Benefit of Backing‐Layer Compliance in Fibrillar Adhesive Patches—Resistance to Peel Propagation in the Presence of Interfacial Misalignment

Abstract

Investigations of backing‐layer effects in bioinspired fibrillar adhesives have shown that increased compliance is detrimental to the strength of fibril arrays under normal loading due to an increase in severity of a circumferential load concentration. In this work, the impact of misalignment on the performance of fibrillar adhesive patches contacting smooth flat surfaces is examined, demonstrating that the conditions for circumferential detachment are extremely limited. For an array of fibrils on a backing layer of varying thickness, normal adhesion tests are performed against a flat surface that maintains a fixed angle of misalignment with respect to the adhesive surface. In the aligned state the detachment is circumferential and the detachment force is highest for the thinnest, least compliant backing layer. However, for misalignment angles on the order of just 0.1°, peel‐like detachments are observed. The thickest backing layer, being 210% more compliant than the thinnest, yields a 43% increase in the adhesive strength at a misalignment angle of 0.4°. This suggests that out‐with conditions of precise alignment, backing‐layer compliance is beneficial to strength under normal loading. A mechanical model is presented, revealing the mechanism behind enhanced resistance to peel propagation is deformation of the backing layer at the detachment front which reduces differential stretching of fibrils.

Statistical properties of defect-dependent detachment strength in bioinspired dry adhesives

“…In regard to the latter, backing layer effects are expected to become more pronounced as the fibril length is reduced, and as the array size is increased [26]. Contact height differences due to inhomogeneities in the backing layer or roughness at the fibril scale [23,32,40], as well as loading imperfections [41,42], will become more pronounced with respect to the fibril length. Statistical models may, therefore, have to account for nonequal load sharing, as has been required in the study of the failure of fibres in composite materials [43].…”

“…The sequence is based upon the position relative to the array centre, from furthest to closest. The peel-like detachments are characteristic of misalignment [41,42], occurring according to position along a single axis. Two such detachment sequences are shown, corresponding to alignment with the axis along which the fibril separation is d and the axis along which it is ffiffi ffi 2 p d, respectively.…”

Contact Aging Enhances Adhesion of Micropatterned Silicone Adhesives to Glass Substrates

“…Introducing microstructures with high length to diameter ratio inherently opens up a strategy for reversibly switching between low and high adhesive states by compressive buckling of these microstructures. [ 7,8 ] Splitting the surface adhering to a substrate into individual contacts further introduces a higher tolerance against defects and misalignment [ 9–12 ] and may even generate adhesion to rough surfaces. [ 13,14 ]…”

Optimal Load Sharing in Bioinspired Fibrillar Adhesives: Asymptotic Solution

“…A stiffer BL would give instead more effective results but only in the presence of negligible misalignment. This is because a softer BL has been observed to better resist interfacial misalignment (θ ≠ 0) [13,17]. Finally, one can reduce the ratio R/h by incrementing the overall fibril length h or by reducing the size of the array, via reduction of R. The former produces a significant loss in bending stiffness, as explained above, while the latter requires a reduction of the area of adhesive contact with the consequent reduction of the maximum detachment force.…”

“…This strategy basically relies on a stiffness gradient along the longitudinal axis of the fibrils. There is evidence to suggest that such a stiffness gradient exists throughout the setae of the adhesive pads of insects, which soft tips and a compliant backing layer (BL) have the additional ability to better conform to the surface roughness [11][12][13]. A similar approach has also been investigated for synthetic mimics to define new engineering design principles for these adhesives to enter a flaw-tolerant regime [14][15][16].…”