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We develop and harness a phase field simulation method to study liquid filling on grooved surfaces. We consider both short-range and long-range liquid-solid interactions, with the latter including purely attractive and repulsive interactions, as well as those with short-range attraction and long-range repulsion. This allows us to capture complete, partial and pseudo-partial wetting states, demonstrating complex disjoining pressure profiles over the full range of possible contact angles as previously proposed in the literature. Applying the simulation method to study liquid filling on grooved surfaces, we compare the filling transition for the three different classes of wetting states as we vary the pressure difference between the liquid and gas phases. The filling and emptying transition is reversible for the complete wetting case, while significant hysteresis is observed for the partial and pseudo-partial cases. In agreement with previous studies, we also show that the critical pressure for the filling transition follows the Kelvin equation for the complete and partial wetting scenarios. Finally, we find the filling transition can display a number of distinct morphological pathways for the pseudo-partial wetting cases, as we demonstrate here for varying groove dimensions., 11 pages, 6 figures, accepted in the Journal of Chemical Physics

Reactive molecular dynamics simulations of MoS2 crystallization from amorphous precursor materials showed that crystal domain size decreased because of excess S or O, relative to the stoichiometric...

Economical CO 2 capture demands low-energy separation strategies. We use a liquid-infused surface (LIS) approach to immobilize reactive liquids, such as amines, on a textured and thermally conductive solid substrate with high surface-area to volume ratio (A/V) continuum geometry. The infused, micrometer-thick liquid retains that high A/V and directly contacts the gas phase, alleviating mass transport resistance typically encountered in mesoporous solid adsorbents. We name this LIS class “solid with infused reactive liquid” (SWIRL). SWIRL-amine requires no water dilution or costly mixing unlike the current liquid-based commercial approach. SWIRL–tetraethylenepentamine (TEPA) shows stable, high capture capacities at power plant CO 2 concentrations near flue gas temperatures, preventing energy-intensive temperature swings needed for other approaches. Water vapor increases CO 2 capacity of SWIRL-TEPA without compromising stability.

Development of low friction surface materials matched with low friction lubricants is an important route to improving engine efficiency and reducing emissions. Diamond-like carbon (DLC) is a material of interest because it combines the durability of diamond with the low friction properties of graphite. Consequently, various forms of DLC have been studied as durable, low friction surface coatings. At the same time, friction modifiers (FMs) are often used in engine lubricants to produce low friction surface layers. Molybdenum-based friction modifiers are commonly used additives that mechanochemically decompose to cover surfaces with MoS2, a solid lubricant. FMs can also reduce friction on DLC surfaces. However, molybdenum can be harmful to DLC and cause rapid degradation and breakthrough of the DLC surface layer. We have found that the wear debit of molybdenum FMs trends with surface affinity of the additive. The adsorption isotherms of 4 Mo-based FMs on boron-doped diamond-like carbon (BDLC) were measured with a quartz crystal microbalance (QCM). The strongest adsorbate, Mo dithiophosphate (MoDTP), which also forms the thickest film, protects the BDLC surface enabling low friction while keeping wear low. Theoretical predictions based on molecular dynamics simulations combined with molecular-thermodynamic theory (MD-MTT) show that adsorption of MoDTP on BDLC deviates significantly from Langmuir behavior and free energy of adsorption for the first monolayer is significantly more negative than the multilayer adsorption free energy observed experimentally. The relationship between wear and adsorption behavior is consistent with experimental observations in the literature. These results also suggest that additive performance in a fully formulated lubricant is related to fundamental adsorption properties measured on the neat additive.

We study the effect of gravity on barrel-shaped droplets on fibers using theoretical analyses, numerical simulations, and experiments. By performing a perturbation analysis in the limit of small Bond numbers and small dimensionless droplet volumes, we formulate a nearly axisymmetric solution that describes the shape of the droplet. The leading-order solution yields the axisymmetric profile and the first-order correction incorporates the effect of gravity. We validate our solution by comparing droplet shape and contact line predictions with experiments and Surface Evolver simulations.

