Pull-off Force Research Articles

Extending the Double-Hertz Model to Allow Modeling of an Adhesive Elliptical Contact

An adhesive elliptical contact is normally found in microscale applications that involve cylindrical solids, crossing at an angle between 0° and 90°. Currently, only one model is available to describe the elliptical contact’s surface interaction: the approximate Johnson–Kendall–Roberts (JKR) model which is limited to soft materials. In this paper, a new adhesive elliptical model is developed for a wide range of adhesive contacts by extending the double-Hertz theory, where adhesion is modeled by the difference between two Hertzian pressure distributions. Both Hertzian pressures are assumed to have an equivalent shape of contact areas, the only difference being in size. Assuming that the annular adhesive region is obtained by the area difference between the two Hertzian contact areas, the pull-off force curves can be calculated. In the limiting case of an adhesive circular contact, the results are very close to results from the existing models. However, for an adhesive elliptical contact in the JKR domain, lower pull-off forces are predicted when compared to the JKR values. Unlike the developed model, the shape of the JKR contact area varies throughout contact. Results show, particularly for conditions close to the JKR domain, that it is important to take into account that the adhesive region is the result of the two Hertzian contact areas having a non-equivalent shape.

Two-Dimensional Adhesive Contact Problem for Graded Coating Based on a New Multi-Layer Model

In this paper, the contact problem with adhesion for graded coating indented by a cylindrical indenter is considered. A multi-layered model is developed to model functionally graded material (FGM) coating with arbitrarily varying elastic modulus. By using the transfer matrix method and Fourier integral transform technique, the problem is reduced to a Cauchy singular integral equation. The numerical calculation is conducted to achieve the effect of the gradient index of the FGM coating and the adhesive parameter on the pull-off force and the relation between the normal load and the maximum indentation depth. The results show that the gradient index of coating can effectively change the behavior of adhesive contact.

On the Sensitivity of Adhesion between Rough Surfaces to Asperity Height Distribution

There has been a long debate about the validity of asperity models in the contact between rough surfaces, much of it concentrated on relatively minor aspects of the solution for the special case of Gaussian random processes for roughness, like the exact value of the area-load slope or the extent of the linear regime. It is shown here that in the case of adhesion, the behavior is extremely sensitive to the shape of the height distribution. We show for example results for Weibull distributions, which has been suggested in a number of practical cases from macroscopic to nanoscopic roughness. Pull-off force is found to vary by several orders of magnitude both lower and higher than in the Gaussian case, whereas the "stickiness" criterion on the adhesion parameter changes by an order of magnitude. Additionally, in some operations like chemical-mechanical polishing, tails are almost completely removed and a sharp peak develops instead of a tail: modeling this with contact on the bounded side of the Weibull distribution, stickiness seems to occur for any level of roughness. Pome qualitative comparison with recent numerical experiments is attempted.

On the DMT adhesion theory: from the first studies to the modern applications in rough contacts

In the last years, increasing interest has been devoted to the study of the adhesion between rough media. The Derjaguin, Muller & Toporov (DMT) theory is one of the most known models to describe adhesion between hard elastic solids with long range adhesive interactions. However, many versions of the DMT theory can be found in the literature.In the first part of the present work, we try to make order about the various versions of the DMT theory appeared over the years. All models, often confusedly called with the same name ”DMT theory”, are based on the same assumption of neglecting deformations due to the adhesive forces. They predicts the same value of the detachment force at the pull-off, but only the so-called DMT force approach correctly captures the evolution trend of the adhesive force during the contact.In the second part of the work, we address the problem to include adhesion in the contact of rough surfaces according to the DMT theory. Specifically, we compare the Maugis’ idea to add adhesion in the Greenwood and Williamson asperity theory, with more recent models: the Interacting and Coalescing Hertzian Asperities (ICHA) one and the Persson’s theory. The two models, which both take account of adhesion according to the DMT force approach, give very similar results, showing that the pull-off force is negligibly affected by the shortest wavelength components of the surface roughness.On the contrary, the Maugis’ model strongly underestimates adhesion and predicts a vanishing pull-off force when the upper cut-off spatial frequency of the surface is increased.

