Robert W. Carpick

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–SiO 2 contact, the boundary friction coefficient, μ, is reduced from ∼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–SiO 2 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 SiO 2 tip. For a hard Al 2 O 3 –SiO 2 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.

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.

Atomistic simulations enhanced to capture force-induced distortion of reactant species enable exploration of how macroscopic stress states affect mechanochemical reactivity.

Graphene, a 1-atom-thick sheet of carbon, has remarkable strength and flexibility, but its fracture behavior at atomic steps remains poorly understood. Graphene’s strength is weaker at its edges which can limit its performance. This project uses atomic force microscopy to characterize fracture at graphene step edges and graphene-SiO₂ edges under varying applied forces. Preliminary results indicate a positive correlation between fracture probability and normal force, though more data needs to be collected. These findings provide insight into graphene’s fracture strength and inform potential applications where its durability under stress is critical.

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 (Δ V = 5.6 ± 1.4 Å 3 ) 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.

a, the average absolute deviation of the height from the mean line (at some, not necessarily known or specified, lateral length scale). However, other parameters, particularly those that are scale-dependent, influence surface and interfacial properties; for example the local surface slope is critical for visual appearance, friction, and wear. The present Surface-Topography Challenge was launched to raise awareness for the need of a multi-scale description, but also to assess the reliability of different metrology techniques. In the resulting international collaborative effort, 153 scientists and engineers from 64 research groups and companies across 20 countries characterized statistically equivalent samples from two different surfaces: a "rough" and a "smooth" surface. The results of the 2088 measurements constitute the most comprehensive surface description ever compiled. We find wide disagreement across measurements and techniques when the lateral scale of the measurement is ignored. Consensus is established through scale-dependent parameters while removing data that violates an established resolution criterion and deviates from the majority measurements at each length scale. Our findings suggest best practices for characterizing and specifying topography. The public release of the accumulated data and presented analyses enables global reuse for further scientific investigation and benchmarking. Supplementary Information: The online version contains supplementary material available at 10.1007/s11249-025-02014-y.

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 $\mathrm{\ensuremath{\Delta}}V$---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 $\mathrm{\ensuremath{\Delta}}V$ between microscale and nanoscale atomic force microscope experiments. Our findings permit more accurate measurement of $\mathrm{\ensuremath{\Delta}}V$, which is crucial for understanding the stress-assisted thermal activation kinetics involved in mechanochemistry and tribochemistry.

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 Ru 2 P, RuP, Rh 2 P, 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 Ru 2 P and Rh 2 P are excellent choices for avoiding friction-induced polymerization, making them promising contact materials for MEMS/NEMS switches.

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msup> <a:mrow> <a:mi>sp</a:mi> </a:mrow> <a:mn>2</a:mn> </a:msup> </a:math> - and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msup> <b:mrow> <b:mi>sp</b:mi> </b:mrow> <b:mn>3</b:mn> </b:msup> </b:math> -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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msup> <c:mrow> <c:mi>sp</c:mi> </c:mrow> <c:mn>3</c:mn> </c:msup> </c:math> carbon atoms is a distribution found approximately 1 eV higher than the binding energy distribution of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msup> <d:mrow> <d:mi>sp</d:mi> </d:mrow> <d:mn>2</d:mn> </d:msup> </d:math> -hybridized carbons. However, the introduction of hydrogen in the a-C network reduces the distance between the characteristic signals of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msup> <e:mrow> <e:mi>sp</e:mi> </e:mrow> <e:mn>3</e:mn> </e:msup> </e:math> - and <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msup> <f:mrow> <f:mi>sp</f:mi> </f:mrow> <f:mn>2</f:mn> </f:msup> </f:math> -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.

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.

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.

Abstract The solution rheology of a fully synthetic, monodisperse mucin that mimics the glycosylated domains of natural mucins, poly(β‐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.

