Interfacial processes underlying the temperature-dependence of friction and wear of calcite single crystals.

HYPOTHESIS: This study posits that thermal effects play a substantial role in influencing interfacial processes on calcite, and consequently impacting its mechanochemical properties. EXPERIMENTS: This work interrogates the temperature-dependence of friction and wear at nanoscale contacts with calcite single crystals at low air humidity (≤ 3-10 % RH) by AFM. FINDINGS: Three logarithmic regimes for the velocity-dependence of friction are identified. BelowTc âˆ¼ 70 °C, where friction increases with T, there is a transition from velocity-weakening (W1) to velocity-strengthening friction (S1). AboveTc âˆ¼ 70 °C, where friction decreases with T, a second velocity-strengthening friction regime (S0) precedes velocity-weakening friction (W1). The low humidity is sufficient to induce atomic scale changes of the calcite cleavage plane due to dissolution-reprecipitation, and more so at higher temperature and 10 % RH. Meanwhile, the surface softens above Tc -likely owing to lattice dilation, hydration and amorphization. These interfacial changes influence the wear mechanism, which transitions from pit formation to plowing with increase in temperature. Furthermore, the softening of the surface justifies the appearance of the second velocity-strengthening friction regime (S0). These findings advance our understanding of the influence of temperature on the interfacial and mechanochemical processes involving calcite, with implications in natural processes and industrial manufacturing.

Surface Curvature Enhances the Electrotunability of Ionic Liquid Lubrication.

Ionic liquids (ILs) are a promising class of lubricants that allow dynamic friction control at electrified interfaces. In the real world, surfaces inevitably exhibit some degree of roughness, which can influence lubrication. In this work, we deposited single-layer graphene onto 20 nm silica nanoparticle films to investigate the effect of surface curvature and electrostatic potential on both the lubricious behavior and interfacial layering structure of 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide on graphene. Normal force and friction force measurements were conducted by atomic force microscopy using a sharp silicon tip. Our results reveal that the friction coefficient at the lubricated tip-graphene contacts significantly depends on surface curvature. Two friction coefficients are measured on graphene peaks and valleys with a higher coefficient measured at lower loads (pressures), whereas only one friction coefficient is measured on smooth graphene. Moreover, the electrotunability of the friction coefficient at low loads is observed to be significantly enhanced in peaks and valleys compared with smooth graphene. This is associated with the promoted overscreening of surface charge on convex interfaces and the steric hindrance at concave interfaces, which leads to more layers of ions (electrostatically) bound to the surface, i.e., thicker boundary films (electrical double layers). This work opens new avenues to control IL lubrication on the nanoscale by combining topographic features and an electric field.

Dynamically tuning friction at the graphene interface using the field effect.

Dynamically controlling friction in micro- and nanoscale devices is possible using applied electrical bias between contacting surfaces, but this can also induce unwanted reactions which can affect device performance. External electric fields provide a way around this limitation by removing the need to apply bias directly between the contacting surfaces. 2D materials are promising candidates for this approach as their properties can be easily tuned by electric fields and they can be straightforwardly used as surface coatings. This work investigates the friction between single layer graphene and an atomic force microscope tip under the influence of external electric fields. While the primary effect in most systems is electrostatically controllable adhesion, graphene in contact with semiconducting tips exhibits a regime of unexpectedly enhanced and highly tunable friction. The origins of this phenomenon are discussed in the context of fundamental frictional dissipation mechanisms considering stick slip behavior, electron-phonon coupling and viscous electronic flow.

Thin-Film Rheology and Tribology of Imidazolium Ionic Liquids.

Ionic liquids (ILs) are organic molten salts with low-temperature melting points that hold promise as next-generation environmentally friendly boundary lubricants. This work examines the relationship between tribological and rheological behavior of thin films of five imidazolium ILs using a surface force apparatus to elucidate lubrication mechanisms. When confined to films of a few nanometers, the rheological properties change drastically as a function of the number of confined ion layers; not only the viscosity increases by several orders of magnitude but ILs can also undergo a transition from Newtonian to viscoelastic fluid and to an elastic solid. This behavior can be justified by the confinement-induced formation of supramolecular clusters with long relaxation times. The quantized friction coefficient is explained from the perspective of the strain relaxation via diffusion of these supramolecular clusters, where higher friction correlates with longer relaxation times. A deviation from this behavior is observed only for 1-ethyl-3-methylimidazolium ethylsulfate ([C2C1Im][EtSO4]), characterized by strong hydrogen bonding; this is hypothesized to restrict the reorganization of the confined IL into clusters and hinder (visco)elastic behavior, which is consistent with the smallest friction coefficient measured for this IL. We also investigate the contrasting influence of traces of water on the thin-film rheology and tribology of a hydrophobic IL, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [C2C1Im][FAP], and a hydrophilic IL, [C2C1Im][EtSO4]. [C2C1Im][EtSO4] remains Newtonian under both dry and humid conditions and provides the best lubrication, while [C2C1Im][FAP], characterized by a prominent solid-like behavior under both conditions, is a poor lubricant. The results of this study may inspire molecular designs to enable efficient IL lubrication.

