The tribological response of mucus is critical for organisms and in emerging applications and depends on the properties of mucins, understood to be the primary functional protein in mucus. The complexity of natural mucus makes correlating mucin structure to mucus function challenging. To address this, we study pure solutions of a synthetic mucin homopolymer, poly(Gal-Thr)(22) which mimics the structure of natural mucins' glycosylated bottlebrush domains. This permits directly correlating specific nanostructured molecular features with lubricity. For a macroscale PDMS-SiO2 contact, the boundary friction coefficient, mu, is reduced from similar to 1 in pure water to <0.1 at 50 mg/mL poly(Gal-Thr)(22), with more modest lubrication at lower concentrations. For a microscale PDMS-SiO2 contact measured using colloidal atomic force microscopy, which directly probes nanoscale contact mechanics, friction is also concentration-dependent: at 1 mg/mL, friction reduction vs water of 40% is attributed to reduced adhesion; at 10 and 50 mg/mL, friction reduction by 80% and 98%, respectively, is attributed to a lubricious nanoscale tribofilm that forms on the SiO2 tip. For a hard Al2O3-SiO2 microscale contact, friction falls below detection limits due to tribofilm formation under high nanoscale contact stresses. Infrared spectroscopy shows the tribofilm is a compacted structure rich in hydrogen bonds. While passively adsorbed mucin-rich films or pellicles are generally understood to provide lubricity in nature, little is known about their response to stress. Solutions of our synthetic mucin lubricate modestly when passively adsorbed, but at sufficient concentrations, with continued sliding a denser, highly lubricious tribofilm forms. Its durability permits the interface to retain lubricity when sliding in mucin-free solution. This demonstrates that a short, glycosylated homopolymer without specific surface anchoring groups substantially lubricates through stress-induced formation of an intrinsically lubricious tribofilm. This motivates further interrogation of the structure of mucins subjected to stress since until now, surface characterization in mucin lubrication studies has focused on the passively adsorbed pellicle.

Mechanochemical nano-writing uses localized mechanical fields to drive interfacial reactions and form ultrathin functional materials. Recent experiments demonstrated this approach by using an atomic force microscope (AFM) to drive the solid-state formation of superconducting Pd7MoTe2 within hBN-encapsulated MoTe2/Pd heterostructures at temperatures far below those required for conventional thermal synthesis. Here, we develop a finite element method (FEM) framework to identify the mechanical conditions governing activation and thereby enable control over the onset and spatial extent of nano-writing. The model resolves stress and strain fields at the reactive MoTe2/Pd interface under experimentally relevant AFM contact conditions. Results show that thinner hBN enhances in-plane stress and strain by reducing elastic confinement, while bilayer MoTe2 produces greater bending-induced in-plane stress and strain than monolayer MoTe2 due to its larger bending stiffness. Lowering the friction between materials also increases stress and strain by allowing more relative tangential motion between adjacent layers which reduces the lateral constraint on the MoTe2. These parameters control both the critical load for mechanochemical activation, defined using an experimentally derived in-plane strain threshold, and the width of the activated region. Among the parameters considered, critical load and width are most sensitive to hBN thickness such that thinner hBN can provide more precise control over the dimensions of written nanostructures. These results reveal how elastic confinement, bending, and interfacial shear constraint govern mechanical localization at reactive interfaces and provide quantitative guidance for controlling mechanochemical nano-writing.

Mechanical energy accelerates many physicochemical processes, including materials syntheses that are hard to produce with thermal energy alone. However, physical understanding connecting applied mechanical forces with internal stresses and ensuing reaction mechanisms is lacking. Here we demonstrate mechanical force-enabled synthesis and nanoscale patterning to metallize a two-dimensional (2D) material, producing an atomically-thin superconducting material. Localized force applied by atomic force microscope tips to van der Waals (vdW) encapsulated stacks of 2D bilayer MoTe2 and adjacent source Pd guides 2D Pd7MoTe2 growth with 50 nm lateral resolution. Force accelerates reaction kinetics exponentially per Eyring's stress-assisted thermal activation model, reducing synthesis temperatures from 200 °C to near-room temperature. Finite element simulations, density functional theory, and ab-initio grand canonical Monte Carlo calculations show that tip-induced compression facilitates Pd chemisorption to tensile-strained MoTe2 that converts to uniform Pd7MoTe2. This demonstrates a new, generalizable paradigm for nanoscale synthesis of quantum materials, and high-precision engineering of superconductivity.

