Showing papers on "Contact area published in 2018"

Tailoring force sensitivity and selectivity by microstructure engineering of multidirectional electronic skins

Abstract: Electronic skins (e-skins) with high sensitivity to multidirectional mechanical stimuli are crucial for healthcare monitoring devices, robotics, and wearable sensors. In this study, we present piezoresistive e-skins with tunable force sensitivity and selectivity to multidirectional forces through the engineered microstructure geometries (i.e., dome, pyramid, and pillar). Depending on the microstructure geometry, distinct variations in contact area and localized stress distribution are observed under different mechanical forces (i.e., normal, shear, stretching, and bending), which critically affect the force sensitivity, selectivity, response/relaxation time, and mechanical stability of e-skins. Microdome structures present the best force sensitivities for normal, tensile, and bending stresses. In particular, microdome structures exhibit extremely high pressure sensitivities over broad pressure ranges (47,062 kPa−1 in the range of <1 kPa, 90,657 kPa−1 in the range of 1–10 kPa, and 30,214 kPa−1 in the range of 10–26 kPa). On the other hand, for shear stress, micropillar structures exhibit the highest sensitivity. As proof-of-concept applications in healthcare monitoring devices, we show that our e-skins can precisely monitor acoustic waves, breathing, and human artery/carotid pulse pressures. Unveiling the relationship between the microstructure geometry of e-skins and their sensing capability would provide a platform for future development of high-performance microstructured e-skins. Customizing piezoresistive sensors with different microscale geometries makes it easier for ‘electronic skin’ devices to sense forces in different directions. Hyunhyub Ko and colleagues from South Korea’s Ulsan National Institute of Science and Technology report that carbon nanotube/silicone elastomer composites fabricated into three shapes—micro-domes, pyramids, and pillars—have unique responses to mechanical stress and deformations. Experiments with interlocked pairs of micropatterned films revealed hemispherical shapes were best at sensing tensile and bending stresses, as well as minute changes to pressure. Micropillars, on the other hand, exhibited strong sensitivity to shear stress. With help from computer simulations, the researchers identified changes in contact area and localized stress as the critical factors needed to guide design of multidirectional force sensitivity. Prototype e-skin devices containing the interlocked microshapes successfully monitored bio-signals including breath patterns, spoken words, and arterial blood pressure. We present piezoresistive electronic skins with tunable force sensitivity and selectivity in response to multidirectional forces (normal, shear, tensile, bending) by engineering microstructure geometries (dome, pyramid, pillar). Microdome structures present the best force sensitivities for normal, tensile, and bending stresses. On the other hand, for shear stress, micropillar structures exhibit the highest sensitivity. As proof-of-concept demonstrations, the e-skins are used for wearable healthcare devices to precisely monitor various bio-signals including sound, human breath, and artery/carotid pulse pressures.

Evolution of real contact area under shear and the value of static friction of soft materials.

Abstract: The frictional properties of a rough contact interface are controlled by its area of real contact, the dynamical variations of which underlie our modern understanding of the ubiquitous rate-and-state friction law. In particular, the real contact area is proportional to the normal load, slowly increases at rest through aging, and drops at slip inception. Here, through direct measurements on various contacts involving elastomers or human fingertips, we show that the real contact area also decreases under shear, with reductions as large as 30%, starting well before macroscopic sliding. All data are captured by a single reduction law enabling excellent predictions of the static friction force. In elastomers, the area-reduction rate of individual contacts obeys a scaling law valid from micrometer-sized junctions in rough contacts to millimeter-sized smooth sphere/plane contacts. For the class of soft materials used here, our results should motivate first-order improvements of current contact mechanics models and prompt reinterpretation of the rate-and-state parameters.

Molecular probes reveal deviations from Amontons' law in multi-asperity frictional contacts.

Abstract: Amontons' law defines the friction coefficient as the ratio between friction force and normal force, and assumes that both these forces depend linearly on the real contact area between the two sliding surfaces. However, experimental testing of frictional contact models has proven difficult, because few in situ experiments are able to resolve this real contact area. Here, we present a contact detection method with molecular-level sensitivity. We find that while the friction force is proportional to the real contact area, the real contact area does not increase linearly with normal force. Contact simulations show that this is due to both elastic interactions between asperities on the surface and contact plasticity of the asperities. We reproduce the contact area and fine details of the measured contact geometry by including plastic hardening into the simulations. These new insights will pave the way for a quantitative microscopic understanding of contact mechanics and tribology.