Water in the Electrical Double Layer of Ionic Liquids on Graphene.

The performance of electrochemical devices using ionic liquids (ILs) as electrolytes can be impaired by water uptake. This work investigates the influence of water on the behavior of hydrophilic and hydrophobic ILs─with ethylsulfate and tris(perfluoroalkyl)trifluorophosphate or bis(trifluoromethyl sulfonyl)imide (TFSI) anions, respectively─on electrified graphene, a promising electrode material. The results show that water uptake slightly reduces the IL electrochemical stability and significantly influences graphene's potential of zero charge, which is justified by the extent of anion depletion from the surface. Experiments confirm the dominant contribution of graphene's quantum capacitance (CQ) to the total interfacial capacitance (Cint) near the PZC, as expected from theory. Combining theory and experiments reveals that the hydrophilic IL efficiently screens surface charge and exhibits the largest double layer capacitance (CIL ∼ 80 µF cm-2), so that CQ governs the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (CIL ∼ 6-9 µF cm-2), which governs Cint already at small potentials. An increase in the total interfacial capacitance is observed at positive voltages for humid TFSI-ILs relative to dry ones, consistent with the presence of a satellite peak. Short-range surface forces reveal the change of the interfacial layering with potential and water uptake owing to reorientation of counterions, counterion binding, co-ion repulsion, and water enrichment. These results are consistent with the charge being mainly stored in a ∼2 nm-thick double layer, which implies that ILs behave as highly concentrated electrolytes. This knowledge will advance the design of IL-graphene-based electrochemical devices.

Insight into the assembly of lipid-hyaluronan complexes in osteoarthritic conditions.

Interactions between molecules in the synovial fluid and the cartilage surface may play a vital role in the formation of adsorbed films that contribute to the low friction of cartilage boundary lubrication. Osteoarthritis (OA) is the most common degenerative joint disease. Previous studies have shown that in OA-diseased joints, hyaluronan (HA) not only breaks down resulting in a much lower molecular weight (MW), but also its concentration is reduced ten times. Here, we have investigated the structural changes of lipid-HA complexes as a function of HA concentration and MW to simulate the physiologically relevant conditions that exist in healthy and diseased joints. Small angle neutron scattering and dynamic light scattering were used to determine the structure of HA-lipid vesicles in bulk solution, while a combination of atomic force microscopy and quartz crystal microbalance was applied to study their assembly on a gold surface. We infer a significant influence of both MW and HA concentrations on the structure of HA-lipid complexes in bulk and assembled on a gold surface. Our results suggest that low MW HA cannot form an amorphous layer on the gold surface, which is expected to negatively impact the mechanical integrity and longevity of the boundary layer and could contribute to the increased wear of the cartilage that has been reported in joints diseased with OA.

Intrinsic and Extrinsic Tunability of Double-Network Hydrogel Strength and Lubricity.

Double-network (DN) hydrogels are promising materials for tissue engineering due to their biocompatibility, high strength, and toughness, but understanding of their microstructure-property relationships still remains limited. This work investigates a DN hydrogel comprising a physically crosslinked agarose, as the first network, and a chemically crosslinked copolymer with a varying ratio of acrylamide and acrylic acid, as the second network. The charge, intrinsic to most DN hydrogels, introduces a responsive behavior to chemical and electrical stimuli. The DN strengthens agarose hydrogels, but the strengthening decreases with the swelling ratio resulting from increasing acrylic acid content or reducing salt concentration. Through careful imaging by atomic force microscopy, the heterogenous surface structure and properties arising from the DN are resolved, while the lubrication mechanisms are elucidated by studying the heterogeneous frictional response to extrinsic stimuli. This method reveals the action of the first (agarose) network (forming grain boundaries), copolymer-rich and poor regions (in grains), charge and swelling in providing lubrication. Friction arises from the shear of the polymeric network, whereas hydrodynamic lift and viscoelastic deformation become more significant at higher sliding velocities. We identify the copolymer-rich phase as the main source of the stimulus-responsive behavior. Salt concentration enhances effective charge density and reduces viscoelastic deformation, while electric bias swells the gel and improves lubrication. This work also demonstrates the dynamic control of interfacial properties like hydrogel friction and adhesion, which has implications for other areas of study like soft robotics and tissue replacements.

