Dry friction of microstructured polymer surfaces inspired by snake skin

Designing metainterfaces with specified friction laws

Many devices, including touchscreens and robotic hands, involve frictional contacts. Optimizing these devices requires fine control of the interface's friction law. We lack systematic methods to create dry contact interfaces whose frictional behavior satisfies preset specifications. We propose a generic surface design strategy to prepare dry rough interfaces that have predefined relationships between normal and friction forces. Such metainterfaces circumvent the usual multiscale challenge of tribology by considering simplified surface topographies as assemblies of spherical asperities. Optimizing the individual asperities' heights enables specific friction laws to be targeted. Through various centimeter-scaled elastomer-glass metainterfaces, we illustrate three types of achievable friction laws, including linear laws with a specified friction coefficient and unusual nonlinear laws. This design strategy represents a scale- and material-independent, chemical-free pathway toward energy-saving and adaptable smart interfaces.

Influence of Surface Texturing on the Dry Tribological Properties of Polymers in Medical Devices

There is a constant need to improve patient comfort and product performance associated with the use of medical devices. Efforts to optimise the tribological characteristics of medical devices usually involve modifying existing devices without compromising their main design features and functionality. This article constitutes a state-of-the-art review of the influence of dry friction on polymeric components used in medical devices, including those having microscale surface features. Surface tribology and contact interactions are discussed, along with alternative forms of surface texturing. Evident gaps in the literature, and areas warranting future research are highlighted; these include friction involving polymer Vs polymer surfaces, information regarding which topologies and feature spacings provide the best performing textured surfaces, and design guidelines that would assist manufacturers to minimise or maximise friction under non-lubricated conditions.

Frictional Anisotropic Locomotion and Adaptive Neural Control for a Soft Crawling Robot

Crawling animals with bendable soft bodies use the friction anisotropy of their asymmetric body structures to traverse various substrates efficiently. Although the effect of friction anisotropy has been investigated and applied to robot locomotion, the dynamic interactions between soft body bending at different frequencies (low and high), soft asymmetric surface structures at various aspect ratios (low, medium, and high), and different substrates (rough and smooth) have not been studied comprehensively. To address this lack, we developed a simple soft robot model with a bioinspired asymmetric structure (sawtooth) facing the ground. The robot uses only a single source of pressure for its pneumatic actuation. The frequency, teeth aspect ratio, and substrate parameters and the corresponding dynamic interactions were systematically investigated and analyzed. The study findings indicate that the anterior and posterior parts of the structure deform differently during the interaction, generating different frictional forces. In addition, these parts switched their roles dynamically from push to pull and vice versa in various states, resulting in the robot's emergent locomotion. Finally, autonomous adaptive crawling behavior of the robot was demonstrated using sensor-driven neural control with a miniature laser sensor installed in the anterior part of the robot. The robot successfully adapted its actuation frequency to reduce body bending and crawl through a narrow space, such as a tunnel. The study serves as a stepping stone for developing simple soft crawling robots capable of navigating cluttered and confined spaces autonomously.

The Tribological Adaptability for Ventral Scales of Dinodon rufozonatum in Dry/Wet/Rough Environments

The ventral scales of Dinodon rufozonatum were investigated to understand the outstanding tribological adaptability in various environments. The coefficient of friction (COF) of ventral scales was measured and changed with the contact conditions. It was discovered that the COF of scales under water-lubrication conditions (WLC) was larger than that under dry conditions (DC). More interestingly, the COF increased first and then decreased as the substrate roughness reduced. The abrasion marks on scales were then observed. The results indicated that the scales in DC wore more gently than that in WLC. Moreover, the degree of wear reduced with the decrease of substrate roughness. The frictional performance of ventral scales enabled the snakes to move more efficiently, quickly, and flexibly in multiple environments.

