What Puts the "Tribo" in Triboelectricity?

An enduring question in science has been why sliding plays a major role in the triboelectric generation of static electricity-the "tribo" in triboelectricity. We provide here a general explanation which is rooted in established science. When sliding is taking place, there is symmetry breaking due to elastic shear, so the front of the sliding body experiences different elastic strains from the back. Consequently the polarization and associated charges at the front and back are not the same, and the difference between the two leads to current flow similar to the difference in air pressure above and below a plane's wing leading to lift. Specific calculations are provided which show good agreement with prior experimental measurements of size and shape dependencies, and reasonable quantitative agreement with experimental current measurements.

Effects of interfacial debonding on the stability of finitely strained periodic microstructured elastic composites.

Predicting failure initiation in nonlinear composite materials, often referred to as metamaterials, is a fundamental challenge in nonlinear solid mechanics. Microstructural failure mechanisms encompass fracture, decohesion, cavitation, compression-induced contact and instabilities, affecting their unconventional static and dynamic performances. To fully take advantage of these materials, especially in extreme applications, it is imperative to predict their nonlinear behaviour using reliable, accurate and computationally efficient numerical methodologies. This study presents an innovative nonlinear homogenization-based theoretical framework for characterizing the failure behaviour of periodic reinforced hyperelastic composites induced by reinforcement/matrix decohesion and interaction between contact mechanisms and microscopic instabilities. Debonding and unilateral contact between different phases are incorporated by employing an enhanced cohesive/contact model, which features a special nonlinear interface constitutive law and an accurate contact formulation within the context of finite strain continuum mechanics. The theoretical formulation is demonstrated using periodically layered composites subjected to macroscopic compressive loading conditions along the lamination direction. Numerical results illustrate the ways in which debonding phenomena, in conjunction with fibre microbuckling, may influence the critical loads of the examined composite solid. The sensitivity of the results obtained through the proposed contact-cohesive model at finite strain with respect to its implementation is also explored. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 1)'.

Modelling of mixed-mechanism stimulation for the enhancement of geothermal reservoirs.

Hydraulic stimulation is a critical process for increasing the permeability of fractured geothermal reservoirs. This technique relies on coupled hydromechanical processes induced through pressurized fluid injection into the rock formation. The injection of fluids causes poromechanical stress changes that can lead to fracture slip and shear dilation, as well as tensile fracture opening and propagation, so-called mixed-mechanism stimulation. The effective permeability of the rock is particularly enhanced when new fractures connect with pre-existing fractures. While hydraulic stimulation can significantly improve the productivity of fractured geothermal reservoirs, the process is also related to induced seismicity. Hence, understanding the coupled physics is central, for both reservoir engineering and seismic risk mitigation. This article presents a modelling approach for simulating the deformation, propagation and coalescence of fractures in porous media under the influence of anisotropic stress and fluid injection. It uses a coupled hydromechanical model for poroelastic, fractured media. Fractures are governed by contact mechanics and a fracture propagation model. For numerical solutions, we employ a two-level approach, combining a finite volume method for poroelasticity with a finite element method for fracture propagation. The study investigates the impact of injection rate, matrix permeability and stress anisotropy on stimulation outcomes.This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'.

Mechanics of two filaments in tight orthogonal contact.

Networks of flexible filaments often involve regions of tight contact. Predictively understanding the equilibrium configurations of these systems is challenging due to intricate couplings between topology, geometry, large nonlinear deformations, and friction. Here, we perform an in-depth study of a simple, yet canonical, problem that captures the essence of contact between filaments. In the orthogonal clasp, two filaments are brought into contact, with each centerline lying in one of a pair of orthogonal planes. Our data from X-ray tomography (µCT) and mechanical testing experiments are in excellent agreement with finite element method (FEM) simulations. Despite the apparent simplicity of the physical system, the data exhibit strikingly unintuitive behavior, even when the contact is frictionless. Specifically, we observe a curvilinear diamond-shaped ridge in the contact-pressure field between the two filaments, sometimes with an inner gap. When a relative displacement is imposed between the filaments, friction is activated, and a highly asymmetric pressure field develops. These findings contrast to the classic capstan analysis of a single filament wrapped around a rigid body. Both the µCT and FEM data indicate that the cross-sections of the filaments can deform significantly. Nonetheless, an idealized geometrical theory assuming undeformable tube cross-sections and neglecting elasticity rationalizes our observations qualitatively and highlights the central role of the small, but nonzero, tube radius of the filaments. We believe that our orthogonal clasp analysis provides a building block for future modeling efforts in frictional contact mechanics of more complex filamentary structures.