Metallic Bond-Enabled Wetting Behavior at the Liquid Ga/CuGa2 Interfaces

Abstract: Interface interaction can strongly modify contact angle, adsorption energy, interfacial tension, and composition of the contact area. In particular, the interfaces between gallium-based liquid metal (LM) and its intermetallic layer present many mysterious and peculiar wetting phenomena, which have not been fully realized up to now. Here in this study, we found that a gallium-based liquid metal droplet can quickly transform into a puddle on the CuGa2 surface through a spreading–wetting procedure. The mechanism lying behind this phenomenon can be ascribed to the formation of an intermetallic CuGa2 on Cu plate surface, which provides a stable metallic bond to induce the wetting behavior. For a quantitative evaluation of the interface force, a metallic bond-enabled wetting model is established on the basis of the density functional theory. The first-principles density functional calculations are then performed to examine the work function, density of states, and adsorption energy. The predicted results show t...

Highly sensitive flexible three-axis tactile sensors based on the interface contact resistance of microstructured graphene

Abstract: The lack of high-performance tactile sensors, especially for pressure/force, is a huge obstacle for the widespread application of intelligent robots. Current pressure sensors are often operated in the high range of pressure and normal direction, showing a little ability in the low range of pressure and three-axis direction simultaneously. Herein, a highly sensitive flexible tactile sensor with three-axis force sensing capacity is presented by combining microstructured polydimethylsiloxane (PDMS) arrays and a reduced graphene oxide (rGO) film. The deformation of microstructured rGO/PDMS results in a change in the contact area between the rGO film and electrode, leading to a high sensitivity of -1.71 kPa-1 in the low range pressure of 0-225 Pa with a fast response time of 6 ms at a large feature size of 100 μm. To realize three-axis sensing, a sensing unit was built up, which was composed of the adjacent four parts of patterns and electrodes underneath a bump. A mechanical model of the exerted spatial force was established to calculate each axis force component via the deformation of the rGO/PDMS pattern. The experimental results show that the current difference between the adjacent two parts has a strong relationship with the applied force. As a proof of concept, we have demonstrated a 3 × 3 array sensor for arbitrary force sensing. Our tactile sensor would be used in transmitting information from a gentle spatial force and would exhibit broad applications as e-skin in integrated robots.

Elastic Contact Mechanics of Randomly Rough Surfaces: An Assessment of Advanced Asperity Models and Persson’s Theory

Abstract: In this work, we discuss important improvements of asperity models. Specifically, we assess the predictive capabilities of a recently developed multiasperity model, which differs from the original Greenwood and Williamson model by (i) including the coupling between the elastic fields generated by each contact spot, and (ii) taking into account the coalescence among the contact areas, occurring during the loading process. Interaction of the elastic field is captured by summing the contributions, which are analytically known, of the elastic displacements in a given point of the surface due to each Hertzian-like contact spot. The coalescence is instead considered by defining an equivalent contact spot in such a way to guarantee conservation of contact area during coalescence. To evaluate the accuracy of the model, a comparison with fully numerical ‘exact’ calculations and Persson’s contact mechanics theory of elastic rough surfaces is proposed. Results in terms of contact area versus load and separation versus load show that the three approaches give almost the same predictions, while traditional asperity models neglecting coalescence and elastic coupling between contact regions are unable to correctly capture the contact behavior. Finally, very good results are also obtained when dealing with the probability distribution of interfacial stresses and gaps.

Electrical devices from top-down structured platinum diselenide films

Abstract: Platinum diselenide (PtSe2) is an exciting new member of the two-dimensional (2D) transition metal dichalcogenide (TMD) family. It has a semimetal to semiconductor transition when approaching monolayer thickness and has already shown significant potential for use in device applications. Notably, PtSe2 can be grown at low temperature making it potentially suitable for industrial usage. Here, we address thickness-dependent transport properties and investigate electrical contacts to PtSe2, a crucial and universal element of TMD-based electronic devices. PtSe2 films have been synthesized at various thicknesses and structured to allow contact engineering and the accurate extraction of electrical properties. Contact resistivity and sheet resistance extracted from transmission line method (TLM) measurements are compared for different contact metals and different PtSe2 film thicknesses. Furthermore, the transition from semimetal to semiconductor in PtSe2 has been indirectly verified by electrical characterization in field-effect devices. Finally, the influence of edge contacts at the metal–PtSe2 interface has been studied by nanostructuring the contact area using electron beam lithography. By increasing the edge contact length, the contact resistivity was improved by up to 70% compared to devices with conventional top contacts. The results presented here represent crucial steps toward realizing high-performance nanoelectronic devices based on group-10 TMDs. Transport measurements on channels of layered PtSe2 give insight into the realization of high-performance nanoelectronic PtSe2 devices. A team led by Niall McEvoy at Trinity College Dublin investigated the electrical contact properties of PtSe2 channels with controlled dimensions and thicknesses. Electron beam lithography was used to fabricate structures with different contact metals and different PtSe2 film thicknesses, and the corresponding contact resistivity and sheet resistance of the PtSe2 devices were extracted from transmission line method measurements. The charge-transport characteristics of the PtSe2 devices revealed that edge-contacted structures are able reduce the contact resistivity when compared to conventional devices with top contacts, thanks to enhancement of the carrier injection at the contacts. These results may pave the way to optimal design of PtSe2 nanoelectronic devices.

