Peel Front Research Articles

Modelling the non-steady peeling of viscoelastic tapes

We present a model to study the non-steady V-shaped peeling of a viscoelastic thin tape adhering to a rigid flat substrate. Geometry evolution and viscoelastic creep in the tape are the main features involved in the process, which allows to derive specific governing equations in the framework of energy balance. Finally, these are numerically integrated following an iterative scheme to calculate the process evolution assuming different controlling conditions (peeling front velocity, peeling force, tape tip velocity). Results show that the peeling behavior is strongly affected by viscoelasticity. Specifically, for a given applied force, the peeling can either be prevented, start and stop after some while, or endlessly propagate, depending on the original undeformed tape geometry. Viscoelasticity also entails that the interface toughness strongly increases when the tape tip is fast pulled, which agrees to recent experimental observations on tougher adhesion of natural systems under impact loads, such as see waves and wind gusts.

Learning the effective adhesive properties of heterogeneous substrates

Adhesion is a fundamental phenomenon that plays a role in many engineering and biological applications. This paper concerns the use of machine learning to characterize the effective adhesive properties when a thin film is peeled from a heterogeneous substrate. There has been recent interest in the use of machine learning in multiscale modeling where macroscale constitutive relations are learnt from data gathered from repeated solution of the microscale problem. We extend this approach to peeling; this is challenging because peeling from heterogeneous substrates is characterized by pinning where the peel front gets stuck at a heterogeneity followed by an abrupt depinning. This results in a heterogeneity dependent critical force and a singular peel force vs. overall peel rate relationship. We propose a neural architecture that is able to accurately predict both the critical peel force and the singular nature of the peel force vs. overall peel rate relationship from the heterogeneous adhesive pattern. Similar issues arise in other free boundary and free discontinuity problems, and the methods we develop are applicable in those contexts.

Harnessing strong and asymmetric adhesion by combining film heterogeneity and substrate morphology

We use a combined theoretical and experimental approach to investigate the quasi-static peeling behaviors of heterogeneous films from corrugated substrates. The heterogeneity is involved in films through two different methods, i.e., local stiffening and kirigami patterning, and the substrate surface is assumed to adopt a sinusoidal profile. Our results show that the spatial variation in film's bending stiffness, caused by the elastic heterogeneity with period form, can significantly enhance the peak peel force and give rise to asymmetric adhesion performance. The substrate's roughness can further amplify such adhesion enhancement and asymmetry by affecting the local peel angle at the peel front. The adhesion performance depends sensitively on the relative position between the heterogeneous films and corrugated substrates, and our results show that a combination of film heterogeneity and substrate undulation can enhance the adhesive strength by more than 100 times, compared with the case of homogeneous film and flat substrate. This study provides a theoretical framework to reveal multiple peeling mechanisms in the problem and can guide the design of film-substrate systems with tunable and anisotropic adhesion.

Out-of-contact peeling caused by elastohydrodynamic deformation during viscous adhesion.

We report on viscous adhesion measurements conducted in sphere-plane geometry between a rigid sphere and soft surfaces submerged in silicone oils. Increasing the surface compliance leads to a decrease in the adhesive strength due to elastohydrodynamic deformation of the soft surface during debonding. The force-displacement and fluid film thickness-time data are compared to an elastohydrodynamic model that incorporates the force measuring spring and finds good agreement between the model and data. We calculate the pressure distribution in the fluid and find that, in contrast to debonding from rigid surfaces, the pressure drop is non-monotonic and includes the presence of stagnation points within the fluid film when a soft surface is present. In addition, viscous adhesion in the presence of a soft surface leads to a debonding process that occurs via a peeling front (located at a stagnation point), even in the absence of solid-solid contact. As a result of mass conservation, the elastohydrodynamic deformation of the soft surface during detachment leads to surfaces that come closer as the surfaces are separated. During detachment, there is a region with fluid drainage between the centerpoint and the stagnation point, while there is fluid infusion further out. Understanding and harnessing the coupling between lubrication pressure, elasticity, and surface interactions provides material design strategies for applications such as adhesives, coatings, microsensors, and biomaterials.

