Low-friction Surface Research Articles

Optimizing two-photon polymerization for rapid and high-resolution prototyping of low-friction microtextured surfaces

Abstract Low-friction surfaces enhance performance across various sectors, such as boosting fuel efficiency in transportation and augmenting precision, safety, and comfort in medical devices. Given these benefits, there is a pressing need to investigate how the geometric characteristics of microtextured surfaces can effectively reduce friction, a process that demands precise geometric control. This study explores the application of Two-Photon Polymerization (TPP) as a high-resolution additive manufacturing technique for prototyping microtextured surfaces used in friction studies. TPP’s ability to create precise microstructures makes it an ideal tool for understanding the impact of geometrical factors on friction. However, a significant trade-off exists between print time and quality, particularly for large samples. The TPP parameters, including laser power, scan speed, hatch, and slice distances, were fine-tuned to cater to friction application and achieve a balance between resolution and efficiency. A hybrid fill mode combining solid, shell and scaffold printing was developed to reduce print time while maintaining surface integrity. Friction tests demonstrated that the TPP-printed microdimpled samples effectively reduced friction, highlighting the potential of TPP as a prototyping tool for tribological applications.

Water-Enabled Electricity Generation by a Smooth Liquid-Like Semiconductor Coating Surface.

Water energy-converting techniques that focus on interfacial charge separation and transfer have aroused significant attention. However, the water-repelling nature leads to a less dense liquid layer and a sharp gradient of liquid velocity, which limits its output performance. Here, a water sliding generator (WSG) based on a smooth liquid-like/semiconductor surface (SLSS) is developed that harnesses the full advantage of liquid sliding friction. The prepared SLSS not only retained the slippery surface's close contact with liquid droplets and the characteristic of sliding without residue but also exhibited an enhanced friction effect on the low-friction surface. The smooth liquid-like/semiconductor surface water sliding generator (SLSS-WSG) exerts outstanding liquid sliding friction energy harvesting with high output (≈16V and ≈60µA) demonstrated, capability in series connection, dual operation of power generation and self-cleaning effect, and high physical and chemical stability (continuous current scour and sun exposure). The prepared surface can be integrated with photovoltaic panels, enabling them to generate electricity from water-sliding energy during rainy days, compensating for the reduced output of photovoltaic panels during overcast and rainy weather. Furthermore, it allows for energy collection even during rainy nights. The prepared surface can be potentially applied in various fields, showing great potential for the development of water-based clean energy.

Selective Plasma Treatment Endowing Low-Friction and Wear-Resistant Surface of Polycrystalline Diamond against Copper.

Despite its unrivaled hardness, polycrystalline diamond can be severely worn under the interaction with softer copper during the ultrafine copper wire drawing process. The influence of the polycrystalline diamond surface state on its friction behavior is investigated in this work, and argon, oxygen, and hydrogen plasma treatments are adopted to regulate the surface state of diamond. When sliding against copper, the original diamond plate exhibits a coefficient of friction (COF) exceeding 0.4, copper wears by transferring to the diamond, and diamond wears owing to the carbon cluster removal, as confirmed by the TOF-SIMS analyses of the obvious wear tracks. After hydrogen plasma treatment, the tribological performance of diamond sliding against copper is most significantly improved, in which the COF is more stable and reaches 0.18 with minimal wear debris. Hydrogen plasma treatment generates more hydrogen termination on the diamond surface as well as a hydrogen-containing amorphous carbon layer, which provides an isolated lubrication effect and strongly reduces adhesive wear. Argon plasma treatment also decreases the level of COF to a certain extent, which is attributed to its polishing and cleaning action that removes surface contamination and reduces roughness. In contrast, oxygen plasma treatment induces a pronounced etching effect, resulting in groove-like protrusions on the diamond surface that exacerbates abrasive wear. Our research provides a more sustainable and residue-free solution for minimizing friction and wear of polycrystalline diamond wire drawing dies.

Degradation of lubricating molecules in synovial fluid alters chondrocyte sensitivity to shear strain.

