Biomimetic wall-shaped hierarchical micro-structure: numerical simulation of sliding inception.

A finite element model is developed to study the behaviour of adhesion-governed contact of soft elastic wall-shaped projections and a rigid flat under normal and tangential loading. Bi-linear cohesive zone material is implemented to model the adhesive interaction of the contact, and Tresca's law of friction is used to model the tangential response. An assessment of maximum tangential load capacity of attachment devices based on dry adhesive contact is performed using the described model and the effects of geometrical properties are investigated. The value of maximum shear stress in the contact of PVS and a silicon flat is identified. The model reveals that the tangential load capacity of the wall-shaped projection is independent of normal load, and that increasing the length of the adhesive flap, as well as decreasing its thickness provides better tangential load capacity.

Flying couplers above spinning resonators generate irreversible refraction.

Creating optical components that allow light to propagate in only one direction-that is, that allow non-reciprocal propagation or 'isolation' of light-is important for a range of applications. Non-reciprocal propagation of sound can be achieved simply by using mechanical components that spin1,2. Spinning also affects de Broglie waves 3 , so a similar idea could be applied in optics. However, the extreme rotation rates that would be required, owing to light travelling much faster than sound, lead to unwanted wobbling. This wobbling makes it difficult to maintain the separation between the spinning devices and the couplers to within tolerance ranges of several nanometres, which is essential for critical coupling4,5. Consequently, previous applications of optical6-17 and optomechanical10,17-20 isolation have used alternative methods. In hard-drive technology, the magnetic read heads of a hard-disk drive fly aerodynamically above the rapidly rotating disk with nanometre precision, separated by a thin film of air with near-zero drag that acts as a lubrication layer 21 . Inspired by this, here we report the fabrication of photonic couplers (tapered fibres that couple light into the resonators) that similarly fly above spherical resonators with a separation of only a few nanometres. The resonators spin fast enough to split their counter-circulating optical modes, making the fibre coupler transparent from one side while simultaneously opaque from the other-that is, generating irreversible transmission. Our setup provides 99.6 per cent isolation of light in standard telecommunication fibres, of the type used for fibre-based quantum interconnects 22 . Unlike flat geometries, such as between a magnetic head and spinning disk, the saddle-like, convex geometry of the fibre and sphere in our setup makes it relatively easy to bring the two closer together, which could enable surface-science studies at nanometre-scale separations.