Friction anisotropy with respect to topographic orientation

Anisotropic Stick-Slip Frictional Surfaces via Titania Nanorod Patterning

Nanoscale or microscale surface texturing is an effective technique to tailor the tribological properties between two surfaces that are rubbed against each other. In order to achieve the desired frictional properties by a patterned surface, one needs an in-depth understanding of the underlying mechanisms. Here, we demonstrate anisotropic stick-slip friction achieved via a nanotextured surface of tilted titania nanorods (TiNRs). The surface was developed by using the glancing angle deposition (GLAD) technique, and exhibited load-dependent variations in stick-slip friction as well as frictional anisotropy in different sliding directions. For studying the frictional properties of the newly developed surface, lateral force microscopy (LFM) was performed in three different reciprocal orientations (0° rotated, 45° rotated, 90° rotated) using a custom-made colloidal alumina atomic force microscopy (AFM) probe. The frictional behavior was found to vary significantly with the orientation. At 0° rotated position) a prominent "stick-slip" was observed when scanning opposite to the tilt direction, whereas the phenomenon reduced significantly when the nanotextured surface was scanned along the tilt direction or rotated to different angles (45 and 90°) with respect to the sliding direction of the AFM cantilever supporting the probe. The experimental findings were interpreted based on the classical solution for large deflections of tilted elastic rods. Overall, the textured surface, LFM-based frictional measurement, and the quantitative analysis presented here provide a fundamental understanding of how friction can be significantly varied on a surface patterned with tilted TiNRs at a length scale of about 1 μm, which can be comprehensively applied to nanorod patterns of other materials on different substrates.

Tribology of Pore-Textured Hard Surfaces under Physiological Conditions: Effects of Texture Scales

Micro- and nanotexturing on hard biomaterials have shown advantages for tissue engineering and antifouling applications. However, a growing number of studies have also shown that texturing may cause an increase in friction, demanding further research on the tribological effects of texturing under physiological conditions. This study investigates the tribological effects of micro- and nanopore patterns on hard hydrophilic silicon sliding against soft hydrophobic polydimethylsiloxane (PDMS) immersed in aqueous liquids with various viscosities, simulating the sliding of a textured implant surface against soft tissues. The experimental results show that silicon surfaces with pore textures at both micro- and nanoscale feature sizes confer a higher coefficient of friction (COF) than an untextured one. It is attributed to the texture's edge effect caused by the periodic pore patterns between the two sliding objects with a large difference in material stiffness. For the same solid area fraction, nanopored surfaces show a higher COF than micropored surfaces because of the significantly higher texture edge length per unit area. For micropored surfaces with a similar length of texture edge length per unit area, the COF increases more significantly with the increase in pore size because of the greater stress at the rims of the larger pores. The COFs of both micro- and nanoscale pores generally decrease from ∼10 to 0.1 with an increase in the surrounding aqueous viscosity, indicating the transition from a boundary lubrication to a mixed lubrication regime while mostly remaining in boundary lubrication. In contrast, the COF of an untextured surface decreases from ∼1 to 0.01, indicating that it mostly remains in the mixed lubrication regime while showing the tendency toward hydrodynamic lubrication. Compared to a hydrophilic hard probe sliding against a textured hydrophobic soft substrate, the hydrophobic soft probe sliding against a textured hydrophilic hard substrate produces a significantly higher COF under similar physiological conditions due to the larger edge effect.

Tribological Behavior of Bioinspired Surfaces

Energy losses due to various tribological phenomena pose a significant challenge to sustainable development. These energy losses also contribute toward increased emissions of greenhouse gases. Various attempts have been made to reduce energy consumption through the use of various surface engineering solutions. The bioinspired surfaces can provide a sustainable solution to address these tribological challenges by minimizing friction and wear. The current study majorly focuses on the recent advancements in the tribological behavior of bioinspired surfaces and bio-inspired materials. The miniaturization of technological devices has increased the need to understand micro- and nano-scale tribological behavior, which could significantly reduce energy wastage and material degradation. Integrating advanced research methods is crucial in developing new aspects of structures and characteristics of biological materials. Depending upon the interaction of the species with the surrounding, the present study is divided into segments depicting the tribological behavior of the biological surfaces inspired by animals and plants. The mimicking of bio-inspired surfaces resulted in significant noise, friction, and drag reduction, promoting the development of anti-wear and anti-adhesion surfaces. Along with the reduction in friction through the bioinspired surface, a few studies providing evidence for the enhancement in the frictional properties were also depicted.

Friction control of elastic materials on glass by means of textured surfaces

To investigate the friction behaviors of elastomer and polyacetal writing tips sliding on various textured glass surfaces, the influences of the pitch size and height of sub-millimeter to millimeter sized texture on friction were examined via reciprocating friction tests. The friction coefficients of each writing tip could be systematically varied by changing the pitch and height of the texture. These changes in friction were based on the relationship between the convex-concave shapes and the contact parts of the writing tip, and hence, influence the adhesive, abrasive, and deformation frictions. By inducing a surface texture with a pitch smaller than the contact area of the writing tip, the friction coefficient could be reduced effectively. By inducing a surface texture with a larger height, the friction coefficient of the elastomer could be increased due to deformation friction. These behaviors indicate the possibility of controlling the friction by changing the parameters such as the pitch and height of the textured glass surfaces.