Articular joints facilitate motion and transfer loads to underlying bone through a combination of cartilage tissue and synovial fluid, which together generate a low-friction contact surface. Traumatic injury delivered to cartilage and the surrounding joint capsule causes secretion of proinflammatory cytokines by chondrocytes and the synovium, triggering cartilage matrix breakdown and impairing the ability of synovial fluid to lubricate the joint. Once these inflammatory processes become chronic, posttraumatic osteoarthritis (PTOA) development begins. However, the exact mechanism by which negative alterations to synovial fluid leads to PTOA pathogenesis is not fully understood. We hypothesize that removing the lubricating macromolecules from synovial fluid alters the relationship between mechanical loads and subsequent chondrocyte behavior in injured cartilage. To test this hypothesis, we utilized an ex vivo model of PTOA that involves subjecting cartilage explants to a single rapid impact followed by continuous articulation within a lubricating bath of either healthy synovial fluid, phosphate-buffered saline (PBS), synovial fluid treated with hyaluronidase, or synovial fluid treated with trypsin. These treatments degrade the main macromolecules attributed with providing synovial fluid with its lubricating properties; hyaluronic acid and lubricin. Explants were then bisected and fluorescently stained to assess global and depth-dependent cell death, caspase activity, and mitochondrial depolarization. Explants were tested via confocal elastography to determine the local shear strain profile generated in each lubricant. These results show that degrading hyaluronic acid or lubricin in synovial fluid significantly increases middle zone chondrocyte damage and shear strain loading magnitudes, while also altering chondrocyte sensitivity to loading.

Acceleration Slip Regulation Control Method for Distributed Electric Drive Vehicles under Icy and Snowy Road Conditions

To achieve a rapid and stable dynamic response of the drive anti-slip system for distributed electric vehicles on low-friction surfaces, this paper proposes an adaptive acceleration slip regulation control strategy based on wheel slip rate. An attachment coefficient fusion estimation algorithm based on an improved singular value decomposition unscented Kalman filter is designed. This algorithm combines Sage–Husa with the unscented Kalman filter for adaptive improvement, allowing for the quick and accurate determination of the road friction coefficient and, subsequently, the optimal slip rate. Additionally, a slip rate control strategy based on dynamic adaptive compensation sliding mode control is designed, which introduces a dynamic weight integral function into the control rate to adaptively adjust the integral effect based on errors, with its stability proven. To verify the performance of the road estimator and slip rate controller, a model is built with vehicle simulation software, and simulations are conducted. The results show that under icy and snowy road conditions, the designed estimator can reduce estimation errors and respond rapidly to sudden changes. Compared to traditional equivalent controllers, the designed controller can effectively reduce chattering, decrease overshoot, and shorten response time. Especially during road transitions, the designed controller demonstrates better dynamic performance and stability.

Growth of thick amorphous carbon films on alumina for improved tribological properties

Growth of thick amorphous carbon films on alumina for improved tribological properties

The influence of occupational footwear on slip responses

This study investigated how walking in occupational footwear (OF) affects slip outcome and slip recovery strategies in response to an unexpected slip. Participants walked along a walkway while either barefoot (BF; n = 13) or in OF (n = 12). The first five walking trials consisted of the no-slip condition, where a high friction sheet was placed halfway down the walkway to ensure a low probability of a slip. Prior to the sixth walking trial and without the participant’s knowledge, the sheet was replaced with a low friction surface to induce an unexpected slip. Results obtained during the no-slip trials indicated that compared to the BF group, the OF group walked with a 15% smaller required coefficient of friction (p = 0.04), a 12° greater dorsiflexion at heel strike (p = 0.007) and a 7° more extended knee position at 100 ms following heel strike (p = 0.008). The OF group also demonstrated greater electromyographic (EMG) activity in the hamstrings of the stance limb and the gastrocnemius of the contralateral limb prior to and after heel strike. When the low friction surface was unexpectedly encountered, a slip was induced in both groups. But compared to the BF group, the OF group experienced a less severe slip, with the slip being 42% shorter (p = 0.004) and 75% slower (p < 0.001), and exhibited a 19–71% lesser EMG activation when responding to the slip. Although these results provide initial evidence for the benefits of wearing OF to minimise the consequence of a slip in the workplace, further research is needed to determine whether the altered walking patterns associated with wearing OF (e.g., greater dorsiflexion, reduced knee range of motion, etc.) may have contributed to the decrease in slip severity.

