Friction Force Research Articles

Coupled wind-induced response of inter-story isolated eccentric tall buildings with friction-pendulum bearings

The isolation technology is widely used to protect structures against strong earthquake. For the inter-story isolated tall building located in strong wind-prone area, the soft isolation layer prolongs the structural period, which can make the wind effects significant. This study presents an analytical framework for assessing three-dimensional coupled wind-induced response of inter-story isolated eccentric tall buildings employing friction-pendulum bearings. The framework utilizes a biaxial Bouc-Wen hysteresis model and a velocity-dependent friction coefficient to describe the coupled frictional forces at each bearing. To enhance the computational efficiency, a simplified model with reduced degrees of freedom is proposed to represent the building. This study investigates eighty combinations of stiffness and mass eccentricities of lower and upper structures relative to isolation layer to analyze their impact on building acceleration. A spherical and two aspheric sliding surfaces of the bearing are examined to understand their effects on building response. The findings suggest that the stiffness eccentricity of either lower or upper building solely affects its own response without influencing other part of the structure. Conversely, the mass eccentricity of upper building induces significant mode coupling, resulting in noticeable impact on the accelerations of both upper and lower buildings. Moreover, the acceleration of the story conner farther from the mass eccentric position experiences significantly amplification. The proposed aspherical bearing demonstrates the capability to substantially reduce the bearing displacement, although with a limited increase in other building responses not exceeding 20 %.

Improving Archard’s Wear Model: An Energy-Based Approach

Archard’s wear law encounters challenges in accurately predicting wear damage and volumes, particularly in complex situations like asperity–asperity collisions. A modified model is proposed and validated, showcasing its ability to predict wear in adhesive contacts with better accuracy than the original Archard’s wear law. The model introduces an improved wear coefficient linked to deformation energy, creating a spatially varying relationship between wear volume and load and imparting a non-linear characteristic to the problem. The improved wear model is coupled with the Boundary Element Method (BEM), assuming that the interacting surfaces are semi-infinite and flat. The deformation energy is calculated from the normal contact pressure and displacements, which are the common outputs of BEM. By relying solely on these outputs, the model can efficiently predict the correct shape and volume of the adhesive wear particle, without resorting to large and often slow models. An important observation is that the wear coefficient is expected to increase based on the accumulated deformation energy along the direction of frictional force. This approach enhances the model’s capability to capture complex wear mechanisms, providing a more accurate representation of real-world scenarios.

On the Design, Fabrication, and Characterization of a Novel Thin-Film Electrode Array for Use in Cochlear Implants.

Thin-film electrode arrays (TFEAs) have been developed as an alternative to conventional electrode arrays (CEAs) used in cochlear implants. However, TFEAs produced by microfabrication techniques have not yet been used clinically because their structural and mechanical properties are far from those of CEAs. The aim of this study is to design, fabricate, and investigate the mechanical and tribological behavior and evaluate the performance of different TFEA designs. Finite Element Analysis (FEA) is performed to determine the elastic properties of several designs. A custom-build experimental setup is designed to observe the tribological behavior in different speeds and environments where frictional (lateral) and vertical force (normal force) are measured on a flat surface and within artificial cochlea. According to the FEA results, the maximum stiffness of the CEA is 37.93 mN/mm and 0.363 mN/mm and TFEA-4 has a maximum stiffness of 39.08 mN/mm and 0.306 mN/mm in the longitudinal and transverse axes, respectively. It is shown experimentally that adding a dummy wire to the carrier of the EA enhances both its longitudinal and transverse stiffness, thereby postponing the initiation of dynamic sliding due to the elevated buckling limit. It is also revealed that the type of TFEA support structure affects both normal and frictional forces, as well as the coefficient of friction.

