characteristics of different particle systems. For the cases studied, it is found that experimental mixing results are representative of the results of the DEM simulations.

Adhesion mechanisms on solar glass: Effects of relative humidity, surface roughness, and particle shape and size

To better understand and quantify soiling rates on solar panels, we are investigating the adhesion mechanisms between dust particles and solar glass. In this work, we report on two of the fundamental adhesion mechanisms: van der Waals and capillary adhesion forces. The adhesion was determined using force versus distance (F-z) measurements performed with an atomic force microscope (AFM). To emulate dust interacting with the front surface of a solar panel, we measured how oxidized AFM tips, SiO2 glass spheres, and real dust particles adhered to actual solar glass. The van der Waals forces were evaluated by measurements performed with zero relative humidity in a glove box, and the capillary forces were measured in a stable environment created inside the AFM enclosure with relative humidity values ranging from 18% to 80%. To simulate topographic features of the solar panels caused by factors such as cleaning and abrasion, we induced different degrees of surface roughness in the solar glass. We were able to 1) identify and quantify both the van der Waals and capillary forces, 2) establish the effects of surface roughness, relative humidity, and particle size on the adhesion mechanisms, and 3) compare adhesion forces between well-controlled particles (AFM tips and glass spheres) and real dust particles.

A novel Lagrangian agglomerate structure model

Agglomeration of fine particles in fluid flows is important for many processes in powder handling or production. In order to accurately perform numerical calculations of such processes models are needed which realistically describe particle agglomeration. For applications in the frame of an Euler/Lagrange approach such a model was developed which allows predicting the agglomerate structure and its behaviour in the point-particle approximation. The detection of collisions between primary particles or particles and agglomerates is realised by the stochastic collision model. The collision point on the particle or agglomerate surface is generated by accounting for the impact efficiency. The developed structure model is based on storing location vectors from the reference primary particle to all other particles collected in the agglomerate which is still treated as point-mass. Thereby the structure and porosity of the agglomerate is known and an equivalent agglomerate diameter (e.g. gyration diameter) is obtained. With this information the fluid dynamic forces acting on the agglomerates can be predicted correctly as well as the appropriate collision cross-section is used in the collision modelling. For testing the novel agglomerate structure model, fine particle agglomeration in homogeneous isotropic turbulence is calculated considering different properties of the primary particles or droplets. In the case of dry solid particles the van der Waals adhesion force is considered and based on an energy balance the possibility of particle sticking or rebound is calculated. Moreover, high viscous droplets are considered as primary particles, a situation also found in spray dying. For such cases the penetration of two primary particles or a primary particle and a stochastically selected primary particle within an agglomerate is modelled. The structure of the agglomerates produced under these different situations and for mono- and poly-sized primary particles are analysed and compared, using for example gyration diameter, porosity of convex hull and fractal dimension. These results demonstrate that the developed novel agglomerate structure model is applicable in the frame of an Euler/Lagrange approach and provides appropriate results.

Size of nanoparticle agglomerates in fluidization

AbstractA model based on force balance was proposed to estimate the equilibrium size of agglomerates formed in the fluidization process of nanoparticles. This model was derived based on the balance between drag, collision, and gravity forces as separation forces, and the van der Waals force as an adhesion force. The effects of permeability of agglomerates and addition of isopropanol to the gas‐solid suspension on the agglomerate size were investigated for the first time. Results showed that the average size of agglomerates decreased when increasing the gas velocity. In addition, the average agglomerate sizes decreased in the presence of isopropanol due to the weakening of van der Waals adhesion force. An overall agreement with an average error of 20 % was observed between the estimated agglomerate sizes and the experimental data.

Thermal effects on van der Waals adhesive forces

We present an experimental and theoretical study on how thermal energy alters van der Waals adhesion forces in nanoscale contacts stretched by mechanical probes. The force follows a distribution whose density function is an asymmetric bell-shaped curve presenting a temperature-dependent negative skewness. With increasing temperature the asymmetry increases whereas the most probable force value decreases. Using a 2--8 Lennard-Jones interaction potential within the reaction rate theory, we offer a theoretical framework permitting an evaluation of the microscopic parameters governing adhesion in a van der Waals nanocontact subjected to mechanical fluctuations.