A Comprehensive Numerical Study of a Wedge-Shaped Textured Convergent Oil Film Gap

The modification of surface geometries to reduce friction is an omnipresent topic of research. In nature, different low-friction surfaces, such as fish skins, exist. To transfer this knowledge to technical applications, for example, to journal or plain bearings, many numerical and experimental studies of textured surfaces have been performed. In this work, the influence of the geometric parameters (texture length l, width b, angle α and start position xstart) of a wedge-shaped texture on three different convergent oil film gaps was analyzed in full-film lubrication and compared with untextured oil film gaps. With the aid of a CFD (computational fluid dynamics) model, a comprehensive variation study was conducted, and the best-performing wedge-shaped texture was determined. The results show that an open texture at the inlet provides the largest improvement. Furthermore, it can be observed that the optimal relative texture width and absolute inlet height for the three investigated oil film gaps are similar. In contrast to the volume flow of the untextured geometry, the volume flow of the textured one is significantly higher, especially that perpendicular to the movement direction.

The influence from PTFE on surface and sub-surface damages of glass fiber reinforced PPS

The influence from PTFE on surface and sub-surface damages of glass fiber reinforced PPS

Enhanced Vehicle Dynamics and Safety through Tire–Road Friction Estimation for Predictive ELSD Control under Various Conditions of General Racing Tracks

This study focuses on the tire–road friction estimation for the predictive control strategy of electronically limited slip differential (ELSD) to improve the handling and acceleration performance of front-wheel drive cars, which typically suffer from excessive understeer and inner drive wheel spin during acceleration while turning due to reduced vertical load on the wheel. To mitigate this, we propose a control logic for ELSD that enhances course followability and acceleration by pre-transferring the driving torque from the inside to the outside wheel, considering the estimated traction potential for rapid response. It is essential to improve the control accuracy of wheel spin prediction by predicting the friction coefficient of the road surface. Furthermore, this study extends to the analysis of vehicle dynamics during lane-change maneuvers on low-friction surfaces, emphasizing the role of accurate tire–road friction estimation in vehicle safety. A CarSim 2023-based simulation study was conducted to investigate the vehicle response on snowy roads with low friction coefficients (μ = 0.2) and low temperatures (−5 °C). The results demonstrated that even minimal steering input could result in significant side-slip angles, highlighting the nonlinear vehicle behavior and the critical need for robust traction estimation in such challenging conditions of general racing tracks. The proposed friction-estimation method was evaluated through vehicle testing and has been substantiated by patents for its originality in control and friction-estimation approaches. The outcomes of these combined methodologies underline the critical importance of tire–road friction coefficient estimation in both the effectiveness of the ELSD system and the broader context of active safety systems.

The Effect of Electroless Nickel–Polytetrafluoroethylene Coating on the Frictional Properties of Orthodontic Wires

In orthodontic treatment, to achieve efficient tooth movement, it is important to reduce the frictional force between the wire and the bracket, especially the binding friction that occurs when the angle between the wire and the bracket is large. Electroless nickel–polytetrafluoroethylene (Ni-PTFE) coating is a coating technology used to deposit PTFE particles with a low coefficient of friction on the coating surface to provide a low-friction surface for metallic materials. The purpose of this study was to investigate the effect of Ni-PTFE-coated orthodontic wires on the frictional force between brackets. The surface morphology, surface roughness, and frictional properties of Ni-PTFE-coated stainless steel wires and Ni-Ti wires were evaluated. The results demonstrate that the Ni-PTFE coating reduced the frictional force between the orthodontic wires and brackets, despite the increased surface roughness. Even when the angle between the wire and bracket was increased, assuming binding friction, the frictional force was reduced by the Ni-PTFE coating. This suggests that the friction between the wire and the bracket was suppressed by the PTFE particles deposited on the wire surface in contact with the bracket.

Path Tracking Control with Constraint on Tire Slip Angles under Low-Friction Road Conditions

This paper presents a method to design a path tracking controller with a constraint on tire slip angles under low-friction road conditions. On a low-friction road surface, a lateral tire force is easily saturated and decreases as a tire slip angle increases by a large steering angle. Under this situation, a path tracking controller cannot achieve its maximum performance. To cope with this problem, it is necessary to limit tire slip angles to a value where the maximum lateral tire force is achieved. The most commonly used controllers for path tracking, linear quadratic regulator (LQR) and model predictive control (MPC), are adopted as a controller design methodology. The control inputs of LQR and MPC are front and rear steering angles and control yaw moment, which have been widely used for path tracking. The constraint derived from tire slip angles is imposed on the steering angles of LQR and MPC. To fully verify the performance of the path tracking controller with the constraint on tire slip angles, a simulation is conducted on vehicle simulation software. From the simulation results, it is shown that the path tracking controller with the constraint on tire slip angles presented in this paper is quite effective for path tracking on low-friction road surface.