Towards yarn-to-yarn friction behavior in various architectures during the manufacturing of engineering woven fabrics

Friction plays an important role in ensuring the quality of fiber-reinforced composites. Understanding and evaluating friction properties across different fabric architectures and yarn pre-tensions pose significant challenges. In this current research, the influence of fabric architectures and yarn pre-tensions on friction properties during the weaving process were investigated. A novel micro-meso theoretical model based on Peirce’s geometrical model to predict friction properties was developed. The theoretical model shows strong agreement with experimental results, particularly regarding fabric architectures. It can be found that yarn pre-tensions, a critical parameter, increased friction force across all tested architectures, directly impacting yarn friction performance. Additionally, the theoretical model was extended to predict the friction properties of yarns during the manufacturing of 3D fabrics. The findings indicate that fabric architecture significantly influences friction behavior, with the configuration cell depending not only on yarn dimensions but also on preform parameters such as the number of layers, thickness, and binding pattern.

Friction Force Mapping of Molecular Ordering and Mesoscopic Phase Transformations in Layered-Crystalline Organic Semiconductor Films.

It is critical to understand molecular ordering processes in small-molecule organic semiconductor (OSC) films in optimizing electronic device applications, although it is difficult to observe and investigate the ordering characteristics at a mesoscopic or device scale. Here, we report that friction force microscopy (FFM) allows visualizing the ordering transformation process from a thermodynamically metastable phase to a stable phase at a mesoscopic scale. We utilized 2-octyl-benzothieno[3,2-b]naphtho[2,3-b]thiophene (2-C8-BTNT) as a typical highly layered-crystalline OSC. We found that the friction force between an AFM tip and spin-coated OSC films significantly depends on whether local film states are in metastable monolayer phase or stable bilayer-type herringbone (b-LHB) phase that exhibits high carrier mobility. The formation of the stable b-LHB phase leads to lower friction than the metastable monolayer phase, clearly visualizing the molecular order. Force map (Fmap) analysis indicates that the lower friction in the b-LHB phase should be associated with the reduction of interfacial adhesion force. Notably, the observed results demonstrate that the spin-coated thin film changes from continuous film with the monolayer phase to rugged microcrystal grains with the b-LHB phase when left at ambient conditions. By contrast, an appropriate post-thermal annealing process facilitates the phase transformation without inducing such morphological changes. The technique provides a unique and effective tool for revealing the relationship between processing conditions and device performance in polycrystalline OSC films.

Atomic-scale tribological behavior of pressure-induced MoO3(0 1 0)/Ag(1 1 1) heterogeneous sliding interface

The atomic-scale tribological properties of the MoO3(010)/Ag(111) heterogeneous sliding interface were studied by first-principles calculation method. The interface binding energy, sliding potential energy, friction force, charge density difference, partial density of states, Mulliken bond population and HiRshfeld charge were analyzed, and the essence of the friction was revealed. The results show that the sliding potential energy and friction exhibit anisotropy for the pressure-induced MoO3(010)/Ag(111) heterogeneous sliding interface. The friction force can be described by the change of the sliding potential energy, and the charge redistribution between interface atoms is the intrinsic reason for the change of tribological properties. This work provides atomic and electronic level insights for understanding the tribological characteristics between two-dimensional materials and soft metals.

The effect of age on the attachment ability of stick insects (Phasmatodea).

Many insect species have found their way into ageing research as small and easy-to-keep model organisms. A major sign of ageing is the loss of locomotory functions due to neuronal disorders or tissue wear. Soft and pliable attachment pads on the tarsi of insects adapt to the substrate texture to maximize their real contact area and, thereby, generate attachment during locomotion. In the majority of stick insects, adhesive microstructures covering those pads support attachment. Stick insects do not molt again after reaching the imaginal stage; hence, the cuticle of their pads is subject to continuous ageing. This study aims to quantify how attachment ability changes with age in the stick insect Sungaya aeta Hennemann, 2023 and elucidate the age effects on the material and microstructure of the attachment apparatus. Attachment performance (adhesion and friction forces) on substrates with different roughnesses was compared between two different age groups, and the change of attachment performance was monitored extending over a larger time frame. Ageing effects on the morphology of the attachment pads and the autofluorescence of the cuticle were documented using light, scanning electron, and confocal laser scanning microscopy. The results show that both adhesion and friction forces decline with age. Deflation of the pads, scarring of the cuticle, and alteration of the autofluorescence, likely indicating stiffening of the cuticle, were observed to accumulate over time. This would reduce the attachment ability of the insect, as pads lose their pliant properties and cannot properly maintain sufficient contact area with the substrate.