Effect of topology on the adhesive forces between electrospun polymer fibers using a T-peel test

Electrospinning provides an effective methodology to obtain high aspect ratio polymer fibers for biomimetic applications. In this article, we evaluate the effect of topology on adhesion between aligned fibers. Polycaprolactone is electrospun using two different setups: (i) a tip collector and (ii) a flat collector. The tip collector enables the fibers to self-align. When a fiber reaches the tip collector, the next fiber is repelled by the charge they carry, forcing the fibers to deposit in a parallel arrangement. The flat collector allows the fibers to deposit at random. The adhesion between the fiber mats is measured using a T-peel test. Adhesion strength (758.7 ± 211.7 kPa) changes marginally with the peeling rate and applied pressure on the membranes. Aligned fibers exhibit higher adhesion strength between the membranes in comparison to randomly oriented nonwovens (613.1 ± 79.9 kPa). The estimated Johnson–Kendall–Roberts contact energy (83.1 ± 32.5 mJ/m2) is consistent with the range of van der Waals adhesion forces. This work shows how the adhesion between two polymer membranes can be modulated by surface topology, based on a T-peel testing setup. POLYM. ENG. SCI., 53:2219–2227, 2013. © 2013 Society of Plastics Engineers

(Invited) Dry Adhesives for MEMS Assembly, Manipulation and Integration: Progress and Challenges

Bioinspired synthetic dry adhesives offer unique potential in the field of micro assembly and manipulation. These adhesives, which take their basic operating principle from animals like geckos, use van der Waals adhesion forces and mechanical fiber optimization to produce significant forces. For several years, these materials have been of interest for climbing robots and adhesive surfaces due to potential characteristics like self-cleaning or anti-fouling behavior, anisotropic adhesion strengths and non-transferring materials. These same strengths would make these materials an ideal mechanism for handling delicate parts or manipulating structures in the field of micro-electro-mechanical systems (MEMS) packaging. In this paper, I review the specific fabrication processes developed by our group and examine some of the challenges in their integration with MEMS assembly and packaging processes. Improving normal adhesion strength and directionality of adhesives has been completed and work is beginning to focus on reducing polymer transfer and improving the anti-static compatibility of the structural materials used in the adhesives.

Multiscale modelling and simulation of the deformation and adhesion of a single gecko seta

A 3D multiscale model is presented which describes the adhesion and deformation of a gecko seta. The multiscale approach combines three models at different length scales: at the top level, on the order of several micrometers, a nonlinear finite element beam model is chosen to capture the branched microstructure of the gecko seta. At the intermediate level, on the order of several nanometers, a second finite element model is used to capture the detailed behaviour of the seta tips, the so-called spatulae. At the lowest level, on the order of a few angstroms, a molecular interaction potential is used to describe the van der Waals adhesion forces between spatulae and substrate. Coarse-graining techiques are used to bridge the scale between the model levels. To illustrate and validate the proposed gecko seta model, numerical pull-off simulations are shown and compared to experimental data from the literature.

Gecko adhesion pad: a smart surface?

Recently, it has been shown that humidity can increase the adhesion of the spatula padsthat form the outermost (adhesive) surface of the tokay gecko feet by 50% relative to themain adhesion mechanism (i.e. van der Waals adhesive forces), although the mechanismby which the enhancement is realized is still not well understood. A change inthe surface hydrophobicity of a gecko setal array is observed when the array,which supports the spatulae, is exposed to a water drop for more than 20 min,suggesting a change in the hydrophilic–lyophilic balance (HLB), and thereforeof the conformation of the surface proteins. A surface force apparatus (SFA)was used to quantify these changes, i.e. in the adhesion and friction forces, whileshearing the setal array against a silica surface under (i) dry conditions, (ii) 100%humidity and (iii) when fully immersed in water. The adhesion increased in thehumid environment but greatly diminished in water. Although the adhesion forceschanged significantly, the friction forces remained unaffected, indicating that thefriction between these highly textured surfaces is ‘load-controlled’ rather than‘adhesion-controlled’. These results demonstrate that the gecko adhesive pads havethe ability to exploit environmental conditions to maximize their adhesion andstabilize their friction forces. Future designs of synthetic dry adhesives inspired bythe gecko can potentially include similar ‘smart’ surfaces that adapt to theirenvironment.

