Low Normal Force Research Articles

A Low Normal Force Interference Piezoelectric Friction Force Sensor Based on Spatial Polarization Field Design

A Low Normal Force Interference Piezoelectric Friction Force Sensor Based on Spatial Polarization Field Design

Insights into the mechanism of plastics’ fragmentation under abrasive mechanical forces: An implication for agricultural soil health

AbstractThe application of agricultural plastic products such as mulch, greenhouse covers, and silage films is increasing due to their economic benefits in providing an early and better‐quality harvest. However, mechanical abrasion of these plastic materials by soil particles could result in generation of microplastic (MP) pollutants that could harm soil organisms and impact food safety. This study aims to better understand the physicochemical mechanisms resulting in the fragmentation of low‐density polyethylene (LDPE). Herein, we used pellets and films to study the impacts of abrasive wear forces on their surface morphology variations and fragmentation behavior. An innovative laboratory approach was developed to abrade the plastic surface under controlled normal loadings and abrasion durations. The investigation of the plastics’ surface morphology variations due to the abrasion process revealed microcutting as the dominant process at low normal force (4 N). However, a combination of microploughing and microcutting occurred for new LDPE films by increasing the normal force to 8 N. Despite the significant surface morphology variations of the new LDPE film due to the abrasion process; the water contact angle did not alter. Furthermore, the fragmentation behavior of photodegraded LDPE pellets and films was compared to the new plastics. The extent of MPs (3 µm < dp < 162 µm) generation due to fragmentation was studied using fluorescence microscopy imaging. The localized stress and strains at the contact sites of plastic and sand particles resulted in abrasion of the plastic surface. According to the results, the normal loadings and duration of abrasion played a significant role in the degree of fragmentation of plastics. Increasing the normal loading applied during the abrasion process from 2 to 8 N linearly increased the number of generated plastic fragments by more than five times for pellets and more than three times for film. Photodegradation significantly enhanced the extent of MPs fragmentation. Moreover, the limitations of this study and the implications for agricultural soil health were discussed.

Calculation and Characterization of Braking Performance for Rail Eddy Current Brake With AC Excited Ring-Winding Armature

With the continuous increases of the train speed, the braking device that relies on the wheel-rail adhesion has approached its acceptable speed limit. Compared with traditional friction brakes, eddy current brakes (ECBs) have the advantages of non-wheel-rail contact, fast response, low noise, wide application range, and no pollution. This study describes an ECB with AC excitation based on Faraday's law. The two-dimensional analytical model of ECB is established by the sub-domain method. Then, the experiment system of rail ECB is designed and built to test the braking characteristics. The authenticity of mathematical model is verified by experimental results. The research indicated that braking force increased at first and reached a maximum value, then slowly decreased with the increase of speed, while normal attractive force was always reduced. This characteristic matches the requirements of high-speed trains with large braking force and low normal force, which makes it a great option for auxiliary braking mechanism in the high-speed trains.

Rheology of graphene oxide stabilized Pickering emulsions.

Pickering emulgels stabilized by graphene oxide (GO) with didodecyldimethylammonium bromide (DDAB) as an auxiliary surfactant and liquid paraffin as the oil phase have proved to be an excellent 3D printable ink. This paper elucidates the structure of such emulgels by a combination of microscopy before and after intensive shear as well as broadband dielectric spectroscopy and rheology in the linear and nonlinear regime. An increase of the DDAB surfactant and GO-contents leads to a systematic increase of modulus and viscosity, a reduction of the limits of the nonlinear regime and a more complicated variation of the normal forces, with negative normal forces at high shear rate  for low GO-contents and positive normal forces at high GO-contents. The interfacial jamming behavior studied by morphology, rheology and dielectric spectroscopy is explained based on droplet deformation, jamming and recovery phenomena.

Encouraging and Detecting Preferential Incipient Slip for Use in Slip Prevention in Robot-Assisted Surgery.

