Peel-off Force Research Articles

Exploiting interfacial instability during peeling a flexible plate from elastic films

Adhesive interactions between soft materials are prevalent in both biological systems and various engineering applications, including soft robots, flexible electronics, and antifouling coatings. Many studies have demonstrated that cavitation and fingering instabilities emerge at the adhesive interface between rigid objects and soft films, owing to the geometric attributes of the contact region. However, in the context of peeling configurations, defining the geometric features is challenging, resulting in relatively scant exploration of interfacial instabilities. Hence, the modulation of instability patterns during the peeling process of a flexible plate from a thin elastic film, alongside the consequential effects on mechanical responses, remains poorly understood. To elucidate the mechanisms underlying interfacial instability during peeling process and its impacts on peel-off force, we use finite element methods to simulate the evolution of interface separation. Consistent with previous experimental observations, we find that the interfacial instability will occur when the bending stiffness of the flexible plate is bigger than a critical value. We show that the interfacial instability is mainly induced by the competition between the adhesion energy and the strain energy of the film, and the incompressibility of the thin film is critical for the appearance of the interfacial instability. Combining theory and finite element simulation, we propose the scaling laws for the critical peel-off force for stable and unstable peelings, respectively, and show that the critical peel-off force will decrease when the interfacial instability occurs. Finally, we demonstrate that weakening the tangential adhesion strength and loosening the constraints between the film and the rigid substrate effectively suppress fingering instability. Collectively, our findings elucidate the pivotal factors influencing interfacial instability, offering invaluable insights for the design of structures or systems involving soft materials.

Ultrasonic fortification of interfiber autohesive contacts in meltblown nonwoven materials

Autohesion is a unique class of adhesion that enables the bonding of two identical surfaces by establishing intimate contact at interfaces. Creating intimacy between two identical surfaces poses a challenging task, often constrained by the presence of surface roughness and chemical heterogeneity. To surmount this challenge, we document a variety of autohesive traits in polypropylene-based meltblown nonwovens, accomplished through a facile, scalable, energy-efficient, and cost-effective ultrasonic bonding process. The mean work of autohesion for a single polypropylene bond, serving as a figure of merit, has been computed by extending the classical Johnson−Kendall−Roberts (JKR) theory by factoring in peel strength along with key fiber and structural parameters of nonwoven materials. Achieving a high figure of merit in ultrasonically bonded nonwovens hinges on the synergistic interplay of key process parameters, including static force, power, and welding speed, with the fiber and structural properties acting in concert. In this regard, peel-off force analysis has also been conducted on a series of twenty-seven ultrasonically bonded meltblown nonwovens prepared using a 33 full factorial design by systematically varying process parameters (static force, power, and welding speed) across three levels and extension rate. X-ray microcomputed tomography (microCT) analysis has been performed on select ultrasonically bonded nonwoven samples to discern their bulk characteristics. A broad spectrum of mean work of autohesion for a single polypropylene bond, ranging from 1.88 to 9.93 J/m², has been ascertained by modulating key process parameters.

Flexible Printed Circuit Board Strain Sensor Embedded in a Miniaturized Pneumatic Finger

A miniaturized pneumatic finger allows for safer grasping without damaging the surface of an object. A flexible strain sensor was developed and embedded in a miniaturized pneumatic finger to sense the finger’s self-deformation as a feedback control signal. The proposed flexible strain sensor comprised a Zr-based metallic glass sensing material and a flexible printed circuit board (FPCB) substrate. The properties of fabricated strain sensors with different patterns and with metallic glass thin films of different thicknesses were measured. The miniaturized pneumatic finger was fabricated from a highly elastic polymer by demolding. To integrate the flexible strain sensor and the miniaturized pneumatic finger, surface treatment was performed to improve the bonding strength. Measurements of the contact angle and peel-off force reveal that the treated surfaces have lower contact angles and higher bonding strength. This higher bonding strength between the flexible strain sensor and the miniaturized pneumatic finger prevented the destruction of the finger during bending. The developed flexible strain sensor embedded in the miniaturized pneumatic finger could sense the finger’s self-deformation by measuring changes in resistance.