Abstract The high strength and flexibility of two-dimensional materials enable their use in flexible devices and structures with a high curvature. However, thin films adhered to curved structures can wrinkle, fold, buckle, delaminate, or crack, depending on the curvature, film thickness, film mechanical properties, and film-substrate adhesion. How adhesion relates to 2D film properties such as crystallinity, defects, and substrate interactions is not understood due to the multiphysical, complex interplay between chemistry, structure, and adhesion. Moreover, it is intrinsically challenging to observe 2D materials and their interfaces with substrates laterally with atomic resolution in complex geometries. Here, we address these challenges using customized transmission electron microscopy (TEM) methodology to examine the atomically resolved structures of nanocrystalline few-layer molybdenum disulfide (MoS2) films deposited onto silicon (Si) tips with nanoscale curvature. We observe a transition from conformal film growth to film delamination with a substantial degree of structural defects below a critical tip radius of curvature (Rc) of approximately 5 nm. Continuum mechanics approximations based on membrane bending and stretching energies overestimate Rc and fail to explain the nature of the observed transition. We thus conducted molecular dynamics (MD) simulations of nanocrystalline MoS2 films on nanoscale Si hemispheres. The predicted Rc from MD is in excellent agreement with the experimental findings when the grain size is accounted for. This suggests an interdependence among film mechanical properties, grain size, adhesion, and curvature. In particular, defects, in this case, structural disorder at grain boundaries, enable curvature exceeding that predicted by continuum mechanics-based membrane models.

The solution rheology of a fully synthetic, monodisperse mucin that mimics the glycosylated domains of natural mucins, poly(beta-Gal-Thr)22, is studied to systematically explore relationships between polymer structure, solution conditions, and rheological properties. Using standard cone-plate rheometry, shear thinning is observed over a range of concentrations, with an apparent yield stress-typical for gels-evident at the highest concentrations. This is surprising given the dilute, weakly interacting nature of the solutions and the lack of observable structure in cryogenic electron microscopy and particle tracking microrheology. However, interfacial rheometry demonstrates that the gel-like behavior is attributable to a thin structured layer at the air-water interface, without any bulk gelation. This is attributed to an interfacial layer formed by inter-mucin H-bonds that yields when sheared. A computational model using kinetic Monte Carlo (kMC) simulations qualitatively reproduces the yield stress response of such a network through an intermolecular bonding potential. An analytical model of stochastic bond formation and breaking, validated by the kMC simulations, demonstrates that having multiple bonding sites per mucin with a force-dependent debonding rate aligns with experiments, consistent with intermolecular interactions for other mucin proteins. This suggests that in mucin solutions, gelation may begin at the air-water interface, and emphasizes the need for multitechnique validation when exploring structural cues of mucus gelation through rheometry.

Point-contact studies of interfacial chemical reactions have revealed that activation barriers can depend strongly on applied stress. However, discrepancies exist in reported values of the activation volume Delta V -the rate at which stress alters activation barriers-hindering its physical interpretation. We show that two contact mechanics effects-the spatially nonuniform stress, and the effect of load on reaction area-can lead to large errors. We derive a corrected model for Hertzian contacts as an example, and validate it using the growth kinetics of zinc dialkyldithiophosphate tribofilms. The model fully resolves disagreements in Delta V between microscale and nanoscale atomic force microscope experiments. Our findings permit more accurate measurement of Delta V , which is crucial for understanding the stress-assisted thermal activation kinetics involved in mechanochemistry and tribochemistry.

