Peeling an architected interface: roles of softness and fractoadhesive length in adhesion toughening

Abstract

Soft adhesion has been rapidly studied and developed for various applications in recent years. Compared to existing toughening mechanisms based on the adherend or adhesive materials themselves, building architectures or patterns in soft adhesion offers an attractive way of enhancing adhesion without modifying the intrinsic material properties. However, despite the recent progress in soft architected adhesion, the fundamental interplay between the geometry and material properties remains largely unexplored. This results in questions about the geometric conditions for effective toughening and the roles of intrinsic material parameters in governing these conditions. Here we explore the geometry-elasticity interplay in toughening a soft architected bilayer with one-dimensional rectangular interfacial pillars. Using finite element simulations on 90-degree peel, we investigate effects of the adherend modulus, pillar aspect ratio, and interfacial contact ratio on the peel strength. We show that compared to a uniform interface, soft interfacial pillars (shear modulus ~ 0.6 MPa) with a high aspect ratio (> 4) can enhance the peel strength to more than 4 times, while stiff pillars (shear modulus ~ 1.5 MPa) only provide a limited enhancement (up to 1.5 times). Such enhancement is further amplified by increasing the interfacial contact ratio, where the best enhancement occurs when pillars are closely packed like a cross-cut surface (100% in contact yet architected). We develop a theory and scaling for the effective adhesion toughness and identify the fractoadhesive length of architected adhesion. We show that the fractoadhesive length provides a lower bound of the architecture feature size for effective toughening, while a large stretch at debonding in pillars further amplifies the toughening. Using an Ashby plot of the relevant architecture feature size and the fractoadhesive length in various architected adhesion systems, we conclude that macroscale architectures are necessary for effective toughening of soft adhesion with large fractoadhesive lengths.