Bio-inspired solution for optimal adhesive performance

In recent years there has been a growing interest into high performance bioinspired adhesives. This communication focuses on the adhesive behavior of a rigid cylinder that indents an elastic layer coated on a rigid substrate. With the assumption of short range adhesive interactions (JKR type) the adhesive solution is obtained very easily starting from the adhesiveless one. We show that ultrastrong adhesion (up to theoretical material strength) can be reached in line contact by reducing the thickness of the layer, typically down to the nanoscale size, which suggests a new possible design for ”optimal adhesion”. Adhesion enhancement occurs as an increase of the actual pull-off force, which is further enhanced by Poisson’s ratio effects in the case of nearly incompressible layer. The system studied could be an interesting geometry for an adhesive system, but also a limit case of the more general class of layered systems, or FGMs (Functionally Graded Materials). The model is well suited for analyzing the behavior of polymer layers coated on metallic substrates.

Bacterial Adhesion to Graphene Oxide (GO)-Functionalized Interfaces Is Determined by Hydrophobicity and GO Sheet Spatial Orientation

The potential of graphene oxide (GO) in environmental applications, such as the development of antimicrobial materials and low-fouling membranes, has thus far been hindered by an incomplete understanding of bioadhesion mechanisms on GO interfaces. Using atomic force microscopy (AFM)-based single-cell force spectroscopy, we investigate the adhesion of single Pseudomonas fluorescens cells on GO-functionalized interfaces possessing distinct morphologies. Specifically, we investigate Si-GO surfaces, in which Langmuir–Blodgett GO films are transferred to Si wafers by dip-coating, forming an immobilized layer of horizontally arranged GO nanosheets, and PLL-GO surfaces, where GO nanosheets, edge-tethered to poly-l-lysine, form an interface characterized by morphological and conformational disorder. We observe strong adhesion forces on both Si-GO and PLL-GO surfaces; analysis of the pull-off forces in terms of the worm-like chain model reveals that adhesion is driven by hydrophobic interactions between proteinace...

Influence of substrate moisture state and roughness on interface microstructure and bond strength: Slant shear vs. pull-off testing

There are conflicting views in the literature concerning the optimum moisture state for an existing substrate prior to the application of a repair material. Both saturated-surface-dry (SSD) and dry substrates have been found to be preferable in a variety of studies. One confounding factor is that some studies evaluate bonding of the repair material to the substrate via pull-off (direct tension) testing, while others have employed some form of shear specimens as their preferred testing configuration. Available evidence suggests that dry substrate specimens usually perform equivalently or better in shear testing, while SSD ones generally exhibit higher bond strengths when a pull-off test is performed, although exceptions to these trends have been observed. This paper applies a variety of microstructural characterization tools to investigate the interfacial microstructure that develops when a fresh repair material is applied to either a dry or SSD substrate. Simultaneous neutron and X-ray radiography are employed to observe the dynamic microstructural rearrangements that occur at this interface during the first 4 h of curing. Based on the differences in water movement and densification (particle compaction) that occur for the dry and SSD specimens, respectively, a hypothesis is formulated as to why different bond tests may favor one moisture state over the other, also dependent on their surface roughness. It is suggested that the compaction of particles at a dry substrate surface may increase the frictional resistance when tested under slant shear loading, but contribute relatively little to the bonding when the interface is submitted to pull-off forces. For maximizing bond performance, the fluidity of the repair material and the roughness and moisture state of the substrate must all be given adequate consideration.

Biomimetic wall-shaped adhesive microstructure for shear-induced attachment: the effects of pulling angle and preliminary displacement.

To date, a handful of different gecko-like adhesives inspired by spatula-shaped attachment hairs have been suggested based on wedge and flap geometry of contact elements. However, while these surface designs have been shown to have directionality in adhesion, high friction, long lifetime and the ability to work in vacuum, an experimental verification of the very basic concept of the pulling angle effect has not yet been reported. To close this gap, here we use wall-shaped adhesive microstructures of three different flap heights to systematically study the effect of pulling angle on the normal and tangential components of the pull-off force tested at different preliminary tangential displacements. In accord with the prediction of the Kendall model for the normal component of peeling force, there is an optimal normal force that is required to detach the wall-shaped adhesive microstructure. The optimum is obtained at about half the distance needed to initiate sliding and at pulling angles that range within 60-90°, which suggests that the wall-shaped microstructure can tolerate relatively large inaccuracies in the loading direction. The increase of the attachment force with increasing flap height is found to correlate with the flap thickness, which decreased with increasing flap height.

