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. Multiple loading configurations are possible, ranging from static normal loading of the roller to free rolling and rolling with a propulsive or a braking torque. Instrument functions, elements, and design are presented in detail and its capabilities are demonstrated by obtaining measurements such as width of the contact interface under normal loading, strain fields of the roller sidewall and contact interface under normal loading, and the roller's resistance to motion for free and forced rolling.

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. Our results demonstrate that while the activated and disactivated pull-off forces decrease, the amplification factor increases as the Young's modulus increases. Given that both the activated pull-off force and the amplification factor should be maximized for the best performance, the Young's modulus has to be carefully chosen based on the characteristics of the objects to be manipulated.

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. The objective of this project was to quantify the static and kinetic coefficients of friction (COF) of various brands of prophylactic dressings under the stresses experienced in situ, while the dressings are in contact with bed linen materials. The COF of six commercial dressings were calculated using tribometer measurements. The ranges of static COF were 0.333-0.542 and kinetic COF were 0.333-0.513. Four dressings exhibited COF that were consistent with skin-linen values reported in the literature and all dressing COF appear to be lower than the COF of moist skin against linen.

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. Measured pull-off forces do not differ between hydrophilic and lipophilic surfaces, ranging between 58 and 186 mN, and resulting in an adhesive strength up to 204 kPa. The mechanical and chemical properties of secretion resemble those of pressure-sensitive adhesives. The Raman, infrared, and nuclear magnetic resonance spectra show the clear presence of saturated aliphatic hydrocarbons, esters, free carboxylic acids, as well as minor amounts of amides and aromatic compounds. We suggest a multi-component material (aliphatic hydrocarbon resin), including alkanes, fatty acids, amides, and tackifying terpenoids embedded in a fluid matrix (fatty acids) comprising nonpolar and polar portions serving the universal and robust adhesive properties. These characteristics matter for ecological-evolutionary aspects and can inspire innovative designs of multifunctional, biomimetic pressure-sensitive adhesives and varnishes.

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.

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. A simple gripper based on a manual center-clamping vise is shown to enable pick-and-place manipulation of various flat and curved objects of everyday use.

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. Our results clearly demonstrate that the adhesion- and friction-driven attachment of the wall-shaped microstructure degrades, regardless of the surface waviness, when the surface roughness increases. Second, splitting the wall-shaped microstructure indeed helps to mitigate the negative effect of the increasing surface unevenness by allowing the split microstructure to adapt more easily to the surface waviness and by reducing the effective average peeling angle. These findings can be used to guide the development of biomimetic shear-actuated adhesives suitable for operation not only on smooth but also on rough surfaces.

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. In accord with the prediction of the Kendall model for the normal component of peeling force, there is an optimal normal force that is required to detach the wall-shaped adhesive microstructure. The optimum is obtained at about half the distance needed to initiate sliding and at pulling angles that range within 60-90°, which suggests that the wall-shaped microstructure can tolerate relatively large inaccuracies in the loading direction. The increase of the attachment force with increasing flap height is found to correlate with the flap thickness, which decreased with increasing flap height.

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

Similar to other leaf beetles, rosemary beetles Chrysolina americana 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. Pull-off force measurements with a custom-made tribometer featuring a self-aligning sample holder confirmed stronger attachment to female elytra compared with glass in both males and females; corresponding to 45 and 30 times the body weight, respectively. In line with previous studies, males generated significantly higher forces than females on convex elytra and flat glass, 1.2 times and 6.8 times, respectively. Convex substrates like elytra seem to improve the attachment ability of rosemary beetles, because they can hold more strongly due to favourable shear angles of legs, tarsi and adhesive setae. A self-aligning sample holder is found to be suitable for running force measurement tests with living biological samples.

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. This method overcomes the inherent disadvantages of photolithography techniques and opens wide perspectives for future production of gecko-like attachment systems. Advanced smart-performance surfaces featuring thin-film-based hierarchical shear-activated elements are fabricated and found capable of generating friction force of several tens of times the contact load, which makes a significant step forward towards a true gecko-like adhesive.

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. When slid against lubricated human skin, the hexagonal surface texture is capable of generating about twice the friction of its technical competitors, which is related to it being much more effective at channelling of the lubricant fluid out of the contact zone. The draining channel shape and contact area fraction are found to be the most important geometrical parameters governing the fluid drainage rate.

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. It was found that classical roughness parameters, such as average roughness (Ra) and others, cannot be used to explain topography-related variation in pull-off force. This has led us to the development of an integrated roughness parameter capable of explaining results of pull-off measurements. Using this parameter, we have also found that there is a critical roughness, above which neither smooth nor microstructured surface could generate any attachment force, which may have important implications on design of both adhesive and anti-adhesive surfaces.

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. Our experiments also suggest a change in visco-elastic properties of the structured surfaces compared with the bulk material. Based on the results obtained, it is assumed that this adhesive may be suitable in dynamic pick-and-drop processes even under vacuum conditions at which sufficiently high adhesive capability is maintained.