Nanoindentation of substrate-supported graphene can produce auto-kirigami (AK) structures: spontaneously folded and extended self-tearing nanoribbons up to several micrometers in length. However, the mechanisms governing their formation and yield are poorly understood. Here, we study graphene AK through statistical analysis of high-throughput experiments involving hundreds-fold arrays of indents on highly uniform regions of exfoliated monolayer and bilayer graphene, with no applied oscillation (in contrast with prior work). Post-mortem atomic force microscopy analysis reveals a baseline AK formation rate of 13-61% for monolayers and 0-17% for bilayers depending on inter-indent pitch. Force-distance curves of each type of nanostructure showed no appreciable differences. Moreover, graphene can remain intact after indentation, permitting formation of unbroken graphene suspended over or conformed within indents. Inter-indent pitch affects the absolute and relative formation rates of these nanostructures, attributed to indentation-induced tensile graphene strain. This advances the understanding of mechanisms for controlled formation of nanostructures, including twisted bilayers of graphene and other van der Waals materials.

Molybdenum disulfide (MoS2) holds great potential in a wide range of applications, including electronics, photodetectors, light-emitting diodes (LEDs), and solar cells due to its unique two-dimensional (2D) structure. This structure enables innovative functionalities, particularly in flexible and wearable technologies. However, a significant knowledge gap remains regarding MoS2's interfacial adhesion, a critical aspect for advancing next-generation devices. To address this, we conducted a comprehensive study investigating the interaction forces originating from the bonding between atoms that govern the adhesion of ultra-thin 2D MoS2. Our pioneering in situ experiments, utilizing TEM-based nanoindentation, provided precise imaging and force monitoring of MoS2's interaction with a diamond. We employed four MoS2-coated AFM tips with varying radii and preparation methods, with films prepared on two Si wafers subjected to different oxidation protocols. Our findings, validated by Raman and X-ray photoelectron spectroscopy, reveal unique insights into MoS2's interfacial behavior. We observed a decreased structural order in MoS2 on sharper tips, particularly those without pre-deposition oxidation. These results underscore the importance of residual stress between the MoS2 film and substrate and the influence of curvature-induced residual stress in fostering less-ordered MoS2 structures with heightened work of adhesion. Importantly, this is the first study to report the work of adhesion for MoS2-diamond contact. Our findings highlight the crucial role of covalent bonding at contact points in the material transfer processes involving 2D materials. This is a critical insight for developing precise and reliable methods for manipulating 2D materials, which could significantly advance our understanding and application of materials science, particularly in nanotechnology and device fabrication.

Two-dimensional (2D) van der Waals materials exhibit exceptional in-plane mechanical and transport properties, yet leveraging these properties in three dimensions (3D) remains a fundamental challenge. Here, we introduce a high-throughput method for the spontaneous formation of three-dimensional auto-kirigami, self-fractured and self-folded structures that evolve during indentation of thin (<100 nm) flakes of graphite and hexagonal boron nitride. These 3D structures provide direct access to in-plane properties via out-of-plane fractured surfaces, demonstrating enhanced electrical conductance along these edges. The 3D auto-kirigami consist of 2-4 plates, or "leaflets", that form by elastic buckling facilitated by in-plane fracture. By analyzing hundreds of leaflet geometries, we demonstrate that leaflet length correlates with buckling load, enabling a real-time predictor of the leaflet morphology. These 3D auto-kirigami provide a high-yield, deformation-driven platform for 3D van der Waals structures that can leverage in-plane properties of 2D materials.