The ability to predict liquid transport rates on textured surfaces is key to the design and optimization of devices and processes such as oil recovery, coatings, reaction-separation, high-throughput screening, and thermal management. In this work we develop a fully analytical model to predict the propagation coefficients for liquids hemiwicking through micropillar arrays. This is carried out by balancing the capillary driving force and a viscous resistive force and solving the Navier-Stokes equation for representative channels. The model is validated against a large data set of experimental hemiwicking coefficients harvested from the literature and measured in-house using high-speed imaging. The theoretical predictions show excellent agreement with the measured values and improved accuracy compared to previously proposed models. Furthermore, using lattice Boltzmann (LB) simulations, we demonstrate that the present model is applicable over a broad range of geometries. The scaling of velocity with texture geometry, implicit in our model, is compared against experimental data, where good agreement is observed for most practical systems. The analytical expression presented here offers a tool for developing design guidelines for surface chemistry and microstructure selection for liquid propagation on textured surfaces.

We have explored the contact conditions under which zinc dialkyl dithiophosphate (ZDDP) is corrosive and, in the presence of carbon black, increases wear. It was found that the corrosivity of ZDDP to steel is due to a load dependent mechanochemical reaction and leads to the antagonism between ZDDP and carbon black which is also likely to be load dependent. Depth-profiled Auger spectroscopic analysis of wear scars produced with ZDDP shows that the iron sulfide layer thickness depends on contact load during the wear process. Higher loading produces a thicker iron sulfide layer but, surprisingly, the thickness is relatively insensitive to time. Indenter nanoscratching measurements demonstrate that the highly corroded surface is less scratch resistant than lightly corroded surfaces made under lower load (or of the native steel), explaining increased wear by abrasion of carbon particles ( e.g. carbon black or soot) under highly loaded conditions.

Wear tests are inherently destructive and wear surfaces are often consumed before the worn volume can be detected with traditional methods. Elucidating the subtler features of incipient wear requires improvements in our ability to quantify, locate, and follow wear at the asperity scale. The topographic difference method provides a possible solution, but its use is limited by the difficulty in perfectly repositioning samples and the uncertain effect of imperfect repositioning on the wear measurement. This paper quantified the detection limits of the topographic difference method under the conditions of typical repositioning errors, surface topographies, and measurement approaches. With repositioning errors on the order of 2 μm, the raw/uncorrected topographic difference method reliably detected worn volumes as small as 10 μm3, which rivals the most sensitive macroscale wear measurements reported in the literature. Following virtual posttest realignment, wear volumes as small as 0.1 μm3 were detected. Using standard topographic difference methods in an interrupted wear test allowed us to map asperity-scale wear onto features of the unworn surface profile, observe a complex distribution of real contact areas, and follow the gradual removal of individual asperities with a resolution of 0.1 μm3. To our knowledge, these are the first direct observations of the asperity-scale wear process within a multiasperity macroscale contact.

Engine valve train wear protection has been observed despite the absence of classical tribofilm formation. This was found with a lubricant containing zinc dialkyldithiophosphate (ZDDP), despite conventional understanding that ZDDP reduces wear by forming a thick, protective, inorganic phosphate tribofilm. We suggest that an antiwear additive can function by forming non-adherent triboproduct via a mechanically-stimulated chemical reaction. This triboproduct provides antiwear performance while entrained in the contact and is eventually expelled. In a reciprocating sliding tribometer it is observed as a pile of debris on the side of the contact.

Scattering of the incident electron beam in the variable pressure scanning electron microscope (VPSEM) affects the ability to perform quantitative chemical measurements. However, the manner in which the sum of the elastic and inelastic scattering cross sections varies as a function of gas type and accelerating voltage in the VPSEM is not well understood. A dual Faraday cup was constructed to measure the scattered fraction of the primary beam as a function of gas pressure, working distance, and accelerating voltage in air, water vapor, and argon environments. Experimentally measured values of the scattering cross section agree with previous experimental work, and agree within a factor of two with those values calculated carefully from theory.