Reducing Torque and Drag in Extended-Reach Wells Using Thermoplastic Polymers for Protective Sliding Rings

Extended-reach drilling represents an advanced way of drilling and accessing reservoirs that were previously economically not feasible to drain, impossible to reach or in an environmentally sensitive area. One of the main issues that appears while drilling such wells is caused by the high values of friction factor which cause high values of torque and drag. One of the suggested solutions is to use a protective sliding ring made from materials such as POM, Teflon and PA6 in combination with lubricants added to a polymer mud system. First, measurements were conducted on a lubricity tester to choose the best material and, after finishing, a mechanical wear test was conducted on a specially designed device to measure mechanical wear. Results showed that Teflon showed lower values of friction factor in comparison to steel and the mechanical wear was minimal. The lowest value of friction force was recorded for blocks made from Teflon in tested mud systems. It is also noticeable that, in polymer mud with weighting additives and lubricant, the value of the friction force is higher than in polymer mud with lubricant only.

Bioinspired film-terminated ridges for enhancing friction force on lubricated soft surfaces

Enhancing friction force in lubricated, compliant contacts is of particular interest due to its wide application in various engineering and biological systems. In this study, we have developed bioinspired surfaces featuring film-terminated ridges, which exhibit a significant increase in lubricated friction force compared to flat samples. We propose that the enhanced sliding friction can be attributed to the energy dissipation at the lubricated interface caused by elastic hysteresis resulting from cyclic terminal film deformation. Furthermore, increasing inter-ridge spacing or reducing terminal film thickness are favorable design criteria for achieving high friction performance. These findings contribute to our understanding of controlling lubricated friction and provide valuable insights into surface design strategies for novel functional devices.

Influence of activation energy and multidiffusive quadratic convective nanoliquid flow past a cone and wedge

The present problem aims to examine the impact of quadratic buoyancy-driven flow nanofluid flow on two specific geometries such as cone and wedge in the presence of binary chemical reactions with activation energy, using liquid hydrogen species. This analysis is relevant for various industrial applications and are widely used in thermal energy storage devices for cooling or heating processes. The fluid flow model's governing equations are numerically solved using a fifth-order boundary value approach in matrix laboratory (MATLAB). In order to obtain the numerical solution, we initially transformed the coupled equations of the flow model and the partial differential equations into ordinary differential equations using similarity variables. The graphs and tables illustrate the importance of different physical factors that impact fluid flow and heat transfer. The quadratic combined convection parameter leads to an augmentation in fluid velocity while also inducing an elevation in frictional forces between the object's surface and the fluid. The increased in values of Schmidt number cause a decrease in mass diffusivity, which in turn leads to lower concentration profiles and improved mass transport efficiency. The activation energy is directly proportional to the decreased amplitude of the concentration profiles of liquid hydrogen species.

Numerical analysis for Cattaneo-Christov heat flux on convective viscous non-Newtonian fluid flow through porous medium with nonuniform heat source

This work investigates the Ohmic heating, nonuniform heat generation, and Hall effects with Cattaneo-Christov model (CCM) on flow and heat transfer of power-low non-Newtonian fluids (NNFs) past stretching surface embedded in a porous medium. Runge-Kutta method is used to solve non-linear ODEs numerically using a shooting procedure. We study non-Newtonian fluids for power law values of 0.7 and 1.2, respectively. Innovation of this work lies in studying the effect of Hall currents in presence of buoyancy force and an irregular heat source on slip flow of NNFs moving through Darcy porous medium. Varying velocities, surface frictional forces, and Nusselt numbers are examined. Resulting entropy is also examined using computational flow problem investigation. Model-simulated results suggested that various factors play critical role in constructing thermal systems. It is asserted that Hall parameter, mixed convection parameter, Biot numbers, and thermal relaxation time improve heat transference rates by directly affecting fluid molecules temperature.