Peeling of a film from a flexible cantilever substrate

The peeling behavior of the film peeling off from the fixed end to the free end (FIFR) and from the free end to the fixed end (FRFI) of the flexible cantilever substrate are studied theoretically. The analysis results indicate the film is more easily peeled off from FIFR of the flexible cantilever substrate than that from FRFI of the flexible cantilever substrate. And the peel-off force of FIFR decreases with the decreasing of flexural rigidity of substrate B, adhesion energy G, the elastic modus E and thickness h of the film, while increases with the decreasing the arc length l between the fixed end and the peeling front. However, the variation of the peel-off force of FRFI is opposite to FIFR. In addition, the peel-off force of FIFR keeps mostly unchangeable when B and Eh are greater than their respective thresholds. Furthermore, there exists a critical value of peeling angle for FRFI, which is related to G, B and l. Beyond this critical value, the peel-off force is relatively large and independent of the peeling angle.

INFLUENCE AND REGULATION OF INTERFACIAL ADHESION PROPERTIES OF A MAGNETIC SENSITIVE FILM/SUBSTRATE BY MAGNETIC FORCE AND FILM'S CURVATURE

The controllable interface adhesion of attachment and detachment has important application requirements in climbing devices, adhesion switches and mechanical grippers. In present paper, the influence mechanism of external magnetic field and film's initial curvature on the interfacial adhesion of a magnetic sensitive film/substrate is studied. The peel-test of the magnetic sensitive thin film with initial curvature on a substrate as well as the corresponding theoretical study are respectively carried out. Both the experimental and theoretical results indicate that the interfacial adhesion force of the magnetic sensitive thin film/substrate increases with increasing the initial curvature of the film, and the external magnetic field can enhance the interfacial adhesion force. Compared with the steady-state peel-off force of a flat thin film that is independent on the bending stiffness, the bending stiffness would decrease the steady-state peel-off force of the film with initial curvature. The interface effective adhesion energy is further considered from the energy point of view, which can disclose the comparing mechanisms of the film's bending energy, the potential energy of external magnetic field and the adhesion energy. Lastly, based on the experimental and theoretical results, a simply mechanical gripper controlled by both the magnetic field and film's initial curvature is proposed, which can continuously realize the gripping, transport and release of an object. The results obtained in the present paper can not only be helpful for understanding the interface reversible adhesion mechanism actuated by multi-field, but also provide a novel approach to design functional devices with controllable interface adhesion.

Angle-independent optimal adhesion in plane peeling of thin elastic films at large surface roughnesses

Adhesive peeling of a thin elastic film from a substrate is a classic problem in mechanics. However, many of the investigations on this topic to date have focused on peeling from substrates with flat surfaces. In this paper, we study the problem of peeling an elastic thin film from a rigid substrate that has periodic surface undulations. We allow for contact between the detached part of the film with the substrate. We give analytical results for computing the equilibrium force given the true peeling angle, which is the angle at which the detached part of the film leaves the substrate. When there is no contact we present explicit results for computing the true peeling angle from the substrate’s profile and for determining an equilibrium state’s stability solely from the substrate’s surface curvature. The general results that we derive for the case involving contact allow us to explore the regime of peeling at large surface roughnesses. Our analysis of this regime reveals that the peel-off force can be made to become independent of the peeling direction by roughening the surface. This result is in stark contrast to results from peeling on flat surfaces, where the peel-off force strongly depends on the peeling direction. Our analysis also reveals that in the large roughness regime the peel-off force achieves its theoretical upper bound, irrespective of the other particulars of the substrate’s surface profile.

Liquid-assisted, etching-free, mechanical peeling of 2D materials

Mechanical peeling is a well-known route to transfer a single piece of two-dimensional (2D) materials from one substrate to another one, yet heavily relies on trial and error methods. In this work, we propose a liquid-assisted, etching-free, mechanical peeling technique of 2D materials and systemically conduct a theoretical study of the peeling mechanics for various 2D materials and substrates in a liquid environment. The surface wettability of 2D materials and substrates and surface tension of liquids have been incorporated into the peeling theory to predict the peel-off force. The theoretical model shows that the peel-off force can be significantly affected by liquid solvents in comparison with that in dry conditions. Moreover, our analysis reveals that the mechanical peeling-induced selective interface delamination in multilayered 2D materials can be achieved by employing a liquid environment. These theoretical results and demonstrations have been extensively confirmed by comprehensive molecular dynamics simulations and good agreement is obtained between them. The present work in theory provides a new approach of peeling 2D materials from substrates and can also be extended for peeling thin films and membranes.