Influence of chemical bonding on the variability of diamond-like carbon nanoscale adhesion

Diamond-like-carbon (DLC) is a promising material for tribological applications such as hard disk, automotive, machine tool, and aerospace coatings. We performed in-situ transmission electron microscopy (TEM) and molecular dynamics (MD) studies of nanoscale single-asperities made of tetrahedral amorphous carbon (ta-C, a type of DLC with high strength) contacting single-crystal diamond, to understand the factors controlling adhesion. Visualization of the contacts in TEM enabled us to correlate the asperity's geometry and the adhesion measurements. MD simulations allowed the atomic-scale mechanisms of adhesion to be elucidated and correlated with the TEM observations. Experimentally-determined pull-in forces show less scatter than pull-off forces. The magnitude of the pull-in forces is consistent with adhesion arising from van der Waals (VDW) interactions, allowing us to estimate the ta-C/diamond Hamaker constant. MD simulations with the AIREBO potential confirmed that including VDW interactions leads to less scatter in adhesive forces in approach than in separation. MD simulations with the REBO+S potential demonstrate that the large scatter in pull-off forces observed experimentally arises from the complex nature of covalent bonding between substrate and tip, influenced by the local energy landscape, hydrogen coverage, and the number of repeated contact events. The scatter in pull-off force also tends to decrease with increasing roughness.

Effect of surface tension on the behavior of adhesive contact based on Lennard–Jones potential law

The present study explores the effect of surface tension on adhesive contact behavior where the adhesion is interpreted by long-range intermolecular forces. The adhesive contact is analyzed using the equivalent system of a rigid sphere and an elastic half space covered by a membrane with surface tension. The long-range intermolecular forces are modeled with the Lennard‒Jones (L‒J) potential law. The current adhesive contact issue can be represented by a nonlinear integral equation, which can be solved by Newton‒Raphson method. In contrast to previous studies which consider intermolecular forces as short-range, the present study reveals more details of the features of adhesive contact with surface tension, in terms of jump instabilities, pull-off forces, pressure distribution within the contact area, etc. The transition of the pull-off force is not only consistent with previous studies, but also presents some new interesting characteristics in the current situation.

Rubber adhesion below the glass transition temperature: Role of frozen-in elastic deformation

We have studied how the adhesion between rubber and a flat countersurface depends on temperature. When the two solids are separated at room temperature negligible adhesion is detected, which is due to the elastic deformation energy stored in the rubber, which is given back during pull-off and help to break the adhesive bonds. When the system is cooled down below the glass transition temperature, the elastic deformation imposed on the system at room temperature is “frozen-in” and the stored-up elastic energy is not given back during separation at the low temperature. This results in a huge increase in the pull-off force. This study is crucial for many applications involving rubber at low temperatures, e.g., rubber seals for cryogenic or space applications.

Mechanical modeling and characteristic study for the adhesive contact of elastic layered media

This paper investigates the adhesive contact between a smooth rigid sphere and a smooth elastic layered medium with different layer thicknesses, layer-to-substrate elastic modulus ratios and adhesion energy ratios. A numerical model is established by combining elastic responses of the contact system and an equation of equivalent adhesive contact pressure which is derived based on the Hamaker summation method and the Lennard–Jones intermolecular potential law. Simulation results for hard layer cases demonstrate that variation trends of the pull-off force with the layer thickness and elastic modulus ratio are complex. On one hand, when the elastic modulus ratio increases, the pull-off force decreases at smaller layer thicknesses, decreases at first and then increases at middle layer thicknesses, while increases monotonously at larger layer thicknesses. On the other hand, the pull-off force decreases at first and then increases with the increase in the layer thickness. Furthermore, a critical layer thickness above which the introduction of hard layer cannot reduce adhesion and an optimum layer thickness under which the pull-off force reaches a minimum are found. Both the critical and optimum layer thicknesses become larger with an increase in the Tabor parameter, while they tend to decrease with the increase in the elastic modulus ratio. In addition, the pull-off force increases sublinearly with the adhesion energy ratio if the layer thickness and elastic modulus ratio are fixed.