Mechanical modeling of friction phenomena in social systems based on friction force

Friction is not an inherent attribute in natural science. Indeed, there are various friction phenomena present in social systems. In this study, the researchers developed the general concept of social friction and distinguished the two-dimensional structure of explicit-implicit social friction phenomena. Specifically, explicit friction includes institutional friction, economic friction, behavioral friction, and migration friction. Implicit friction includes cultural friction, cognitive friction, interpersonal friction, technological friction, and information friction. Physical theories such as friction force, classical mechanics, and functional principles have been introduced to construct a social friction force model. This paper describes the emergence and evolution of social friction phenomena from the mechanics perspective and clarifies the various characteristics of social friction force in different areas. This study also provides a new theoretical perspective for examining social friction phenomena and adds innovative content to intersection studies in physics and the social sciences.

Thermal Properties of Athermal Granular Materials.

Dry granular materials consist of a vast ensemble of discrete solid particles interacting through complex frictional forces at the contact points. The particles are so large that these systems are believed to be completely athermal. Here, we arrest the dynamics of a flowing granular material in a steady-state-flow configuration, enabling an isolated examination of aging at the particle contacts without granular rearrangements. Our findings reveal that the evolution of interparticle forces within the arrested athermal granular network results in the spontaneous increase of the system's yield stress. This strengthening process is logarithmic in time with a rate that depends on the temperature. We demonstrate that the material's stress relaxation exhibits similar time- and temperature-dependent behavior, suggesting a shared origin for aging and stress relaxation in these systems governed by thermal molecular processes at the scale of the grain contacts.

Frictional resistance and delamination mechanisms in 2D tungsten diselenide revealed by multi-scale scratch and in-situ observations

Friction phenomena in two-dimensional (2D) materials are conventionally studied at atomic length scales in a few layers using low-load techniques. However, the advancement of 2D materials for semiconductor and electronic applications requires an understanding of friction and delamination at a few micrometers length scale and hundreds of layers. To bridge this gap, the present study investigates frictional resistance and delamination mechanisms in 2D tungsten diselenide (WSe2) at 10 µm length and 100–500 nm depths using an integrated atomic force microscopy (AFM), high-load nanoscratch, and in-situ scanning electron microscopic (SEM) observations. AFM revealed a heterogenous distribution of frictional resistance in a single WSe2 layer originating from surface ripples, with the mean increasing from 8.7 to 79.1 nN as the imposed force increased from 20 to 80 nN. High-load in-situ nano-scratch tests delineated the role of the individual layers in the mechanism of multi-layer delamination under an SEM. Delamination during scratch consists of stick-slip motion with friction force increasing in each successive slip, manifested as increasing slope of lateral force curves with scratch depth from 10.9 to 13.0 (× 103) Nm−1. Delamination is followed by cyclic fracture of WSe2 layers where the puckering effect results in adherence of layers to the nanoscratch probe, increasing the local maximum of lateral force from 89.3 to 205.6 µN. This establishment of the interconnectedness between friction in single-layer and delamination at hundreds of layers harbors the potential for utilizing these materials in semiconductor devices with reduced energy losses and enhanced performance.