Attractive forces slow contact formation between deformable bodies underwater.

Thermodynamics tells us to expect underwater contact between two hydrophobic surfaces to result in stronger adhesion compared to two hydrophilic surfaces. However, the presence of water changes not only energetics but also the dynamic process of reaching a final state, which couples solid deformation and liquid evacuation. These dynamics can create challenges for achieving strong underwater adhesion/friction, which affects diverse fields including soft robotics, biolocomotion, and tire traction. Closer investigation, requiring sufficiently precise resolution of film evacuation while simultaneously controlling surface wettability, has been lacking. We perform high-resolution in situ frustrated total internal reflection imaging to track underwater contact evolution between soft-elastic hemispheres of varying stiffness and smooth-hard surfaces of varying wettability. Surprisingly, we find the exponential rate of water evacuation from hydrophobic-hydrophobic (adhesive) contact is three orders of magnitude lower than that from hydrophobic-hydrophilic (nonadhesive) contact. The trend of decreasing rate with decreasing wettability of glass sharply changes about a point where thermodynamic adhesion crosses zero, suggesting a transition in mode of evacuation, which is illuminated by three-dimensional spatiotemporal height maps. Adhesive contact is characterized by the early localization of sealed puddles, whereas nonadhesive contact remains smooth, with film-wise evacuation from one central puddle. Measurements with a human thumb and alternatively hydrophobic/hydrophilic glass surface demonstrate practical consequences of the same dynamics: adhesive interactions cause instability in valleys and lead to a state of more trapped water and less intimate solid-solid contact. These findings offer interpretation of patterned texture seen in underwater biolocomotive adaptations as well as insight toward technological implementation.

Partial weight-bearing and range of motion limitation significantly reduce the loads at medial meniscus posterior root repair sutures in a cadaveric biomechanical model.

PURPOSE: The aim of this study was to investigate the influence of medial meniscus posterior root avulsion (MMPRA) before and after surgical treatment on the biomechanics of the knee joint, including suture repair forces during daily and crutch-assisted gait movements. METHODS: MMPRA were investigated in eight human cadaver knee joint specimens by a dynamic knee joint simulator with daily (normal gait, gait with additional rotational movement, standing up, sitting down) and rehabilitation-associated movements (crutch-assisted gait with limited flexion range of motion [30°] and 30% [toe-touch weight-bearing, TTWB] and 50% of body weight [partial weight-bearing, PWB]) with simulated physiologic muscle forces. Each specimen was tested in intact, torn and repaired (transtibial suture) state. The biomechanical parameters were: medial mean contact pressure and area, knee joint kinematics, medial displacement of the posterior meniscus horn and loading on the anchoring suture. RESULTS: Significant reduction of the contact area due to the avulsion was observed in all movements except for PWB and sitting down. MMPRA repair significantly increased the contact areas during all movements, bringing them to levels statistically indistinguishable from the initial state. MMPRA resulted in a medial displacement up to 12.8 mm (sitting down) and could be reattached with a residual displacement ranging from 0.7 mm (PWB) to 5.7 mm (standing up), all significantly (p < 0.001) reduced compared to the torn state. The mean peak anchoring suture load increased from TTWB (77 N), PWB (91 N) to normal gait (194 N), gait rotation (207 N), sitting (201 N; p < 0.01) and to standing up (232 N; p = 0.03). CONCLUSION: Surgical treatment of MMPRA allows restoration of physiological knee joint biomechanics. Crutch-assisted movements reduce the loading of the repair suture, thus likewise the risk for failure. From a biomechanical point of view, crutch-assisted movements are recommended for the early rehabilitation phase after MMPRA repair. LEVEL OF EVIDENCE: Level V.