Functional consequences of convergently evolved microscopic skin features on snake locomotion

The small structures that decorate biological surfaces can significantly affect behavior, yet the diversity of animal-environment interactions essential for survival makes ascribing functions to structures challenging. Microscopic skin textures may be particularly important for snakes and other limbless locomotors, where substrate interactions are mediated solely through body contact. While previous studies have characterized ventral surface features of some snake species, the functional consequences of these textures are not fully understood. Here, we perform a comparative study, combining atomic force microscopy measurements with mathematical modeling to generate predictions that link microscopic textures to locomotor performance. We discover an evolutionary convergence in the ventral skin structures of a few sidewinding specialist vipers that inhabit sandy deserts-an isotropic texture that is distinct from the head-to-tail-oriented, micrometer-sized spikes observed on a phylogenetically broad sampling of nonsidewinding vipers and other snakes from diverse habitats and wide geographic range. A mathematical model that relates structural directionality to frictional anisotropy reveals that isotropy enhances movement during sidewinding, whereas anisotropy improves movement during slithering via lateral undulation of the body. Our results highlight how an integrated approach can provide quantitative predictions for structure-function relationships and insights into behavioral and evolutionary adaptations in biological systems.

Nanoimprint Replication of Biomimetic, Multilevel Undercut Nanostructures

The nanoimprint replication of biomimetic nanostructures can be interesting for a wide range of applications. We demonstrate the process chain for Morpho-blue-inspired nanostructures, which are especially challenging for the nanoimprint process, since they consist of multilayer undercut structures, which typically cannot be replicated using nanoimprint lithography. To achieve this, we used a specially made, proprietary imprint material to firstly allow successful stamp fabrication from an undercut master structure, and secondly to enable UV-based nanoimprinting using the same material. Nanoimprinting was performed on polymer substrates with stamps on polymer backplanes to be compatible with roller-based imprinting processes. We started with single layer undercut structures to finally show that it is possible to successfully replicate a multilayer undercut stamp from a multilayer undercut master and use this stamp to obtain multilayer undercut nanoimprinted samples.

Fabrication and friction characteristics of arbitrary biosurfaces

There are many different types of surfaces found in nature which can increase or reduce friction, such as the well-studied frog toe or lotus leaf. However, methods for replicating these surfaces on a large scale for use in industrial applications are needed in order to take advantage of this natural friction engineering. Most replication processes rely on molding that requires an input surface size comparable to the desired output surface. We present a novel approach of replicating large-scale biosurfaces using a laser scanning confocal microscope for surface digitization and 3D two-photon lithography for the fabrication of the digitized surface. Two different natural surfaces (banana skin and daffodil petal) were replicated. An intermediary tiling process was used to cover a target area of arbitrary size independent of the input texture size. The surfaces were coated with a thin layer of ZnO, and the frictional and wettability characteristics of the replicated surfaces were then examined, demonstrating significant friction reduction up to 42% and increased hydrophobicity due to the presence of texture.

Variation of the frictional anisotropy on ventral scales of snakes caused by nanoscale steps

The ventral scales of most snakes feature micron-sized fibril structures with nanoscale steps oriented towards the snake's tail. We examined these structures by microtribometry as well as atomic force microscopy (AFM) and observed that the nanoscale steps of the micro-fibrils cause a frictional anisotropy, which varies along the snake's body in dependence of the height of the nanoscale steps. A significant frictional behavior is detected when a sharp AFM tip scans the nanoscale steps up or down. Larger friction peaks appear during upward scans (tail to head direction), while considerably lower peaks are observed for downward scans (head to tail direction). This effect causes a frictional anisotropy on the nanoscale, i.e. friction along the head to tail direction is lower than in the opposite direction. The overall effect increases linearly with the step height of the micro-fibrils. Although the step heights are different for each snake, the general step height distribution along the body of the examined snakes follows a common pattern. The frictional anisotropy, induced by the step height distribution, is largest close to the tail, intermediate in the middle, and lower close to the head. This common distribution of frictional anisotropy suggests that snakes even optimized nanoscale features like the height of micro-fibrils through evolution in order to achieve optimal friction performance for locomotion. Finally, ventral snake scales are replicated by imprinting their micro-fibril structures into a polymer. As the natural prototype, the artificial surface exhibits frictional anisotropy in dependence of the respective step height. This feature is of high interest for the design of tribological surfaces with artificial frictional anisotropy.

