Pressurized interfacial failure of soft adhesives

Interfacial separation of soft, often viscoelastic, materials typically cause the onset of instabilities, such as cavitation and fingering. These instabilities complicate the pathways for interfacial separation, and hence hinder the quantitative characterization of bulk and interfacial contributions to soft material adhesion. To overcome these challenges, we developed a method termed pressurized interfacial failure (PIF), in which the interfacial separation is controlled by applying a positive pressure at the contact interface between a rigid, annular probe and a thin adhesive. We conducted experiments on model and commercially-available acrylic adhesives. Surprisingly, all the materials studied here fail by an inside-out growth of an interfacial cavity and show similar trends in the interrelationship between the cavity radius, applied pressure and change of contact force. In contrast, the force-displacement relationships of the same materials measured by conventional tack tests vary significantly. Accordingly, we conclude that the PIF method allows for controlling the interfacial failure mechanism. Furthermore, we have applied a linear elastic fracture mechanics framework and conducted finite element analysis to develop analytical models to calculate the critical energy release rate for interfacial separation, G_c. For model acrylic adhesives and commercially available adhesives, the values of G_c are similar to values determined by sphere-probe tack tests. Collectively, the herein introduced PIF method and analysis work provide a new foundation for quantitatively decoupling the interfacial and bulk contributions to soft polymer adhesion.