A Real-Time Supervisory Control Strategy for Roll-to-Roll Dry Transfer of 2D Materials and Printed Electronics

With the advancement of manufacturing technology in transfer printing and two-dimensional materials fabrication, roll-to-roll (R2R) dry transfer by mechanical peeling is becoming a promising process for efficient transfer of thin film materials from a donor substrate to a target substrate for device fabrication. As a key process variable, the peeling angle determines the loading condition of the R2R dry transfer process and thus the quality of the transferred material. Controlling the peeling angle is challenging in a R2R process because of the adhesion energy and peeling front velocity in addition to the flexible substrates involved in the peeling process. In this article, a real-time supervisory control strategy is developed for the R2R mechanical peeling process. A strain energy-based peeling process model is employed to estimate the peeling front velocity and the adhesion energy. The real-time estimation is then used to generate reference signals for web tension controllers to track the desired peeling angles. The proposed control strategy is demonstrated with both simulation and experimental results.

Multiple kinds of peeling processes and instabilities in heterogeneous film peeling

Although it has been demonstrated that heterogeneous film can enhance adhesion properties without modifying the interface, there is still a dearth of comprehensive studies in the literature regarding the peeling process of such film and the potential instability that may occur during the peeling process. To address these issues, a peeling model based on the cohesion zone model and Euler-Bernoulli beam equation is proposed in this study. Our analysis revealed four previously unknown peeling behaviors: Firstly, there are nine kinds of peeling processes for heterogeneous film. Only one is stable, static peeling process, while the other eight exhibit instability when the peeling front reaches a certain position. Secondly, instability may occur at the interface from the stiff section to the compliant section of the homogeneous film, as well as at stiff section or compliant section. Moreover, two of these peeling processes experience two instabilities within one peeling period – one at the interface from the stiff section to the compliant section and another at stiff section. Thirdly, an optimal length fraction of the stiff section for enhancing adhesive performance exists and its value is largely determined by the strength of the interface material. Specifically, higher interfacial strength results in smaller values of the optimal length fraction. Finally, the maximum peeling force is positively correlated with the bending stiffness ratio, interfacial strength, interfacial toughness, and period length of heterogeneous film. But it is inversely related to the bending stiffness. These findings are crucial for understanding the peeling behavior of heterogeneous film and strongly adhesive interfaces found in nature, and can be applied to optimize adhesive design and performance in practical engineering applications.

Examination of gecko-inspired dry adhesives for heritage conservation as an example of iterative design and testing process for new adhesives

AbstractRarely within the conservation of cultural heritage have conservation professionals been lucky enough to have materials custom-designed to meet their requirements. Most of the time the field must adapt solutions developed for other applications. The research presented here was initiated as part of a long-term aim to develop new adhesives for heritage conservation. Gecko-inspired dry adhesives (GDAs) are polymer tapes with micropatterns that are based on the adhesive properties of the pads of gecko lizard feet: they have strong normal and shear adhesion with low peel adhesion. They present potentially versatile and reversible adhesives for heritage conservation applications; which do not require solvents for activation or for removal, and do not migrate or off-gas. In nature, geckos can adhere with their feet to any surface they walk on. This is possible because of micro- and nanostructures on their feet that attach to surface via van der Waals forces. Practical studies aimed at comprehensively assessing their properties, and biomimetic solutions in heritage conservation are still sparse at present. This research has the objective of assessing GDAs properties by mechanical testing of adhesive joints, as well as physical and chemical characterisation of the materials used. The research has also included a museum case study and a two-year natural ageing test. The testing has shown that the GDAs can perform very well as an adhesive patch on the reverse of gelatine-based photographs, achieving shear forces between 0.80 N and 48.10 N on 8 cm2 lap joints (depending on the type of the GDA) and peel forces between 0.20 N and 0.47 N over 2 cm of peel front. This is lower than forces exceeding 1 N recorded in widely available pressure-sensitive tapes. This shows that GDAs may have the potential of a sufficiently strong, yet easily removable adhesive that works well on materials widely present in museum collections.

