Ellipticity Enhances Adhesion Strength for Contacts under Shear Loads

Adhesion under a shear load parallel to the contact interface is a common issue in engineering and biological systems, such as when insects and adhesion devices crawl on vertical walls. A question of interest is whether and how the shear load influences the adhesion behavior between an elliptical flat punch and an elastic medium. Here, we derive lower- and upper-bound limiting analytical solutions to this problem based on the local and global energy balances. Numerical results based on our analytical solution indicate that the lower- and upper-bounds are very close to each other, and their ratio depends only on the ellipse eccentricity in the absence of shear load. The lower-bound solution yields the location of the initial detachment, which is influenced by the contact area, ellipse eccentricity, shear load, and Poisson’s ratio, but not Young’s modulus. The presence of a shear load reduces the pull-off force and can also alter the position where the detachment is nucleated. Furthermore, there exists an optimal elliptical shape that gives rise to the maximum adhesion strength in the presence of a shear load. The theoretical findings are validated through experiments involving the pull-off of a rigid punch from an adhesive polymer substrate subjected to a shear load. The present study sheds insight and may provide helpful guidelines for designing biomimetic adhesive systems.