Kinetic Friction Force Research Articles

Persistence of Structural Lubricity on Contaminated Graphite: Rejuvenation, Aging, and Friction Switches.

Using atomic force microscopy experiments and molecular dynamics simulations of gold nanoislands on graphite, we investigate why ultralow friction commonly associated with structural lubricity can be observed even under ambient conditions. Measurements conducted within a few days after sample synthesis reveal previously undiscovered phenomena in structurally lubric systems: rejuvenation, a drop in kinetic friction of an order of magnitude shortly after the onset of sliding; aging, a significant increase in kinetic friction forces after a rest period of 30 min or more; and switches, spontaneous jumps between distinct friction branches. These three effects are drastically suppressed a few weeks later. Imaging of a contamination layer and simulations provide a consistent picture of how single- and double-layer contamination underneath the gold nanoislands as well as contamination surrounding the nanoislands affect structural lubricity without leading to its breakdown.

Analysis Of The Effect Of Mass Variation On The Coefficient Of Friction On Wood

The purpose of this study is to determine the coefficients of kinetic and static friction in order to identify areas where we can reduce unwanted friction, thereby enhancing the performance and efficiency of various devices or machines we use daily. This research employs a quantitative approach with an experimental design or practical activities conducted in a laboratory or controlled environment, where specific variables can be accurately measured. The collected data will be analyzed in numerical or statistical form. The values indicate that the kinetic friction coefficient between wood and wood is the smallest, at 0.14. The kinetic friction coefficient between metal and wood ranges from 0.28 to 0.30. This means that the kinetic friction force acting on a wooden block on a wooden surface is the smallest compared to other materials. The static friction coefficient between wood and wood is the highest, at 0.40. The static friction coefficient between metal and wood ranges from 0.23 to 0.32. This indicates that the static friction force required to initiate the movement of a metal object on a wooden surface varies depending on the mass of the metal.

The Role of Friction in Athletic Performance: A Descriptive Analysis

Friction is a fundamental force that has a substantial impact on athletic performance across all sports disciplines. This descriptive research investigates the varied impact of friction in building athletic skill. This study looks at how friction affects running, jumping, throwing, and other sports activities by examining static and kinetic frictional forces, as well as elements that influence frictional interactions. The research examines the interaction of athletes, footwear, surfaces, and equipment to explain how friction governs propulsion, stability, and efficiency in sports activity. The findings of this study have important significance for players, coaches, and sports scientists, as they provide avenues for optimizing training techniques, equipment design, and performance tactics for increased athletic achievement. Keywords Friction , Athletic Performance , Role , Descriptive Analysis , Surface Friction , Footwear Friction , Sports Science , Biomechanics , Performance Optimization , Surface Interaction , Traction , Stability , Movement Efficiency , Injury Prevention ,Training Strategies

Static and Kinetic Friction of 3D Printed Polymers and Composites

The work examines the characteristics of the static and kinetic friction of five types of polymer materials and composites obtained by 3D printing, which are united into 2 groups: group A: PLA, Tough PLA (Technical Polylactic acid), and SteelFill (PLA with stainless steel powder approx. w80%15) and group B: PETG (Polyethylene terephthalate): PETG and CarbonFilTM (PETG with carbon fibers). Results were obtained and analyzed for the variation of the static and kinetic friction force, the static and kinetic coefficient of friction (COF), and the difference between the static and kinetic COF (a jump in COF, known as Stick-slip) from the magnitude of the normal load in two cases of contact with a steel counter-body - without lubricant and with SAE15W40 oil.