Structure and Frictional Properties of the Leg Joint of the Beetle Pachnoda marginata (Scarabaeidae, Cetoniinae) as an Inspiration for Technical Joints

The efficient locomotion of insects is not only inspiring for control algorithms but also promises innovations for the reduction of friction in joints. After previous analysis of the leg kinematics and the topological characterization of the contacting joint surfaces in the beetle Pachnoda marginata, in the present paper, we report on the measurement of the coefficient of friction within the leg joints exhibiting an anisotropic frictional behavior in different sliding directions. In addition, the simulation of the mechanical behavior of a single microstructural element helped us to understand the interactions between the contact parts of this tribological system. These findings were partly transferred to a technical contact pair which is typical for such an application as joint connectors in the automotive field. This innovation helped to reduce the coefficient of friction under dry sliding conditions up to 17%.

In-Liquid Lateral Force Microscopy of Micropatterned Surfaces in a Fatty Acid Solution under Boundary Lubrication

This study aims to investigate the influence of surface morphology on boundary-lubricated friction in a stearic acid solution. The surface morphology was controlled by fabricating submicrometer line-and-space patterns on Si(100) surface via photolithography. The boundary-lubricated friction on the patterns was measured by in-liquid lateral force microscopy for both transverse and longitudinal ridges, with respect to the sliding direction; the highest friction was observed on longitudinal ridges and grooves, which is in agreement with the tendency observed in our previous friction studies on steel surfaces. To further investigate this phenomenon, some additional patterns having different submicrometer morphologies were prepared and their friction characteristics were investigated. On the patterns not allowing the fluid to flow along the grooves, the frictional forces were equivalent for transverse and longitudinal grooves and ridges. Therefore, the high friction observed on the longitudinal ridges was caused by flowing out of fluid along the grooves, and it was possible to conclude that the fluidity around the submicrometer ridges and grooves influences the friction-reducing effect of stearic acid in boundary lubrication regime.

Friction in the contact between skin and a soft counter material: Effects of hardness and surface finish

The interaction behaviour of skin with a counter surface depends strongly on the surface roughness of the counter surface. For relatively hard surfaces this effect is described in various literature, but for soft, or compliant, materials this is much less studied. Inside the contact, the protuberances on the surface will deform substantially. In order to gain insights into the effect of surface roughness and hardness on the frictional behaviour between skin and a soft counter surface a range of experiments were performed using artificial skin and various silicone compounds which are commonly used in medical devices that interact with the human skin. Using these results, a 'friction map' was created that shows the friction behaviour as a function of the elastic modulus and the surface roughness. When the surface roughness is increased the friction coefficient decreases due to the reduction in the real area of contact, which weakens the adhesion between the two surfaces. A minimum coefficient of friction was observed at a surface roughness of approximately 4 µm. For the softest compounds tested there was minimal effect of surface roughness on friction because the roughness protuberances inside the contact will be flattened. Silicone compounds with increased hardness showed a larger sensitivity of the friction to the surface roughness, because these harder surface roughness protuberances are more resistant against deformation. The friction map provides a tool when designing products that require certain frictional properties: for products that are required to adhere to skin a smooth and soft material is recommended, whereas for products that require a low coefficient of friction a harder compound with a surface roughness of approximately 4 µm is recommended.

Evidence of friction reduction in laterally graded materials

In many biological structures, optimized mechanical properties are obtained through complex structural organization involving multiple constituents, functional grading and hierarchical organization. In the case of biological surfaces, the possibility to modify the frictional and adhesive behaviour can also be achieved by exploiting a grading of the material properties. In this paper, we investigate this possibility by considering the frictional sliding of elastic surfaces in the presence of a spatial variation of the Young's modulus and the local friction coefficients. Using finite-element simulations and a two-dimensional spring-block model, we investigate how graded material properties affect the macroscopic frictional behaviour, in particular, static friction values and the transition from static to dynamic friction. The results suggest that the graded material properties can be exploited to reduce static friction with respect to the corresponding non-graded material and to tune it to desired values, opening possibilities for the design of bio-inspired surfaces with tailor-made tribological properties.

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.

