Soft Adhesion & Friction: Compliance, Hysteresis, and Swelling

Friction and adhesion are ubiquitous in daily life, playing important and essential roles in many natural processes and engineered technologies. Significant recent efforts have been devoted to understanding and mimicking adhesion mechanisms found in many organisms in nature, such as geckos and insects. In addition, soft and extensible materials have been used in state-of-the-art technologies, such as wearable sensors and stretchable electronics, in which the materials can deform under relatively small amounts of load, owing to a high effective compliance resulting from either low modulus or geometric design. Therefore it is necessary to understand the role of effective axial compliance on the adhesive and frictional properties of materials, from both experimental and theoretical points of view. In this talk, we will discuss the roles that compliance, hysteresis, and wet interfaces play in controlling soft adhesion and soft friction. In particular, we will examine the role of effective axial compliance on the friction of extensible strips and on the force capacity of adhesive bonds. Beginning with the classical fracture mechanics approach, we show how the characteristic shear lag length affects the frictional response of soft materials, and changes the failure mechanism of adhesive bonds from progressive to catastrophic. We will address the role of adhesion hysteresis on the kinetic coefficient of friction of soft elastomers. Finally, we will discuss how the adhesion of a soft interface is modified in the presence of fluid that swells the material - a process that changes both the bulk and interfacial properties of the material over time.