Pull-off Force Research Articles

Non-linear double-peeling: Experimental vs. theoretical predictions

ABSTRACTThe double peeling of detachment of non-linear adhesive tapes from a flat Poly(methyl methacrylate) (PMMA) surface has been investigated from both experimental and theoretical point of view. Double peeling tests show that, as the detachment process advances, the peeling angle stabilizes on a limiting value θlim corresponding to a critical pull-off force Fc above which the tape is completely detached from the substrate. This observed behavior is in good agreement with results obtained following the new theory of multiple peeling and taking into account the hardening-softening non-linear behavior of the experimentally tested adhesive tapes and clarifies some aspects of the experimental data. In particular, the theoretical model shows that the value of the limiting peeling angle depends on the geometry of the adhesive tape as well as on the stiffness properties and on the interfacial energy ∆γ. Finally, theoretical predictions confirm that solutions with a peeling angle lower than θlim are unstable.

Nanometer-Sized Water Bridge and Pull-Off Force in AFM at Different Relative Humidities: Reproducibility Measurement and Model Based on Surface Tension Change.

This article deals with the analysis of the relationship between the pull-off force measured by atomic force microscopy and the dimensions of water bridge condensed between a hydrophilic silicon oxide tip and a silicon oxide surface under ambient conditions. Our experiments have shown that the pull-off force increases linearly with the radius of the tip and nonmonotonically with the relative humidity (RH). The latter dependence generally consists of an initial constant part changing to a convex-concave-like increase of the pull-off force and finally followed by a concave-like decrease of this force. The reproducibility tests have demonstrated that the precision limits have to be taken into account for comparing these measurements carried out under atmospheric conditions. The results were fitted by a classical thermodynamic model based on water-bridge envelope calculations using the numerical solution of the Kelvin equation in the form of axisymmetric differential equations and consequent calculation of adhesive forces. To describe the measured data more precisely, a decrease of the water surface tension for low RH was incorporated into the calculation. Such a decrease can be expected as a consequence of the high surface curvature in the nanometer-sized water bridge between the tip and the surface.

Solvent-Free Patterning of Colloidal Quantum Dot Films Utilizing Shape Memory Polymers

Colloidal quantum dots (QDs) with properties that can be tuned by size, shape, and composition are promising for the next generation of photonic and electronic devices. However, utilization of these materials in such devices is hindered by the limited compatibility of established semiconductor processing techniques. In this context, patterning of QD films formed from colloidal solutions is a critical challenge and alternative methods are currently being developed for the broader adoption of colloidal QDs in functional devices. Here, we present a solvent-free approach to patterning QD films by utilizing a shape memory polymer (SMP). The high pull-off force of the SMP below glass transition temperature (Tg) in conjunction with the conformal contact at elevated temperatures (above Tg) enables large-area, rate-independent, fine patterning while preserving desired properties of QDs.

Sticking properties of ice grains

We study the size dependence of pull-off forces of water ice in laboratory experiments and numerical simulations. To determine the pull-off force in our laboratory experiments, we use a liquid nitrogen cooled centrifuge. Depending on its rotation frequency, spherical ice grains detach due to the centrifugal force which is related to the adhesive properties. Numerical simulations are conducted by means of molecular dynamics simulations of hexagonal ice using a standard coarse-grained water potential. The pull-off force of a single contact between two spherical ice grains is measured due to strain controlled simulations. Both, the experimental study and the simulations reveal a dependence between the pull-off force and the (reduced) particle radii, which differ significantly from the linear dependence of common contact theories.

Cohesive detachment of an elastic pillar from a dissimilar substrate

The adhesion of micron-scale surfaces due to intermolecular interactions is a subject of intense interest spanning electronics, biomechanics and the application of soft materials to engineering devices. The degree of adhesion is sensitive to the diameter of micro-pillars in addition to the degree of elastic mismatch between pillar and substrate. Adhesion-strength-controlled detachment of an elastic circular cylinder from a dissimilar substrate is predicted using a Dugdale-type of analysis, with a cohesive zone of uniform tensile strength emanating from the interface corner. Detachment initiates when the opening of the cohesive zone attains a critical value, giving way to crack formation. When the cohesive zone size at crack initiation is small compared to the pillar diameter, the initiation of detachment can be expressed in terms of a critical value Hc of the corner stress intensity. The estimated pull-off force is somewhat sensitive to the choice of stick/slip boundary condition used on the cohesive zone, especially when the substrate material is much stiffer than the pillar material. The analysis can be used to predict the sensitivity of detachment force to the size of pillar and to the degree of elastic mismatch between pillar and substrate.