Friction reduction through biologically inspired scale-like laser surface textures

Reducing friction forces is a major challenge in many engineering applications involving moving parts. For the past 50 years, the morphological texturing of surfaces for improving tribological properties has been investigated. Only recently, the application of biologically inspired surface features, like scales found on lizards and snakes, has come to the attention of tribologists. Here, we present results of the lubricated and unlubricated performance of biologically inspired scale-like textures applied with laser light to the surface of bearing steel pins. These were paired in unidirectional sliding against metallic (100Cr6), polymeric (PEEK) and ceramic (Al₂O₃) counter bodies. Additionally, a possible size effect was investigated by changing the scale diameter between 13 and 150 µm under dry sliding contact against sapphire. Our results demonstrate that depending on the contact conditions a biologically inspired surface morphology has the potential to reduce friction forces by more than 80%. However, under certain conditions, especially for slow-moving lubricated steel-on-steel and steel-on-ceramic contacts, these surface morphologies may increase friction as well. Similar to classical laser surface textures, such as round dimples, these biologically inspired morphologies need to be carefully optimized for each tribological system in which they are intended to be applied. There is no standard solution for all sliding conditions. The results presented here demonstrate that such efforts have the potential to yield significant reduction in friction forces and are expected to spark future research in the field of biologically inspired surface morphologies applied to tribological contacts.

Inversion of friction anisotropy in a bio-inspired asymmetrically structured surface

Friction anisotropy is an important property of many surfaces that usually facilitate the generation of motion in a preferred direction. Such surfaces are very common in biological systems and have been the templates for various bio-inspired materials with similar tribological properties. So far friction anisotropy is considered to be the result of an asymmetric arrangement of surface nano- and microstructures. However, here we show by using bio-inspired sawtooth-structured surfaces that the anisotropic friction properties are not only controlled by an asymmetric surface topography, but also by the ratio of the sample-substrate stiffness, the aspect ratio of surface structures, and by the substrate roughness. Systematically modifying these parameters, we were able to demonstrate a broad range of friction anisotropies, and for specific sample-substrate combinations even an inversion of the anisotropy. This result highlights the complex interrelation between the different material and topographical parameters on friction properties and sheds new light on the conventional design paradigm of tribological systems. Finally, this result is also of great importance for understanding functional principles of biological materials and surfaces, as such inversion of friction anisotropy may correlate with gait pattern and walking behaviour in climbing animals, which in turn may be used in robotic applications.

Surface structure and tribology of legless squamate reptiles

Squamate reptiles (around 10,000 species of snakes and lizards) comprise a myriad of distinct terrestrial vertebrates. The diversity within this biological group offers a great opportunity for customized bio-inspired solutions that address a variety of current technological problems especially within the realm of surface engineering and tribology. One subgroup within squamata is of interest in that context, namely the legless reptiles (mainly snakes and few lizards). The promise of that group lies within their functional adaptation as manifested in optimized surface designs and locomotion that is distinguished by economy of effort even when functioning within hostile tribological environments. Legless reptiles are spread over a wide range in the planet, this geographical diversity demands customized response to local habitats. Customization, in turn, is facilitated through specialized surface design features. In legless reptiles, micro elements of texture, their geometry and topological layout advance mitigation of frictional effects both in locomotion and in general function. Lately, the synergy between functional traits and intrinsic surface features has emerged as focus of research across disciplines. Many investigations have sought to characterize the structural as well as the tribological response of legless species from an engineering point of view. Despite the sizable amount of data that have accumulated in the literature over the past two decades or so, no effort to review the available information, whence this review. This manuscript, therefore, endeavors to assess available data on surface metrology and tribological behavior of legless reptiles and to define aspects of that performance necessary to formulate an advanced paradigm for bio-inspired surface engineering.

Numerical investigation of the tribological performance of micro-dimple textured surfaces under hydrodynamic lubrication

Surface texturing is an important approach for controlling the tribological behavior of friction pairs used in mechanical and biological engineering. In this study, by utilizing the method of three-dimensional computational fluid dynamics (CFD) simulation, the lubrication model of a friction pair with micro-dimple array was established based on the Navier-Stokes equations. The typical pressure distribution of the lubricant film was analyzed. It was found that a positive hydrodynamic pressure is generated in the convergent part of the micro-dimple, while a negative hydrodynamic pressure is generated in the divergent part. With suitable parameters, the total integration of the pressure is positive, which can increase the load-carrying capacity of a friction pair. The effects of the micro-dimple parameters as well as fluid properties on tribological performance were investigated. It was concluded that under the condition of hydrodynamic lubrication, the main mechanism for the improvement in the tribological performance is the combined effects of wedging and recirculation. Within the range of parameters investigated in this study, the optimum texture density is 13%, while the optimum aspect ratio varies with the Reynolds number. For a given Reynolds number, there exists a combination of texture density and aspect ratio at which the optimum tribological performance could be obtained. Conclusions from this study could be helpful for the design of texture parameters in mechanical friction components and even in artificial joints.