A recently installed synchrotron radiation near-edge X-ray absorption fine structure (NEXAFS) full field imaging electron spectrometer was used to spatially resolve the chemical changes of both counterfaces from an ultra- nanocrystalline diamond (UNCD) tribological contact. A silicon flat and Si3N4 sphere were both coated with UNCD, and employed to form two wear tracks on the flat in a linear reciprocating tribometer. The first wear track was produced using a new, unconditioned sphere whose surface was thus conditioned during this first experiment. This led to faster run-in and lower friction when producing a second wear track using the conditioned sphere. The large depth of field of the magnetically guided NEXAFS imaging detector enabled rapid, large area spectromicroscopic imaging of both the spherical and flat surfaces. Laterally resolved NEXAFS data from the tribological contact area revealed that both substrates had an as-grown surface layer that contained a higher fraction of sp2-bonded carbon and oxygen which was mechanically removed. Unlike the flat, the film on the sphere showed evidence of having graphitic character, both before and after sliding. These results show that the graphitic character of the sphere is not solely responsible for low friction and short run-in. Rather, conditioning the sphere, likely by removing asperities and passivating dangling bonds, leads to lower friction with less chemical modification of the substrate in subsequent tests. The new NEXAFS imaging spectroscopy detector enabled a more complete understanding of the tribological phenomena by imaging, for the first time, the surface chemistry of the spherical counterface which had been in continual contact during wear track formation.

Mechanisms of microscale wear in silicon-based microelectromechanical systems (MEMS) are elucidated by studying a polysilicon nanotractor, a device specifically designed to conduct friction and wear tests under controlled conditions. Photoelectron emission microscopy (PEEM) was combined with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and atomic force microscopy (AFM) to quantitatively probe chemical changes and structural modification, respectively, in the wear track of the nanotractor. The ability of PEEM–NEXAFS to spatially map chemical variations in the near-surface region of samples at high lateral spatial resolution is unparalleled and therefore ideally suited for this study. The results show that it is possible to detect microscopic chemical changes using PEEM–NEXAFS, specifically, oxidation at the sliding interface of a MEMS device. We observe that wear induces oxidation of the polysilicon at the immediate contact interface, and the spectra are consistent with those from amorphous SiO2. The oxidation is correlated with gouging and debris build-up in the wear track, as measured by AFM and scanning electron microscopy (SEM).

The morphology, composition, and bonding character (carbon hybridization state) of continuous, ultra-thin (thickness ∼60 nm) nanocrystalline diamond (NCD) membranes are reported. NCD films were deposited on a silicon substrate that was pretreated using an optimized, two-step seeding process. The surface after each of the two steps, the as-grown NCD topside and the NCD underside (revealed by etching away the silicon substrate) is examined by X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM) combined with X-ray absorption near edge structure (XANES) spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The first step in the seeding process, a short exposure to a hydrocarbon plasma, induces the formation of SiC at the diamond/Si interface along with a thin, uniform layer of hydrogenated, amorphous carbon on top. This amorphous carbon layer allows for a uniform, dense layer of nanodiamond seed particles to be spread over the substrate in the second step. This facilitates the growth of a homogeneous, continuous, smooth, and highly sp 3 -bonded NCD film. We show for the first time that the underside of this film possesses atomic-scale smoothness (RMS roughness: 0.3 nm) and N98% diamond content, demonstrating the effectiveness of the two-step seeding method for diamond film nucleation. © 2007 Elsevier B.V. All rights reserved.