Aggregate growth and breakup in particulate suspension flow through a micro-nozzle

A computational study is reported on the growth of aggregates in flow of a particulate suspension through a micro-nozzle. The study employs a soft-sphere discrete element method (DEM) with van der Waals adhesion force between the particles in two-dimensional, incompressible channel flow. A new computational approach for particle transport in complex domains is developed which uses a background Cartesian grid for efficient flow field interpolation at the particle locations, together with a level-set method to represent the nozzle boundaries in the particle computation. Three mechanisms for the growth or breakup of particulate aggregates in the micro-nozzle are examined: (1) enhanced particle collision due to lateral compression as fluid elements pass through the nozzle, (2) stretching of aggregates due to axial stretching of fluid elements, and (3) collision and intermittent adhesion of particles to the nozzle wall. The first of these mechanisms leads to aggregate growth, and the second to aggregate breakup. The wall collision and adhesion mechanism can enhance either aggregate growth or breakup, but it is found in most cases to be a primary agent in the breakup of incident aggregates as part of the aggregate attaches to the nozzle wall and is torn from the remainder of the aggregate due to the high shear near the walls. Simplified models for these processes are developed and used to interpret the trends observed in the DEM simulations. The effects of particle adhesion parameter, particle size and density, particle concentration, and nozzle geometry are examined. It is found that passage of a particulate suspension through a nozzle can lead to either a substantial decrease in aggregate size or a modest increase under different conditions, depending in part on the size of the incident aggregates.

Megasonic cleaning of wafers in electrolyte solutions: Possible role of electro-acoustic and cavitation effects

Investigations have been conducted on the feasibility of removal of particles from silicon wafers in electrolyte solutions of different ionic strengths irradiated with megasonic waves. Cleaning experiments have been performed using potassium chloride (KCl) as a model electrolyte and silica particles as model contaminant particles. Particle removal efficiency (PRE) increases with KCl concentration and transducer power density and much lower power densities may be used at higher KCl concentration for a comparable level of cleaning. Enhanced cleaning in KCl solutions has been explained as due to two types of electro-acoustic potentials, namely, ionic vibration potential (IVP) and colloidal vibration potential (CVP) that arise when the sound wave propagates through the electrolyte solution. Theoretical computations have shown that the removal forces due to CVP are much larger in magnitude than those due to IVP and are comparable to van der Waals adhesion forces. The effect of ionic strength on cavitation has been investigated through the measurement of acoustic pressure in solutions using a hydrophone. Using Fourier transformation of time dependent pressure data, the size distribution of stable bubbles in KCl solutions of different concentration has been obtained.

Adhesive surface design using topology optimization

Tailoring adhesive properties between surfaces is of great importance for micro-scale systems, ranging from managing stiction in MEMS devices to designing wall-scaling gecko-like robots. A methodology is introduced for designing adhesive interfaces between structures using topology optimization. Structures subjected to external loads that lead to delamination are studied for situations where displacements and deformations are small. Only the effects of adhesive forces acting normal to the surfaces are considered. An interface finite element is presented that couples a penalty contact formulation and a Lennard–Jones model of van der Waals adhesive forces. Two- and three dimensional design optimization problems are presented in which adhesive force distributions are designed such that load-displacement curves of delaminating structures match target responses. The design variables describe the adhesive energy per area of the interface between the surfaces, as well as the geometry of the delaminating structure. A built-in length scale in the formulation of the adhesion forces eliminates the need for filtering to achieve comparable optimal adhesive designs over a range of mesh densities. The resulting design problem is solved by gradient based optimization algorithms evaluating the design sensitivities by the adjoint method. Results show that the delamination response can be effectively manipulated by the method presented. Varying simultaneously both adhesive and geometric parameters yields a wider range of reachable target load-displacement curves than in the case varying adhesive energy alone.

Stick Slip Contact Mechanics between Dissimilar Materials: Effect of Charging and Large Friction

Measurements of the contact radius as a function of applied force between a mica surface and a silica surface (mica/silica) in air are reported. The load/unload results show that the contact radius generally increases with applied force. Because of the presence of charging due to contact electrification, both a short-range van der Waals adhesion force and longer-range electrostatic adhesive interaction contribute to the measured force. The results indicate that approximately 20% of the pull-off force is due to van der Waals forces. The contact radius versus applied force results can be fit to Johnson-Kendall-Roberts (JKR) theory by considering that only the short-range van der Waals forces contribute to the work of adhesion and subtracting a constant longer-range electrostatic force. Also, an additional and unexpected step function is superimposed on the contact radius versus applied force curve. Thus, the contact diameter increases in a stepped dependence with increasing force. The stepped contact behavior is seen only for increasing force and is not observed when symmetric mica/mica or silica/silica contacts are measured. In humid conditions, the contact diameter of the mica/silica contact increases monotonically with applied force. Friction forces between the surfaces are also measured and the shear stress of a mica/silica interface is 100 times greater than the shear stress of a mica/mica interface. This large shear stress retards the increase in contact area as the force is increased and leads to the observed stepped contact mechanics behavior.