Adhesion tunable bio-inspired dry adhesives by twisting

We report on a control method by twisting the adhesion property of a bio-inspired dry adhesive with mushroom-shaped micro structures. The fabricated artificial dry adhesive has a strong adhesive force in all directions; however, this adhesive is considerably strong to detach simply for reversible usages. This study proposes twisting a mushroom-shaped artificial dry adhesive for easy a detachment method that makes a pull-off force decrease from 14 N/cm 2 to nearly 0. In addition, the optimal process parameters of the twisting detachment are studied by experimentally comparing the pull-off force, net torque, and required time depending on the thickness of the adhesive and angular velocity of twisting. Result indicates that thin adhesive (1.2 mm) and high angular velocity (0.0436 rad/s) provide a good detachment performance. For the feasible application of the proposed method, a glass transportation robot was used for demonstration and a glass substrate was transported several times.

How the sucker comes to grip

Biomimetics![Figure][1] Making bio-inspired suction pads PHOTO: WANG ET AL. Remoras have an oval-shaped first dorsal fin with a pad that allows them to adhere to other sea creatures without causing them harm. Through a detailed study of the pad found on a shark remora, Wang et al. were able to fabricate an analog pad using components that had stiffnesses similar to those of natural fish spines, bones, and fibrous tissue. Carbon-fiber spines, providing the strength and stability, were patterned into a mix of rigid and soft printed acrylate polymers that could be manipulated using soft actuators. The engineered pad attached to different surfaces and generated pull-off force up to 340 times the weight of the pad. Sci. Robot. 10.1126/scirobotics.aan8072 (2017). [1]: pending:yes

Measuring the Pull-Off Force of an Individual Fiber Using a Novel Picoindenter/Scanning Electron Microscope Technique.

We employed a novel picoindenter (PI)/scanning electron microscopy (SEM) technique to measure the pull-off force of an individual electrospun poly(vinylidene fluoride) (PVDF) fibers. Individual fibers were deposited over a channel in a custom-designed silicon substrate, which was then attached to a picoindenter. The picoindenter was then positioned firmly on the sample stage of the SEM. The picoindenter tip laterally pushed individual fibers to measure the force required to detach it from the surface of substrate. SEM was used to visualize and document the process. The measured pull-off force ranged between 5.8 ± 0.2 μN to ~17.8 ± 0.2 μN for individual fibers with average diameter ranging from 0.8 to 2.3 μm. Thus, this study, a first of its kind, demonstrates the use of a picoindenter to measure the pull-off force of a single micro/nanofiber.

How tight are beetle hugs? Attachment in mating leaf beetles.

Similar to other leaf beetles, rosemary beetles Chrysolina americana exhibit a distinct sexual dimorphism in tarsal attachment setae. Setal discoid terminals occur only in males, and they have been previously associated with a long-term attachment to the female's back (elytra) during copulation and mate guarding. For the first time, we studied living males and females holding to female's elytra. Pull-off force measurements with a custom-made tribometer featuring a self-aligning sample holder confirmed stronger attachment to female elytra compared with glass in both males and females; corresponding to 45 and 30 times the body weight, respectively. In line with previous studies, males generated significantly higher forces than females on convex elytra and flat glass, 1.2 times and 6.8 times, respectively. Convex substrates like elytra seem to improve the attachment ability of rosemary beetles, because they can hold more strongly due to favourable shear angles of legs, tarsi and adhesive setae. A self-aligning sample holder is found to be suitable for running force measurement tests with living biological samples.