ADHESIVE CONTACT BETWEEN SILICON-BASED MEMS TOOTH SURFACES MODELLED BY THE MULTISCALE MULTI-BLOCK MODEL

The contact interactions between microgear silicon-based MEMS teeth working in a clean and a vacuum environment are under consideration. A new approach has used to determine the friction force and the coefficient of friction over the whole meshing surfaces of the teeth. In this approach, the dry friction force is calculated through the energy dissipated during sliding contact between two meshed micro-tooth elastic rough surfaces. The energy dissipated may be caused by the different physical and chemical interactions between the counterparts surfaces. Due to the vacuum environment, these mechanisms reduced to the energy lost due to the dissociation of chemical and van der Waals bonds, and the energy lost through the elastic interlocking between the asperities located on the meshing micro-tooth surfaces. There is no plastic deformation of the microgear tooth surface asperities due to their size and the Polonsky-Keer effect. A multiscale hierarchical elastic structure (a multiscale block) is used to model the surface asperities. The tooth block roughness has modelled at two scales specified by the character of interactions: atomic level, where chemical interactions occur, and adhesive subscale, where van der Waals interactions are significant. The adhesion layer is defined similarly to Maugis approximation. The adhesion force of each nanoasperity has assumed to be equal to the pull-off force in the Boussinesq-Kendall model and corrected by the Borodich no-slip coefficient. Atomic Force Microscopy (AFM) techniques have been used to measure the tooth roughness. It is argued that there be a high probability for stiction between the clean silicon surfaces due to very high values of the friction force between the micro-conjunctions. On the other hand, the tooth surfaces having functionalized carbon-based layers are much less prone to stiction. However, due to wear of the functionalized coating the probability of stiction will start to increase. The results of the simulation for both the non-functionalized and functionalized micro-tooth surfaces (silicon-based MEMS surfaces) are presented.

Probing the adhesion properties of alginate hydrogels: a new approach towards the preparation of soft colloidal probes for direct force measurements.

The adhesion of alginate hydrogels to solid surfaces was probed by atomic force microscopy (AFM) in the sphere/plane geometry. For this purpose a novel approach has been developed for the immobilization of soft colloidal probes onto AFM-cantilevers, which is inspired by techniques originating from cell biology. The aspiration and consecutive manipulation of hydrogel beads by micropipettes allows the entire manipulation sequence to be carried-out in situ. Hence, any alteration of the hydrogel beads upon drying can be excluded. The adhesive behaviour of alginate hydrogels was first evaluated by determining the distribution of pull-off forces on self-assembled monolayers (SAMs) terminating in different functional groups (-CH3, -OH, -NH2, -COOH). It was demonstrated that solvent exclusion plays practically no role in the adhesion process, in clear difference to solid colloidal probes. The adhesion of alginate beads is dominated by chemical interactions rather than solvent exclusion, in particular in the case of amino-terminated SAMs. The data set acquired on the SAMs provided the framework to relate the adhesion of alginate beads on recombinant spider silk protein films to specific functional groups. The preparation of soft colloidal probes and the presented approach in analysing the adhesive behaviour is not limited to alginate hydrogel beads but can be generally applied for probing and understanding the adhesion behaviour of hydrogels on a wide range of substrates, which would be relevant for various applications such as biomedical surface modification or tissue engineering.

A note on the possibility of roughness enhancement of adhesion in Persson's theory

In an attempt to model the observed enhancement of adhesion in some classical experiments in the 1970–1980's, Persson introduced in his theory of adhesion between rough solids an “effective adhesion energy” term which is increased due to roughness-induced area increase. In the old experiments, the adhesion enhancement was measured to be up to one order of magnitude in terms of rolling resistance (and hence adhesion hysteresis), whereas it is generally smaller (up to 30–40%) for pull-off force. Here, an estimate the area increase in those experiments shows Persson's postulate is not supported since a discrepancy of at least one order of magnitude is found. Other explanations of adhesion enhancement come from more recent studies involving special axisymmetric indenters confirmed by experimental findings with negligible area increase, or by a model with well separated gaps in a fully random surface contact. However, even these models are very limited by their assumptions, and in the most general case of adhesion of random rough surfaces a comprehensive model remains elusive, and many questions remain open, requiring a detailed understanding of loading and unloading processes, of the possible effects of range of adhesive forces, of effects of roughness anisotropy, and so on.