Development of a piezoelectric hybrid-driven nanopositioner using combined leaf-spring-shaped and C-shaped flexure hinge mechanism

This paper presents a novel piezoelectric hybrid-driven nanopositioner using combined leaf-spring-shaped and C-shaped flexure hinge mechanism. The piezoelectric hybrid-driven nanopositioner combines the piezoelectric stick-slip-driven nanopositioner and piezoelectric scanner in a single and compact device. This advance can decrease the size of nanoscratch-AFM hybrid system and make nanoscratch-AFM hybrid system compatible with the limited space of SEM vacuum chamber. A flexure hinge combining leaf-spring-shaped mechanism and C-shaped mechanism is developed to improve the load capability of piezoelectric stick-slip-driven nanopositioner. Unlike existing methods of improving load-capability by decreasing kinetic friction force value, the proposed flexure hinge employs C-shaped structure to decrease the retraction motion time of kinetic friction force, which can achieve high-load and compact structure simultaneously. Finite element analysis is implemented to optimize the thickness of the leaf-spring-shaped mechanism and diameter of the C-shaped mechanism. A prototype is fabricated and its experimental system is established. The mechanical output experiments show that the piezoelectric scanner achieved a maximum travel range of 4.9 μm, and piezoelectric stick-slip-driven nanopositionerm achieves a maximum load of 2 Kg. Experimental results of the nanoscratch-AFM hybrid system inside a standard SEM HITACHI SU5000 demonstrate that the proposed piezoelectric hybrid-driven nanopositioner is capable of nanoscratch positioning.

Unambiguous coupling between acoustic and electromagnetic emissions in plastically deformed crystals

An evident coupling between acoustic (AE) and electromagnetic (EME) emissions has been proved experimentally during plastic deformation of LiF ionic monocrystals under uniaxial compression with simultaneous recording of both AE and EME. The strong correlation between AE and EME demonstrate clearly that the observed EME is caused by dynamical dislocations and charged vacancies in the ionic lattice during work hardening. The theoretical interpretation proposed to explain the observable EME is based on the well-known Stepanov effect that means sweeping-up the charged vacancies of a preferable sign by gliding edge dislocations and formation of charged Cottrell clouds. During work hardening dislocation pile-ups are formed, and a certain nonequilibrium charge density is accumulated at their heads, resulting to the dynamic electric polarization of the deformed crystal. As the external loading increases, a locked dislocation pile-up bursts through the stoppers and quickly loses its bound charge. The relaxation of this charge produces intrinsic polarization currents generating electric pulses strongly correlated with dynamic dislocation process during plastic deformation. To build the theoretical model, it is assumed that the relaxation current can be described as an athermic viscous motion of vacancies under the kinetic friction force ∼Bυ (B is the friction coefficient and υ is the vacancy velocity) in a self-consistent electric field determined by the distribution of the total charge density. The electrical signal generated by an acting slip system has been calculated. By comparing the calculated and experimentally measured electric signal patterns, the friction coefficient for the linear chain of vacancies (the analogue of an edge dislocation extra-plane) in LiF has been estimated to be B≃ 0.9⋅10–5 g cm–1⋅s–1. This value is in accordance with the corresponding coefficient for dislocations in ionic lattices.

Cutting characteristics of viscoelastic membranes under hypodermic needle insertion

Cutting characteristics of thin membranes and the mechanics of their interactions with sharp aggressors like needles are critical in many industrial and medical applications. In this study, we performed needle insertion experiments to study the cutting behaviour of membranes made with polyethylene (PE) and nitrile butadiene rubber (NBR) and investigated different stages of the procedure by analyzing the needle reaction force. Moreover, using a detailed parametric study, we investigated the effect of different insertion and membrane parameters on the most important features of the insertion process. The results showed that, in most cases, the crack size, maximum insertion force, puncture displacement, kinetic frictional force, and static frictional force increase with the increase of needle diameter, increase of membrane thickness, or inclining of the needle. On the other hand, the effect of the velocity on features of the insertion process is negligible except for the kinetic frictional force which increases with the insertion velocity. We also showed although lubrication cannot considerably change the maximum needle insertion force into thin membranes, it can substantially decrease the puncture displacement and frictional forces. Furthermore, we observed that the maximum insertion force, puncture displacement, and frictional forces for PE are greater than the values obtained for NBR, while the crack sizes obtained for PE are smaller than those for NBR. Additionally, we proposed a combined experimental-numerical approach using finite-element (FE) modelling to calculate fracture toughness. The fracture toughness values obtained for PE and NBR are 6.66 ± 1.16 kJ/m2 and 3.98 ± 0.56 kJ/m2, respectively. We also exploited the developed finite-element model to predict the whole-field membrane deformation and its mechanical stress.