Limits to Crystallization Pressure.

Crystallization pressure drives deformation and damage in monuments, buildings, and the Earth's crust. Even though the phenomenon has been known for 170 years, there is no agreement between theoretical calculations of the maximum attainable pressure and experimentally measured pressures. We have therefore developed a novel experimental technique to image the nanoconfined crystallization process while controlling the pressure and applied it to calcite. The results show that displacement by crystallization pressure is arrested at pressures well below the thermodynamic limit. We use existing molecular dynamics simulations and atomic force microscopy data to construct a robust model of the disjoining pressure in this system and thereby calculate the absolute distance between the surfaces. On the basis of the high-resolution experiments and modeling, we formulate a novel mechanism for the transition between damage and adhesion by crystallization that may find application in Earth and materials sciences and in conservation of cultural heritage.

Velocity-weakening and -strengthening friction at single and multiasperity contacts with calcite single crystals.

SignificanceThe empirical nature of rate-and-state friction (RSF) equations remains a drawback to their application to predict earthquakes. From nanoscale friction measurements on smooth and rough calcite crystals, a set of parameters is analyzed to elucidate microscopic processes dictating RSF. We infer the influence of roughness on the velocity dependence of friction in dry environment and that atomic attrition leads to stick-slip instabilities at slow velocities. In fault dynamics, stick-slip is associated with seismic slips. The aqueous environment eliminates atomic attrition and stick-slip and dissolves calcite under pressure. This yields remarkable lubrication, even more so in rough contacts, and suggests an alternative pathway for seismic slips. This work has implications for understanding mechanisms dictating fault strength and seismicity.

Rheological Characteristics of Ionic Liquids under Nanoconfinement.

While the dynamic properties of ionic liquids (ILs) in nanoconfinement play a crucial role in the performance of IL-based electrochemical and mechanical devices, experimental work mostly falls short at reporting "solid-like" versus "liquid-like" behavior of confined ILs. The present work is the first to conduct frequency-sweep oscillatory-shear rheology on IL nanofilms, reconciling the solid-versus-liquid debate and revealing the importance of shear rate in the behavior. We disentangle and analyze the viscoelasticity of nanoconfined ILs and shed light on their relaxation mechanisms. Furthermore, a master curve describes the scaling of the dynamic behavior of four (non-hydrogen-bonding) ILs under nanoconfinement and reveals the role of the compressibility of the flow units.

Using Patterned Self-Assembled Monolayers to Tune Graphene-Substrate Interactions.

Graphene has unique mechanical, electronic, and optical properties that make it of interest for an array of applications. These properties can be modulated by controlling the architecture of graphene and its interactions with surfaces. Self-assembled monolayers (SAMs) can tailor graphene-surface interactions; however, spatially controlling these interactions remains a challenge. Here, we blend colloidal lithography with varying SAM chemistries to create patterned architectures that modify the properties of graphene based on its chemical interactions with the substrate and to study how these interactions are spatially arrayed. The patterned systems and their resulting structural, nanomechanical, and optical properties have been characterized using atomic force microscopy, Raman and infrared spectroscopies, scattering-type scanning near-field optical microscopy, and X-ray photoelectron spectroscopy.

Nanoscale insight into the relation between pressure solution of calcite and interfacial friction.

Pressure solution of carbonate-based rocks participates in many geophysical and geochemical processes, but fundamental knowledge of the interfacial processes is still lacking. By concurrently pressing and sliding two single calcite crystals past each other, the pressure solution rate and the friction force between the crystals were concurrently measured in calcium-carbonate saturated water with an extended surface forces apparatus. These studies reveal that both a decrease and an increase in frictional strength can originate from the pressure-solution of calcite single crystals. By conducting nanoscale force measurements with an atomic force microscope, ion specific effects were unveiled at the level of a single asperity. Pressure solution is promoted when the interfacial water layers of calcite remain undisturbed under stress (e.g. with Ca(II)) and the dissolved ions and water lubricate the interface - a phenomenon called pressure-solution facilitated slip. The mechanically induced disruption of the hydration layers of the calcite surface (e.g. with Mg(II) and low Ni(II) concentration) correlates with the more fluid-like and lubricious behavior of the confined fluid in the absence of pressure solution. Charge neutralization of the calcite surface leads to an abrupt change of calcite's hydration layers, which promotes pressure-solution facilitated slip. This work advances the fundamental understanding of physicochemical interactions occurring at confined surfaces of stressed calcite.

