Mechanics of Soft Interfaces
Soft interfaces have unique mechanical properties that are critical to understanding and properly designing adhesives, coatings, robotic grippers, and composite materials. This talk will primarily focus on creating tunable adhesives through a composite post geometry, however the conclusion will also examine future research directions for studying the mechanics of soft interfaces. Tunable adhesion is the ability for the same surface to have high adhesion under one set of conditions and low adhesion under another. It has a variety of applications, including transfer printing of micro- and nano-scale components, climbing and perching robots, and material handling in manufacturing. Approaches to tunable adhesion often rely on van der Waals forces to achieve dry adhesion. Previous strategies for dry tunable adhesives have generally exploited complex fibrillar structures that are inspired by nature. This work investigates a different strategy for enhanced and tunable adhesion based on composite structures with simple geometries. These composite posts, consisting of stiff insets surrounded by a compliant shell, are used for achieving enhanced and tunable adhesion. This composite structure has a high effective adhesion strength under normal loading and low adhesion when shear is applied. Experiments as well as finite element (FE) analysis are used to understand the mechanics of these posts under both types of loading. The adhesion of composite posts is affected by the stress distribution at the contacting surface. Homogeneous posts have concentrated stress near the edge, facilitating crack initiation, while the composite post can result in a redistribution of this stress towards the center, resulting in higher adhesion. The basic mechanics of these posts are demonstrated through experiments on mm-scale posts. The composite mm-scale composite posts have 3x higher adhesion than homogeneous posts under normal loading and shear displacement was shown to significantly decrease the effective adhesion strength. Micro-scale posts are studied and used in micro-transfer printing applications. These posts have an effective adhesion strength of 1.5 MPa, and the pull-off force of the composite post is 8x that of a homogeneous post. In both the mm-scale and micro-scale studies, the experimental results are supported by FE simulations. This work established the mechanics of composite posts for achieving enhanced and tunable adhesion.