Peeling of Finite-Length Plates From an Elastomeric Foundation: A 1D Cylindrical Bending Solution

Abstract Quasi-static peeling of a finite-length, flexible, horizontal, one-dimensional (1D) plate (strip, thin film) from a horizontal, thin, elastomeric layer (foundation) is considered. The displaced end of the plate is subjected to an upward deflection or to a rotation. The top of the interlayer is perfectly bonded to the plate, and its lower surface is bonded to a rigid, flat substrate. A transversality (debonding) condition is derived for peeling, based on the common fracture mechanics approach. Whereas debonding from a Winkler foundation can be expressed in terms of the displacement (or equivalently the foundation stress2) at the bond termination, the sixth-order formulation required for elastomeric foundations involves a more complex debonding criterion. Transversality relationships are used to describe this limit state (here the onset of debonding) in terms of co-state variables, herein the deflection and slope at the peel front. In the analysis, bending is assumed to be paramount, linear Kirchhoff–Love (classical) plate theory is used to model the deformation, and therefore displacements are assumed to be small. The foundation is linearly elastic and incompressible. The effects of the work of adhesion, the length of the plate, and the initial nonbonded length of the plate are investigated. The results are compared to those for a Winkler foundation. By replacing the shear modulus of the interlayer by viscosity, and displacements by their time derivatives, the results are expected to apply to viscous liquid interlayers as well.

The role of a pre-wetting layer in resolving the contact line paradox of thin films in infinite elastic domains

Hydraulic fracturing for oil and gas production from shale formations, as well as natural geological phenomena, involve the propagation of thin viscous films within elastic media. For viscous fluids, stress diverges as the thickness of the film tends to zero, arresting the propagation of the film, and thus implying the contact line paradox. For free-surface films, this paradox is resolved by considering a precursor film, leading to Tanner's law. This approach was extended recently for viscous films between a thin elastic plate and a rigid solid, allowing calculation of the film propagation rate. In this work, we examine the effect of a pre-wetting layer on the rate of propagation of a viscous flow within an infinitely deep and long domain. We analyse the linear and nonlinear dynamic problems, and perform a self-similarity analysis. We find that peeling front propagation scales as time to the power of $1/9$ and $1/3$ for thin and thick pre-wetting layer limits, respectively. Our results contribute to the understanding of the contact line paradox in elastic media and the crucial role of the pre-wetting layer in resolving it.

A Tape-Peeling Model for Spatiotemporal Pattern Formation by Deformed Adhesives

We propose a new model for pattern formation in peeling of an adhesive tape based on the equation of motion for the displacement of deformed adhesives in the peel front. The spatiotemporal patterns obtained from the model are consistent with those from previous models and experiments. Moreover, dynamical and statistical properties of the patterns are investigated.

Toughening of hydrogel adhering interface based on soft/hard heterogeneous structures

Emerging technologies like hydrogel based stretchable devices call for new interfacial toughening strategies for the achievement of reliable hydrogel adhering interface. Here, we propose a soft/hard composite design to enhance the average peel force for stretchable hydrogel adhesion. We fabricate a hydrogel/elastomer composite with alternate structures where the hydrogel acts as the soft component and the elastomer acts as the hard component. The elastomer is embedded in the hydrogel, with a thin hydrogel layer coated on its surfaces to work as the interfacial hydrogel layers for adhesion. When two pieces of the adhering composites are separated in a 180°peeling test, the peeling front will get stuck when it extends from a soft phase to an adjacent hard phase. The interfacial hydrogel layers between the entire symmetric elastomer units near the peeling front are stretched and the elastomer bends steadily until a sudden debonding occurs, which releases the stored energy and improves the overall adhesion properties. We adjust the structural parameters and obtain an average peak peel force of 330 N/m, which is almost 5 times the peel force of 70 N/m required to peel homogeneous hydrogels. We establish a beam model to study the peeling behaviors of the hydrogel/elastomer composite in the 180°peeling test. The numerical results agree with the experimental results quantitatively. We also show that the peeling performance of the hydrogel/elastomer composite can be tuned by controlling the patterning of the composite structures.

Understanding the role of crosslink density and linear viscoelasticity on the shear failure of pressure-sensitive-adhesives.