Molybdenum disulfide (MoS 2 ) 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 MoS 2 '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 MoS 2 . Our pioneering in situ experiments, utilizing TEM-based nanoindentation, provided precise imaging and force monitoring of MoS 2 's interaction with a diamond. We employed four MoS 2 -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 MoS 2 's interfacial behavior. We observed a decreased structural order in MoS 2 on sharper tips, particularly those without pre-deposition oxidation. These results underscore the importance of residual stress between the MoS 2 film and substrate and the influence of curvature-induced residual stress in fostering less-ordered MoS 2 structures with heightened work of adhesion. Importantly, this is the first study to report the work of adhesion for MoS 2 -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.

Abstract Antiwear additives permit energy-efficient lubrication of gearboxes, bearings, and other tribological interfaces. We study zirconia (ZrO 2 ) nanocrystal additives, which readily form protective tribofilms in tribological contacts. Our prior work demonstrated cooperative antiwear performance between ZrO 2 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 ZrO 2 tribofilms, including the influence of the co-additives. In the boundary lubrication regime for mixed rolling-sliding contacts, the initial phase of ZrO 2 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 ZrO 2 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. Graphical abstract

Abstract This work examines the effect of environmental humidity on rate-and-state friction behavior of nanoscale silica-silica nanoscale 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 ( $$\ll 1\% RH$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>≪</mml:mo> <mml:mn>1</mml:mn> <mml:mo>%</mml:mo> <mml:mi>R</mml:mi> <mml:mi>H</mml:mi> </mml:mrow> </mml:math> ), 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-catalyzed interfacial Si–O-Si bond formation. At the highest humidities examined (&gt; 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 bond-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.

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Abstract 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% additives in polyethylene glycol base oil (MW 200; PEG 200), were tested in the lubrication of ASTM 52100 bearing steel contacts in extreme pressure conditions (under mixed lubrication with a Hertzian pressure of 1.12 GPa) using a mini traction machine (MTM). Due to the poor resistance to corrosion of bearing steel, a semi-ester of succinic acid derivative corrosion inhibitor (Lanxess RC 4801) was added to the mixtures at a 1 wt% concentration. The ILs 1-hexyl-methylimidazolium trifluoromethanesulfonate ([C 6 mim][TfO]) and 1-hexyl-4-picolinium trifluoromethanesulfonate ([C 6 -4-pic][TfO]) revealed promising results in terms of surface protection of bearing steel. In contrast, 4-picolinium hydrogen sulfate ([4-picH][HSO 4 ]) as 2-wt% additive to PEG 200 + 1% RC 4801 did not show any improvement in wear performance compared to neat PEG 200 + 1% RC 4801. PEG 200 + 2% [C 6 mim][TfO] + 1%RC 4801 allowed for a decrease in wear up to ~ 76% and PEG 200 + 2% [C 6 -4-pic][TfO] + 1%RC 4801 up to ~ 46% when compared with neat PEG 200 + 1% RC 4801. Optical microscopy images suggest the formation of an adsorbed layer, which was further supported by chemical analysis via x-ray photoelectron spectroscopy (XPS) data for [C 6 mim][TfO]. Graphical abstract

Abstract 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. Graphical abstract

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.

Low-dimensional materials, such as MoS 2, 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 MoS 2 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 MoS 2 were 604 ± 323 mJ/m 2 and 932 ± 647 mJ/m 2, respectively, significantly higher than previously reported values for mechanically exfoliated MoS 2 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 MoS 2 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 MoS 2 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.

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 (${\mathrm{Ti}}_{n}{\mathrm{O}}_{2n\ensuremath{-}1}$, Magn\'eli phases) are good candidates for NEMS applications because of their metallic conductivity, environmental resistance, and low cost, as compared with other conductive oxides like ${\mathrm{RuO}}_{2}$. 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\'eli phase Ti suboxides for NEMS switch applications.

Faculty and staff can and do influence the climate of a department and achievement of students. Research shows the positive effects of choosing to implement evidence-based teaching practices like active learning and inclusive teaching However, research also shows that the local climate in a department could cause students of color to be driven from STEM And while the focus of Diversity, Equity, and Inclusion (DEI) efforts tends to be on women and under-represented minorities (URMs, defined as non-white, non-Asian), populations with representation at or above the demographics of the general population (typically Asian) face their own challenges Additionally, part of supporting all students includes mitigating disenfranchisement in majority populations (typically white males) In this paper, we describe recent efforts in the Mechanical Engineering and Applied Mechanics (MEAM) Department at the University of Pennsylvania (UPenn) to address these issues. Most of our initial efforts in this area have focused on the undergraduate population as well as their intersection with faculty and staff. It is our aim that sharing these early efforts may encourage others to take on similar endeavors, and will also provide a reference point for future work of ours in this area.