Preload-responsive adhesion of microfibre arrays to rough surfaces

Adhesion of bio-inspired microfibre arrays to a rough surface is studied theoretically. The array consists of vertical elastic rods fixed on a rigid backing layer, and the surface is modeled by rigid steps with a normally distributed height. Analytical expressions are obtained for the adhesion forces in both the approach and retraction processes. It is shown that, with the increasing preload, the pull-off force increases at first and then attains a plateau value. The results agree with the previous experiments and are expected helpful in adhesion control of the array in practical applications.

Sexual dimorphism in the attachment ability of the ladybird beetle Coccinella septempunctata on soft substrates

Many insects possess adhesive foot pads, which enable reliable attachment to diverse and unpredictable substrates. The function of these adhesive organs was shown to be affected by environmental conditions such as substrate roughness, chemistry, and ambient humidity. So far, the attachment ability of insects and also that of spiders and geckos has been tested on rigid substrates only. However, the natural habitats of climbing animals may provide a variety of substrate stiffness ranging from rigid rock surfaces to soft, biofilm covered substrates. In order to test the effect of different substrate stiffness on the attachment ability of insects, we have performed friction experiments with female and male ladybird beetles Coccinella septempunctata on smooth silicone elastomer substrates of different stiffness, using a centrifugal force tester. Whereas in females, the attachment ability was not affected by the substrate stiffness within the range of tested stiffness, males showed decreasing attachment ability with decreasing substrate stiffness. This sexual dimorphism in attachment ability is explained by the presence of a specialized, discoidal seta type in males, which is not present in females. It is argued that discoidal setae, when softer if compared to the substrate, may show an advantageous peak-free interfacial stress distribution when being pulled off the substrate. For such setae being stiffer if compared the substrate, they potentially show increased edge stress concentration. In this case, lower pull-off forces are expected, in agreement with the experimentally obtained results. With the present study, we demonstrate for the first time that the substrate stiffness may have an effect on the attachment ability of climbing animals, which may also be of relevance for technical and medical applications involving adhesion to soft substrates.

Assessment of Head Displacement and Disassembly Force With Increasing Assembly Load at the Head/Trunnion Junction of a Total Hip Arthroplasty Prosthesis

BackgroundMost femoral components used now for total hip arthroplasty are modular, requiring a strong connection at assembly. The aim of this study was to assess the effect of assembly force on the strength of head-trunnion interface and to measure the initial displacement of the head on the trunnion with different assembly forces. MethodsThree assembly load levels were assessed (A: 2 kN, B: 4 kN, C: 6 kN) with 4 implants in each group. The stems were mounted in a custom rig and the respective assembly loads were applied to the head at a constant rate of 0.05 kN/s (ISO7260-10:2003). Load levels were recorded during assembly. Head displacement was measured with a laser sensor. The disassembly force was determined by a standard pull-off test. ResultsThe maximum head displacement on the trunnion was significantly different between the 2 kN group and the other 2 groups (4 kN, 6 kN, P = .029), but not between the 4 kN and 6 kN groups (P = .89). The disassembly forces between the 3 groups were significantly different (mean ± standard deviation, A: 1316 ± 223 kN; B: 2224 ± 151 kN; C: 3965 ± 344 kN; P = .007), with increasing assembly load leading to a higher pull-off force. For the 4 kN and 6 kN groups, a first peak of approximately 2.5 kN was observed on the load recordings during assembly before the required assembly load was eventually reached corresponding to sudden increase in head displacement to approximately 150 μm. ConclusionAn assembly force of 2 kN may be too low to overcome the frictional forces needed to engage the head and achieve maximum displacement on the trunnion and thus an assembly load of greater than 2.5 kN is recommended.

THE JKR-ADHESIVE NORMAL CONTACT PROBLEM OF AXISYMMETRIC RIGID PUNCHES WITH A FLAT ANNULAR SHAPE OR CONCAVE PROFILES

The JKR-adhesive frictionless normal contact problem is solved for the flat annular and the conical or spherical concave rigid punch indenting an elastic half space. The adhesive solution can be derived analytically from the non-adhesive one, the latter one being calculated by the boundary element method. It is found that the annular flat punch will always start to detach at the outer boundary. The pull-off forces for both concave punch shapes almost do not depend on the pull-off boundary regime and can be significantly larger than the pull-off force for the cylindrical flat punch.