Polymer carbon nitride nanosheet-based lamellar membranes inspired by "couple hardness with softness" for ultrafast molecular separation in organic solvents.

Membranes with ultrafast molecular separation ability in organic solvents can offer unprecedented opportunities for efficient and low-cost solvent recovery in industry. Herein, a graphene-like polymer carbon nitride nanosheet (PCNN) with a low-friction surface was applied as the main membrane building block to boost the ultrafast transport of the solvent. Meanwhile, inspired by the concept of "couple hardness with softness", soft and flexible graphene oxide (GO) was chosen to fix the random stack of the rigid PCNN and tailor the lamellar structure of the PCNN membrane. The optimal PCNN/GO lamellar membrane shows a remarkable methanol permeance of 435.5 L m-2 h-1 bar-1 (four times higher than that of the GO membrane) while maintaining a high rejection for reactive black (RB, 98.9% in ethanol). Molecular dynamics simulations were conducted to elucidate the ultrafast transport mechanism of the PCNN/GO membrane. This study reveals that PCNN is a promising building block for lamellar membranes and may open up new avenues for high-performance molecular separation membranes.

Hypotrochoidal scaffolds for cartilage regeneration

The main function of articular cartilage is to provide a low friction surface and protect the underlying subchondral bone. The extracellular matrix composition of articular cartilage mainly consists of glycosaminoglycans and collagen type II. Specifically, collagen type II fibers have an arch-like organization that can be mimicked with segments of a hypotrochoidal curve. In this study, a script was developed that allowed the fabrication of scaffolds with a hypotrochoidal design. This design was investigated and compared to a regular 0-90 woodpile design. The mechanical analyses revealed that the hypotrochoidal design had a lower component Young's modulus while the toughness and strain at yield were higher compared to the woodpile design. Fatigue tests showed that the hypotrochoidal design lost more energy per cycle due to the damping effect of the unique microarchitecture. In addition, data from cell culture under dynamic stimulation demonstrated that the collagen type II deposition was improved and collagen type X reduced in the hypotrochoidal design. Finally, Alcian blue staining revealed that the areas where the stress was higher during the stimulation produced more glycosaminoglycans. Our results highlight a new and simple scaffold design based on hypotrochoidal curves that could be used for cartilage tissue engineering.

Characterization of nanocomposite tetrahedral amorphous carbon coated drill bits for improved drilled-hole quality in carbon fiber-reinforced polymer/Al7075-T6 stacks

The use of hybrid composite stacks is kept on increasing in the aircraft industry due to their numerous advantages in this sector. Therefore, improving the drilled-hole quality is also sought over time. The present work focuses on the effect of metallic elements-doped tetrahedral amorphous carbon coated drill bits on the drilled-hole quality while performing single-shot drilling of CFRP/Al7075-T6 stacks. This was evaluated from CFRP exit delamination and Al7075-T6 exit burr height. Mechanical and tribological properties of three different tetrahedral amorphous carbon (ta-C) coating-types, i.e. micro-, chromium-doped (:Cr) and titanium-doped (:Ti) were compared to uncoated tool ones. Bonding strength of the selected coatings with the tool was also calculated. Process capability six pack analysis was used to validate the experimental results obtained for drilled-hole quality. It was shown that micro-ta-C coating-type exhibited maximum hardness. The hardness value directly influences the exit delamination. It was found that CFRP exit delamination decreased as the hardness of the drill bit increased. Moreover, the multi-material stacks drilled with ta-C:Cr coated tool presented the lowest burr height (less than 150 µm), compared to the other tested coating-types. Low-friction surfaces seem to promote low exit burr height.

Preparation and Properties of PDMS Surface Coating for Ultra-Low Friction Characteristics.

Polydimethylsiloxane (PDMS) has excellent physical-chemical properties and good biocompatibility. Thus, PDMS has been widely applied in biomedical applications. However, the low surface free energy and surface hydrophobicity of PDMS can easily lead to adverse symptoms, such as tissue damage and ulceration, during medical treatment. Therefore, the construction of a hydrophilic low-friction surface on the PDMS surface could be helpful for alleviating patient discomfort and would be of great significance for broadening the application of PDMS in the field of interventional medical catheters. Existing surface modification methods such as hydrogel coatings and chemical grafting suffer from several deficiencies including uncontrollable thickness, surface fragility, and low surface strength. In this study, a hydrophilic surface with ultra-low friction properties was prepared on the surface of PDMS by an ultraviolet light (UV) curing method. The monomer acrylamide (AM) was induced by a photoinitiator to form a coating on the surface of the silicone rubber by in situ polymerization. The surface roughness of the as-prepared coatings was regulated by adding different concentrations of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to the monomer solution, and the coating properties were systematically characterized. The results indicated that the roughness and thickness of the as-prepared coatings decreased with increasing AMPS concentration and the as-prepared coatings had good hydrophilicity and low-friction properties. The Coefficient of Friction (CoF) was as low as 0.0075 in the deionized water solution, which was 99.7% lower than that of the unmodified PDMS surface. Moreover, the coating with a lower surface roughness exhibited better low-friction properties. The results reported herein provide new insight into the preparation of hydrophilic, low-friction coatings on polymer surfaces.