Structural superlubricity at homogenous interface of penta-graphene

Two-dimensional (2D) van der Waals layered materials have been widely used as lubricant. Penta-graphene (PG), a 2D carbon allotrope exclusively composed of irregular carbon pentagons has recently been predicted to have superlubricating property. In the present study, by combining the molecular dynamics simulation and first-principles calculations, we investigated the frictional property of PG in both commensurate and incommensurate contacts. Our calculations show the ultra-low friction at the interface of relatively rotated bilayer PG with twist angles of more than 10° away from the commensurate configuration. Meanwhile, our calculations demonstrate the isotropy of the ultra-low friction at the interface of incommensurate contact, in contrast to the anisotropic of the commensurate contacting interface. Additionally, the evolution of friction force and the fluctuation of potential energy along sliding path correlate closely with the interface’s structure. The energetics and charge density explain the difference between the friction at the interfaces of the commensurate and incommensurate contacts. Not only that, we found the correlation between the intrinsic structural feature and interlayer binding energy. Importantly, our findings on the retainment of the ultra-low friction under work conditions indicates that the superlubricating state of PG has good practical adaptability.

Synergistic effects of interfacial friction on shear performance of stud-NSC composite connection

Synergistic effects of friction force on static and fatigue performance of stud-normal strength concrete (NSC) composite connection are clarified in the present study. Push-out specimens with greased interface are used to degrade the interfacial friction force. The static testing and the fatigue testing are carried out to reveal the synergistic effects of friction force on shear performance of stud-NSC connection. The frictional mechanism between stud connected steel-NSC interface is discussed and illustrated. Results show that interfacial friction synergizes with the stud to bear the shearing force and improves the shear performance of stud-NSC connection. In present test, the greased interface-induced friction degradation decreases the shear strength about 8.7 %, the shear stiffness about 13.2 % and the fatigue life about 3.9 % as compared to the case without friction degradation. Friction force between steel-NSC interface is a secondary force and influenced by shearing force from stud, which is easily to be weakened as the repeated wearing between steel plate and concrete during the fatigue loading cycles. The synergistic effects of friction force on shear performance of stud-NSC connection is much slighter than that of stud-UHPC connection, especially its influences on fatigue performance of stud-NSC connection are almost negligible.

Shape memory polymer composite hinges for solar sails

Smart hinges for self-deployable solar sails have been prototyped by using shape memory polymer composites (SMPCs). During hinge lamination, a shape memory (SM) epoxy resin is deposited between carbon fiber reinforced (CFR) plies, and consolidation is obtained by an out-of-autoclave (OOA) process. The smart hinges have been assembled with CFR booms to prototype a small self-deployable square sail. Polymeric supports, made by fused deposition modelling (FDM), have been added for correct assembly and to drive the recovery of the SMPC hinges. Flexible heaters have been used as heating elements for SMPC hinge activation. The sail consisted of a polyimide membrane which was connected with the composite booms by flexible joints. The solar sail prototype size has been limited to 260 × 260 mm2 for on-Earth testing, with 4 hinges at the middle length of the composite booms. Recovery tests have shown the ability of the smart hinges to overcome the sail weight and additional friction forces, and the small sail has been successfully deployed in laboratory. Full recovery has been achieved in less than 3 min under different configurations.

Cracking modes and force dynamics in the insertion of neural probes into hydrogel brain phantom

Objective. The insertion of penetrating neural probes into the brain is crucial for advancing neuroscience, yet it involves various inherent risks. Prototype probes are typically inserted into hydrogel-based brain phantoms and the mechanical responses are analyzed in order to inform the insertion mechanics during in vivo implantation. However, the underlying mechanism of the insertion dynamics of neural probes in hydrogel brain phantoms, particularly the phenomenon of cracking, remains insufficiently understood. This knowledge gap leads to misinterpretations and discrepancies when comparing results obtained from phantom studies to those observed under the in vivo conditions. This study aims to elucidate the impact of probe sharpness and dimensions on the cracking mechanisms and insertion dynamics characterized during the insertion of probes in hydrogel phantoms. Approach. The insertion of dummy probes with different shank shapes defined by the tip angle, width, and thickness is systematically studied. The insertion-induced cracks in the transparent hydrogel were accentuated by an immiscible dye, tracked by in situ imaging, and the corresponding insertion force was recorded. Three-dimensional finite element analysis models were developed to obtain the contact stress between the probe tip and the phantom. Main results. The findings reveal a dual pattern: for sharp, slender probes, the insertion forces remain consistently low during the insertion process, owing to continuously propagating straight cracks that align with the insertion direction. In contrast, blunt, thick probes induce large forces that increase rapidly with escalating insertion depth, mainly due to the formation of branched crack with a conical cracking surface, and the subsequent internal compression. This interpretation challenges the traditional understanding that neglects the difference in the cracking modes and regards increased frictional force as the sole factor contributing to higher insertion forces. The critical probe sharpness factors separating straight and branched cracking is identified experimentally, and a preliminary explanation of the transition between the two cracking modes is derived from three-dimensional finite element analysis. Significance. This study presents, for the first time, the mechanism underlying two distinct cracking modes during the insertion of neural probes into hydrogel brain phantoms. The correlations between the cracking modes and the insertion force dynamics, as well as the effects of the probe sharpness were established, offering insights into the design of neural probes via phantom studies and informing future investigations into cracking phenomena in brain tissue during probe implantations.