Coupled effects of the temperature and the relative humidity on gecko adhesion

To explain the inconsistent results of experiments on temperature-dependent gecko adhesion, a theoretical peeling model is established wherein a nano-thin film is adopted to simulate a gecko spatula. The model considers not only the respective effects of temperature and environmental humidity on the peel-off force but also the coupled effect of both factors. Increasing temperature is found to lead to a decreasing peel-off force if the environmental humidity is uncontrolled. However, if the environmental humidity is constant, the peel-off force is insensitive to the temperature and remains almost constant. The synthetic theoretical analysis demonstrates that the seemingly contradictory results of temperature-dependent gecko adhesion experiments are actually consistent under their respective experimental conditions. This inconsistency is mainly due to the environmental humidity, which varies with the changing temperature if it is not artificially controlled. The results cannot only reasonably explain the different experimental results for the effect of temperature on gecko adhesion but can also facilitate the design of temperature-controlled or humidity-controlled adhesion sensors by tuning the environmental humidity or temperature.

Thermally assisted peeling of an elastic strip in adhesion with a substrate via molecular bonds

A statistical model is proposed to describe the peeling of an elastic strip in adhesion with a flat substrate via an array of non-covalent molecular bonds. Under an imposed tensile peeling force, the interfacial bonds undergo diffusion-type transition in their bonding state, a process governed by a set of probabilistic equations coupled to the stretching, bending and shearing of the elastic strip. Because of the low characteristic energy scale associated with molecular bonding, thermal excitations are found to play an important role in assisting the escape of individual molecular bonds from their bonding energy well, leading to propagation of the peeling front well below the threshold peel-off force predicted by the classical theories. Our study establishes a link between the deformation of the strip and the spatiotemporal evolution of interfacial bonds, and delineates how factors like the peeling force, bending rigidity of the strip and binding energy of bonds influence the resultant peeling velocity and dimensions of the process zone. In terms of the apparent adhesion strength and dissipated energy, the bond-mediated interface is found to resist peeling in a strongly rate-dependent manner.

Effect of relative humidity on the peeling behavior of a thin film on a rigid substrate.

Inspired by gecko adhesion in humid environments, a modified Kendall's model is established in order to investigate the effect of relative humidity on the interfacial peeling behavior of a thin film adhering on a rigid substrate. When the humidity is less than 90%, a monolayer of water molecules adsorbed on the substrate surface induces a strong disjoining pressure at the interface. As a result, the steady-state peel-off force between the thin film and substrate is significantly enhanced. When the humidity is greater than 90%, water molecules condense into water droplets. Four different peeling models are established on this occasion, depending on the surface wettability of the film and substrate. It is found that the steady-state peel-off force is influenced by the water meniscus in a complicated manner, which is either enhanced or reduced by the water capillarity comparing to that predicted by the classical Kendall's model, i.e., a dry peeling model. It should be noted that, at the vicinity of the wetting transition, the peel-off force of the four models can be reduced to an identical one, which means the four peeling models can transit from one to another continuously. The present model, as an extension of the classical Kendall's one, should be useful not only for understanding gecko adhesion in humid environments, but also for analyzing interface behaviors of a film-substrate system in real applications.

Technological Investigation of Clad Sheet Bonding by Hot Rolling

In this study the bonding properties of three layer-plated aluminum sheets are investigated. The alloys applied in specific layers were as follows: AlMn1Si0.8 (core alloy) and AlSi10 (liner). The bonding was performed on a Von Roll experimental roll mill using hot rolling. The experimental temperatures were 460, 480 and 500 °C, respectively. To qualify bond development, T-peel test was used. The test was performed using an Instron universal material testing machine. T-peel test can be well used for the qualification of bond strength as the peel-off force and bond value developed on contacting surfaces are proportional. In addition to T-peel test, optical micrographs and SEM micrographs were also captured, in which typical bond faults were sought. The study aims at modelling the technology used in industry and exploring some typical bond faults as well as suggesting the causes generating these and their remedy. The impact of surface roughening before heating was studied as well. Also, the study aimed at confirming the suitability of T-peel test to qualify bond strength.

Peeling behavior of a thin-film on a corrugated surface

The peeling behavior of a thin-film perfectly adhering on a corrugated substrate is investigated theoretically. Unlike the usually adopted average method of introducing an effective adhesion energy, the effect of substrate roughness is considered directly in this paper and an accurate closed-form solution to the peel-off force under quasi-static peeling process is achieved. Comparing to the results obtained by the average method and those of a smooth substrate case shows that the peel-off force in the present model varies periodically, similar to the roughness of substrates. Furthermore, it is interesting to find that the peeling strength (defined by the maximal peel-off force) of the corrugated interface can be significantly improved with the increase of substrate roughness, while the peel-off force obtained by the average method was found to decrease monotonically or increase first and then decrease with the increasing surface roughness. Spontaneous detachment happens locally at the valley or crest of each asperity when the substrate roughness is large enough, but it does not influence the enhanced trend of the maximal peel-off force. The effect of mode-mixity dependent interface adhesion energy on the peel-off force is also considered, by which the interface peeling strength is further improved. The results in this paper should be helpful for deep understanding of the interface behavior between a film and a rough substrate and be useful for the design of film/substrate interfaces with high interface quality in nano-devices.

