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.     

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. It appears that the fibrillar microstructure can be used in applications where a total attachment force should be generated in a binary on/off state and, most surprisingly, is suitable to stabilize and minimize elastomer friction.     

Theoretical Substantiation of the Slip Index Approach to Fretting

A theoretical basis is presented for a previous fully empirical development of the unified approach to fretting that was based on a new similarity criterion termed slip index. It is shown that the slip index can be analytically derived from any friction loop geometry and hence, can be used for the characterization of any fretting system.     

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 (R_a) 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.     

Slip Index: A New Unified Approach to Fretting

A new similarity criterion, termed slip index, is introduced. This slip index replaces conventional fretting maps in determining the different fretting regimes as well as the transition from fretting to reciprocal sliding. The slip index is derived from a dimensional analysis of the parameters that govern sliding conditions and provides a unified approach to fretting that is valid for any particular system. The validity of this approach is verified by comparison of fretting data from different test rigs and on different scales. A more accurate definition of fretting is offered based on the present approach.     

Tribometer for In Situ Scanning Electron Microscopy of Microstructured Contacts

Here, we report on a special tribometer built to operate inside an environmental scanning electron microscope enabling charge-free imaging of non-conductive and/or hydrated materials. The device is intended to be used for simultaneous testing and in situ visual inspection of biological and biomimetic patterned surfaces during contact formation, pulling-off, peeling, and shearing modeling the behavior of biological microstructured attachment systems in nature. To demonstrate its performance, a simple array of hexagonal elastomer micropillars is tested. The results obtained show that direct link between precise data on the contact forces and images of the contact elements deformed by these forces indeed allows getting an insight into how contact surface patterns function when in contact.     

Table Tennis Rubber: Tribological Characterization

To characterize rubber coverings, three parameters, namely, speed, spin, and control, are normally used. However, all three of them appear to be the characteristics of the ball being hit and not those of the racket covering itself. The expert??s method of valuation complicates the rubber characterization even more, so the speed-spin-control tables found in the distributors?? catalogs are traditionally referred to as ??mystic and non-objective?? ones. Based on a more scientific approach, here we report on a reliable tribological method allowing to characterize the frictional behavior of rubber racket covering unequivocally using three simple non-dimensional parameters. To demonstrate its performance, 8 different popular rubbers, 4 pimples-in and 4 pimples-out ones, are tested.     

Elimination of Stick-Slip Motion in Sliding of Split or Rough Surface

Here, we present a mass-less quasi-static model of stick-slip phenomenon built exclusively on the difference between higher static and lower kinetic friction force. The model allows explaining the disappearance of stick-slip motion when elastic surface slid in contact with rigid counter-face bears large amount of small outgrowths. Adjusting the model parameters, it is also possible simulating systems with different transient responses. The results obtained may also be helpful in understanding the variety of sliding behavior of different materials.