Contact mechanics between the human finger and a touchscreen under electroadhesion

Abstract: 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.

Asperity-Level Origins of Transition from Mild to Severe Wear.

Abstract: Wear is the inevitable damage process of surfaces during sliding contact. According to the well-known Archard's wear law, the wear volume scales with the real contact area and as a result is proportional to the load. Decades of wear experiments, however, show that this relation only holds up to a certain load limit, above which the linearity is broken and a transition from mild to severe wear occurs. We investigate the microscopic origins of this breakdown and the corresponding wear transition at the asperity level. Our atomistic simulations reveal that the interaction between subsurface stress fields of neighboring contact spots promotes the transition from mild to severe wear. The results show that this interaction triggers the deep propagation of subsurface cracks and the eventual formation of large debris particles, with a size corresponding to the apparent contact area of neighboring contact spots. This observation explains the breakdown of the linear relation between the wear volume and the normal load in the severe wear regime. This new understanding highlights the critical importance of studying contact beyond the elastic limit and single-asperity models.

Heart Valve Flow Computation with the Space–Time Slip Interface Topology Change (ST-SI-TC) Method and Isogeometric Analysis (IGA)

Abstract: We present a heart valve flow computation with the Space–Time Slip Interface Topology Change (ST-SI-TC) method and Isogeometric Analysis (IGA). The computation is for a realistic heart valve model with actual contact between the valve leaflets. The ST-SI-TC method integrates the ST-SI and ST-TC methods in the framework of the ST Variational Multiscale (ST-VMS) method. The ST-VMS method functions as a moving-mesh method, which maintains high-resolution boundary layer representation near the solid surfaces. The ST-TC method was introduced for moving-mesh computation of flow problems with TC, such as contact between the leaflets of a heart valve. It deals with the contact while maintaining high-resolution representation near the leaflet surfaces. The ST-SI method was originally introduced to addresses the challenge involved in high-resolution representation of the boundary layers near spinning solid surfaces. The mesh covering a spinning solid surface spins with it, and the SI between that mesh and the rest of the mesh accurately connects the two sides. This maintains the high-resolution representation near solid surfaces. In the context of heart valves, the SI connects the sectors of meshes containing the leaflets, enabling a more effective mesh moving. In that context, the ST-SI-TC method enables high-resolution representation even when the contact is between leaflets that are covered by meshes with SI. It also enables dealing with contact location change or contact and sliding on the SI. With IGA, in addition to having a more accurate representation of the surfaces and increased accuracy in the flow solution, the element density in the narrow spaces near the contact areas is kept at a reasonable level. Furthermore, because the flow representation in the contact area has a wider support in IGA, the flow computation method becomes more robust. The computation we present for an aortic-valve model shows the effectiveness of the ST-SI-TC-IGA method.

A Novel Surface Texture Shape for Directional Friction Control

Abstract: An experimental study is presented to evaluate the influence of anisotropically shaped textures on the behaviour of sliding friction and sensitivity to sliding direction. The plate samples were textured with triangular sloped dimples using an ultrafast laser surface texturing technique. Reciprocating cylinder-on-plate tests were conducted with steel sliding pairs using mineral base oil as a lubricant to compare the tribological performance of reference non-textured specimen and dimpled samples. The dimples were designed with varying converging angles in the transverse y–z plane and top-view x–y plane. In this study, no dimple was fully covered in the contact area since the dimples size is much larger than the Hertzian line contact width. Stribeck style dynamic friction curves across boundary, mixed and hydrodynamic lubrication regimes were used to determine the benefit or antagonism of texturing. Observation of the directional friction effect of the anisotropic textures indicated that the converging shapes are beneficial for friction reduction, and the dimpled specimens have a lower friction coefficient particular under prevailing boundary lubrication conditions. It was also found that the real contact length variation rate is a major factor controlling the local friction response. The sloped bottoms of the textures produce effective converging wedge action to generate hydrodynamic pressure and contribute to the overall directional friction effects.

Mechanism of Antiwear Property Under High Pressure of Synthetic Oil-Soluble Ultrathin MoS2 Sheets as Lubricant Additives.