Robotic surgical platforms have helped to improve minimally invasive surgery; however, limitations in their force feedback and force control can result in undesirable tissue trauma or tissue slip events. In this paper, we investigate a sensing method for the early detection of slip events when grasping soft tissues, which would allow surgical robots to take mitigating action to prevent tissue slip and maintain stable grasp control while minimising the applied gripping force, reducing the probability of trauma. The developed sensing concept utilises a curved grasper face to create areas of high and low normal, and thus frictional, force. In the areas of low normal force, there is a higher probability that the grasper face will slip against the tissue. If the grasper face is separated into a series of independent movable islands, then by tracking their displacement it will be possible to identify when the areas of low normal force first start to slip while the remainder of the tissue is still held securely. The system was evaluated through the simulated grasping and retraction of tissue under conditions representative of surgical practice using silicone tissue simulants and porcine liver samples. It was able to successfully detect slip before gross slip occurred with a 100% and 77% success rate for the tissue simulant and porcine liver samples, respectively. This research demonstrates the efficacy of this sensing method and the associated sensor system for detecting the occurrence of tissue slip events during surgical grasping and retraction.

Natural two-dimensional pyrophyllite: Nanoscale lubricant, electrical insulator and easily-machinable material

Pyrophyllite, with the chemical formula Al2Si4O10(OH)2, is a naturally occurring and abundant van der Waals mineral belonging to the group of phyllosilicates. It is very soft, layered crystal used for sculpting and an excellent electrical and thermal insulator aimed for the operation at high pressure and temperature. Here, for the first time, two-dimensional (2D) pyrophyllite obtained by both mechanical and liquid phase exfoliation is presented and investigated at the nanoscale. The layered structure provides low friction coefficient of around 0.1 as measured by friction force microscopy. The wear properties, studied by atomic force microscope (AFM) based scratching, are distinctly different from graphene. Since the wear is initiated at low normal forces, 2D pyrophyllite can be routinely carved by the AFM tip and it is suitable for scratching based nanolithography. According to our optical measurements, 2D pyrophillite is an insulator with a band gap of ∼5.2eV. Local current measurements by conductive AFM reveal that 2D pyrophyllite flakes behave as efficient electrical insulators with a breakdown voltage of around 6MV/cm. Therefore, the obtained results indicate possible applications of 2D pyrophyllite as a low-cost electric insulator and lubricant, as well as an easily-machinable material at the nanoscale.

Analysis of Linear Hybrid Excited Flux Switching Machines with Low-Cost Ferrite Magnets

Linear hybrid excited flux switching machines (LHEFSM) combine the features of permanent magnet flux switching machines (PMFSM) and field excited flux switching machines (FEFSM). Because of the widespread usage of rare-earth PM materials, their costs are steadily rising. This study proposes an LHEFSM, a dual stator LHEFSM (DSLHEFSM), and a dual mover LHEFSM (DMLHEFSM) to solve this issue. The employment of ferrite magnets rather than rare-earth PM in these suggested designs is significant. Compared to traditional designs, the proposed designs feature greater thrust force, power density, reduced normal force, and a 25% decrease in PM volume. A yokeless primary structure was used in a DSLHEFSM to minimize the volume of the mover, increasing the thrust force density. In DMLHELFSM, on the other hand, a yokeless secondary structure was used to lower the secondary volume and the machine’s total cost. Single variable optimization was used to optimize all of the proposed designs. By completing a 3D study, the electromagnetic performances acquired from the 2D analysis were confirmed. Compared to conventional designs, the average thrust force in LHEFSM, DSLHEFSM, and DMLHEFSM was enhanced by 15%, 16.8%, and 15.6%, respectively. Overall, the presented machines had a high thrust force density, a high-power density, a high no-load electromotive force, and a low normal force, allowing them to be used in long-stroke applications.

Numerical and Experimental Studies on the Effects of the TBM Cutter Profile on Rock Cutting

This paper explores the effect of the profile of tunnel boring machine (TBM) cutters on the rock cutting performance via the use of a particle flow discrete element model. Two of the most commonly used cutters, namely a flat-tip cutter and a circular-tip cutter, are considered in the study. Reduced-scale rotary rock cutting experiments are performed to verify the key conclusions obtained by the numerical analysis. The results from both the simulations and experiments indicate that, at the same penetration depth, the volumes of rock chips produced by the two cutters are similar, but the normal force of the circular-tip cutter is much smaller than that of the flat-tip cutter, leading to a lower specific energy. Contact analysis implemented by a finite element model further reveals that the stress field caused by the circular-tip cutter has a higher magnitude and is concentrated in a smaller area, indicating that the circular-tip cutter could penetrate the rock with lower normal force and produce less rock powder, which could avoid excessive rock broken.