Additive explanation for anti-wear Additives in oil are vital for protecting engines from wear by forming films at sliding interfaces. Zinc dialkydithiophosphate (ZDDP) has been used for decades to reduce engine wear. Now there is a strong incentive for finding a replacement for ZDDP: Its breakdown products shorten catalytic converter lifetime. Gosvami et al. examined exactly how ZDDP produces an anti-wear film under high stress or elevated temperature (see the Perspective by Schwarz). Understanding these mechanisms will help in the development of higher-performance and more effective additives. Science , this issue p. 102 ; see also p. 40

This paper reports the recent development and applications of conductive boron-doped ultrananocrystalline diamond (BD-UNCD). The authors have determined that BD-UNCD can be synthesized with an H-rich gaseous chemistry and a high CH4/H2 ratio, which is opposite to previously reported methods with Ar-rich or H-rich gas compositions but utilizing very low CH4/H2 ratios. The BD-UNCD reported here has a resistivity as low as 0.01 ohm cm, with low roughness (

We demonstrate the ability to visualize nanoparticle dissolution while simultaneously providing chemical signatures that differentiate between citrate-capped silver nanoparticles (AgNPs), AgNPs forced into dissolution via exposure to UV radiation, silver nitrate (AgNO3), and AgNO3/citrate deposited from aqueous solutions and suspensions. We utilize recently developed inkjet printing (IJP) protocols to deposit the different solutions/suspensions as NP aggregates and soluble species, which separate onto surfaces in situ, and collect mass spectral imaging data via time-of-flight secondary ion mass spectrometry (TOF-SIMS). Resulting 2D Ag(+) chemical images provide the ability to distinguish between the different Ag-containing starting materials and, when coupled with mass spectral peak ratios, provide information-rich data sets for quick and reproducible visualization of NP-based aqueous constituents. When compared to other measurements aimed at studying NP dissolution, the IJP-TOF-SIMS approach offers valuable information that can potentially help in understanding the complex equilibria in NP-containing solutions and suspensions, including NP dissolution kinetics and extent of overall dissolution.

Wear in self-mated tetrahedral amorphous carbon (ta-C) films is studied by molecular dynamics and near-edge X-ray absorption fine structure spectroscopy. Both theory and experiment demonstrate the formation of a soft amorphous carbon (a-C) layer with increased sp2 content, which grows faster than an a-C tribolayer found on self-mated diamond sliding under similar conditions. The faster $$\hbox{sp}^{3} \rightarrow\,\hbox{ sp}^{2}$$ transition in ta-C is explained by easy breaking of prestressed bonds in a finite, nanoscale ta-C region, whereas diamond amorphization occurs at an atomically sharp interface. A detailed analysis of the underlying rehybridization mechanism reveals that the $$\hbox{sp}^{3}\, \rightarrow\hbox{ sp}^{2}$$ transition is triggered by plasticity in the adjacent a-C. Rehybridization therefore occurs in a region that has not yet experienced plastic yield. The resulting soft a-C tribolayer is interpreted as a precursor to the experimentally observed wear.

Despite best efforts at controlling nanoparticle (NP) surface chemistries, the environment surrounding nanomaterials is always changing and can impart a permanent chemical memory. We present a set of preparation and measurement methods to be used as the foundation for studying the surface chemical memory of engineered NP aggregates. We attempt to bridge the gap between controlled lab studies and real-world NP samples, specifically TiO(2), by using well-characterized and consistently synthesized NPs, controllably producing NP aggregates with precision drop-on-demand inkjet printing for subsequent chemical measurements, monitoring the physical morphology of the NP aggregate depositions with scanning electron microscopy (SEM), acquiring "surface-to-bulk" mass spectra of the NP aggregate surfaces with time-of-flight secondary ion mass spectrometry (ToF-SIMS), and developing a data analysis scheme to interpret chemical signatures more accurately from thousands of data files. We present differences in mass spectral peak ratios for bare TiO(2) NPs compared to NPs mixed separately with natural organic matter (NOM) or pond water. The results suggest that subtle changes in the local environment can alter the surface chemistry of TiO(2) NPs, as monitored by Ti(+)/TiO(+) and Ti(+)/C(3)H(5)(+) peak ratios. The subtle changes in the absolute surface chemistry of NP aggregates vs. that of the subsurface are explored. It is envisioned that the methods developed herein can be adapted for monitoring the surface chemistries of a variety of engineered NPs obtained from diverse natural environments.