Abstract: Wear occurs between two rubbing surfaces. Severe wear due to seizure under high pressure leads to catastrophic failures of mechanical systems and raises wide concerns. In this paper, a kind of synthetic oil-soluble ultrathin MoS2 sheets is synthesized and investigated as lubricant additives between steel surfaces. It is found that, with the ultrathin MoS2 sheets, the wear can be controlled under the nominal pressure of about 1 GPa, whereas the bearable nominal pressure for traditional lubricants is only a few hundred megapascals. It is found that when wear is under control, the real pressure between the asperities agrees with the breaking strength of ultrathin MoS2. Therefore, it is believed that, because of the good oil solubility and ultrasmall thickness, the ultrathin MoS2 sheets can easily enter the contact area between the contacting asperities. Then, the localized seizure and further wear are prevented because there will be no metal-to-metal contact as long as the real pressure between the asperitie...

Do uniform tangential interfacial stresses enhance adhesion

Abstract: We present theoretical arguments, based on linear elasticity and thermodynamics, to show that interfacial tangential stresses in sliding adhesive soft contacts may lead to a significant increase of the effective energy of adhesion. A sizable expansion of the contact area is predicted in conditions corresponding to such scenario. These results are easily explained and are valid under the assumptions that: (i) sliding at the interface does not lead to any loss of adhesive interaction and (ii) spatial fluctuations of frictional stresses can be considered negligible. Our results are seemingly supported by existing experiments, and show that frictional stresses may lead to an increase of the effective energy of adhesion depending on which conditions are established at the interface of contacting bodies in the presence of adhesive forces.

Experimental Investigation of the Peak Shear Strength Criterion Based on Three-Dimensional Surface Description

Abstract: The three-dimensional (3D) morphology of joints is enormously important for the shear mechanical properties of rock. In this study, three-dimensional morphology scanning tests and direct shear tests are conducted to establish a new peak shear strength criterion. The test results show that (1) surface morphology and normal stress exert significant effects on peak shear strength and distribution of the damage area. (2) The damage area is located at the steepest zone facing the shear direction; as the normal stress increases, it extends from the steepest zone toward a less steep zone. Via mechanical analysis, a new formula for the apparent dip angle is developed. The influence of the apparent dip angle and the average joint height on the potential contact area is discussed, respectively. A new peak shear strength criterion, mainly applicable to specimens under compression, is established by using new roughness parameters and taking the effects of normal stress and the rock mechanical properties into account. A comparison of this newly established model with the JRC–JCS model and the Grasselli’s model shows that the new one could apparently improve the fitting effect. Compared with earlier models, the new model is simpler and more precise. All the parameters in the new model have clear physical meanings and can be directly determined from the scanned data. In addition, the indexes used in the new model are more rational.

A flexible comb electrode triboelectric–electret nanogenerator with separated microfibers for a self-powered position, motion direction and acceleration tracking sensor

Abstract: In this paper, we report a flexible comb electrode triboelectric–electret coupling nanogenerator using a separated friction microfiber object for self-powered position, motion direction and acceleration tracking sensing and its energy harvesting. The power was generated from the coupling of the electrostatic and triboelectric effects among a separated triboelectric object, a polytetrafluoroethylene (PTFE) film and interdigital electrodes. Under an acceleration of 1 m s−2, we found that the corona charged PTFE film with a sliding motion reached ∼3 times more short-circuit current (Isc) and ∼6 times more open-circuit voltage (Voc) than the PTFE film without corona charges, respectively. The device can be a good self-powered acceleration tracking sensor, where the reasons were a stability voltage out during the process of separated components frictions and a numerical relationship of Isc with increased accelerations. Moreover, potential applications using several different separated materials such as a bulk of carbon microfibers, a finger sliding with a nylon glove, and water microdrops were used to show the energy harvesting of various friction materials and their effective contact area. It was found that a much larger contact area “finger” sliding showed a Voc of ∼45 V, whereas a smaller contact area “microdrop” sliding on the prototype presented a maximum peak Voc of ∼1.8 V.

Perception of partial slips under tangential loading of the fingertip.

Abstract: During tactile exploration, partial slips occur systematically at the periphery of fingertip-object contact prior to full slip. Although the mechanics of partial slips are well characterized, the perception of such events is unclear. Here, we performed psychophysical experiments to assess partial slip detection ability on smooth transparent surfaces. In these experiments, the index fingertip of human subjects was stroked passively by a smooth, transparent glass plate while we imaged the contact slipping against the glass. We found that subjects were able to detect fingertip slip before full slip occurred when, on average, only 48% of the contact area was slipping. Additionally, we showed that partial slips and plate displacement permitted slip detection, but that the subjects could not rely on tangential force to detect slipping of the plate. Finally, we observed that, keeping the normal contact force constant, slip detection was impeded when the plate was covered with a hydrophobic coating dramatically lowering the contact friction and therefore the amount of fingertip deformation. Together, these results demonstrate that partial slips play an important role in fingertip slip detection and support the hypothesis that the central nervous system relies on them to adjust grip force during object manipulation.