Effects of Layering and Supporting Substrate on Liquid Slip at the Single-Layer Graphene Interface.

Understanding modulation of liquid molecule slippage along graphene surfaces is crucial for many promising applications of two-dimensional materials, such as in sensors, nanofluidic devices, and biological systems. Here, we use force measurements by atomic force microscopy (AFM) to directly measure hydrodynamic, solvation, and frictional forces along the graphene plane in seven liquids. The results show that the greater slip lengths correlate with the interfacial ordering of the liquid molecules, which suggests that the ordering of the liquid forming multiple layers promotes slip. This phenomenon appears to be more relevant than solely the wetting behavior of graphene or the solid-liquid interaction energy, as traditionally assumed. Furthermore, the slip boundary condition of the liquids along the graphene plane is sensitive to the substrate underneath graphene, indicating that the underlying substrate affects graphene's interaction with the liquid molecules. Because interfacial slip can have prominent consequences on the pressure drop, on electrical and diffusive transport through nanochannels, and on lubrication, this work can inspire innovation in many applications through the modulation of the substrate underneath graphene and of the interfacial ordering of the liquid.

Nanoheterogeneity of LiTFSI Solutions Transitions Close to a Surface and with Concentration.

Water-in-salt (WIS) electrolytes composed of 21 m LiTFSI have recently emerged as a safe and environmentally friendly alternative to conventional organic electrolytes in Li-ion batteries. Several studies have emphasized the relation between the high conductivity of WIS electrolytes and their nanoscale structure. Combining force measurements with a surface forces apparatus and atomic force microscopy, this study describes the nanoheterogeneity of LiTFSI solutions as a function of concentration and distance from a negatively charged (mica) surface. We report various nanostructures coexisting in the WIS electrolyte, whose size increases with concentration and is influenced by the proximity of the mica surface. Two key concentration thresholds are identified, beyond which a transition of behavior is observed. The careful scrutinization on the concentration-dependent nanostructures lays groundwork for designing novel electrolytes in future energy storage devices.

Transient stiffening of cartilage during joint articulation: A microindentation study.

As a mechanoactive tissue, articular cartilage undergoes compression and shear on a daily basis. With the advent of high resolution and sensitive mechanical testing methods, such as micro- and nanoindentation, it has become possible to assess changes in small-scale mechanical properties due to compression and shear of the tissue. However, investigations on the changes of these properties before and after joint articulation have been limited. To simulate articular loading of cartilage in the context of human gait, a previously developed bioreactor system was used. Immediately after bioreactor testing, the stiffness was measured using microindentation. Specifically, we investigated whether the mechanical response of the tissue was transient or permanent, dependent on counterface material, and an effect limited to the superficial zone of cartilage. We found that cartilage surface stiffness increases immediately after articular loading and returns to baseline values within 3 hr. Cartilage-on-cartilage stiffening was found to be higher compared to both alumina- and cobalt chromium-on-cartilage stiffening, which were not significantly different from each other. This stiffening response was found to be unique to the superficial zone, as articular loading on cartilage with the superficial zone removed showed no changes in stiffness. The findings of this study suggest that the cartilage superficial zone may adapt its stiffness as a response to articular loading. As the superficial zone is often compromised during the course of osteoarthritic disease, this finding is of clinical relevance, suggesting that the load-bearing function deteriorates over time.

Correction: Nanoscale insight into the degradation mechanisms of the cartilage articulating surface preceding OA.

Correction for 'Nanoscale insight into the degradation mechanisms of the cartilage articulating surface preceding OA' by Tooba Shoaib, et al., Biomater. Sci., 2020, 8, 3944-3955, DOI: 10.1039/D0BM00496K.

Nanoscale insight into the degradation mechanisms of the cartilage articulating surface preceding OA.

Osteoarthritis (OA) is a degenerative joint disease and a leading cause of disability globally. In OA, the articulating surface of cartilage is compromised by fissures and cracks, and sometimes even worn away completely. Due to its avascular nature, articular cartilage has a poor self-healing ability, and therefore, understanding the mechanisms underlying degradation is key for OA prevention and for optimal design of replacements. In this work, the articulating surface of bovine cartilage was investigated in an environment with enhanced calcium concentration -as often found in cartilage in relation to OA- by combining atomic force microscopy, spectroscopy and an extended surface forces apparatus for the first time. The experimental results reveal that increased calcium concentration irreversibly weakens the cartilage's surface layer, and promotes stiction and high friction. The synergistic effect of calcium on altering the cartilage surface's structural, mechanical and frictional properties is proposed to compromise cartilage integrity at the onset of OA. Furthermore, two mechanisms at the molecular level based on the influence of calcium on lubricin and on the aggregation of the cartilage's matrix, respectively, are identified. The results of this work might not only help prevent OA but also help design better cartilage replacements.