The contact between nanoscale single-crystal silicon asperities and substrates terminated with -H and -OH functional groups is simulated using reactive molecular dynamics (MD). Consistent with previous MD simulations for self-mated surfaces with -H terminations only, adhesion is found to be low at full adsorbate coverages, be it self-mated coverages of mixtures of -H and -OH groups, or just -OH groups. As the coverage reduces, adhesion increases markedly, by factors of ∼5 and ∼6 for -H-terminated surfaces and -OH-terminated surfaces, respectively, and is due to the formation of covalent Si-Si bonds; for -OH-terminated surfaces, some interfacial Si-O-Si bonds are also formed. Thus, covalent linkages need to be broken upon separation of the tip and substrate. In contrast, replacing -H groups with -OH groups while maintaining complete coverage leads to negligible increases in adhesion. This indicates that increases in adhesion require unsaturated sites. Furthermore, plane-wave density functional theory (DFT) calculations were performed to investigate the energetics of two Si(111) surfaces fully terminated by either -H or -OH groups. Importantly for the adhesion results, both DFT and MD calculations predict the correct trends for the relative bond strengths: Si-O > Si-H > Si-Si. This work supports the contention that prior experimental work observing strong increases in adhesion after sliding Si-Si nanoasperities over each other is due to sliding-induced removal of passivating species on the Si surfaces.

Abstract Faculty and staff can and do influence the climate of a department and achievement of students. Research shows the positive effects of choosing to implement evidence-based teaching practices like active learning and inclusive teaching, and having a growth mindset in relation to the abilities of students. However, research also shows that the local climate in a department could cause students of color to be driven from STEM, or that a chilly climate could have a disproportionate impact on female students. And while the focus of Diversity, Equity, and Inclusion (DEI) efforts tends to be on women and under-represented minorities (URMs, defined as non-white, non-Asian), populations with representation at or above the demographics of the general population face their own challenges. In this paper, we describe recent efforts in the Mechanical Engineering and Applied Mechanics (MEAM) Department at the University of Pennsylvania to address these issues. Most of our initial efforts in this area have focused on the undergraduate population as well as their intersection with faculty and staff. We have started exploring the departmental structures and practices and have some initial demographic data on students and faculty. We are interested in exploring how retention, graduation, and achievement in general overlap and intersect with gender, race, and socio-economic status. We have also recently implemented a DEI Scholars program that further engages undergraduate and graduate students in this process. This initial work establishes baseline numbers and describes the first cohort we will track from acceptance through graduation. It is our aim that sharing these early efforts may encourage others to take on similar endeavors, and will also provide a reference point for future work of ours in this area.

MoS2 is 2D material applicable for electronics, photo-detectors, light-emitting diodes, and solar cells. Since these applications operate at elevated temperatures, determining and controlling the environmental stability of MoS2 is essential. This study uses Raman spectroscopic experiments at elevated temperatures to observe reversible and irreversible changes in 2D MoS2 films with varying thickness, morphology, and substrate to explore the limits of its thermal stability. Molecular dynamics simulations and in-situ X-ray diffraction confirm that thermal expansion causes the Raman reversible spectral shifts. Further analysis reveals irreversible spectral shifts associated with oxidation and strain effects caused by oxidation with two distinct modes. The former is due to the general homogenous thinning of layers from the top layer down through randomly spaced sulfur vacancies. The latter is due to inhomogeneous oxidation, which known to occur as localized pitting at grain boundaries, void formation from nucleation centers, or stress corrosion cracking starting from edges under strain. Segregation of doping and strain contributions confirms higher strain in the edges. Annealing increases the spread of the strain distribution.These results highlight the critical role of strain, resulting from thermal expansion coefficient mismatch between substrates and 2D MoS2 films, on their environmental stability.