Pressure-sensitive-adhesives (PSAs) are ubiquitous in electronic, automobile, packaging, and biomedical applications due to their ability to stick to numerous surfaces without undergoing chemical reactions. Although these materials date back to the 1850s with the development of surgical tapes based on natural rubber, their resistance to shear loads remains challenging to predict from molecular design. This work investigates the role of crosslink density on the shear resistance of model PSAs based on poly(2-ethylhexyl acrylate-co-acrylic acid) physically crosslinked with aluminum acetylacetonate. The key result is that crosslinking PSAs leads to notable stress concentrations ahead of the peel front, as well as a transition from cohesive to adhesive failure. The shear stress distributions, as evaluated by means of a linearly viscoelastic shear lag model, suggest that this transition is related to the evolution of the ratio of the load transfer length to the bond length as dictated by the mechanical properties of the backing and adhesive layers, and the geometry of the tape.

Peeling of linearly elastic sheets using complex fluids at low Reynolds numbers

We investigate the transient, fluid–structure interaction (FSI) of a non-Newtonian fluid peeling two Hookean sheets at low Reynolds numbers ( R e ≪ 1 ). Two different non-Newtonian fluids are considered – a simplified Phan–Thien–Tanner (sPTT) model fluid, and an inelastic fluid with shear-thinning viscosity (generalized Newtonian fluid). In the limit of small gap between the sheets, we invoke a lubrication approximation and numerically solve for the gap height between the two sheets during the start-up of a pressure-controlled flow. What we observe is that for an impulse pressure applied to the sheet inlet, the peeling front moves diffusively ( x f ∼ t 1 / 2 ) towards the end of the sheet when the fluid is Newtonian. However, when one examines a complex fluid with shear-thinning, the propagation front moves sub-diffusively in time ( x f ∼ t α , α < 0 . 5 ), but ultimately reaches the end faster due to an order of magnitude larger prefactor for the propagation speed. We provide scaling analyses and similarity solutions to delineate several regimes of peeling based on the sheet elasticity, flow Weissenberg number (for sPTT fluid), and shear-thinning exponent (for generalized Newtonian fluids). To conclude, this study aims to afford to the experimentalist a system of knowledge to a priori delineate the peeling characteristics of a certain class of complex fluids. • Modeled fluid–structure interaction of a complex fluid peeling two Hookean sheets. • Examined two different fluid constitutive rheological models. • Developed simulations and similarity solutions for the peeling front during start-up. • Quantified peeling time effects due to shear thinning. • Peeling time for a shear-thinning fluid is smaller than a Newtonian fluid.

Multi-peeling states of elastic film from flexible substrate

Application of an external tensile displacement at the end of an elastic film induces its peeled from the free (fixed) end of the cantilever flexible substrate to the fixed (free) end of it. This work proposes a theoretical model for studying this simple elastic system. The analysis results for the film peeled from free end to fixed end (FRFI) demonstrate a number of unknown and unexpected behaviors: (i) There are five basic types of peeling form depending on the flexural rigidity of the substrate, the elastic modulus and thickness of film and adhesive energy; (ii) Some peeling forms have two peeling states (the substrate has two deformed shapes and the pulling force has two values when the film is peeling off) for certain peeling front; (iii) When the film is peeled to a certain place, unstable dynamic peeling will occur. In comparison to FRFI, the results of the film peeled from the fixed to free end (FIFR) appear natural. The effect of flexural rigidity and adhesive energy on the peeling behavior of FIFR is larger.

A method to estimate adhesion energy of as-grown graphene in a roll-to-roll dry transfer process

The interfacial adhesion energy of as-grown graphene on its metal growth substrate is an important variable in designing, monitoring, and controlling a roll-to-roll (R2R) graphene transfer process using mechanical peeling. In this study, we develop a novel method to estimate the adhesion energy of as-grown graphene during the R2R dry transfer process. An energy balance model is established to derive the adhesion energy based on web tension and bending curvature measurements. Experiments were conducted under various mechanical peeling conditions. The adhesion energy of as-grown graphene on copper foil was determined to be from 1.22 J/m2 to 2.58 J/m2 depending on the peeling front geometry. The developed adhesion energy estimation method is compatible with the R2R process and can be used to monitor and control the large-scale graphene transfer process with in-process measurements.

