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Open Access Publications from the University of California

Multiscale mechanics of bioinspired dry adhesives

  • Author(s): Booth, Jamie Alexander
  • Advisor(s): Foster, Kimberly L
  • et al.
Abstract

The adhesive systems of climbing animals have served as inspiration for a new class of temporary adhesive utilizing surface microstructure in place of intrinsically soft and viscoelastic materials. These have the potential to address requirements for robust, releasable, and reusable bonding. Efforts to characterize synthetic dry adhesives, as well as to scale adhesive patches to large areas while maintaining performance, necessitate consideration of features of the system across length scales. This work addresses two topics which require that the behavior of individual microfibrils be accounted for explicitly within large-scale loading configurations.

Under ideal conditions the strength of fibril arrays is known to be controlled by an array edge load concentration associated with compliance of the backing layer. Laboratory experiments have revealed that the strength is sensitive to the alignment of the adhesive and substrate surfaces, however no systematic investigation of the response to these perturbations in the loading configuration has been performed. A contact mechanics model is developed, considering the role of backing layer compliance in addition to misalignment. A monotonic decay in the adhesive strength of the array with increasing misalignment angle is confirmed. More interestingly, regimes of dominance of backing layer compliance and misalignment in control of the adhesive strength are revealed. Where circumferential detachment gives way to peel-like detachment, compliance of the backing layer is found to be beneficial to performance. This is the result of shielding of the peel-front load concentration by backing layer deformation. Subsequent experimental characterization of a mushroom-tipped synthetic fibril array shows that this regime is dominant for misalignment angles of just ~ 0.2° over a patch size of 2 mm. These results can be utilized to anticipate the performance of fibrillar adhesive patches on flat surfaces without precise control of alignment, or of sub-arrays within a larger hierarchy where surface undulations may lead to local misalignment.

While the potential importance of the variability in fibril adhesive strength in controlling the performance of microstructure arrays has been highlighted in past work, there has been no effort to systematically characterize the strength distribution or understand its effect on performance further. The capabilities of an experimental platform with in-situ contact visualization are leveraged to provide strength data on a fibril-by-fibril basis. A framework is developed, based upon the statistical theory of fracture, allowing for the decoupling of two defect populations and assessment of the impact of fabrication imperfections on performance. A subsequent theoretical investigation is performed with a view to understanding the combined effect of variability in fibril adhesive strength and load concentrations at the array scale. It is shown that, dependent on the severity of the load concentration, increased variability in fibril adhesive strength can modulate the influence of load concentrations and lead to independence of the adhesive strength of the array from properties such as backing layer compliance or substrate curvature. This is highly significant, given that the severity of load concentrations is a key factor in designing hierarchical structures for adhesive strength.

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