Hierarchical Spring-Block Model for Multiscale Friction Problems

A primary issue in biomaterials science is to design materials with ad hoc properties, depending on the specific application. Among these properties, friction is recognized as a fundamental aspect characterizing materials for many practical purposes. Recently, new and unexpected frictional properties have been obtained by exploiting hierarchical multiscale structures, inspired by those observed in many biological systems. In order to understand the emergent frictional behavior of these materials at the macroscale, it is fundamental to investigate their hierarchical structure, spanning across different length scales. In this article, we introduce a statistical multiscale approach, based on a one-dimensional formulation of the spring-block model, in which friction is modeled at each hierarchical scale through the classical Amontons-Coulomb force with statistical dispersion on the friction coefficients of the microscopic components. By means of numerical simulations, we deduce the global statistical distributions of the elementary structure at micrometric scale and use them as input distributions for the simulations at the next scale levels. We thus study the influence of microscopic artificial patterning on macroscopic friction coefficients. We show that it is possible to tune the friction properties of a hierarchical surface and provide some insight on the mechanisms involved at different length scales.

Static and dynamic friction of hierarchical surfaces

Hierarchical structures are very common in nature, but only recently have they been systematically studied in materials science, in order to understand the specific effects they can have on the mechanical properties of various systems. Structural hierarchy provides a way to tune and optimize macroscopic mechanical properties starting from simple base constituents and new materials are nowadays designed exploiting this possibility. This can be true also in the field of tribology. In this paper we study the effect of hierarchical patterned surfaces on the static and dynamic friction coefficients of an elastic material. Our results are obtained by means of numerical simulations using a one-dimensional spring-block model, which has previously been used to investigate various aspects of friction. Despite the simplicity of the model, we highlight some possible mechanisms that explain how hierarchical structures can significantly modify the friction coefficients of a material, providing a means to achieve tunability.

Mechanical interlocking of cotton fibers on slightly textured surfaces of metallic cylinders

Mechanical interlocking is widely applied in industry and general lives of human beings. In this work, we realized the control of locking or sliding states of cotton fibers on the metal surfaces with slightly different textures through traditional machining. Three types of sliding states, i.e., locking, one-way sliding, and two-way sliding have been achieved. It is found that the locking or sliding of the cotton fibers on the metallic cylinder depends on the friction coefficient and the ratio of cotton fiber diameter, 2r, to the height of the rough peaks, h, of metal surfaces. When the critical ratio h/r exceeds 1, the cotton fibers could tightly attach to the metallic surface through mechanical interlocking. This work provided a convenient and universal method for the control of interlocking or sliding of fiber-based materials on textured surfaces.

Bio-inspired scale-like surface textures and their tribological properties

Friction, wear and the associated energy dissipation are major challenges in all systems containing moving parts. Examples range from nanoelectromechanical systems over hip prosthesis to off-shore wind turbines. Bionic approaches have proven to be very successful in many engineering problems, while investigating the potential of a bio-inspired approach in creating morphological surface textures is a relatively new field of research. Here, we developed laser-created textures inspired by the scales found on the skin of snakes and certain lizards. We show that this bio-inspired surface morphology reduced dry sliding friction forces by more than 40%. In lubricated contacts the same morphology increased friction by a factor of three. Two different kinds of morphologies, one with completely overlapping scales and one with the scales arranged in individual rows, were chosen. In lubricated as well as unlubricated contacts, the surface texture with the scales in rows showed lower friction forces than the completely overlapping ones. We anticipate that these results could have significant impact in all dry sliding contacts, ranging from nanoelectromechanical and micro-positioning systems up to large-scale tribological contacts which cannot be lubricated, e.g. because they are employed in a vacuum environment.