Study on shear fracture flow capacity of hard brittle rocks

It is of great significance for the construction and operation of high head pumped storage power station, high dam project and deep tunnel project to master the seepage characteristics and laws of rock mass shear fracture. In this paper, the seepage characteristics of marble shear fractures are summarized as follows. (1) In the radial seepage test, the seepage process conforms to darcy’s law, and there is a linear relationship between head pressure and flow. (2) The characteristics of local tensile failure in the cracks generated under low normal force during the shear process hinder the seepage. (3) The normal force has a significant influence on the fracture seepage. In the process of the normal force increasing from 36kN to 48kN, the seepage characteristics show a sudden change, and the contact form of the shear fracture surface changes, indicating that part of the seepage channel in this stage appears a closed phenomenon with the increase of the normal force, and a large area of closure is formed in the fracture.

Friction maps and wear maps of Ag/MoS2/WS2 nanocomposite with different sliding speed and normal force

The friction maps and wear maps for Ag/MoS2/WS2 nanocomposite under different sliding speeds and normal forces were constructed using the Response Surface Classification Study Method. The investigation found that the lower normal force and the lower sliding speed led to the ploughing wear, and the higher normal force and the higher sliding speed resulted in the oxidation delamination. The lower normal force and the higher sliding speed facilitated the formation of the transfer film. However, the sliding speed of 1.0 m/s and the normal force of 0.25 N showed a stable low friction coefficient and wear rate. The wear mechanism, friction coefficient and wear rate of Ag/MoS2/WS2 nanocomposite were predicted by the wear maps with different sliding speeds and normal forces.

Slow speed friction behaviour of a-C:H with different fs-laser micro-patterns against diamond tip in hyaluronic acid

Slow speed and different normal forces were chosen for investigation of the friction under short impact sliding. The base material is a hydrogenated diamond-like carbon coating (a-C:H). The coating was modified by femtosecond laser micro-patterning (µ-patterning). Three different µ-patterns were generated in the hydrogenated diamond-like carbon (DLC) coating and compared with a-C:H in the “as-deposited” condition. The sliding behaviour was demonstrated without an intermediate medium and with fluid addition. In the latter case, hyaluronic acid gel (HYA) was added. Gentle or deep dimpling by local femtosecond laser ablation resulted in a significantly more variable coefficient of friction (COF) curvature. During dry sliding of the diamond tip against the laser µ-patterned a-C:H, a zig-zag shape of the COF curve was observed. For deep laser µ-patterned DLC under high normal force of 10 N, sudden switches to very low COF down to 0.02 occurred when HYA was added. For hydrogenated DLC with gentle laser µ-patterning, the lowest COF were observed in this study under hyaluronic gel lubrication and low normal forces of 1 N. A volume expansion of the DLC and negative coefficients of friction were observed after loading with 1 N for some coating variants. For laser-induced periodic surface structured (LIPSS) DLC, at low loads of 1 N the COF became higher while at high normal forces like 10 N, a positive trend of LIPSS on the friction behaviour were seen under hyaluronic gel addition. Laser µ-patterning effected graphitization of small or large regions of the a-C:H coating. The defect-rich graphite sections on laser µ-patterned DLCs influenced the crystallisation behaviour of HYA and its distribution.

Taber abrasive wear resistance of organic offshore wind power coatings at varying normal forces

Thirteen organic coatings with three base polymers (epoxy, polysiloxane, polyurethane) were tested in a load-controlled Taber abrasion tester at different normal force levels (2.5 to 25 N). Abrasive wear functions, as well as two partial abrasive wear resistance coefficients, were estimated. Results of scanning electron microscopy (SEM) investigations indicated that both plastic deformation mechanisms and fracture mechanisms caused material removal during the abrasive wear of the materials. The predominant and rate-controlling mechanism depended on normal force and polymer type. Abrasive wear in terms of coating layer thickness loss, as well as the probability of fracture/cracking-based material removal mechanisms, increased with increasing normal force. The ranking of abrasive wear resistance was as follows: epoxy > polysiloxane > polyurethane. The relationship between abrasive wear and normal force followed a power law with power exponents between 0.45 and 1.4. The power exponents were found to depend on the polymer types. The type of polymer was very important for low normal forces, whereas the importance of polymer variation vanished for the higher normal forces.