Friction force-based measurements for simultaneous determination of the wetting properties and stability of superhydrophobic surfaces

HypothesisContact angle and sliding angle measurements are widely used to characterize superhydrophobic surfaces because of the simplicity and accessibility of the technique. We hypothesize that dynamic friction measurements, with increasing pre-loads, between a water drop and a superhydrophobic surface is more accurate because this technique is less influenced by local surface inhomogeneities and temporal surface changes. ExperimentsA water drop, held by a ring probe which is connected to a dual-axis force sensor, is sheared against a superhydrophobic surface while maintaining a constant preload. From this force-based technique, static and kinetic friction forces measurements are used to characterize the wetting properties of the superhydrophobic surfaces. Furthermore, by applying increased pre-loads to the water drop while shearing, the critical load at which the drop transitions from the Cassie-Baxter to Wenzel state is also measured. FindingsThe force-based technique predicts sliding angles with reduced standard deviations (between 56 and 64%) compared to conventional optical-based measurements. Kinetic friction force measurements show a higher accuracy (between 35 and 80%) compared to static friction force measurements in characterizing the wetting properties of superhydrophobic surfaces. The critical loads for the Cassie-Baxter to Wenzel state transition allows for stability characterization between seemingly similar superhydrophobic surfaces.

Force-Based Characterization of the Wetting Properties of LDPE Surfaces Treated with CF4 and H2 Plasmas.

Low-density polyethylene (LDPE) films are widely used in packaging, insulation and many other commodity applications due to their excellent mechanical and chemical properties. However, the water-wetting and water-repellant properties of these films are insufficient for certain applications. In this study, bare LDPE and textured LDPE (T-LDPE) films were subjected to low-pressure plasmas, such as carbon tetrafluoride (CF4) and hydrogen (H2), to see the effect of plasma treatment on the wetting properties of LDPE films. In addition, the surface of the LDPE film was textured to improve the hydrophobicity through the lotus effect. The LDPE and T-LDPE films had contact angle (θ) values of 98.6° ± 0.6 and 143.6° ± 1.0, respectively. After CF4 plasma treatments, the θ values of the surfaces increased for both surfaces, albeit within the standard deviation for the T-LDPE film. On the other hand, the contact angle values after H2 plasma treatment decreased for both surfaces. The surface energy measurements supported the changes in the contact angle values: exposure to H2 plasma decreased the contact angle, while exposure to CF4 plasma increased the contact angle. Kinetic friction force measurements of water drops on LDPE and T-LDPE films showed a decrease in friction after the CF4 plasma treatment, consistent with the contact angle and surface energy measurements. Notably, the kinetic friction force measurements proved to be more sensitive compared to the contact angle measurements in differentiating the wetting properties of the T-LDPE versus 3× CF4-plasma-treated LDPE films. Based on Atomic Force Microscopy (AFM) images of the flat LDPE samples, the 3× CF4 plasma treatment did not significantly change the surface morphology or roughness. However, in the case of the T-LDPE samples, Scanning Electron Microscopy (SEM) images showed noticeable morphological changes, which were more significant at sharp edges of the surface structures.