Mechanochemical reactions are increasingly studied using molecular dynamics simulations to understand mechanically activated chemical transformations. However, accurately capturing reactivity under mechanochemical conditions using classical potentials remains a challenge because standard models inhibit force-induced distortion of reactant species. In this study, we used the REACTER protocol, a method for simulating reactive events via dynamic bond changes, with a classical potential modified to allow the molecular distortion observed in first-principles calculations of a 4 + 2 Diels-Alder cycloaddition reaction. The approach was used to simulate the reaction in non-mechanochemical conditions with a solvent and no external stress, as well as in mechanochemical conditions. Mechanochemical simulations were run at hydrostatic stresses of 0.1 MPa and 2.5 GPa, both with and without shear applied, to investigate how the stress state influences reactivity. Relative to the non-mechanochemical reference case, hydrostatic stress and shear stress increased reaction yield. This increase was due to molecular distortion, the primary mechanism by which mechanical force activates chemical reactions, that could only be modeled using the modified classical potential. However, some of the increase in reaction yield was attributable to secondary mechanochemical activation mechanisms. Specifically, hydrostatic stress decreased the distance between reactants and shear stress facilitated alignment of reactants in the direction of imposed shear. This work provides new insight into how the stress state affects mechanochemical reaction mechanisms and establishes a generally applicable framework for improving classical potential-based simulations for organic reactions.

The quantitative evaluation of the carbon hybridization state by x-ray photoelectron spectroscopy (XPS) has been a surface-analysis problem for the last three decades due to the challenges associated with the unambiguous identification of the characteristic binding energy values for sp2-and sp3-bonded carbon. Here, we computed the binding energy values of C(1s) core electrons on the absolute energy scale for model structures of amorphous carbon (a-C) using density functional theory (DFT). The DFT calculations show that in the case of hydrogen-free a-C, the C(1s) binding energy for sp3 carbon atoms is a distribution found approximately 1 eV higher than the binding energy distribution of sp2-hybridized carbons. However, the introduction of hydrogen in the a-C network reduces the distance between the characteristic signals of sp3-and sp2-bonded carbon due to the increased ability to screen the core hole by neighboring hydrogen atoms as compared to carbon atoms. This effect hinders the unambiguous quantification of the carbon hybridization state on the basis of C(1s) XPS data alone. This work can assist surface scientists in the use of XPS for the accurate characterization of carbon-based materials.

Transition metal phosphides (TMPs) have aroused widespread research interest in the past decade due to their excellent electrical and mechanical properties. Nonetheless, their application in micro- and nanoelectromechanical systems (MEMS and NEMS) has not been investigated. Here, we use density functional theory (DFT) to explore the potential of four transition-metal phosphides to act as contact materials of MEMS/NEMS switches. Specifically, we first investigate the thermodynamic stability of Ru2P, RuP, Rh2P, and TiP under an oxygen environment. Then, using benzene as the background gas, the mechanical contact cycle is modeled to examine the process of tribopolymer formation on the surface of the contacts, which has been reported as the major reason for conductance loss after repeated actuation. The results show that Ru2P and Rh2P are excellent choices for avoiding friction-induced polymerization, making them promising contact materials for MEMS/NEMS switches.

Contact-induced reactions of interfacially confined molecules represent a widespread yet poorly understood class of mechanochemical phenomena, with broad implications for surface chemistry, tribology, and nanotechnology. Tribopolymerization─stress-induced polymerization of organic adsorbates into insulating nanolayers─causes conductance loss and limits the reliability of electrical contacts across length scales, particularly in nanoelectromechanical systems (NEMS). Using atomic force microscopy (AFM), we investigate how stress and voltage drive tribopolymer growth from ambient-adsorbed molecules in Pt/Pt nanocontacts. The measured kinetics follow a stress-assisted thermal activation model, confirming its mechanochemical origin. We develop a new contact-mechanics-corrected model that combines stress-dependent reaction kinetics with realistic contact mechanics. Using power-law tip geometries, this model accounts for inevitable wear-induced nonstandard tip shapes by integrating local reaction rates over the full, nonuniform stress distribution within the contact region. This enables accurate extraction of a unified activation volume (ΔV = 5.6 ± 1.4 Å3) across two decades of both contact area and stress, in sharp contrast to conventional analyses that neglect contact geometry and yield widely scattered activation volumes spanning 2 orders of magnitude. We further show that applied voltage accelerates tribopolymerization in a manner similar to stress, described through a newly introduced activation parameter and a field-induced bond-stretching model. Together, these results provide a general approach for quantifying coupled stress- and field-driven mechanochemical reactions at nanoscale interfaces, and offer mechanistic insights into tribopolymerization-induced electrical degradation of nanocontacts critical to device reliability.