High-Precision Tribometer for Studies of Adhesive Contacts.

Herein, we describe the design of a laboratory setup operating as a high-precision tribometer. The whole design procedure is presented, starting with a concept, followed by the creation of an exact 3D model and final assembly of all functional parts. The functional idea of the setup is based on a previously designed device that was used to perform more simple tasks. A series of experiments revealed certain disadvantages of the initial setup, for which pertinent solutions were found and implemented. Processing and correction of the data obtained from the device are demonstrated with an example involving backlash and signal drift errors. Correction of both linear and non-linear signal drift errors is considered. We also show that, depending on the research interests, the developed equipment can be further modified by alternating its peripheral parts without changing the main frame of the device.

When Flexoelectricity Drives Triboelectricity.

Triboelectricity has been known since antiquity, but the fundamental science underlying this phenomenon lacks consensus. We present a flexoelectric model for triboelectricity where contact deformation induced band bending at the nanoscale is the driving force for charge transfer. This framework is combined with first-principles and finite element calculations to explore charge transfer implications for different contact geometry and materials combinations. We demonstrate that our ab initio based formulation is compatible with existing empirical models and experimental observations including charge transfer between similar materials and size/pressure dependencies associated with triboelectricity.

Band Bending and Ratcheting Explain Triboelectricity in a Flexoelectric Contact Diode.

Triboelectricity was recognized millennia ago, but the fundamental mechanism of charge transfer is still not understood. We have recently proposed a model where flexoelectric band bending due to local asperity contacts drives triboelectric charge transfer in non-metals. While this ab initio model is consistent with a wide range of observed phenomena, to date there have been no quantitative analyses of the proposed band bending. In this work we use a Pt0.8Ir0.2 conductive atomic force microscope probe to simultaneously deform a Nb-doped SrTiO3 sample and collect current-bias data. The current that one expects based upon an analysis including the relevant flexoelectric band bending for a deformed semiconductor quantitively agrees with the experiments. The analysis indicates a general ratcheting mechanism for triboelectric transfer and strong experimental evidence that flexoelectric band bending is of fundamental importance for triboelectric contacts.

Prediction of Model Generated Patellofemoral Joint Contact Forces Using Principal Component Prediction and Reconstruction.

It is not currently possible to directly and noninvasively measure in vivo patellofemoral joint contact force during dynamic movement; therefore, indirect methods are required. Simple models may be inaccurate because patellofemoral contact forces vary for the same knee flexion angle, and the patellofemoral joint has substantial out-of-plane motion. More sophisticated models use 3-dimensional kinematics and kinetics coupled to a subject-specific anatomical model to predict contact forces; however, these models are time consuming and expensive. We applied a principal component analysis prediction and regression method to predict patellofemoral joint contact forces derived from a robust musculoskeletal model using exclusively optical motion capture kinematics (external approach), and with both patellofemoral and optical motion capture kinematics (internal approach). We tested this on a heterogeneous population of asymptomatic subjects (n = 8) during ground-level walking (n = 12). We developed equations that successfully capture subject-specific gait characteristics with the internal approach outperforming the external. These approaches were compared with a knee-flexion based model in literature (Brechter model). Both outperformed the Brechter model in interquartile range, limits of agreement, and the coefficient of determination. The equations generated by these approaches are less computationally demanding than a musculoskeletal model and may act as an effective tool in future rapid gait analysis and biofeedback applications.

[Effect of prosthetic joint line installation height errors on insert wear in unicompartmental knee arthroplasty].