Assessment of magnetic field interactions and heating for cerebral aneurysm flow diverters during 7T MRI.

Flow diverters (FDs) are utilized for a wide range of aneurysms, but show safety issues such as adverse interactions with static magnetic fields (displacement force and torque) and radiofrequency-induced heating during magnetic resonance imaging (MRI). The present study aimed to assess these adverse interactions in a 7-tesla (7T) static magnetic field and radiofrequency-induced heating during a 7T MRI for two types of FD. Displacement force and magnetically induced torque were assessed using the deflection angle method and low friction surface method, respectively. To assess heating, each FD was set in a phantom filled with gelled-saline mixed with polyacrylic acid and underwent a 7T MRI using a three-dimensional fast spin echo method. Displacement force and magnetically induced torque in the 7T static magnetic field were undetectable, and radiofrequency-induced heating during 7T MRI remained ≤ 0.6 °C for both types of FD, suggesting that magnetic field interactions and heating on FDs during a 7T MRI are acceptable from a safety perspective.

Lubricant skin on diverse biomaterials with complex shapes via polydopamine-mediated surface functionalization for biomedical applications.

Implantable biomedical devices require an anti-biofouling, mechanically robust, low friction surface for a prolonged lifespan and improved performance. However, there exist no methods that could provide uniform and effective coatings for medical devices with complex shapes and materials to prevent immune-related side effects and thrombosis when they encounter biological tissues. Here, we report a lubricant skin (L-skin), a coating method based on the application of thin layers of bio-adhesive and lubricant-swellable perfluoropolymer that impart anti-biofouling, frictionless, robust, and heat-mediated self-healing properties. We demonstrate biocompatible, mechanically robust, and sterilization-safe L-skin in applications of bioprinting, microfluidics, catheter, and long and narrow medical tubing. We envision that diverse applications of L-skin improve device longevity, as well as anti-biofouling attributes in biomedical devices with complex shapes and material compositions.

Comparing a friction-based uni-directional treadmill and a slip-style omni-directional treadmill on first-time HMD-VR user task performance, cybersickness, postural sway, posture angle, ease of use, enjoyment, and effort

Comparing a friction-based uni-directional treadmill and a slip-style omni-directional treadmill on first-time HMD-VR user task performance, cybersickness, postural sway, posture angle, ease of use, enjoyment, and effort

Boxes-Based Representation and Data Sharing of Road Surface Friction for CAVs

Vehicles can easily lose control unexpectedly when encountering unforeseen hazardous road friction conditions. With automation and connectivity increasingly available to assist drivers, vehicle performance can significantly benefit from a road friction preview map, particularly to identify where and how friction ahead of a vehicle may be suddenly decreasing. Although many techniques enable the vehicle to measure the local friction as driving upon a surface, these encounters limit the ability of a vehicle to slow down before a low-friction surface is already encountered. Using the connectivity of connected and autonomous vehicles (CAVs), a global road friction map can be created by aggregating information from vehicles. A challenge in the creation of these global friction maps is the very large quantity of data involved, and that the measurements populating the map are generated by vehicle trajectories that do not uniformly cover the grid. This paper presents a road friction map generation strategy that aggregates the measured road-tire friction coefficients along the individual trajectories of CAVs into a road surface grid. In addition, through clustering the friction grids further, an insight of this work is that the friction map can be represented compactly by rectangular boxes defined by a pair of corner coordinates in space, a friction value, and a confidence interval within the box. To demonstrate the method, a simulation is presented that integrates traffic simulations, vehicle dynamics and on-vehicle friction estimators, and a highway road surface, where friction is changing in space, particularly over a bridge segment. The experimental results indicate that the road friction distribution can be measured effectively by collecting and aggregating the friction data from CAVs.