Peeling of polydimethylsiloxane adhesives at low velocities: Cohesive failure

Unsteady peeling accompanied by oscillations of adhesive tape from a soft adherend

Peel angle dependence of the peel force was investigated using cross-linked polydimethylsiloxane (PDMS) covered with a pressure-sensitive adhesive tape. In our previous report, the peeling behavior at peel angles below 90° was discussed. The study revealed that an unsteady peeling occurs due to the extreme effects of the shear force on the adhered portion of the tape before the peeling starts. In this paper, we studied the peeling behavior and unsteady peeling accompanied by oscillations at peel angles higher than 90°, which is different from unsteady peeling at peel angles lower than 90°. The appearance of unsteady peeling accompanied by oscillations depended on the experimental conditions and material properties. Specifically, unsteady peeling was observed when the peel velocity, peel angle, and the thickness of PDMS were increased. Additionally, a decrease in the degree of cross-linking in PDMS also resulted in unsteady peeling. The periodic length of the oscillation was obtained from the peel force curves, which depended on the thickness and elasticity of the adherend. This unsteady peeling resulted from the large deformation of the adherend and the buckling of the tape due to the shear and compressive forces. In this paper, the mechanism of unsteady peeling was discussed on the basis of the classical models of mechanics.

A review of Winkler's foundation and its profound influence on adhesion and soft matter applications

Few advanced mechanics of materials solutions have found broader and more enduring applications than Emil Winkler's beam on elastic foundation analysis, first published in 1867. Now, 150 years after its introduction, this concept continues to enjoy widespread use in its original application field of civil engineering, and has also had a profound effect on the field of adhesion mechanics, including for soft matter adhesion phenomena. A review of the model is presented with a focus on applications to adhesion science, highlighting classical works that utilize the model as well as recent usages that extend its scope. The special case of the behavior of plates on incompressible (e.g., elastomeric and viscous liquid) foundations is reviewed because of the significant relevance to the behavior of soft matter interlayers between one or more flexible adherends.

Understanding Surface Adhesion in Nature: A Peeling Model

Nature often exhibits various interesting and unique adhesive surfaces. The attempt to understand the natural adhesion phenomena can continuously guide the design of artificial adhesive surfaces by proposing simplified models of surface adhesion. Among those models, a peeling model can often effectively reflect the adhesive property between two surfaces during their attachment and detachment processes. In the context, this review summarizes the recent advances about the peeling model in understanding unique adhesive properties on natural and artificial surfaces. It mainly includes four parts: a brief introduction to natural surface adhesion, the theoretical basis and progress of the peeling model, application of the peeling model, and finally, conclusions. It is believed that this review is helpful to various fields, such as surface engineering, biomedicine, microelectronics, and so on.

Coupled effects of applied load and surface structure on the viscous forces during peeling

Tree frogs are able to take advantage of an array of epithelial cells in their toe pads to modulate their adhesion to surfaces under dry, wet, and flooded environments. It has been hypothesized that the interconnected channels separating the epithelial cells could reduce the hydrodynamic repulsion to facilitate contact under a completely submerged environment (flooded conditions). Using a custom-built apparatus we investigate the interplay between surface structure and loading conditions on the peeling force. By combining a normal approach and detachment by peeling we can isolate the effects of surface structure from the loading conditions. We investigate three surfaces: two rigid structured surfaces that consist of arrays of cylindrical posts and a flat surface as a control. We observe three regimes in the work required to separate the structured surface that depend on the fluid film thickness prior to pull out. These three regimes are based on hydrodynamics and our experimental results are compared with a simple scaling argument that relates the surface features to the different regimes observed. Overall we find that the work of separation of a structured surface is always less than or equal to that for a smooth surface when considering purely viscous contributions.

Whole animal measurements of shear and adhesive forces in adult tree frogs: insights into underlying mechanisms of adhesion obtained from studying the effects of size and scale

This allometric study of adhesion in 15 Trinidadian tree frog species investigates how relationships between length, area and mass limit the ability of adult frog species of different sizes to adhere to inclined and overhanging surfaces. Our experiments show that hylid frogs possess an area-based wet adhesive system in which larger species are lighter than expected from isometry and adhere better than expected from their toe pad area. However, in spite of these adaptations, larger species adhere less well than smaller species. In addition to these adhesive forces, tree frogs also generate significant shear forces that scale with mass, suggesting that they are frictional forces. Toe pads detach by peeling and frogs have strategies to prevent peeling from taking place while they are adhering to surfaces, including orienting themselves head-up on slopes. The scaling of tree frog adhesion is also used to distinguish between different models for adhesion, including classic formulae for capillarity and Stefan adhesion. These classic equations grossly overestimate the adhesive forces that tree frogs produce. More promising are peeling models, designed to predict the pull-off forces of adhesive tape. However, more work is required before we can qualitatively and quantitatively describe the adhesive mechanism of tree frogs.

Biomechanics of smooth adhesive pads in insects: influence of tarsal secretion on attachment performance

Many insects possess smooth adhesive pads on their legs, which adhere by thin films of a two-phasic secretion. To understand the function of such fluid-based adhesive systems, we simultaneously measured adhesion, friction and contact area in single pads of stick insects (Carausius morosus). Shear stress was largely independent of normal force and increased with velocity, seemingly consistent with the viscosity-effect of a continuous fluid film. However, measurements of the remaining force 2 min after a sliding movement show that adhesive pads can sustain considerable static friction. Repeated sliding movements and multiple consecutive pull-offs to deplete adhesive secretion showed that on a smooth surface, friction and adhesion strongly increased with decreasing amount of fluid. In contrast, pull-off forces significantly decreased on a rough substrate. Thus, the secretion does not generally increase attachment but does so only on rough substrates, where it helps to maximize contact area. When slides were repeated at one position so that secretion could accumulate, sliding shear stress decreased but static friction remained clearly present. This suggests that static friction which is biologically important to prevent sliding is based on non-Newtonian properties of the adhesive emulsion rather than on a direct contact between the cuticle and the substrate.