Instrument for in situ study of rolling under normal load and torque.

Instabilities that develop at the contact interface of solid rollers or airless tires while in motion can lead to increased energy losses and reduced service life. This manuscript describes an instrument that can give better insight into the origin of such instabilities by monitoring both local and global roller mechanics. This is done by simultaneously obtaining force and displacement data from sensors as well as optical measurements and local deformation fields across two different planes, extracted from images taken by a high-speed camera.

Amplification factor in shear-activated adhesives: effect of elasticity.

In contrast to common adhesives used in joining, directional adhesives are designed for a dynamic attachment/detachment operation and, hence, their performance is assessed not only in an activated mode but also in a disactivated mode. Consequently, in addition to a peak adhesive strength, a ratio of maximum over minimum attachment force, which can be called a shear-driven amplification factor, is utilized for their characterization. Considering that the peak adhesive response and the amplification factor both depend on the elasticity of the microstructured surface, here we report a combined numerical and experimental study of the effect of the Young's modulus on the attachment properties of flap-shaped contact elements drawn from polyurethane.

Friction characteristics of preventative wound dressings under clinically-relevant conditions.

Wound dressings can be used prophylactically or during actual treatment. Preventative dressings have become a standard of care to prevent pressure ulcers in patients while in bed. While the mechanism of the preventative benefit has not been completely explained, the friction between the dressing and linen is hypothesized as being a key performance factor.

Robust, universal, and persistent bud secretion adhesion in horse-chestnut trees.

Buds of horse-chestnut trees are covered with a viscous fluid, which remains sticky after long-term exposure to heat, frost, radiation, precipitation, deposition of aerosols and particles, attacks by microbes and arthropods. The present study demonstrates that the secretion does not dry out under arid conditions, not melt at 50 °C, and not change significantly under UV radiation or frost at a microscopic level. It is slightly swellable under wet conditions; and, it universally wets and adheres to substrates having different polarities.

Comparison of tarsal attachment in two closely related leaf beetle species.

Green dock beetles Gastrophysa viridula and Colorado potato beetles Leptinotarsa decemlineata having distinctly different body mass and gait habits were compared with respect to their tarsal morphology and attachment ability. The focus laid on shapes and dimensions of tenent setae related to the peeling line, i.e.

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.

Drawing-Based Manufacturing of Shear-Activated Reversible Adhesives.

Although biomimetic technologies for dry reversible adhesion seem to be maturing, the costs, complexity, and time expenditures associated with the current template-based molding techniques call for research on other fabrication methods. In this paper, we report a novel cost-effective, simple, and flexible drawing-based technique for manufacturing the soft elastomeric thin-film-based microstructures needed for successful implementation of the principles of biological shear-activated adhesion. Several different types of adhesive microstructures are fabricated, and the best of them demonstrate shear-driven amplification of pull-off force by a factor of 40, which significantly outperforms known molded analogues.

Contact splitting in dry adhesion and friction: reducing the influence of roughness.

Splitting a large contact area into finer, sub-contact areas is thought to result in higher adaptability to rough surfaces, stronger adhesion, and a more uniform stress distribution with higher tolerance to defects. However, while it is widely believed that contact splitting helps to mitigate the negative effects of roughness on adhesion- and friction-based attachment, no decisive experimental validation of this hypothesis has been performed so far for thin-film-based adhesives. To this end, we report on the behavior of original and split, wall-shaped adhesive microstructures on different surfaces ranging across four orders of magnitude in roughness.

Biomimetic wall-shaped adhesive microstructure for shear-induced attachment: the effects of pulling angle and preliminary displacement.

To date, a handful of different gecko-like adhesives inspired by spatula-shaped attachment hairs have been suggested based on wedge and flap geometry of contact elements. However, while these surface designs have been shown to have directionality in adhesion, high friction, long lifetime and the ability to work in vacuum, an experimental verification of the very basic concept of the pulling angle effect has not yet been reported. To close this gap, here we use wall-shaped adhesive microstructures of three different flap heights to systematically study the effect of pulling angle on the normal and tangential components of the pull-off force tested at different preliminary tangential displacements.

How tight are beetle hugs? Attachment in mating leaf beetles.