Effect of trunnion roughness and length on the modular taper junction strength under typical intraoperative assembly forces

Modular hip implants are at risk of fretting-induced postoperative complications most likely initiated by micromotion between adjacent implant components. A stable fixation between ball head and stem-neck taper is critical to avoid excessive interface motions. Therefore, the aim of this study was to identify the effect of trunnion roughness and length on the modular taper strength under typical intraoperative assembly forces.Custom-made Titanium trunnions (standard/mini taper, smooth/grooved surface finish) were assembled with modular Cobalt-chromium heads by impaction with peak forces ranging from 2kN to 6kN. After each assembly process these were disassembled with a materials testing machine to detect the pull-off force as a measure for the taper strength.As expected, the pull-off forces increased with rising peak assembly force (p < 0.001). For low and moderate assembly forces, smooth standard tapers offered higher pull-off forces compared to grooved tapers (p < 0.038). In the case of an assembly force of 2kN, mini tapers showed a higher taper strength than standard ones (p=0.037).The results of this study showed that smooth tapers provided a higher strength for taper junctions. This higher taper strength may reduce the risk of fretting-related complications especially in the most common range of intraoperative assembly forces.

Adhesion measurements for tungsten dust deposited on tungsten surfaces

The first experimental determination of the pull-off force for tungsten dust adhered to tungsten surfaces is reported. Dust deposition is conducted with gas dynamics methods in a manner that mimics sticking as it occurs in the tokamak environment. Adhesion measurements are carried out with the electrostatic detachment method. The adhesion strength is systematically characterized for spherical micron dust of different sizes and planar surfaces of varying roughness. The experimental pull-off force is nearly two orders of magnitude smaller than the predictions of contact mechanics models, but in strong agreement with the Van der Waals formula. A theoretical interpretation is provided that invokes the effects of nanometer-scale surface roughness for stiff materials such as tungsten.

Avoidance, escape and microstructural adaptations of the tea green leafhopper to water droplets.

Rain and dew droplets can dislodge or drown small insects and may be important factors that drive adaptations for avoidance and escape. Studying the microstructure of small insects and insect behaviour can help understand these adaptations. We quantified avoidance behaviour and entrapment of nymphs and adults of the tea green leafhopper (Empoasca onukii) using simulated rainfall onto host plant foliage and made observations of pretarsi and abdomen microstructures. Adults responded rapidly to simulated rainfall and escaped by jumping whilst most young nymphs were washed from water-sprayed leaves though older nymphs tended to remain on leaves and subsequently escaped from water droplets. Adults had denser covering of water-repelling brochosomes on pretarsi and abdomen surface than nymphs, and were able to stand on water film whilst most nymphs had multiple penetrating tarsi. Removal of brochosomes from the abdomen of adults reduced hydrophobicity, demonstrating the hydrophobic significance of brochosomes in the capacity of leafhopper to escape from water droplets. Nymphs exhibited a higher pull-off force than adults. This research is one of the few studies to focus on the wettability and water avoidance of small insect pests and has implications for pest management.

Edge Debonding in Peeling of a Thin Flexible Plate From an Elastomer Layer: A Cohesive Zone Model Analysis

A cohesive zone modeling (CZM) approach with a bilinear traction-separation relation is used to study the peeling of a thin overhanging plate from the edge of an incompressible elastomeric layer bonded firmly to a stationary rigid base. The deformations are approximated as plane strain and the materials are assumed to be linearly elastic, homogeneous, and isotropic. Furthermore, governing equations for the elastomer deformations are simplified using the lubrication theory approximations, and those of the plate with the Kirchhoff–Love theory. It is found that the peeling is governed by a single nondimensional number defined in terms of the interfacial strength, the interface fracture energy, the plate bending rigidity, the elastomer shear modulus, and the elastomeric layer thickness. An increase in this nondimensional number monotonically increases the CZ size ahead of the debond tip, and the pull-off force transitions from a fracture energy to strength dominated regime. This finding is supported by the results of the boundary value problem numerically studied using the finite element method. Results reported herein could guide elastomeric adhesive design for load capacity and may help ascertain test configurations for extracting the strength and the fracture energy of an interface from test data.