Mixing oil and water with ionic liquids: bicontinuous microemulsions under confinement.

We report the structural transition of a phosphonium ionic liquid-based microemulsion from the bulk to nanoconfined between atomically flat micas. Upon the nanoconfinement, we observed a firmly surface-adsorbed ionic liquid film that stabilizes the nanoconfined microemulsion. Further confinement (<11 nm) induces rearrangements in the microemulsion culminating into two well-ordered layers with slow dynamics.

Influence of Loading Conditions and Temperature on Static Friction and Contact Aging of Hydrogels with Modulated Microstructures.

Biological tribosystems enable diverse functions of the human body by maintaining extremely low coefficients of friction via hydrogel-like surface layers and a water-based lubricant. Although stiction has been proposed as a precursor to damage, there is still a lack of knowledge about its origin and its relation to the hydrogel's microstructure, which impairs the design of soft matter as replacement biomaterials. In this work, the static friction of poly(acrylamide) hydrogels with modulated composition was investigated by colloidal probe lateral force microscopy as a function of load, temperature, and loading time. Temperature-dependent studies enable to build a phase diagram for hydrogel's static friction, which explains stiction via (polymer) viscoelastic and poroelastic relaxation, and a subtle transition from solid- to liquid-like interfacial behavior. At room temperature, the static friction increases with loading time, a phenomenon called contact aging, which stems from the adhesion of the polymer to the colloid and from the drainage-induced increase in contact area. Contact aging is shown to gradually vanish with increase in temperature, but this behavior strongly depends on the hydrogel's composition. This work scrutinizes the relation between the microstructure of hydrogel-like soft matter and interfacial behavior, with implications for diverse areas of inquiry, not only in biolubrication and biomedical applications but also in soft robotics and microelectromechanical devices, where the processes occurring at the migrating hydrogel interface are of relevance. The results support that modulating both the hydrogel's mesh size and the structure of the near-surface region is a means to control static friction and adhesion. This conceptual framework for static friction will foster further understanding of the wear of hydrogel-like materials.

Influence of Water on Structure, Dynamics, and Electrostatics of Hydrophilic and Hydrophobic Ionic Liquids in Charged and Hydrophilic Confinement between Mica Surfaces.

Water is ubiquitous in the environment and is the origin for operational constraints in ionic-liquid based electrolytes for supercapacitors. In this study, the influence of water on the interfacial behavior of hydrophilic (1-ethyl-3-methylimidazolium ethylsulfate, abbr. [EMIM][EtSO4]) and hydrophobic (1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, abbr. [EMIM][FAP] and [EMIM][TFSI], respectively) ionic liquids (ILs) confined between mica surfaces was investigated at separations precisely modulated by a surface force apparatus and at controlled relative humidity between 0% and 50% RH. Diffusion experiments revealed that water spontaneously invades the nanoconfined ILs above a certain humidity threshold and that the confined hydrophobic IL is completely replaced by water at sufficiently high environmental humidity (∼45% here) as a result of surface-induced phase separation. This behavior is expected to be universal for other ILs that are not fully miscible with water when they are confined in hydrophilic nanopores of a few nanometers in width. The effect of environmental humidity on interfacial structure, dynamics, and electrostatics was studied via dynamic force measurements. In the dry state, several layers of ions are immobilized on the mica surface, and the effective viscosity increases by up to 2 orders of magnitude with a decrease in film thickness from ∼10 to ∼3 nm. Based on recent work, it is proposed that nanoconfinement enhances the anion-cation association in highly concentrated electrolytes, thereby justifying the loss of fluidity of the ILs. When phase separation is excluded, water is intercalated in the layered structure of the three ILs, and it leads to a change of the layer thickness compared to the dry state. Furthermore, our results reveal that interfacial water prevents ions from being immobilized on the surface and facilitates the outflow of both hydrophilic and hydrophobic ILs by reducing their effective viscosity in the order [EMIM][FAP] < [EMIM][TFSI] < [EMIM][EtSO4]. The underlying mechanisms are evaluated by considering the roles of water in enhancing ion dissociation through screening of electrostatic interactions and solvation of the selected ILs to different extents. The discussed experimental observations support the recent discoveries made by molecular dynamic simulations and neutron scattering studies that using hydrophilic ILs coupled with water as cosolvent could lead to the enhanced power density of IL-based supercapacitors, and therefore, that water-in-(hydrophilic) ILs is a direction worth exploring as electrolytes for supercapacitors.