The clinical performance and failure issues are significantly influenced by prosthetic malposition in unicompartmental knee arthroplasty (UKA). Uncertainty exists about the impact of the prosthetic joint line height in UKA on tibial insert wear. In this study, we combined the UKA musculoskeletal multibody dynamics model, finite element model and wear model to investigate the effects of seven joint line height cases of fixed UKA implant on postoperative insert contact mechanics, cumulative sliding distance, linear wear depth and volumetric wear. As the elevation of the joint line height in UKA, the medial contact force and the joint anterior-posterior translation during swing phase were increased, and further the maximum von Mises stress, contact stress, linear wear depth, cumulative sliding distance, and the volumetric wear also were increased. Furthermore, the wear area of the insert gradually shifted from the middle region to the rear. Compared to 0 mm joint line height, the maximum linear wear depth and volumetric wear were decreased by 7.9% and 6.8% at -2 mm joint line height, and by 23.7% and 20.6% at -6 mm joint line height, the maximum linear wear depth and volumetric wear increased by 10.7% and 5.9% at +2 mm joint line height, and by 24.1% and 35.7% at +6 mm joint line height, respectively. UKA prosthetic joint line installation errors can significantly affect the wear life of the polyethylene inserted articular surfaces. Therefore, it is conservatively recommended that clinicians limit intraoperative UKA joint line height errors to -2-+2 mm.

Surface topography as a material parameter.

Abstract: Materials science is about understanding the relationship between a material's structure and its properties-in the sphere of mechanical behavior, this includes elastic modulus, yield strength, and other bulk properties. We show in this issue that, analogously, a material's surface structure governs its surface properties-such as adhesion, friction, and surface stiffness. For bulk materials, microstructure is a critical component of structure; for surfaces, the structure is governed largely by surface topography. The articles in this issue cover the latest understanding of these structure-property connections for surfaces. This includes both the theoretical basis for how properties depend on topography, as well as the latest understanding of how surface topography emerges, how to measure and understand topography-dependent properties, and how to engineer surfaces to improve performance. The present article frames the importance of surface topography and its effect on properties; it also outlines some of the critical knowledge gaps that impede progress toward optimally performing surfaces.

Threshold damage mechanisms in brittle solids and their impact on advanced technologies.

Threshold damage mechanisms in brittle covalent-ionic solids are outlined. Fracture and deformation modes are analyzed in terms of classical contact mechanics. Distinctions are made between brittle, ductile and quasiplastic mechanisms in both axial and translational contact. Special attention is devoted to the relatively unexplored subthreshold region where macrofracture is largely suppressed, a region of increasing relevance in the relentless move toward ever smaller devices and precision shaping technologies in the manufacturing sector. Cross-section micrographic images illustrate the fundamental nature of shear events within the hardness deformation zone responsible for crack initiation and propagation. Basic analytical relations for the strengths of surfaces with contact-induced damage in the postthreshold and subthreshold regions are presented, with emphasis on concept rather than fine detail. Strength data for a prototypical brittle material after sharp-indenter damage are presented to highlight the vital role of microstructure in determining transitions between brittle and quasiplastic responses. Pristine defect-free solids are shown to be highly vulnerable to contact damage, even in the subthreshold region. Heterogeneous solids with granular microstructures have lower initial strengths, but are more flaw tolerant. Brittle solids are also highly susceptible to degradation by surface removal processes in wear and machining settings, to a large extent depending again on microstructure. Implications of these findings concerning advanced technological applications of covalent-ionic solids are discussed.

Finite Element Modeling of Planus and Rectus Foot Types for the Study of First Metatarsophalangeal and First Metatarsocuneiform Joint Contact Mechanics.

The foot is a highly complex biomechanical system for which finite element (FE) modeling has been used to evaluate its loading environment. However, there is limited knowledge of first metatarsophalangeal (MTP) and first metatarsocuneiform (MTC) joint contact mechanics. Our goal was to develop a framework for FE modeling of the medial forefoot which could accurately predict experimental measurements of first MTP and first MTC joint loading. Simulations of planus and rectus foot types were conducted for midstance of gait. A custom-built force-controlled cadaveric test-rig was used to derive intracapsular pressure sensor measurements of contact pressure, force, and area during quasi-static loading. The FE model was driven under the same boundary and loading conditions as the cadaver. Mesh sensitivity analyses and best-fit calibrations of moduli for first MTP and first MTC joint cartilage were performed. Consistent with previous experimental research, a lower compressive modulus was best-fit to the first MTP compared to first MTC joint at 10 MPa and 20 MPa, respectively. Mean errors in contact pressures, forces, and areas were 24%, 4%, and 40% at the first MTP joint and 23%, 12%, and 19% at the first MTC joint, respectively. The present developmental framework may provide a basis for future modeling of first MTP and first MTC joint contact mechanics. This study acts as a precursor to validation of realistic physiological loading across gait to investigate joint loading, foot type biomechanics, and surgical interventions of the medial forefoot.

