Design of biomimetic fibrillar interfaces: 1. Making contact.

This paper explores the contact behaviour of simple fibrillar interfaces designed to mimic natural contact surfaces in lizards and insects. A simple model of bending and buckling of fibrils shows that such a structure can enhance compliance considerably. Contact experiments on poly(dimethylsiloxane) (PDMS) fibrils confirm the model predictions. Although buckling increases compliance, it also reduces adhesion by breaking contact between fibril ends and the substrate. Also, while slender fibrils are preferred from the viewpoint of enhanced compliance, their lateral collapse under the action of surface forces limits the aspect ratio achievable. We have developed a quantitative model to understand this phenomenon, which is shown to be in good agreement with experiments

Detachment of stretched viscoelastic fibrils

New experimental results are presented about the final stage of failure of soft viscoelastic adhesives. A microscopic view of the detachment of the adhesive shows that after cavity growth and expansion, well adhered soft adhesives form a network of fibrils connected to expanded contacting feet which fail via a sliding mechanism, sensitive to interfacial shear stresses rather than by a fracture mechanism as sometimes suggested in earlier work. A mechanical model of this stretching and sliding failure phenomenon is presented which treats the fibril as a nonlinear elastic or viscoelastic rod and the foot as an elastic layer subject to a friction force proportional to the local displacement rate. The force on the stretched rod drives the sliding of the foot against the substrate. The main experimental parameter controlling the failure strain and stress during the sliding process is identified by the model as the normalized probe pull speed, which also depends on the magnitude of the friction and PSA modulus. In addition, the material properties, viscoelasticity and finite extensibility of the polymer chains, are shown to have an important effect on both the details of the sliding process and the ultimate failure strain and stress

Design of biomimetic fibrillar interfaces: 2. Mechanics of enhanced adhesion.

This study addresses the strength and toughness of generic fibrillar structures. We show that the stress sigmac required to pull a fibril out of adhesive contact with a substrate has the form sigma(c) = sigma(0)Phi(chi). In this equation, sigma(0) is the interfacial strength, Phi(chi) is a dimensionless function satisfying 0 <or= Phi(chi) <or= 1 and chi is a dimensionless parameter that depends on the interfacial properties, as well as the fibril stiffness and radius. Pull-off is flaw sensitive for chi >> 1, but is flaw insensitive for chi < 1. The important parameter chi also controls the stability of a homogeneously deformed non-fibrillar (flat) interface. Using these results, we show that the work to fail a unit area of fibrillar surface can be much higher than the intrinsic work of adhesion for a flat interface of the same material. In addition, we show that cross-sectional fibril dimensions control the pull-off force, which increases with decreasing fibril radius. Finally, an increase in fibril length is shown to increase the work necessary to separate a fibrillar interface. Besides our calculations involving a single fibril, we study the concept of equal load sharing (ELS) for a perfect interface containing many fibrils. We obtain the practical work of adhesion for an idealized fibrillated interface under equal load sharing. We then analyse the peeling of a fibrillar surface from a rigid substrate and establish a criterion for ELS