MEAM Seminar: “Exploiting Interface Patterning for Adhesion Control”

Surface force-mediated adhesion, e.g. van der Waals forces, is critical for direct bonding of bulk solids in the absence of an adhesive layer. However, no two surfaces are ideally flat or perfectly conformal due to surface roughness or intentional patterning. When in the adhered state, non-zero surface tractions arise wherever the local separation of the surfaces differs from an intrinsic equilibrium separation, for which surface tractions vanish, hence regions of tensile and compressive loads are induced across the interface. A fundamental understanding of such imperfect adhesion is important and unlocks opportunities to control interfacial strength and toughness in various applications including MEMS/NEMS, micro-transfer printing, and processes to manufacture advanced 3D integrated-circuits. The aim of this work is to understand the fundamentals of direct adhesion of non-conformal surfaces by examining the interplay of (1) the intrinsic adhesive properties of traction-separation relation (TSR), (2) interface geometry, and (3) elastic deformation of the adhered bulk solids. The TSR we adopted accounts for strong repulsion when the interface separation is less than the equilibrium separation, and only the normal surface tractions were considered. The effective TSR properties, including the effective adhesion strength and work of separation, are determined from numerical calculations. Simple closed-form solutions are obtained when assuming rigid bulk solids. A finite element model utilizing cohesive elements in a periodic cell was constructed to study cases with non-uniform deformation in the elastic solids under pure normal separation without shear.