Nanomechanical probing of thin-film dielectric elastomer transducers

Dielectric elastomer transducers (DETs) have attracted interest as generators, actuators, sensors, and even as self-sensing actuators for applications in medicine, soft robotics, and microfluidics. Their performance crucially depends on the elastic properties of the electrode-elastomer sandwich structure. The compressive displacement of a single-layer DET can be easily measured using atomic force microscopy (AFM) in the contact mode. While polymers used as dielectric elastomers are known to exhibit significant mechanical stiffening for large strains, their mechanical properties when subjected to voltages are not well understood. To examine this effect, we measured the depths of 400 nanoindentations as a function of the applied electric field using a spherical AFM probe with a radius of (522 ± 4) nm. Employing a field as low as 20 V/μm, the indentation depths increased by 42% at a load of 100 nN with respect to the field-free condition, implying an electromechanically driven elastic softening of the DET. This at-a-glance surprising experimental result agrees with related nonlinear, dynamic finite element model simulations. Furthermore, the pull-off forces rose from (23.0 ± 0.4) to (49.0 ± 0.7) nN implying a nanoindentation imprint after unloading. This embossing effect is explained by the remaining charges at the indentation site. The root-mean-square roughness of the Au electrode raised by 11% upon increasing the field from zero to 12 V/μm, demonstrating that the electrode's morphology change is an undervalued factor in the fabrication of DET structures.

Effect of coating thickness on the stress profile at the epoxy/silicone interface subjected to pull-off loading: A finite element analysis

Previous experimental studies of silicone coatings have shown three distinct types of release behavior in the tensile flat punch test, depending on coating thickness. The mechanical response in the punch test is highly dependent upon the Poisson's ratio of the coating and its confinement ratio (punch radius divided by coating thickness). This study developed a high accuracy finite-element model of the punch test using the adaptive p-method with extensive mesh refinement to produce smooth stress profiles up to the punch edge. Stress distributions were found for a wide range of confinement parameters and Poisson's ratios. At a typical Poisson's ratio of 0.49, the highest center stress occurred for the intermediate thickness coatings—not thin or thick. Also, the thickest coatings demonstrated steadily increasing high stress towards the edge, while other thicknesses showed the steep singularity at the edge with a protective stress depression bordering inside it. The results further help explain why the critical pull-off force continues to increase as the thickness decreases, even with different release mechanisms. The stress profiles for thick coatings have almost no sensitivity to Poisson's ratio, unlike other thicknesses which show high sensitivity. Edge peeling is most likely to occur for all thick coatings, while other debonding modes are most likely for thin and intermediate thickness coatings. Together, results show the stress mechanics of the flat punch test follow three distinct types of confinement.

Development of water surface mobile robot inspired by water striders

This work describes a water surface mobile robot utilising surface tension forces. Recent biological studies on water striders revealed how they stay afloat and move on the surface of water. By using their hairy legs coated with hydrophobic substance produced, they increase the water repellency and float by the surface tension. This work focuses on understanding the static and dynamic interactions between supporting legs and the surface of water. First, microstructured wire legs were fabricated by using femtosecond laser machining to enhance the water repellency. Then the supporting force, the pull-off force and the drag force were measured to find the suitable legs for water strider robot. Finally, by assembling the legs optimised for supporting load and propulsion, a water strider robot weighing 4.39 g was developed. The robot successfully moved on the surface of water at a speed of 59.2 mm/s.

Multivalent Adhesion and Friction Dynamics Depend on Attachment Flexibility

Self-assembled monolayers introduce chemical functionalities to material surfaces, providing a route to tune their equilibrium and dynamical properties. We report on atomic force microscopy measurements and simulations of adhesion and friction forces caused by a macromolecular host–guest system, where the host molecules are attached to silicon oxide surfaces by means of self-assembled silane layers. Different preparation routes for the silane layers lead to different flexibility of the molecular attachment. The velocity dependencies of the work of separation and of friction vary significantly for attachments with different flexibility. Stiff attachment leads to low pull-off forces at low pulling velocity and to vanishing friction forces in the limit of low sliding velocity. Flexible attachment enhances cooperative contribution of multiple molecular bonds to adhesion and friction and causes significant friction at low sliding velocity. The latter observation can be explained by the contribution of intermittent contact aging to the friction force.