Hybrid electrostatic and gecko-inspired gripping pads for manipulating bulky, non-smooth items

We present hybrid electrostatic and gecko-inspired gripping pads that help robots to lift and carry bulky objects. The directional, gecko-inspired adhesive provides a controllable shear force with low normal force, providing a very high effective coefficient of friction so that items like bags full of groceries and heavy cardboard boxes can be lifted using only 1–2 N of squeezing force. The addition of electrostatics helps the pads to conform to non-smooth surfaces, providing substantially higher shear forces than with gecko-inspired adhesives alone. The electrostatic effect is enhanced by doping the silicone rubber adhesive material with calcium copper titanate (CCTO), except at the contacting faces of the adhesive microstructures, where a smooth surface finish is important for adhesion. We describe the multimaterial fabrication process used to create the new hybrid adhesive and present the results of robotic lifting experiments with non-smooth bulky items.

A novel high-shear and low-pressure grinding method using specially developed abrasive tools

In this work, a novel grinding method using a special abrasive tool was first proposed to achieve high tangential grinding force and low normal grinding force. The abrasive tool was developed with flexible composites to remove workpiece materials under “high-shear and low-pressure” grinding mode. The concept of the high-shear and low-pressure grinding method was introduced in detail. Grinding tests were carried out on a precision grinder with the developed abrasive tool for single crystal silicon specimens. The results showed that the grinding force ratio between tangential force and normal force for the proposed grinding method was ranged between 0.9 and 1.3, which was three times larger than that of the conventional grinding method. It was verified that the developed abrasive tool possessed the grinding characteristics of high tangential grinding force and low normal grinding force. After grinding of silicon specimens for 120 min, the value of surface roughness decreased from 429.20 to 32.91 nm under the selected grinding conditions. The surface quality of the silicon specimens was greatly improved after grinding.

Experimental investigation on high-shear and low-pressure grinding process for Inconel718 superalloy

In this work, we reported a novel grinding method with high tangential grinding force and low normal grinding force using specially developed grinding tools. The tools were made of flexible composites based on the principle of liquid body armor and the shear thickening mechanism of non-Newtonian fluid. During grinding, abrasive particles are capable of generating a “hydro-cluster effects” under reverse tangential load, which lead to the decreased normal grinding force and the increased tangential grinding force. Hence, workpiece materials are removed under “high-shear and low-pressure” grinding mode. A serial of grinding experiments were carried out on Inconel718. The results showed that the novel grinding tool had an excellent grinding performance on Inconel718 workpieces. The value of surface roughness decreased from Ra 473.7 nm to Ra 153.0 nm under the optimal grinding parameters, i.e., wheel speed of 1 m/s, workpiece speed of 2000 mm/min, and grinding depth of cut of 180 μm. The surface defects of the Inconel718 workpiece were gradually removed. Meanwhile, the uniformed grinding textures were generated. The surface of the grinding tool had residual wear debris, and there was a little loss of grains after 240 grinding cycles.

Step-Change in Friction Under Electrovibration.

Rendering tactile effects on a touch screen via electrovibration has many potential applications. However, our knowledge on tactile perception of change in friction and the underlying contact mechanics are both very limited. In this article, we investigate the tactile perception and the contact mechanics for a step change in friction under electrovibration during a relative sliding between a finger and the surface of a capacitive touch screen. First, we conduct magnitude estimation experiments to investigate the role of normal force and sliding velocity on the perceived tactile intensity for a step increase and decrease in friction, called rising friction (RF) and falling friction (FF). To investigate the contact mechanics involved in RF and FF, we then measure the frictional force, the apparent contact area, and the strains acting on the fingerpad during sliding at a constant velocity under three different normal loads using a custom-made experimental set-up. The results show that the participants perceived RF stronger than FF, and both the normal force and sliding velocity significantly influenced their perception. These results are supported by our mechanical measurements; the relative change in friction, the apparent contact area, and the strain in the sliding direction were all higher for RF than those for FF, especially for low normal forces. Taken together, our results suggest that different contact mechanics take place during RF and FF due to the viscoelastic behavior of fingerpad skin, and those differences influence our tactile perception of a step change in friction.