Similar to other leaf beetles, rosemary beetles exhibit a distinct sexual dimorphism in tarsal attachment setae. Setal discoid terminals occur only in males, and they have been previously associated with a long-term attachment to the female's back (elytra) during copulation and mate guarding. For the first time, we studied living males and females holding to female's elytra.

Biomimetic wall-shaped hierarchical microstructure for gecko-like attachment.

Most biological hairy adhesive systems involved in locomotion rely on spatula-shaped terminal elements, whose operation has been actively studied during the last decade. However, though functional principles underlying their amazing performance are now well understood, due to technical difficulties in manufacturing the complex structure of hierarchical spatulate systems, a biomimetic surface structure featuring true shear-induced dynamic attachment still remains elusive. To try bridging this gap, a novel method of manufacturing gecko-like attachment surfaces is devised based on a laser-micromachining technology.

Use of biomimetic hexagonal surface texture in friction against lubricated skin.

Smooth contact pads that evolved in insects, amphibians and mammals to enhance the attachment abilities of the animals' feet are often dressed with surface micropatterns of different shapes that act in the presence of a fluid secretion. One of the most striking surface patterns observed in contact pads of these animals is based on a hexagonal texture, which is recognized as a friction-oriented feature capable of suppressing both stick-slip and hydroplaning while enabling friction tuning. Here, we compare this design of natural friction surfaces to textures developed for working in similar conditions in disposable safety razors.

Effect of counterface roughness on adhesion of mushroom-shaped microstructure.

In this study, the effect of the substrate roughness on adhesion of mushroom-shaped microstructure was experimentally investigated. To do so, 12 substrates having different isotropic roughness were prepared from the same material by replicating topography of different surfaces. The pull-off forces generated by mushroom-shaped microstructure in contact with the tested substrates were measured and compared with the pull-off forces generated by a smooth reference.

Suction component in adhesion of mushroom-shaped microstructure.

To shed light on the role of suction in adhesion of microstructure with mushroom-shaped terminal elements, we compared pull-off forces measured at different retraction velocities on structured and smooth surfaces under different pressure conditions. The results obtained allow us to suggest that suction may contribute up to 10 per cent of the pull-off force measured on the structured surfaces at high velocities. We therefore conclude that the attachment ability of this biomimetic adhesive must not be solely based on van der Waals forces.

Close-up of mushroom-shaped fibrillar adhesive microstructure: contact element behaviour.

To analyse the performance of mushroom-shaped fibrillar adhesive microstructure, its behaviour was studied during different stages of attachment-loading-detachment cycle. Visualizing the evolutions of real contact area of single microfibres, it is shown that the mushroom-shaped geometry of contact elements promotes fast and simple generation of reliable adhesion. The mushroom-shaped geometry seems to transform fibrillar contact elements into passive suction devices and makes them tolerant to overload, thus enhancing their robustness and stability.

A beetle-inspired solution for underwater adhesion.

Glue-free reversible adhesion was achieved underwater using a beetle-inspired mushroom-shaped fibrillar microstructure. Structured surfaces reveal a 25% increase in pull-off force when immersed in water and their underwater attachment is 20 times more effective than that of flat surfaces. The van der Waals interaction that underlies the adhesion of the mushroom-shaped fibrillar microstructure is significantly enhanced by a suction effect when underwater.

Shearing of fibrillar adhesive microstructure: friction and shear-related changes in pull-off force.

To characterize the effect of shearing on function of fibrillar adhesive microstructure, friction and shear-related changes in pull-off force of a biomimetic polyvinylsiloxane mushroom-shaped fibrillar adhesive microstructure were studied. In contrast to a control flat surface, which exhibited pronounced stick-slip motion accompanied with high friction, the fibrillar microstructure demonstrated a stable and smooth sliding with a friction coefficient approximately four times lower. The structured contact also manifested zero pull-off force in a sheared state, while the flat surface exhibited highly scattered and unreliable pull-off force when affected by contact shearing.

Biomimetic mushroom-shaped fibrillar adhesive microstructure.

To improve the adhesive properties of artificial fibrillar contact structures, the attachment systems of beetles from the family Chrysomelidae were chosen to serve as a model. Biomimetic mushroom-shaped fibrillar adhesive microstructure inspired by these systems was characterized using a variety of measurement techniques and compared with a control flat surface made of the same material. Results revealed that pull-off force and peel strength of the structured specimens are more than twice those of the flat specimens.