Cassette-like peeling system for testing the adhesion of soft-to-rigid assemblies

A novel cassette-like peeling system is developed to address the limitations of current peeling standards when evaluating bonding quality of soft-to-rigid assemblies. The system transforms the translation of a specimen in the conventional peeling configuration to rotation via a cassette-like spool clamping the specimen. The peeled film is loaded by tension to drive the winding of the spool, thus achieving self-similar crack propagation and a stationary peeling front unrelated to the stiffness of the film. These features enable the system’s compatibility with most universal testers and in situ observation of crack tip morphology with optical instruments. Analysis to derive the intrinsic fracture energy when peeling a soft film is conducted based on Griffith energy balance, making use of which, a parametric study is performed to clarify the related mechanisms. We carry out a comprehensive validation of the cassette-like peeling system by performing a series of peeling tests using our in-house prototype and by comparing the results with those from the conventional system. Owing to its universality and ease-of-use, the proposed cassette-like peeling system can potentially be applied to the development of the next generation of peel test standards.

H∞ Optimal Control for Maintaining the R2R Peeling Front

Dry transfer using Roll-to-Roll (R2R) mechanical peeling could significantly increase the throughput and efficiency of the production of 2D materials such as graphene and flexible electronics. Currently, such a R2R process does not exist in industry. For this dry transfer R2R process to be practical for industrial applications, the peeling angle between the growth substrate and the functional material needs to be precisely controlled. In this paper, a nonlinear state space representation of the R2R dry peeling process is formulated with the peeling front velocity variation as a disturbance input. This state space model is used to construct a linear parameter varying (LPV) representation of the system, and a methodology on how to bound the LPV representation within a convex polytopic linear differential inclusion (PLDI) set is presented. This PLDI representation is then used in a linear matrix inequality (LMI) optimization framework to design a full state feedback controller that minimizes the H∞ gain of the connection between the adhesion energy variation and the peeling front geometry. Simulation results demonstrate that this controller improves the precision of the R2R peeling angle, and this increase in precision enables higher web speed. Thus, this technique can be an enabling tool for making R2R mechanical peeling dry transfer of 2D materials a reality in industrial settings.

Viscous peeling of a nanosheet.

Combining molecular dynamics (MD) and continuum simulations, we study the dynamics of propagation of a peeling front in a system composed of multilayered graphene nanosheets completely immersed in water. Peeling is induced by lifting one of the nanosheet edges with an assigned pulling velocity normal to the flat substrate. Using MD, we compute the pulling force as a function of the pulling velocity, and quantify the viscous resistance to the advancement of the peeling front. We compare the MD results to a 1D continuum model of a sheet loaded with modelled hydrodynamic loads. Our results show that the viscous dependence of the force on the velocity is negligible below a threshold velocity. Above this threshold, the hydrodynamics is mainly controlled by the viscous resistance associated to the flow near the crack opening, while lubrication forces are negligible owing to the large hydrodynamic slip at the liquid-solid boundary. Two dissipative mechanisms are identified: a drag resistance to the upward motion of the edge, and a resistance to the gap opening associated to the curvature of the flow streamlines near the entrance. Surprisingly, the shape of the sheet was found to be approximately independent of the pulling velocity even for the largest velocities considered.

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.

The role of interfacial friction on the peeling of thin viscoelastic tapes

We study the peeling process of a thin viscoelastic tape from a rigid substrate. Two different boundary conditions are considered at the interface between the tape and the substrate: stuck adhesion, and relative sliding in the presence of frictional shear stress. In the case of perfectly sticking interfaces, we found that the viscoelastic peeling behavior resembles the classical Kendall behavior of elastic tapes, with the elastic modulus given by the tape high-frequency viscoelastic modulus. Including the effect of frictional sliding, which occurs at the interface adjacent to the peeling front, makes the peeling behavior strongly dependent on the peeling velocity. Also, at sufficiently small peeling angles, we predict a tougher peeling behavior than the classical stuck cases. This phenomenon is in agreement with recent experimental evidences indicating that several biological systems (e.g. geckos, spiders) exploit low-angle peeling to control attachment force and locomotion.