Control Of Dry Adhesion Via Mechanics And Structuring

Aoyi Luo, University of Pennsylvania

Abstract

Adhesion due to van der Waals forces, known as “dry adhesion”, has multiple advantages including versatility, cleanliness, and reversibility. However, van der Waals forces are relatively weak compared to other types of bonding and thus careful mechanical design is required to use dry adhesion in practical applications. This thesis investigates the mechanics of adhesion to design structures and processes that exploit dry adhesion for applications ranging from robotic grasping to manipulation of semiconductor components.First, the optimum displacement to apply to the top of a thin elastic layer to generate a highly uniform interfacial stress distribution was investigated. From a numerical investigation, it was found that the optimal stress distribution is obtained from a displacement distribution consisting of uniform tension in the center, an increase in tension between the center region and edge, and compression near the edge. Second, the adhesion of composite pillars with non-circular contacts was considered. Finite element modeling and experiments show that a composite pillar with sharp corners can have comparable adhesion to a circular composite pillar through appropriate design. Then, the dynamic control of adhesion via subsurface pressure modulation was investigated by considering the adhesion of a pillar with an embedded annular chamber that can be pressurized. The location and magnitude of the maximum normal stress at the interface can be tuned by pressurization of the embedded chamber, allowing adhesion to be controlled. Next, the mechanics of tunable adhesion in microtransfer printing was examined. When the component to be transferred is sufficiently thin, altering the stress distribution on the stamp-component interface also changes the stress distribution on the component-substrate interface resulting in limited ability to control the process. To provide control in microtransfer printing of thin components, a stamp design, guided by the singularity order of the stress distribution at edge is proposed. Finally, the adhesion of a heterogeneous double cantilever beams was considered. The results show that the inclusion of stiff insets in the beams can enhance the adhesion. Moreover, directional-dependent adhesion can be achieved in this geometry through design of asymmetric stiff insets.