How droplets pin on solid surfaces

HypothesisWhen a droplet starts sliding on a solid surface, the droplet-solid friction force develops in a manner comparable to the solid–solid friction force, showing a static regime and a kinetic regime. Today, the kinetic friction force that acts on a sliding droplet is well-characterized. But the mechanism underlying the static friction force is still less understood. Here we hypothesize that we can further draw an analogy between the detailed droplet-solid and solid–solid friction law, i.e., the static friction force is contact area dependent. MethodsWe deconstruct a complex surface defect into three primary surface defects (atomic structure, topographical defect, and chemical heterogeneity). Using large-scale Molecular Dynamics simulations, we study the mechanisms of droplet-solid static friction forces induced by primary surface defects. FindingsThree element-wise static friction forces related to primary surface defects are revealed and the corresponding mechanisms for the static friction force are disclosed. We find that the static friction force induced by chemical heterogeneity is contact line length dependent, while the static friction force induced by atomic structure and topographical defect is contact area dependent. Moreover, the latter causes energy dissipation and leads to a wiggle movement of the droplet during the static-kinetic friction transition.

Towards neural network‐based numerical friction models

AbstractFriction contacts can be found in almost all mechanical systems and are often of great technical importance. However, they are usually difficult to describe, and their behavior and influence on the whole system are hard to predict accurately. Modern product design and system operation strongly benefit from numerical simulation approaches today, but reliable friction models still represent a major challenge in this context.To tackle this problem, we employ neural network regression to capture the characteristics of frictional contacts and make them accessible for numerical methods in a minimal intrusive fashion. In particular, we test our approach using a Finite Element model of a 2D cantilever beam subject to stick‐slip vibrations induced by a moving conveyor belt at its free end. As a reference solution, we perform a transient analysis based on a simple analytical friction model, where the kinetic friction force only depends on the normal load and the relative sliding velocity. We take the same friction model, add some artificial noise to mimic uncertainties coming with experimental measurements, and pick a limited set of data points to train a regression neural network. The machine learning friction model is then deployed in the Finite Element code to predict the kinetic friction force acting on the beam tip during the slip phases.The deflection curves obtained by the transient numerical analysis using the new neural network friction model agree well with the reference solution based on the underlying analytical model. The results indicate that data‐driven approaches may also be capable of capturing more complex frictional contacts, including effects of temperature, humidity, and load history. The trained neural network friction models can then be employed in numerical simulations in a minimally intrusive manner. This approach opens up new possibilities to predict individual mechanical system behavior as accurately as possible.

Pulling a Spool

A simple model for the kinetic frictional force on an object sliding across a surface is adopted in introductory physics. Its magnitude is given by a constant coefficient of kinetic friction multiplied by the normal force exerted on the object by the surface, and the force points opposite the direction that the object slides relative to the surface. In contrast, the standard model for the static frictional force is more complicated. There is no formula for its magnitude, but instead it can vary from zero up to some maximum value, as required to keep the point of contact of the object at rest relative to the surface it is on. Furthermore, it points in the direction opposite the way in which the object would slip at its point of contact if there were zero friction, but it is not always obvious which way that is. In an attempt to bypass these complications, students can make erroneous assumptions about the magnitude and direction of the static frictional force. To counter them, it is helpful to present some example that shows students that those assumptions need not hold. Such a system is a pulled spool that rolls without slipping on a horizontal table. Since the point of contact between the spool and the table does not slip, any frictional force between them must be static.

The drift motion of a spinning ball on carpet

A ball that rolls on carpet while also spinning around a vertical axis will experience a drift force that acts perpendicular to its velocity, opposite to the tangential velocity component of the front point of the ball. Here, we present a model of this motion based on three assumptions: the ball rolls without slipping around the point of contact directly below its center of mass; the ball experiences rolling resistance due to a forward-shifted normal force; and the ball experiences a forward-shifted kinetic friction (drift) force. This model produces a simple analytic solution that is consistent with experimental data. Our measurements suggest that the kinetic friction force acts near the front of the contact patch between the ball and carpet.