Fabrication of PVDF hierarchical fibrillar structures using electrospinning for dry-adhesive applications

We report the fabrication of hierarchical poly(vinylidene fluoride) (PVDF) fibrous structures using a unique fabrication technique based on electrospinning. Electrospinning was used to fabricate aligned PVDF fibrous membranes. These membranes were then brought in contact with anodized aluminum oxide (AAO) template and then heat-treated above the glass transition temperature (T g) of the polymer to assist the flow of polymer within the cylindrical pores of AAO template. Scanning electron microscopy images confirmed that this approach lead to the growth of nanopillars on the surface of PVDF fibers. Nanoindenter was used to measure the pull-off force that was required to completely detach the indenter from the samples. To investigate the effect of hierarchy, pull-off force required to detach the indenter from neat PVDF fibers was determined and compared with the pull-off force recorded for hierarchical fibers. The effect of indentation depth was also investigated on both PVDF fibers and PVDF fiber with nanopillars. Significant pull-off force recorded indicates that these PVDF hierarchical fibrous structures can be potentially used for dry-adhesive applications.

Boundary element analyses on the adhesive contact between an elastic sphere and a rigid half-space

Boundary element method is used to analyze the adhesive contact between an elastic sphere and a rigid half-space. Lennard-Jones potential is used for the surface traction. In the past, the simulation for the adhesive contact between spheres usually used paraboloid approximation for sphere surface. In this paper, the adhesive contact is investigated using exact sphere. A new algorithm for boundary element method is proposed. The S-shaped load-approach curve and the whole solution can be obtained. The result is compared with the adhesive contact between a rigid paraboloid and an elastic half-space, and that between an elastic paraboloid and a rigid half-space. It is found that the Tabor parameter and the radius of sphere affect the adhesive contact. As the Tabor parameter is larger, the effect of the radius is larger. As the radius is smaller, the pull-off force and the pull-off distance for fixed-grips device are smaller. The adhesive contact for rigid paraboloid/elastic half-space can approximate that for elastic paraboloid/rigid half-space and that for elastic sphere/rigid half-space with small Tabor parameter and large radius. Finally, an approximate equation for the pull-off force vs. Tabor parameter is proposed.

Contact probing of stretched membranes and adhesive interactions: graphene and other two-dimensional materials.

Contact probing is the preferable method for studying mechanical properties of thin two-dimensional (2D) materials. These studies are based on analysis of experimental force–displacement curves obtained by loading of a stretched membrane by a probe of an atomic force microscope or a nanoindenter. Both non-adhesive and adhesive contact interactions between such a probe and a 2D membrane are studied. As an example of the 2D materials, we consider a graphene crystal monolayer whose discrete structure is modelled as a 2D isotropic elastic membrane. Initially, for contact between a punch and the stretched circular membrane, we formulate and solve problems that are analogies to the Hertz-type and Boussinesq frictionless contact problems. A general statement for the slope of the force–displacement curve is formulated and proved. Then analogies to the JKR (Johnson, Kendall and Roberts) and the Boussinesq–Kendall contact problems in the presence of adhesive interactions are formulated. General nonlinear relations among the actual force, displacements and contact radius between a sticky membrane and an arbitrary axisymmetric indenter are derived. The dimensionless form of the equations for power-law shaped indenters has been analysed, and the explicit expressions are derived for the values of the pull-off force and corresponding critical contact radius.

Fluidized-bed measurements of carefully-characterized, mildly-cohesive (Group A) particles

Fluidization is sensitive to mild cohesion levels, as evidenced by the different flow behaviors of Geldart Group A and Group B particles. Additionally, cohesion is extremely sensitive to surface roughness. Here, mildly cohesive (Group A) spherical particles were carefully characterized and used in high-fidelity experiments of defluidization, bed expansion, and bubble characterization. The experiments were conducted such that van der Waals forces were the only source of cohesion, and thus the results presented can be used for validating Discrete Element Method (DEM) and/or continuum models. Particle characterization includes surface roughness and adhesion (pull-off) force, which can be used for modeling the van der Waals force. Additionally, the particle friction coefficient, coefficient of restitution, shape (sphericity), and diameter were measured via a suite of simple experiments. This level of particle characterization, which is critical for the validation of cohesive models (DEM and continuum), has not been reported in previous fluidization studies. Accordingly, the characterization experiments employed here are described in sufficient detail for straightforward adaption elsewhere.