Manipulation of Elastic Deformation in Bio-inspired Wet Adhesion

Johns Hopkins University, Baltimore MD

Investigators

Abstract

The choice of materials to control adhesion depends on the desired application. In robotics, for example, multiple adhesive cycles are necessary for locomotion, gripping, and manipulation. Synthetic hierarchical structures based on the gecko toe pads demonstrated these capabilities in air. Fulfilling these requirements for reversible adhesion in fluid environments (for example underwater), however, is more challenging. In fluids, dynamic effects such as drag and swelling can deform surface structures prior to contact and prevent adhesion. The objective of the work is to develop the fundamental understanding in mechanics necessary for the creation of structured surfaces that can act as reversible adhesives in liquids. The strategy to be followed is the fabrication of porous soft material films with a surface topography inspired by the frog toe pads. These films will be employed to investigate the role played by porosity and deformation on adhesion. The development of coatings for reversible underwater adhesion will advance the manufacturing of robotic components, bandages and wound sealants, and could help understand the mechanisms of biofouling. Beyond underwater adhesion for robotic and larger scale applications, a better understanding of poroelasticity in compliant materials could lead to better performing materials to replace joint cartilage. Outreach efforts include participation of high school and undergraduate students in the laboratory. Students participating in the project will be encouraged to present at conferences and be involved with additional outreach efforts at local elementary schools. The dynamics of friction, adhesion, and fracture of porous compliant materials involve poorly understood phenomena because transport, deformation, and adhesion are highly coupled. To address this issue, new instrumentation will be developed to study the mechanical performance of soft and porous coatings to measure simultaneously dissipative forces and spatiotemporal deformation. The performance (e.g. adhesion and poroelastic relaxation) of polymer coatings fabricated with three different type of anisotropic pillars will be investigated and results will be compared with uniform coatings of the same material. In particular, dissipative force measurements in fluids will be performed to determine the role played by out-of-contact deformation alter the magnitude and directionality of adhesion. Throughout the project, experiments will be compared to existing theories and continuum numerical finite element models for elastohydrodynamics and poroelasticity that are based on lubrication, elasticity, and transport in porous media. The contributions of multi-scale porosity (e.g. mesh scale, pillar spacing, and surface roughness), modulus, and directional anisotropy in surface topography will be characterized in terms of their ability to be engineered toward applications in reversible adhesion in wet environments.