Peeling behavior of a viscoelastic thin-film on a rigid substrate

In order to study the adhesion mechanism of a viscoelastic thin-film on a substrate, peeling experiment of a viscoelastic polyvinylchloride (PVC) thin-film on a rigid substrate (glass) is carried out. The effects of peeling rate, peeling angle, film thickness, surface roughness and the interfacial adhesive on the peel-off force are considered. It is found that both the viscoelastic properties of the film and the interfacial adhesive contribute to the rate-dependent peel-off force. For a fixed peeling rate, the peel-off force decreases with the increasing peeling angle. Increasing film thickness or substrate roughness leads to an increase of the peel-off force. Viscoelastic energy release rate in the present experiment can be further predicted by adopting a recently published theoretical model. It is shown that the energy release rate increases with the increase of peeling rates or peeling angles. The results in the present paper should be helpful for understanding the adhesion mechanism of a viscoelastic thin-film.

On-Chip Parylene-C Microstencil for Simple-to-Use Patterning of Proteins and Cells on Polydimethylsiloxane

Polydimethylsiloxane (PDMS) is widely used as a substrate in miniaturized devices, given its suitability for execution of biological and chemical assays. Here, we present a patterning approach for PDMS, which uses an on-chip Parylene-C microstencil to pattern proteins and cells. To implement the on-chip Parylene-C microstencil, we applied SiOx-like nanoparticle layers using atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) of tetraethyl orthosilicate (TEOS) mixed with oxygen. The complete removal of Parylene-C from PDMS following application of SiOx-like nanoparticle layers was demonstrated by various surface characterization analysis, including optical transparency, surface morphology, chemical composition, and peel-off force. Furthermore, the effects of the number of AP-PECVD treatments were investigated. Our approach overcomes the tendency of Parylene-C to peel off incompletely from PDMS, which has limited its use with PDMS to date. The on-chip Parylene-C microstencil approach that is based on this Parylene-C peel-off process on PDMS can pattern proteins with 2-μm resolution and cells at single-cell resolution with a vacancy ratio as small as 10%. This provides superior user-friendliness and a greater degree of geometrical freedom than previously described approaches that require meticulous care in handling of stencil. Thus, this patterning method could be applied in various research fields to pattern proteins or cells on the flexible PDMS substrate.

Method for controlling surface energies of paper substrates to create paper-based printed electronics

The creation of practical paper-based electronics requires secured conductivity with conductive silver tracks fabricated on a paper substrate. Various paper properties were explored for obtaining the key parameters intimately related to printed circuit qualities. A comparison of the resistance of silver tracks printed by ink-jet with a tetradecane-based ink of silver nanoparticles among four substrates—photo-quality ink-jet paper, matte-type ink-jet paper, coated offset paper, and uncoated laboratory sheets—implied the importance of pore size, porosity, surface roughness, and surface energy. Paper surface layers with small pore sizes and high porosities produced highly conductive, narrow silver tracks because of quick ink absorption, as observed in the photo-quality ink-jet paper. The surface roughness induced a high resistance to peel-off force at the expense of conductivity, and this improvement in the peel-off resistance is considered to be achieved because of the anchor effect of silver nanoparticle inks which fell into dents present on the rough paper surfaces. The widths of the silver tracks were significantly reduced by controlling the surface energies of the paper sheets. This tendency was remarkable, especially for uncoated laboratory sheets, and thus the conductivities of the silver tracks were successfully improved.

Effect of pre-tension on the peeling behavior of a bio-inspired nano-film and a hierarchical adhesive structure

Inspired by the reversible adhesion behaviors of geckos, the effects of pre-tension in a bio-inspired nano-film and a hierarchical structure on adhesion are studied theoretically. In the case with a uniformly distributing pre-tension in a spatula-like nano-film under peeling, a closed-form solution to a critical peeling angle is derived, below or above which the peel-off force is enhanced or reduced, respectively, compared with the case without pre-tension. The effects of a non-uniformly distributing pre-tension on adhesion are further investigated for both a spatula-like nano-film and a hierarchical structure-like gecko's seta. Compared with the case without pre-tension, the pre-tension, no matter uniform or non-uniform, can increase the adhesion force not only for the spatula-like nano-film but also for the hierarchical structure at a small peeling angle, while decrease it at a relatively large peeling angle. Furthermore, if the pre-tension is large enough, the effective adhesion energy of a hierarchical structure tends to vanish at a critical peeling angle, which results in spontaneous detachment of the hierarchical structure from the substrate. The present theoretical predictions can not only give some explanations on the existing experimental observation that gecko's seta always detaches at a specific angle and no apparent adhesion force can be detected above the critical angle but also provide a deep understanding for the reversible adhesion mechanism of geckos and be helpful to the design of biomimetic reversible adhesives.