Involvement of TRPV4 in temperature-dependent perspiration in mice.

Reports indicate that an interaction between TRPV4 and anoctamin 1 (ANO1) could be widely involved in water efflux of exocrine glands, suggesting that the interaction could play a role in perspiration. In secretory cells of sweat glands present in mouse foot pads, TRPV4 clearly colocalized with cytokeratin 8, ANO1, and aquaporin-5 (AQP5). Mouse sweat glands showed TRPV4-dependent cytosolic Ca2+ increases that were inhibited by menthol. Acetylcholine-stimulated sweating in foot pads was temperature-dependent in wild-type, but not in TRPV4-deficient mice and was inhibited by menthol both in wild-type and TRPM8KO mice. The basal sweating without acetylcholine stimulation was inhibited by an ANO1 inhibitor. Sweating could be important for maintaining friction forces in mouse foot pads, and this possibility is supported by the finding that wild-type mice climbed up a slippery slope more easily than TRPV4-deficient mice. Furthermore, TRPV4 expression was significantly higher in controls and normohidrotic skin from patients with acquired idiopathic generalized anhidrosis (AIGA) compared to anhidrotic skin from patients with AIGA. Collectively, TRPV4 is likely involved in temperature-dependent perspiration via interactions with ANO1, and TRPV4 itself or the TRPV4/ANO 1 complex would be targeted to develop agents that regulate perspiration.

Design and evaluation of a pneumatic actuation unit for a wasp-inspired self-propelled needle.

Transperineal laser ablation is a minimally invasive thermo-ablative treatment for prostate cancer that requires the insertion of a needle for accurate optical fiber positioning. Needle insertion in soft tissues may cause tissue motion and deformation, resulting in tissue damage and needle positioning errors. In this study, we present a wasp-inspired self-propelled needle that uses pneumatic actuation to move forward with zero external push force, thus avoiding large tissue motion and deformation. The needle consists of six parallel 0.25-mm diameter Nitinol rods driven by a pneumatic actuation system. The pneumatic actuation system consists of Magnetic Resonance (MR) safe 3D-printed parts and off-the-shelf plastic screws. A self-propelled motion is achieved by advancing the needle segments one by one, followed by retracting them simultaneously. The advancing needle segment has to overcome a cutting and friction force, while the stationary needle segments experience a friction force in the opposite direction. The needle self-propels through the tissue when the friction force of the five stationary needle segments overcomes the sum of the friction and cutting forces of the advancing needle segment. We evaluated the prototype's performance in 10-wt% gelatin phantoms and ex vivo porcine liver tissue inside a preclinical Magnetic Resonance Imaging (MRI) scanner in terms of the slip ratio of the needle with respect to the phantom or liver tissue. Our results demonstrated that the needle was able to self-propel through the phantom and liver tissue with slip ratios of 0.912-0.955 and 0.88, respectively. The prototype is a promising step toward the development of self-propelled needles for MRI-guided transperineal laser ablation as a method to treat prostate cancer.