The spacer layer imaging method (SLIM) is widely used to measure the thickness of additive and lubricant films, in lubricant development and evaluation, and for fundamental research into elastohydrodynamic lubrication and tribofilm formation mechanisms. The film thickness measurement, as implemented on several popular tribometers, provides powerful, non-destructive in-situ mapping of film topography with nanometre-scale height sensitivity. However, the results can be highly sensitive to experimental procedure, machine condition, and image analysis, in some cases reporting unphysical film thickness trends. The prevailing image analysis techniques make it challenging to interrogate these errors, often hiding their multivariate nonlinear behaviour from the user by spatial averaging. Herein, several common ‘silent errors’ in the SLIM measurement, including colour matching to incorrect fringe orders, and colour drift due to the optical properties of the system or film itself, are discussed, with examples. A robust suite of novel a priori and a posteriori methods to address these issues, and to improve the accuracy and reliability of the measurement, are also presented, including a novel, computationally inexpensive circle-finding algorithm for automated image processing. In combination, these methods allow reliable mapping of films up to at least 800 nm in thickness, representing a significant milestone for the utility of SLIM applied to elastohydrodynamic contact.

This work examines the effect of environmental humidity on rate-and-state friction behavior of nanoscale silica-silica single asperity contacts in an atomic force microscope, particularly, its effect on frictional ageing and velocity-weakening vs. strengthening friction from 10 nm/s to 100 µm/s sliding velocities. At extremely low humidities (<< 1% RH ), ageing is nearly absent for up to 100 s of nominally stationary contact, and friction is strongly velocity-strengthening. This is consistent with dry interfacial friction, where thermal excitations help overcome static friction at low sliding velocities. At higher humidity levels (10–40% RH), ageing becomes pronounced and is accompanied by much higher kinetic friction and velocity-weakening behavior. This is attributed to water catalyzing interfacial Si-O-Si bond formation. At the highest humidities examined (> 40% RH), ageing subsides, kinetic friction drops to low levels, and friction is velocity-strengthening again. These responses are attributed to intercalated water separating the interfaces, which precludes interfacial bonding. The trends in velocity-dependent friction are reproduced and explained using a computational multi-bond model. Our model explicitly simulates bond formation and breaking, and the passivation and reactivation of reaction sites across the interface during sliding, where the activation energies for interfacial chemical reactions are dependent on humidity. These results provide potential insights into nanoscale mechanisms that may contribute to the humidity dependence observed in prior macroscale rock friction studies. They also provide a possible microphysical foundation to understand the role of water in interfacial systems with water-catalyzed bonding reactions, and demonstrate a profound change in the interfacial physics near and above saturated humidity conditions.

Low-dimensional materials, such as MoS2, hold promise for use in a host of emerging applications, including flexible, wearable sensors due to their unique electrical, thermal, optical, mechanical, and tribological properties. The implementation of such devices requires an understanding of adhesive phenomena at the interfaces between these materials. Here, we describe combined nanoscale in situ transmission electron microscopy (TEM)/atomic force microscopy (AFM) experiments and simulations measuring the work of adhesion (W-adh) between self-mated contacts of ultrathin nominally amorphous and nanocrystalline MoS2 films deposited on Si scanning probe tips. A customized TEM/AFM nanoindenter permitted high-resolution imaging and force measurements in situ. The W-adh values for nanocrystalline and nominally amorphous MoS2 were 604 +/- 323 mJ/m(2) and 932 +/- 647 mJ/m(2), respectively, significantly higher than previously reported values for mechanically exfoliated MoS2 single crystals. Closely matched molecular dynamics (MD) simulations show that these high values can be explained by bonding between the opposing surfaces at defects such as grain boundaries. Simulations show that as grain size decreases, the number of bonds formed, the W-adh and its variability all increase, further supporting that interfacial covalent bond formation causes high adhesion. In some cases, sliding between delaminated MoS2 flakes during separation is observed, which further increases the W-adh and the range of adhesive interaction. These results indicate that for low adhesion, the MoS2 grains should be large relative to the contact area to limit the opportunity for bonding, whereas small grains may be beneficial, where high adhesion is needed to prevent device delamination in flexible systems.