Highly $s{p}^{3}$-bonded, nearly hydrogen-free carbon-based materials can exhibit extremely low friction and wear in the absence of any liquid lubricant, but this physical behavior is limited by the vapor environment. The effect of water vapor on friction and wear is examined as a function of applied normal force for two such materials in thin film form: one that is fully amorphous in structure (tetrahedral amorphous carbon, or ta-C) and one that is polycrystalline with $l10$ nm grains [ultrananocrystalline diamond (UNCD)]. Tribologically induced changes in the chemistry and carbon bond hybridization at the surface are correlated with the effect of the sliding environment and loading conditions through ex situ, spatially resolved near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. At sufficiently high relative humidity (RH) levels and/or sufficiently low loads, both films quickly achieve a low steady-state friction coefficient and subsequently exhibit low wear. For both films, the number of cycles necessary to reach the steady-state is progressively reduced for increasing RH levels. Worn regions formed at lower RH and higher loads have a higher concentration of chemisorbed oxygen than those formed at higher RH, with the oxygen singly bonded as hydroxyl groups (C-OH). While some carbon rehybridization from $s{p}^{3}$ to disordered $s{p}^{2}$ bonding is observed, no crystalline graphite formation is observed for either film. Rather, the primary solid-lubrication mechanism is the passivation of dangling bonds by OH and H from the dissociation of vapor-phase H${}_{2}$O. This vapor-phase lubrication mechanism is highly effective, producing friction coefficients as low as 0.078 for ta-C and 0.008 for UNCD, and wear rates requiring thousands of sliding passes to produce a few nanometers of wear.

The impressively low friction and wear of diamond in humid environments is debated to originate from either the stability of the passivated diamond surface or sliding-induced graphitization/rehybridization of carbon. We find ultralow friction and wear for ultrananocrystalline diamond surfaces even in dry environments, and observe negligible rehybridization except for a modest, submonolayer amount under the most severe conditions (high load, low humidity). This supports the passivation hypothesis, and establishes a new regime of exceptionally low friction and wear for diamond.

Recent improvements in growth methodologies have decreased the grain sizes and thicknesses of polycrystalline diamond films to the nanometer range, while also increasing the film uniformity and growth rate and preserving the outstanding mechanical properties of diamond. This is rendering such films more technologically and commercially viable. Ultrananocrystalline diamond (UNCD) are the thinnest (

It is shown that electron-phonon interaction provides a natural explanation for the unusual band dispersion of the metallic surface states at the $\mathrm{Si}(111)\mathrm{\text{\ensuremath{-}}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface. Angle-resolved photoemission reveals a discontinuity of the adatom band at a binding energy close to the dominant surface phonon mode at $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}=70\text{ }\text{ }\mathrm{meV}$. This mode has been assigned to adatom vibrations by molecular dynamics calculations. A calculation of the spectral function for electron-phonon interaction with this well-defined Einstein mode matches the data. Two independent determinations of the electron-phonon coupling parameter from the band dispersion and from the temperature-dependent phonon broadening yield similar values of $\ensuremath{\lambda}=1.09$ and $\ensuremath{\lambda}=1.06$.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

We present a novel method for correlating and classifying ion-specific time-of-flight secondary ion mass spectrometry (ToF-SIMS) images within a multispectral dataset by grouping images with similar pixel intensity distributions. Binary centroid images are created by employing a k-means-based custom algorithm. Centroid images are compared to grayscale SIMS images using a newly developed correlation method that assigns the SIMS images to classes that have similar spatial (rather than spectral) patterns. Image features of both large and small spatial extent are identified without the need for image pre-processing, such as normalization or fixed-range mass-binning. A subsequent classification step tracks the class assignment of SIMS images over multiple iterations of increasing n classes per iteration, providing information about groups of images that have similar chemistry. Details are discussed while presenting data acquired with ToF-SIMS on a model sample of laser-printed inks. This approach can lead to the identification of distinct ion-specific chemistries for mass spectral imaging by ToF-SIMS, as well as matrix-assisted laser desorption ionization (MALDI), and desorption electrospray ionization (DESI).