Machine Learning Approach to Analyze the Surface Properties of Biological Materials

Similar to how CRISPR has revolutionized the field of molecular biology, machine learning may drastically boost research in the area of materials science. Machine learning is a fast-evolving method that allows for analyzing big data and unveiling correlations that otherwise would remain undiscovered. It may hold invaluable potential to engineer novel functional materials with desired properties, a field, which is currently limited by time-consuming trial and error approaches and our limited understanding of how different material properties depend on each other. Here, we apply machine learning algorithms to classify complex biological materials based on their microtopography. With this approach, the surfaces of different variants of biofilms and plant leaves can not only be distinguished but also correctly classified according to their wettability. Furthermore, an importance ranking provided by one of the algorithms allows us to identify those surface features that are critical for a successful sample classification. Our study exemplifies how machine learning can contribute to the analysis and categorization of complex surfaces, a tool, which can be highly useful for other areas of materials science, such as damage assessment as well as adhesion or friction studies.

Interfacial Friction Anisotropy in Few-Layer Van der Waals Crystals

Friction anisotropy is one of the important friction behaviors for two-dimensional (2D) van der Waals (vdW) crystals. The effects of normal pressure and thickness on the interfacial friction anisotropy in few-layer graphene, h-BN, and MoSe₂ under constant normal force mode have been extensively investigated by first-principle calculations. The increase of normal pressure and layer number enhances the interfacial friction anisotropy for graphene and h-BN but weakens that for MoSe₂. Such significant deviations in the interfacial friction anisotropy of few-layer graphene, h-BN and MoSe₂ can be mainly attributed to the opposite contributions of electron kinetic energies and electrostatic energies to the sliding energy barriers and different interlayer charge exchanges. Our results deepen the understanding of the influence of external loading and thickness on the friction properties of 2D vdW crystals.

Soft-bodied adaptive multimodal locomotion strategies in fluid-filled confined spaces

Soft-bodied locomotion in fluid-filled confined spaces is critical for future wireless medical robots operating inside vessels, tubes, channels, and cavities of the human body, which are filled with stagnant or flowing biological fluids. However, the active soft-bodied locomotion is challenging to achieve when the robot size is comparable with the cross-sectional dimension of these confined spaces. Here, we propose various control and performance enhancement strategies to let the sheet-shaped soft millirobots achieve multimodal locomotion, including rolling, undulatory crawling, undulatory swimming, and helical surface crawling depending on different fluid-filled confined environments. With these locomotion modes, the sheet-shaped soft robot can navigate through straight or bent gaps with varying sizes, tortuous channels, and tubes with a flowing fluid inside. Such soft robot design along with its control and performance enhancement strategies are promising to be applied in future wireless soft medical robots inside various fluid-filled tight regions of the human body.

Tribological and nanomechanical properties of a lignin-based biopolymer

A research is reported on the nanomechanics and tribology of the Arboblend V2 Nature biopolymer (a 100% bio-based material, biodegradable, or resistant depending of application), being a mixture of different biopolymers such as lignin, polylactic acid, cellulose, biopolyamides, and other natural additives. The specimens were made by an industrial-scale injection molding machine. The nanoindentation characterization have unveiled that an increase in processing temperature from 160°C to 170°C produces a rise in hardness and elastic modulus of ∼20%. Tribological characterization against a bearing-steel counterface has shown that for both processing temperatures, the increase of the applied load or the increase of sliding speed will produce an increase of the friction coefficient (µ) and wear. At an applied load of 1 N (contact pressure of 104 MPa) and tracks in a direction perpendicular to the surface textured lines, the lowest µ ∼ 0.148 are for samples made T = 170°C, while for tracks parallel to the textured lines, the lowest µ ∼ 0.059 is obtained for samples made at T = 160°C. Experiments made at different ambient humidity have established that friction coefficient is higher at 0% RH or at 75% RH than at 33% RH. Our results show that the biopolymers Arboblend V2 Nature is a candidate to substitute some popular fossil-based thermoplastics in numerous tribological industrial applications.

The Topology of the Leg Joints of the Beetle Pachnoda marginata (Scarabaeidae, Cetoniinae) and Its Implication for the Tribological Properties

Locomotion of walking insects is exceptionally efficient. The function of their leg joints in different movement scenarios depends on their kinematics and contacting conditions between moving parts. The kinematics was previously studied in some insects, but contact mechanics within the joints remains largely unknown. In order to understand the complex topology of the contacting surfaces of the leg joints in the Congo rose beetle Pachnoda marginata peregrina (Scarabaeidae, Cetoniinae), we have investigated the shape, the waviness, and the roughness of the joint base and its counter body by applying confocal laser scanning microscopy and white light interferometry. Additionally, we performed nanoindentation tests on the contacting joint surfaces, in order to analyze material properties (elasticity modulus and hardness) of the joint cuticle. We found two topological design principles of the contact surfaces that might be considered as adaptations for reducing frictional drag during leg movements. First, the contact pairs of all leg joints studied consist of convex and concave counterparts. Second, there is a smooth and a rough surface in contact in which microprotuberances are present on the rough surface. These principles might be potentially interesting for technical implications, to design bioinspired joints with both reduced friction and wear rate.