The Effectiveness of Standard Friction Models in Predicting the Behavior of Micropatterned Surfaces

Computational modeling methods were used to explore how well the behavior of a surface with a micropatterned array of uniformly shaped and spaced semi-cylindrical ribs, as predicted through a deterministic model, may be represented using a traditional Coulomb-based bulk-effects friction model. The effects of the numerical solution method, contact enforcement method, material damage model, and the number of asperities considered were first examined when the micropatterned ribs were directly included in the computational domain. The tribological behavior, defined as the static and kinetic friction forces and the associated energy dissipated, was then recreated for a comparable smooth-surface system using a Coulomb-based bulk-effects friction model, exploring the influence of user-input parameters such as the friction coefficients. With properly selected bulk-effects model parameters, the tribological behavior could be matched between the two types of models. However, the bulk-effects model could not capture the local and time-dependent effects of the asperity interactions on the force and energy measures, which are important in designing micropatterned surfaces. Through the understanding of the influences on model function that is gained through this work, a means to determine the appropriateness of each of these interface model types in studying particular phenomena of interest is provided.

Tuning static drop friction

AbstractThe friction force opposing the onset of motion of a drop on a solid surface is typically considered to be a material property for a fixed drop volume on a given surface. However, here we show that even for a fixed drop volume, the static friction force can be tuned by over 30% by preshaping the drop. The static friction usually exceeds the kinetic friction that the drop experiences when moving in a steady state. Both forces converge when the drop is prestretched in the direction of motion or when the drop shows low contact angle hysteresis. In contrast to static friction, kinetic friction is independent of preshaping the drop, that is, the drop history. Kinetic friction forces reflect the material properties.

Anisotropic Wettability of Bioinspired Surface Characterized by Friction Force.

Bioinspired surfaces with special wettabilities attract increasing attention due to their extensive applications in many fields. However, the characterizations of surface wettability by contact angle (CA) and sliding angle (SA) have clear drawbacks. Here, by using an array of triangular micropillars (ATM) prepared by soft lithography, the merits of measuring the friction force of a water droplet on ATM over measurements of CA and SA in characterizing the surface wettability are demonstrated. The CA and SA measurements show ignorable differences in the wettabilities of ATM in opposite directions (1.13%) and that with different periodic parameters under the elongation ranging from 0 to 70%. In contrast, the friction measurement reveals a difference of > 10% in opposite directions. Moreover, the friction force shows a strong dependence on the periodic parameters which is regulated by mechanical stretching. Increasing the elongation from 0 to 50% increases the static and kinetic friction force up to 23.0% and 22.9%, respectively. Moreover, the stick-slip pattern during kinetic friction can reveal the periodic features of the measured surface. The friction force measurement is a sensitive technique that could find applications in the characterization of surface wettabilities.

Effects of Two Fluoride Mouthwashes on Surface Topography and Frictional Resistance of Orthodontic Wires.

Objectives: This study compared the effects of fluoride mouthwashes on surface topography of orthodontic wires, and static and kinetic frictional forces between the stainless steel (SS) orthodontic brackets and SS and nickel-titanium (NiTi) archwires. Materials and Methods: This in vitro, study evaluated 240 standard SS maxillary central incisor brackets and 0.018, and 0.025×0.019 inch NiTi and SS archwires. The wire-bracket sets (different combinations of wire diameters and types) were exposed to artificial saliva (control), 0.05% sodium fluoride (NaF) for 1 minute daily, or 0.2% NaF for 1 minute, weekly (37°C) for 3 months. The wires were pulled in bracket slots by 5 mm in a universal testing machine (10 mm/minute). The static and kinetic forces were measured. The surface topography of wires was inspected under a scanning electron microscope (SEM; x500). Data were analyzed by SPSS 25. Results: The mean static and kinetic frictional forces of 0.025×0.019 inch NiTi wire in 0.05% NaF group were significantly greater than SS wire (P=0.000). The mean kinetic frictional force in 0.05% NaF was significantly greater than 0.2% NaF and artificial saliva for all wires (P=0.001). The mean static and kinetic forces in 0.2% NaF were significantly greater than in artificial saliva (P=0.025). In all groups, larger wires showed higher mean frictional forces (P=0.000). SEM results revealed higher wire surface roughness in 0.05% NaF followed by 0.2% NaF group. Conclusion: Weekly use of 0.2% NaF mouthwash is recommended during sliding mechanics to minimize frictional forces between the SS and NiTi wires and SS brackets.

