Adhesives with a thickness smaller than the fractocohesive length enhance adhesion.

When two materials are adhered by an adhesive, two length scales emerge: the adhesive thickness and the adhesive fractocohesive length. The former is a geometric length, and the latter is a material length, defined as the fracture toughness divided by the work of rupture of the bulk adhesive. The fractocohesive length scales with the size of the process zone around a crack tip. In this paper, we show that the ratio of the adhesive thickness to the fractocohesive length, h/lf, can control adhesion. When h/lf > 1, the process zone is confined in the adhesive layer. The debonding elicits dissipation in the adhesive, and the adhesion energy is on a similar level to the fracture toughness of the adhesive. When h/lf < 1, the process zone extends beyond the adhesive layer to the adherends. The debonding elicits dissipation from both the adhesive and the adherends, and the adhesion energy is amplified on a similar level to the fracture toughness of the adherend. We illustrate this mechanism by bonding tough alginate-polyacrylamide hydrogels using brittle alginate and polyacrylamide adhesives with varied h/lf. We also study debonding speeds on h/lf and adhesion. We further explore adhesion behaviors with non-uniform thickness adhesives. These experiments support our proposed mechanism.