Effect of Graphite and Bronze Fillers on PTFE Tribological Behavior: A Commercial Materials Evaluation

This work presents a tribological evaluation of the influence of graphite and bronze fillers in two commercial composites in a dry sliding pin-on-disc test at the same contact severity ( parameter) and four testing conditions (varying both normal force and sliding velocity). Both fillers, in comparison to pure PTFE, significantly affected the coefficient of friction (COF) and the mass loss. The sliding distance required to reach steady-state for COF was statistically equal for all three materials. The COF experienced a relevant influence of both normal force and sliding velocity. The graphite filler acted as a solid lubricant and exhibited reductions in COF (∼27% on average) and in mass loss (∼4.2 times lower on average). The bronze filler, on the other hand, had a COF increase of up to 5% for conditions with lower normal force but showed a significant reduction on the specific mass loss (∼5.3 times lower on average). Due to its higher thermal conductivity, the composite filled with graphite presented lower tribosystem temperatures in thermography evaluations. Good agreement of the temperature data was found considering a model that proposes the rate of heat generated by friction. With respect to wear of the composites, evidence of adhesion (transfer films to the counterface) and abrasive (microploughing and microcutting) wear mechanisms was found, in addition to the identification of preferential wear.

Nanotribology of hydrogels with similar stiffness but different polymer and crosslinker concentrations

HypothesisThe stiffness has been found to regulate hydrogel performances and applications. However, the key interfacial properties of hydrogels, like friction and adhesion are not controlled by the stiffness, but are altered by the structure and composition of hydrogels, like polymer volume fraction and crosslinking degree. ExperimentsColloidal probe atomic force microscopy has been use to investigate the relationship between tribological properties (friction and adhesion) and composition of hydrogels with similar stiffness, but different polymer volume fractions and crosslinking degrees. FindingsThe interfacial normal and lateral (friction) forces of hydrogels are not directly correlated to the stiffness, but altered by the hydrogel structure and composition. For normal force measurements, the adhesion increases with polymer volume fraction but decreases with crosslinking degree. For lateral force measurements, friction increases with polymer volume fraction, but decreases with crosslinking degree. In the low normal force regime, friction is mainly adhesion-controlled and increases significantly with the adhesion and polymer volume fraction. In the high normal force regime, friction is predominantly load-controlled and shows slow increase with normal force.

Probing of Nanoscale Friction and Mechanical Characteristics of Cotton Fiber’s Surface

The surface topography and nanomechanical attributes of two samples of cotton fibers, namely, A and B, were characterized with various operation modes of an Atomic Force Microscope (AFM). The surface topography and friction images of the fibers were obtained in contact mode. The nanomechanical properties images—i.e., adhesion and deformation—were obtained in force tapping mode. The results indicate that the surface nanomechanical and nanoscale frictional properties of the fibers vary significantly between two samples. The plots of friction versus normal force of the fibers’ surface from both samples are fitted to the equation of single-asperity, adhesion-controlled friction. Nevertheless, within the range of the applied normal force, the friction curves of sample A surfaces show a characteristic transition phase. That is, under low normal forces, the friction curves closely conform with the Hertzian component of friction; after the transition takes place at higher normal forces, the friction curves follow Amontons’ law of friction. We demonstrated that the transition phase corresponds to a state at which the cuticle layer molecules are displaced from the fibers’ surface. The average adhesion force of the samples is consistent with the average friction signal strength collected under low normal forces.

Low-Cost, Continuously Variable, Strain Wave Transmission Using Gecko-Inspired Adhesives

In robotics, high gear reductions are often required when using an electromagnetic motor to drive revolute joints, as in a humanoid robot. Strain wave gears (SWGs), also known as harmonic drives, are often used. However, these transmissions are relatively expensive and have a fixed gear ratio. Here, we present a low-cost transmission that maintains the high reduction of the SWG and enables a continuously variable gear ratio. We achieve this by, first, replacing the teeth of an SWG with a smooth frictional contact to enable continuous gear variation and, second, inverting the gear elements to enable high frictional torque transfer with a low preload via the capstan effect. We choose a gecko-inspired dry adhesive as the frictional material because it has high friction at low normal forces, and is not inherently tacky. We present a model describing the transmission, as well as experimental data gathered from a prototype device that verifies this model and establishes a proof of concept. Our transmission expands the possibilities for low-cost robotic systems.