An accordion model integrating self-cleaning, strong attachment and easy detachment functionalities of gecko adhesion

Recent experiments have shown that gecko adhesion exhibits multifunctionalities including self-cleaning, strong attachment and easy detachment. Currently, there exists no theoretical framework in contact mechanics that is capable of integrating all these features in one unified model. Here we propose an accordion model of adhesion that accomplishes this integration with an adhesive pad consisting of a compliant backbone covered by a foldable hard skin containing hidden adhesives which are initially folded into the structure to avoid adhesion in the stress-free state. At the default, stress-free state, the pad is non-adhesive and self-cleaning. Under a tensile prestress large enough to expose the hidden adhesives, the pad becomes highly stiffened and strongly adhesive. We investigate the adhesion behavior of such a structure and show that strong attachment can be established spontaneously when the pad is dragged on an external surface at a low angle, with prestress trapped in the attached portion of the pad. We then investigate the orientation-dependent peel-off force of an attached accordion pad and show that it can be spontaneously detached above a critical pulling angle, leading to strongly reversible adhesion.

Adhesion Performance of Polymer Nanofibres Under Shear Loading

Geckos have extraordinary wall-climbing ability because of the millions of hairs with micro/nano fibrillar structures on their feet. Mimicking gecko's feet is of scientific and engineering importance for development of physical adhesion materials and devices. The design of gecko-inspired physical adhesives seems to be geometry dominated. In this study, Finite Element Method (FEM) has been used to analyse the vertical peel-off force of polyporpylene (PP) nanofibres having different fibre dimensions, inclining angels and contact areas on a flat glass substrate. It has been found that the main parameters affecting the frictional adhesion are fibre diameter and fibre aspect ratio, the inclining angle between the fibre and the substrate surface, and the intimate contact areas. Our analysis has shown that PP nanofibres with a diameter of less than 200nm can generate less peel-off force than fibres of larger diameters, indicating more stable adhesion with the glass substrate for thinner fibres. A bent fibre with more intimate contact area can bear more shear force than a straight fibre with less contact area. Also, under the same shear loading, fibres with an inclining angle of less than 30° provide a low peel off force.

Pre-tension generates strongly reversible adhesion of a spatula pad on substrate

Motivated by recent studies on reversible adhesion mechanisms of geckos and insects, we investigate the effect of pre-tension on the orientation-dependent adhesion strength of an elastic tape adhering on a substrate. Our analysis shows that the pre-tension can significantly increase the peel-off force at small peeling angles while decreasing it at large peeling angles, leading to a strongly reversible adhesion. More interestingly, we find that there exists a critical value of pre-tension beyond which the peel-off force plunges to zero at a force-independent critical peeling angle. We further show that the level of pre-tension required for such force-independent detachment at a critical angle can be induced by simply dragging a spatula pad along a substrate at sufficiently low angles. These results provide a feasible explanation of relevant experimental observations on gecko adhesion and suggest possible strategies to design strongly reversible adhesives via pre-tension.

PDMS as a sacrificial substrate for SU-8-based biomedical and microfluidic applications

We describe a new fabrication process utilizing polydimethylesiloxane (PDMS) as a sacrificial substrate layer for fabricating free-standing SU-8-based biomedical and microfluidic devices. The PDMS-on-glass substrate permits SU-8 photo patterning and layer-to-layer bonding. We have developed a novel PDMS-based process which allows the SU-8 structures to be easily peeled off from the substrate after complete fabrication. As an example, a fully enclosed microfluidic chip has been successfully fabricated utilizing the presented new process. The enclosed microfluidic chip uses adhesive bonding technology and the SU-8 layers from 10 µm to 450 µm thick for fully enclosed microchannels. SU-8 layers as large as the glass substrate are successfully fabricated and peeled off from the PDMS layer as single continuous sheets. The fabrication results are supported by optical microscopy and profilometry. The peel-off force for the 120 µm thick SU-8-based chips is measured using a voice coil actuator (VCA). As an additional benefit the release step leaves the input and the output of the microchannels accessible to the outside world facilitating interconnecting to the external devices.