Deployment Dynamics of a Tape-Shaped Tethered Satellite

In comparison with conventional thin-cylindrical tethers, tape-shaped tethers have obvious advantages in orbital applications such as electron collection, satellite deorbit, and reliability. There are many differences between them not only in appearance but also in dynamics. Hence, this paper studies the dynamics of a tape-shaped flexible tethered satellite during free deployment. First, an approximate model for the system, in which the tape-shaped tether is divided into a series of rigid elements connected by massless spherical joints, was structured. The bending stiffness of the tether was analyzed, and the tension force along the tether length direction was ignored. In particular, three types of constraint equations for rigid elements were built to help identify tether deployment. Then, the kinetic friction force between the tape-shaped tether and deployment device, as well as the atmospheric drag/lift on the system, were discussed. Finally, numerical simulations demonstrated that the deployment dynamics of the tape-shaped tethered system are sensitive to the bending stiffness, friction force, and orbital altitude.

Analysis of friction and wear processes in an innovative spine stabilization system. Part 1. A study of static and kinetic friction of a metal rod-polymer cord friction joint

Purpose: This analysis is the first part of research that aims to develop a model of the tribological wear of PE-UHMW cord – biometal rod combination. This type of sliding joint is applied in spine stabilization systems that enable the treatment of early-onset idiopathic scoliosis. Methods: The friction tests included force measurements, followed by the determination of static and kinetic friction coefficients as a function of the number of the performed movement cycles, and static friction coefficient with regards to the string tension force FN in the range of 50–300 N. Additionally, the surface roughness and microscopic observations of the metal rods were made. The friction measurements were carried out at a stabilized temperature T = 38 °C in the presence of distilled water and acidic sodium lactate. Results: The measurements confirmed the impact of both the number of completed movement cycles and the value of the force loaded on the cord on the static friction coefficient. Similar values of kinetic friction force occur for the pairs with the titanium alloys rods, as well as for the pairs with the steel and CoCr rod. The type of lubricant affected the obtained measurement results unevenly: (Ti6Al4V and Ti6Al7Nb – slight impact, steel 316L and CoCrMo – large impact). During microscopic observations, numerous wear products, were visible, including harder than the base material large conglomerates. Conclusions: Susceptibility of polymer fibres results in its increased resistance to wear, but it can be also combined with an increase in wear of the surface of the metal rod.

Frictional Properties of the TiNbTaZrO Orthodontic Wire-A Laboratory Comparison to Popular Archwires.

Background. This study aimed to determine the kinetic frictional force (FF) of the recently produced TiNbTaZrO (Gummetal) orthodontic wire and compare it to the widely used wires of stainless steel (SS), nickel-titanium (NiTi), cobalt-chromium (CoCr) and titanium-molybdenum (TiMo) alloys. Methods. Five types of 0.016″ × 0.022″ wires were ligated with elastic ligatures to 0.018″ × 0.025″ SS brackets. The dynamic FFs between the brackets and ligated wires were measured utilizing a specialized tensile tester machine. Prior sample sizes for different archwires were conducted using power analysis for the general linear models. The existence of significant differences in FF between examined materials was initially confirmed by the one-way analysis of variance (ANOVA) with further evidence of pairwise differences by Tukey’s Honest Significant Difference test. Results. The pairwise differences between means of kinetic FFs for NiTi, CoCr, and Gummetal wires were not statistically significant (adjusted p-value > 0.05). Stainless steel alloy presented the lowest FF values significantly different from other groups (adjusted p-value < 0.05). On the contrary, TiMo wires showed significantly greater FFs (adjusted p-value < 0.05) than other alloys. Conclusions. Gummetal orthodontic wire exhibits similar frictional resistance as NiTi and CoCr wires. Bendable TiNbTaZrO wire might be used for sliding mechanics due to its favorable frictional properties.