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Abstract
The natural adhesive setae and the bio-inspired composite fibrils possess decreasing elastic properties along their length. This feature has demonstrated the potential to enhance adhesion on rough substrates. In this study, the viscoelastic contact behavior between a spherical asperity and a soft polymer was firstly studied by the analytical and finite element methods. The numerical results for both methods were compared to verify the reliability of the finite element one in modeling the viscoelastic contact. Then, the finite element method was used to systematically investigate the viscoelastic contact between a spherical asperity and a composite micropillar that consists of a stiff base stalk and a soft top layer. Effects of the thickness of the soft top layer, the radius of the spherical asperity, and the plateau load on contact properties were evaluated. The tensile stress distribution along the contact interface between a flat rigid substrate surface and the composite micropillar was also studied. The results indicate that there exists a critical thickness for the soft top layer of the composite micropillar. The existence of the critical thickness likely correlates with the stiffness of the composite micropillar and the hardening behavior of the soft top layer. It is suggested that the thickness of the soft top layer should not be less than the critical value. The viscoelastic effects of the soft material facilitate the adaptation to rough surfaces. This study can contribute to a better understanding of viscoelastic contact problem for a composite micropillar, and further guide optimal design of composite micropillars for bio-inspired adhesion.