Linking energy loss in soft adhesion to surface roughness.

A mechanistic understanding of adhesion in soft materials is critical in the fields of transportation (tires, gaskets, and seals), biomaterials, microcontact printing, and soft robotics. Measurements have long demonstrated that the apparent work of adhesion coming into contact is consistently lower than the intrinsic work of adhesion for the materials, and that there is adhesion hysteresis during separation, commonly explained by viscoelastic dissipation. Still lacking is a quantitative experimentally validated link between adhesion and measured topography. Here, we used in situ measurements of contact size to investigate the adhesion behavior of soft elastic polydimethylsiloxane hemispheres (modulus ranging from 0.7 to 10 MPa) on 4 different polycrystalline diamond substrates with topography characterized across 8 orders of magnitude, including down to the angstrom scale. The results show that the reduction in apparent work of adhesion is equal to the energy required to achieve conformal contact. Further, the energy loss during contact and removal is equal to the product of the intrinsic work of adhesion and the true contact area. These findings provide a simple mechanism to quantitatively link the widely observed adhesion hysteresis to roughness rather than viscoelastic dissipation.

Polymer-Metal Interfacial Friction Characteristics under Ultrasonic Plasticizing Conditions: A United-Atom Molecular Dynamics Study.

Understanding the properties of polymer-metal interfacial friction is critical for accurate prototype design and process control in polymer-based advanced manufacturing. The transient polymer-metal interfacial friction characteristics are investigated using united-atom molecular dynamics in this study, which is under the boundary conditions of single sliding friction (SSF) and reciprocating sliding friction (RSF). It reflects the polymer-metal interaction under the conditions of initial compaction and ultrasonic vibration, so that the heat generation mechanism of ultrasonic plasticization microinjection molding (UPMIM) is explored. The contact mechanics, polymer segment rearrangement, and frictional energy transfer features of polymer-metal interface friction are investigated. The results reveal that, in both SSF and RSF modes, the sliding rate has a considerable impact on the dynamic response of the interfacial friction force, where the amplitude has a response time of about 0.6 ns to the friction. The high frequency movement of the polymer segment caused by dynamic interfacial friction may result in the formation of a new coupled interface. Frictional energy transfer is mainly characterized by dihedral and kinetic energy transitions in polymer chains. Our findings also show that the ultrasonic amplitude has a greater impact on polymer-metal interfacial friction heating than the frequency, as much as it does under ultrasonic plasticizing circumstances on the homogeneous polymer-polymer interface. Even if there are differences in thermophysical properties at the heterointerface, transient heating will still cause heat accumulation at the interface with a temperature difference of around 35 K.

Photophysics of Fluorescent Contact Sensors Based on the Dicyanodihydrofuran Motif.

Fluorescent molecular rotors have been used for measurements of local mobility on molecular length scales, for example to determine viscosity, and for the visualization of contact between two surfaces. In the present work, we deepen our insight into the excited-state deactivation kinetics and mechanics of dicyanodihydrofuran-based molecular rotors. We extend the scope of the use of this class of rotors for contact sensing with a red-shifted member of the family. This allows for contact detection with a range of excitation wavelengths up to ∼600 nm. Steady-state fluorescence shows that the fluorescence quantum yield of these rotors depends not only on the rigidity of their environment, but - under certain conditions - also on its polarity. While excited state decay via rotation about the exocyclic double bond is rapid in nonpolar solvents and twisting of a single bond allows for fast decay in polar solvents, the barriers for both processes are significant in solvents of intermediate polarity. This effect may also occur in other molecular rotors, and it should be considered when applying such molecules as local mobility probes.

Contact mechanics between the human finger and a touchscreen under electroadhesion.