The protection of steel surfaces from wear under extreme pressure conditions is of major importance in several industries as it provides better performance and longer life of machinery. The motivation for this work was to study the lubrication of steel by ionic liquids (ILs), which have recently emerged as greener alternatives to commercial lubricants and additives. Three ILs based on sulfur-containing anions, used as 2-wt

Antiwear additives permit energy-efficient lubrication of gearboxes, bearings, and other tribological interfaces. We study zirconia (ZrO2) nanocrystal additives, which readily form protective tribofilms in tribological contacts. Our prior work demonstrated cooperative antiwear performance between ZrO2 and the S- and P-based co-additives in fully formulated hydrocarbon gear oils. Here, we extend that work by examining the growth kinetics of the ZrO2 tribofilms, including the influence of the co-additives. In the boundary lubrication regime for mixed rolling-sliding contacts, the initial phase of ZrO2 tribofilm growth is soon overtaken by removal processes, phenomena whose importance has gone unnoticed in prior work. Tribofilm removal affects the steady-state thickness and morphology of the tribofilm as well as its growth kinetics. The S- and P-based co-additives are incorporated into the ZrO2 tribofilm, and alter the competition between the growth and removal processes, increasing initial net growth rates per contact cycle and contributing to a more polished final interface. This work highlights the significance of removal processes in determining tribofilm antiwear performance, and suggests several routes for improving tribofilm growth kinetics using co-additives.

Conducting metal oxides offer many advantages for novel electronics applications, including sensors, fuel cells, piezoelectric devices, and microelectronic circuits, due to their conductivity, hardness, and chemically inert surfaces. In particular, their high electrical conductivity and mechanical properties make these materials suitable for microelectromechanical and nanoelectromechanical system (MEMS/NEMS) devices. NEMS switches have great potential for next-generation electronic computing associated with scalability to small dimensions, low power consumption, and (relatively) high speed. Oxygen-deficient Ti oxides with ordered planes of vacancies (TinO2n-1, Magn & eacute;li phases) are good candidates for NEMS applications because of their metallic conductivity, environmental resistance, and low cost, as compared with other conductive oxides like RuO2. Although Ti suboxides have been produced in crystalline form, various synthesis methods may also produce amorphous material. In this paper, we focus on the structural and electrical transport properties of several Ti suboxides. In particular, we examine the effects of temperature, transition-metal dopants, and amorphization on these structural and electronic properties and the potential applicability of Magn & eacute;li phase Ti suboxides for NEMS switch applications.

Van der Waals heterostructures formed by stacked 2D materials show exceptional electronic, mechanical, and optical properties. Superlubricity, a condition where atomically flat, incommensurate planes of atoms result in ultra-low friction, is a prime example enabling, for example, self-assembly of optically visible graphene nanostructures in air via a sliding auto-kirigami process. Here, it is demonstrated that a subtle but ubiquitous adsorbate stripe structure found on graphene and graphitic surfaces in ambient conditions remains stable within the interface between twisted graphene layers as they slide over each other. Despite this contamination, the interface retains an exceptional superlubricious state with an estimated upper bound frictional shear strength of 10 kPa, indicating that direct atomic incommensurate contact is not required to achieve ambient superlubricity for 2D materials. The results suggest that any phenomena depending on 2D heterostructure interfaces such as exotic electronic behavior may need to consider the presence of stripe adsorbate structures that remain intercalated. A ubiquitous stripe structure adsorbate that has previously been observed on graphene and graphitic surfaces is examined within twisted graphene-graphene interfaces. This adsorbate is observed to be stable during sliding of layers in this interface and does not destroy the low friction (superlubricious) contact at the interface under ambient conditions. image