Challenges in quantifying how force affects bond formation have hindered the widespread adoption of mechanochemistry. We used parallel tip-based methods to determine reaction rates, activation energies, and activation volumes of force-accelerated [4+2] Diels-Alder cycloadditions between surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand. The rate dependences on pressure were unexpectedly strong, and substantial differences were observed between the dienophiles. Multiscale modeling demonstrated that in proximity to a surface, mechanochemical trajectories ensued that were distinct from those observed solvothermally or under hydrostatic pressure. These results provide a framework for anticipating how experimental geometry, molecular confinement, and directed force contribute to mechanochemical kinetics.

Silicon oxide-doped hydrogenated amorphous carbon (a-C:H:Si:O) is an important form of diamond-like carbon (DLC) for tribological applications, primarily because of its enhanced thermal stability and reduced dependence of friction on environmental humidity. As with other DLCs, its mechanisms of lubrication are still an active area of research, though it is now known that surface passivation and tribofilm growth are important factors. In this study, tribofilm formation for a-C:H:Si:O is examined at the microscale by using steel colloid atomic force microscopy probes as the sliding counterface. This approach provides some inherent advantages over macroscale tribology experiments, namely that the tribofilm thickness and stiffness can be tracked in situ and correlated directly with the friction response. The results of these experiments show that the tribofilm grows rapidly on the steel colloid following a period of counterface wear and high friction. The friction drops more than 80% upon nucleation of the tribofilm, which is attributed to a decrease of more than 80% in adhesion combined with a decrease in the estimated interfacial shear strength of at least 65%. Approximately 80% of the friction decrease occurs before the tribofilm reaches a thickness of 2 nm, suggesting that only the near-surface properties of the tribofilm provide the needed functionality for its effective lubrication mechanisms. Graphical abstract

-acrylonitrile) (SAN), we generate unique polymer nanocomposite (PNC) morphologies by balancing the degree of surface enrichment, phase separation, and wetting within the films. Depending on the annealing temperature and time, thin films undergo different stages of phase evolution, resulting in homogeneously dispersed systems at low temperatures, enriched PMMA-NP layers at the PNC interfaces at intermediate temperatures, and three-dimensional bicontinuous structures of PMMA-NP pillars sandwiched between two PMMA-NP wetting layers at high temperatures. Using a combination of atomic force microscopy (AFM), AFM nanoindentation, contact angle goniometry, and optical microscopy, we show that these self-regulated structures lead to nanocomposites with increased elastic modulus, hardness, and thermal stability compared to analogous PMMA/SAN blends. These studies demonstrate the ability to reliably control the size and spatial correlations of both the surface-enriched and phase-separated nanocomposite microstructures, which have attractive technological applications where properties such as wettability, toughness, and wear resistance are important. In addition, these morphologies lend themselves to substantially broader applications, including: (1) structural color applications, (2) tuning optical adsorption, and (3) barrier coatings.

Abstract Silicon oxide-doped hydrogenated amorphous carbon (a-C:H:Si:O) is an important form of diamond like carbon (DLC) for tribological applications, primarily because of its enhanced thermal stability and reduced dependence of friction on environmental humidity. As with other DLCs, its mechanisms of lubrication are still an active area of research, though it is now known that surface passivation and tribofilm growth are important factors. In this study, tribofilm formation for a-C:H:Si:O is examined at the microscale by using steel colloid atomic force microscopy probes as the sliding counterface. This approach provides some inherent advantages over macroscale tribology experiments, namely that the tribofilm thickness and stiffness can be tracked in situ and correlated directly with the friction response. The results of these experiments show that the tribofilm grows rapidly on the steel colloid following a period of counterface wear and high friction. The friction drops more than 80% upon nucleation of the tribofilm, which is attributed to a decrease of more than 80% in adhesion combined with a decrease in the estimated interfacial shear strength of at least 65%. Approximately 80% of the friction decrease occurs before the tribofilm reaches a thickness of 2 nm, suggesting that only the near-surface properties of the tribofilm provide the needed functionality for its effective lubrication mechanisms.