RUI: Multiscale Analysis of Adhesion and Friction Coupling Enabled by Bio-Inspired Anisotropic Fibrillar Adhesives

California State L A University Auxiliary Services Inc., Los Angeles CA

Investigators

Abstract

NON-TECHNICAL SUMMARY Many organisms in nature have adapted microscopic hairs and fibrils as robust adhesives for locomotion purposes and/or securing a strong attachment. Geckos are among the most studied species for developing bio-inspired dry adhesives due to their fairly large body size and sophisticated toe pad structure. Despite great improvements on adhesion strength, the durability and reusability of many lab-scale 'proof-of-principle' artificial adhesives still lag behind their natural counterparts, even being tested on smoother and cleaner substrates. In this project, adhesive arrays with controlled contact shapes and geometries will be fabricated using template-assisted cast molding and advanced micro-fabrication techniques. Adhesive behavior will be characterized to simultaneously measure adhesion and friction under non-idealized conditions (e.g., on non-dust-free surfaces, and at different temperatures and humidity). Computer simulation will be performed to explain the experimental results. Overall, the proposed work provides an alternative approach to study surface and interfacial phenomena of advanced smart superstructures mimicking nature. Students will examine problems at the crossroads of engineering, materials science, physics, chemistry, and other related disciplines. Exploring a broad range of applications by building up state-of-the-art versatile mechanical system and simulation tools will substantially enhance research productivity and provide essential research training to undergraduate and graduate students at Cal State LA and in the Southern California region. The educational plan contributes to Cal State LA's long tradition of educating local students to become the next generation of engineers, scientists, and educators whose vision is to link science priorities to the solving of societal problems. TECHNICAL SUMMARY Efforts to mimic gecko toe pad structure and function seek to develop a new class of advanced adhesives that are not only sticky and easy to unstick, but also non-fouling. Current synthetic dry adhesives suffer from large, required preloading and intolerance of dusty environments, which significantly limit their use for sticking to or manipulating fragile and non-dust-free objects. This project focuses on understanding the engaging and disengaging mechanisms of anisotropic fibrillar adhesive units featuring a variety of contact shapes/geometries and structural/materials gradient across multiple lengths and temporal scales. The proposed dry adhesive design incorporates both the geometrical anisotropy seen at the setal level and the special contact shape of the setal branches at the spatula level. The ultimate goal is to elucidate how adhesion and friction are coupled and may be better controlled by rationally designing the geometry dependent dry adhesive units. Adhesive performance will be evaluated under cyclic loadings and non-dust-free environments, as well as different temperatures and relative humidity, using a custom-designed surface force apparatus. Multiscale anisotropic finite-element cohesive-zone modeling will be implemented to parameterize the contact shape dependent interfacial behaviors, and to analyze the adhesive mechanisms, fracture and failures. This project will yield both basic and applied advances in biomimetic research, e.g., development of switchable and self-cleaning surfaces; micro-/nanomanipulations; and high tolerance dry adhesives and drug delivery systems for robotics, energy and biomedical applications. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.