MEAM Ph.D. Thesis Defense: “Imperfect Adhesion Between Elastic Solids”

Surface force-mediated adhesion, i.e. via van der Waals forces, is critical for the direct bonding of bulk solids in the absence of an adhesive layer. However, no two surfaces are perfectly flat nor conformal and some have intentional patterns, which leads to imperfect adhesion, i.e. interface adhesion strength that is below its ideal strength. In the case of non-conformal patterned interface, regions of tensile and compressive stresses exist in the adhered solids at the equilibrium state. Imperfect adhesion also can arise from edge effects. The understanding of imperfect adhesion is important in controlling interfacial strength and toughness for various applications including MEMS/NEMS, micro-transfer printing, and soft robotic grippers.

The overall goal of this thesis is to investigate the mechanics of surface force-mediated adhesion by examining the interplay between intrinsic traction-separation relation (TSR), interface topography, and elastic bulk properties. The TSR developed accounts for strong repulsion to avoid material interpenetration. The effective interface properties, including the overall adhesion strength and work of separation, are determined from numerical calculations using finite element analysis.

The first study exploits surface patterning for adhesion control. A cohesive model for a periodic unit cell with non-conformal patterned interfaces is developed to analyze the joint effects of non-uniform interface separation and elastic bulk deformation. The second study investigates edge effects. In general, interface shear tractions coupled with Poisson contraction reduces the interface adhesion significantly and result in fracture-based failure. Understanding the interaction between the normal and the shear tractions is important but has not been studied extensively thus far. The second study concerns the detachment of an elastic pillar from a rigid substrate. A non-dimensional parameter is defined to describe the transition between strength-based and fracture-based failure. In a third study, the mechanics that govern interface failure in electroadhesives is investigated using a TSR derived from Coulomb’s law. This last study highlights the importance of interface crack growth in the design of electroadhesives through modeling an elastic cylinder electrostatically adhered to a rigid substrate. The adhesion strength of the electroadhesives becomes imperfect when a non-dimensional parameter reaches a critical value.