The understanding and control of human skin contact against technological substrates is the key aspect behind the design of several electromechanical devices. Among these, surface haptic displays that modulate the friction between the human finger and touch surface are emerging as user interfaces. One such modulation can be achieved by applying an alternating voltage to the conducting layer of a capacitive touchscreen to control electroadhesion between its surface and the finger pad. However, the nature of the contact interactions between the fingertip and the touchscreen under electroadhesion and the effects of confined material properties, such as layering and inelastic deformation of the stratum corneum, on the friction force are not completely understood yet. Here, we use a mean field theory based on multiscale contact mechanics to investigate the effect of electroadhesion on sliding friction and the dependency of the finger-touchscreen interaction on the applied voltage and other physical parameters. We present experimental results on how the friction between a finger and a touchscreen depends on the electrostatic attraction between them. The proposed model is successfully validated against full-scale (but computationally demanding) contact mechanics simulations and the experimental data. Our study shows that electroadhesion causes an increase in the real contact area at the microscopic level, leading to an increase in the electrovibrating tangential frictional force. We find that it should be possible to further augment the friction force, and thus the human tactile sensing, by using a thinner insulating film on the touchscreen than used in current devices.

A Biomechanical Evaluation of the ECRL Tenodesis for Reconstruction of the Scapholunate Ligament.

PURPOSE: Reconstruction of the scapholunate ligament (SLL) in the setting of dynamic instability remains a surgical challenge, with lack of consensus on the best reconstructive procedure. Reconstruction of only the dorsal component may lead to volar gapping and abnormal wrist kinematics. This cadaveric active motion simulation study determined whether scapholunate (SL) motion, angulation, and contact are restored following open reconstruction using the extensor carpi radialis longus (ECRL) tenodesis, which reconstructs both the volar and the dorsal SLL components. METHODS: Seven fresh-frozen cadaveric upper limbs (mean age, 68 ± 10.1 years) underwent a 4-stage protocol of cyclic dart-throw motion and flexion-extension motion (utilizing an active wrist motion simulator that used tendon load/motion-controlled actuation. Scaphoid and lunate motion, relative scaphoid translation, SL angle, and dorsal-volar SL diastasis were measured with (1) wrist ligaments intact, (2) following complete sectioning of the SLL, and (3) following SL reconstruction using the ECRL tenodesis technique. RESULTS: Complete SLL sectioning resulted in a typical pattern of SL instability. Following the ECRL tenodesis, lunate extension was not corrected. Scaphoid flexion, however, was not significantly different from the native state in FEM but remained significantly flexed during dart-throw motion. Differential dorsal and volar gapping did not significantly improve following ECRL tenodesis (dorsal, 1.2-2.3 mm; volar, 1.1-1.7 mm). CONCLUSIONS: This biomechanical study demonstrates that the ECRL tenodesis did not fully restore native carpal kinematics, despite dorsal and volar SLL, and scaphotrapeziotrapezoid reconstruction. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic IV.

In vivo reverse total shoulder arthroplasty contact mechanics.

BACKGROUND: Several in vitro studies have investigated the biomechanics of reverse total shoulder arthroplasty (RTSA); however, few in vivo studies exist. The purpose of this study was to examine in vivo RTSA contact mechanics in clinically relevant arm positions. Our hypothesis was that contact would preferentially occur in the inferior region of the polyethylene liner. METHODS: Forty patients receiving a primary RTSA were recruited for a prospective cohort study. All patients received the same implant design with a nonretentive liner. Stereo radiographs were taken at maximal active range of motion. Model-based radiostereometric analysis was used to identify implant position. Contact area between the polyethylene and glenosphere was measured as the geometric intersection of the 2 components and compared with respect to polyethylene liner size, arm position, and relative position within the liner. RESULTS: There were no differences in the proportion of contact area in any arm position between polyethylene liner sizes, ranging from 30% ± 17% to 38% ± 23% for 36-mm liners and 32% ± 21% to 41% ± 25% for 42-mm liners. Contact was equally distributed between the superior and inferior halves of the liner at each arm position (P = .06-.79); however, greater contact area was observed in the outer radius of the liner when the arm was flexed (P = .002). CONCLUSION: This study highlights that contact mechanics are similar between 36- and 42-mm liners. Contact area is generally equally distributed throughout the liner across the range of motion and not preferentially in the inferior region as hypothesized.