Wettability and friction control of a stainless steel surface by combining nanosecond laser texturing and adsorption of superhydrophobic nanosilica particles

In this work, we present functionalization of AISI 316 L surfaces by nanosecond Nd:YAG laser texturing and adsorption of superhydrophobic fluoroalkylsilane functionalized 30-nm silica nanoparticles. Surface modification by varying the distance between laser-produced micro(μ)-channels leads to different surface roughnesses. After nanosilica coating, the superhydrophilic laser-textured surfaces change into superhydrophobic surfaces with the same μ-roughness. A higher μ-channel density leads to more hydrophobic surfaces after coating. This enables a study of the combined effect of surface wettability and morphology on the friction coefficient and wear resistance. Experiments were performed in dry and water environments. In the case of dry friction, increased μ-roughness leads to a higher friction coefficient, and the water-repellency modification by nanosilica particles has no influence on the tribological behaviour. In contrast, in the water environment, the wettability presents an important contribution to the properties of contact surfaces: hydrophobic surfaces exhibit a lower friction coefficient, especially at higher densities of μ-channels. Energy-dispersive X-ray spectroscopy analysis of surfaces before and after the tribological experiments is performed, revealing the difference in weight % of Si in the worn surface compared to the unworn surface, which varies according to the nature of the surface morphology due to laser texturing in both dry and water environments.

Open System Tribology and Influence of Weather Condition

The tribology of an open system at temperatures ranging between 3 °C and -35 °C, with and without snow, was investigated using a pin-on-disc tribometer mounted in a temperature-controlled environmental chamber. The relationship between the microstructure and ductility of the materials and the tribology at the contacting surfaces was investigated. The study shows that during continuous sliding, pressure causes snow particles to melt into a liquid-like layer, encouraging the generation of oxide flakes on the contact path. The friction coefficient and wear rate are dramatically reduced through an oxidative friction and wear mechanism. In the absence of snow, the tribological process is controlled by the low temperature brittleness of steel in the temperature range from 3 °C to -15 °C. At these temperatures, cracks are prone to form and extend on the worn surfaces, resulting in the spalling of bulk scraps, which are crushed into debris that increases the friction coefficient and wear rate due to strong abrasion. When the temperature falls to -25 °C, an ice layer condenses on the metal surfaces and relaxes the tribological process in the same way as the added snow particles, which significantly decreases the friction and wear.

Friction behavior of a microstructured polymer surface inspired by snake skin

The aim of this study was to understand the influence of microstructures found on ventral scales of the biological model, Lampropeltis getula californiae, the California King Snake, on the friction behavior. For this purpose, we compared snake-inspired anisotropic microstructured surfaces to other microstructured surfaces with isotropic and anisotropic geometry. To exclude that the friction measurements were influenced by physico-chemical variations, all friction measurements were performed on the same epoxy polymer. For frictional measurements a microtribometer was used. Original data were processed by fast Fourier transformation (FFT) with a zero frequency related to the average friction and other peaks resulting from periodic stick-slip behavior. The data showed that the specific ventral surface ornamentation of snakes does not only reduce the frictional coefficient and generate anisotropic frictional properties, but also reduces stick-slip vibrations during sliding, which might be an adaptation to reduce wear. Based on this extensive comparative study of different microstructured polymer samples, it was experimentally demonstrated that the friction-induced stick-slip behavior does not solely depend on the frictional coefficient of the contact pair.

Dry friction of microstructured polymer surfaces inspired by snake skin

The microstructure investigated in this study was inspired by the anisotropic microornamentation of scales from the ventral body side of the California King Snake (Lampropeltis getula californiae). Frictional properties of snake-inspired microstructured polymer surface (SIMPS) made of epoxy resin were characterised in contact with a smooth glass ball by a microtribometer in two perpendicular directions. The SIMPS exhibited a considerable frictional anisotropy: Frictional coefficients measured along the microstructure were about 33% lower than those measured in the opposite direction. Frictional coefficients were compared to those obtained on other types of surface microstructure: (i) smooth ones, (ii) rough ones, and (iii) ones with periodic groove-like microstructures of different dimensions. The results demonstrate the existence of a common pattern of interaction between two general effects that influence friction: (1) molecular interaction depending on real contact area and (2) the mechanical interlocking of both contacting surfaces. The strongest reduction of the frictional coefficient, compared to the smooth reference surface, was observed at a medium range of surface structure dimensions suggesting a trade-off between these two effects.