Contact mechanics of biologically-inspired interface geometries

The mechanics of biological contact are important in many natural processes, motivating extensive study of biological adhesive systems as well as attempts at biomimickry through microfabricated adhesive surfaces. Improved understanding of the physical mechanisms of biological adhesion is essential for further progress in the design of advanced adhesive systems so that natural systems are not just copied, but are ultimately improved upon. The work presented here focuses on two broad classes of biological contact problems related to the key design variables of surface topography and material properties: geometric adhesion enhancement, where the force and/or work required to separate two surfaces are increased solely through the surface geometry, and mixed mode adhesive contact, i.e., combined normal and tangential loading or straining, of viscoelastic or otherwise dissipative adhesive interfaces. An analytical contact model for axisymmetric concave surfaces is shown to predict higher adhesive pull-off forces for a range of these geometries than for flat punches of equal size, and experiments on gelatin validate the predictions of the model. Axisymmetric sinusoidal wavy surfaces are also studied analytically, and it is shown that low-amplitude waviness can enhance adhesion through both strengthening and toughening of the interface. A model is developed for the wavy surface so that the adhesive pull-off forces in the JKRDMT transition regime can be studied, and significant adhesion enhancement due to surface topography is seem to be limited to the JKR regime. Mixed mode adhesive contact is studied by introducing a phenomenological model for increased effective work of adhesion with increasing phase angle of mode mixity, allowing the energy dissipation due to viscoelasticity or irreversible interfacial processes seen in contact experiments of glass spheres on PDMS to be captured. This phenomenological model is incorporated into an analysis of mixed mode contact of axisymmetric wavy surfaces, and the mechanisms which enhance adhesion are shown to be linked to enhanced static sliding resistance. These results highlight the importance of surface topography and material behavior in adhesive contact.