The chemical modification of nanocrystalline diamond (NCD) atomic force microscope (AFM) tips was investigated and used for chemical force spectroscopy (CFS). In contrast to common chemical modification routes for gold or silicon AFM tips, this method creates stable C–C bonding to attach functional moieties to the NCD tip. There have been no previous studies reporting the chemical functionalization of NCD AFM tips. In this study, hydrogen-terminated NCDs (H-NCDs) were deposited on both silicon wafers and silicon AFM tips and subsequently subjected to a photochemical reaction with undecylenic acid (UA) to create UA attached to NCD surface (UA-NCD). The UA-NCD on wafers were used for surface analyses (water contact angle, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) measurements) to confirm the chemical modification. The UA-NCD AFM tips were subjected to fluorescent labelling to confirm the existence of carboxylic acid on the tip and AFM adhesion measurements to assess their performance as a probe for the detection of chemical species on surfaces. These results indicate the promising ability of this method to serve as an ideal platform for CFS, which requires robust chemical functionalities on the AFM tip surfaces.

We perform a quantitative investigation of the energetics of thermally induced sp3→sp2 conversion of carbon-carbon bonds in tetrahedral amorphous carbon (ta-C) films by using near-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy. We investigate the evolution of the bonding configuration in ta-C thin films subjected to high temperature annealing in flowing Argon gas using a rapid thermal annealing furnace over the range of 200–1000 °C. We observe no substantial change in bonding structure below 600 °C. Changes in the NEXAFS and Raman spectra start to appear above 600 °C, and by 1000 °C a significant increase in the sp2 bonding in the film is observed. No oxygen bonding is detected in the NEXAFS spectra, but we do observe an isosbestic point, demonstrating that the thermally driven sp3→sp2 conversion reaction occurs without passing through an intermediate transition state. This allows us to use NEXAFS spectra of thermally annealed ta-C films to quantitatively determine that the activation energy for directly converting the sp3-bonded carbon to the sp2-bonded configuration is 3.5±0.9 eV.

This letter describes the fundamental process underlying the synthesis of ultrananocrystalline diamond (UNCD) films, using a new low-pressure, heat-assisted bias-enhanced nucleation (BEN)/bias enhanced growth (BEG) technique, involving H2∕CH4 gas chemistry. This growth process yields UNCD films similar to those produced by the Ar-rich/CH4 chemistries, with pure diamond nanograins (3–5nm), but smoother surfaces (∼6nm rms) and higher growth rate (∼1μm∕h). Synchrotron-based x-Ray absorption spectroscopy, atomic force microscopy, and transmission electron microscopy studies on the BEN-BEG UNCD films provided information critical to understanding the nucleation and growth mechanisms, and growth condition-nanostructure-property relationships.

The authors report tribochemical changes due to sliding of a silicon nitride (Si3N4) ball against an ultrananocrystalline diamond (UNCD) thin film. Unidirectional sliding wear measurements were conducted for 2000cycles using a ball-on-disk apparatus with a 3∕16in. diameter Si3N4 ball at a sliding speed of 3.3mm∕s and a normal load of 98.0mN (nominal Hertzian stress of 0.6GPa) in a nitrogen environment at 50% relative humidity at room temperature. The wear track produced on the UNCD film was analyzed by X-ray photoelectron emission spectromicroscopy (X-PEEM) combined with X-ray absorption near-edge structure (XANES) spectroscopy to identify and spatially resolve chemical changes inside the wear track, particularly rehybridization of carbon. XANES spectra show that SiOx complexes are deposited within the wear track. Very little rehybridization of the UNCD from its primarily sp3 bonding configuration to sp2 bonding is observed, and there is no observable oxidation of the UNCD, pointing to the impressive stab...