Geckos owe their remarkable stickiness to millions of dry, hard setae on their toes. In this study, we discovered that gecko setae stick more strongly the faster they slide, and do not wear out after 30 000 cycles. This is surprising because friction between dry, hard, macroscopic materials typically decreases at the onset of sliding, and as velocity increases, friction continues to decrease because of a reduction in the number of interfacial contacts, due in part to wear. Gecko setae did not exhibit the decrease in adhesion or friction characteristic of a transition from static to kinetic contact mechanics. Instead, friction and adhesion forces increased at the onset of sliding and continued to increase with shear speed from 500 nm s ⁻¹ to 158 mm s ⁻¹ . To explain how apparently fluid-like, wear-free dynamic friction and adhesion occur macroscopically in a dry, hard solid, we proposed a model based on a population of nanoscopic stick–slip events. In the model, contact elements are either in static contact or in the process of slipping to a new static contact. If stick–slip events are uncorrelated, the model further predicted that contact forces should increase to a critical velocity ( V *) and then decrease at velocities greater than V *. We hypothesized that, like natural gecko setae, but unlike any conventional adhesive, gecko-like synthetic adhesives (GSAs) could adhere while sliding. To test the generality of our results and the validity of our model, we fabricated a GSA using a hard silicone polymer. While sliding, the GSA exhibited steady-state adhesion and velocity dependence similar to that of gecko setae. Observations at the interface indicated that macroscopically smooth sliding of the GSA emerged from randomly occurring stick–slip events in the population of flexible fibrils, confirming our model predictions.

Although poor mechanical properties are usually found in films cast from waterborne colloidal polymers relative to their solvent cast counterparts, these materials offer the opportunity to control structure and composition very precisely at the nanoscale. Here, we introduce a knowledge-based strategy to design what we call a “soft–soft nanocomposite” in which a percolating crosslinked phase contains a second less crosslinked dispersed phase distributed regularly throughout it. This new type of structure uses polymer colloid particles as building blocks in a bottom-up approach to obtain simultaneously a very viscoelastic behaviour at small strains and an elastic behaviour at larger strains, bringing highly desirable properties for adhesives applications. For instance, the adhesion energy of the soft–soft nanocomposite on polyethylene is more than four times greater than that of a commercial material in which the particles are crosslinked and the interfaces are entangled. Our conclusions are broadly applicable to a large class of soft materials based on deformable polymeric networks, such as gels, elastomers and artificial tissues.

The morphology of an elastic strip subject to vertical compressive stress on a frictional rigid substrate is investigated by a combination of theory and experiment. We find a rich variety of morphologies, which—when the bending elasticity dominates over the effect of gravity—are classified into three distinct types of states: pinned, partially slipped, and completely slipped, depending on the magnitude of the vertical strain and the coefficient of static friction. We develop a theory of elastica under mixed clamped-hinged boundary conditions combined with the Coulomb-Amontons friction law and find excellent quantitative agreement with simulations and controlled physical experiments. We also discuss the effect of gravity in order to bridge the difference in the qualitative behaviors of stiff strips and flexible strings or ropes. Our study thus complements recent work on elastic rope coiling and takes a significant step towards establishing a unified understanding of how a thin elastic object interacts vertically with a solid surface.