Atomic Force Microscopy (AFM) Test for Lubricants

The mechanical detection of photothermal expansion from infrared (IR) absorption with an atomic force microscope (AFM) bypasses Abbe's diffraction limit, forming the chemical imaging technique of AFM-IR. Here, we develop a Fourier transform AFM-IR technique with peak force infrared microscopy and broadband femtosecond IR pulses. A Michelson interferometer creates a pair of IR pulses with controlled time delays to generate photothermal signals transduced by AFM to form an interferogram. A Fourier transform is performed to recover IR absorption spectra. We demonstrate the Fourier transform AFM-IR microscopy on a polymer blend and hexagonal boron nitride. An intriguing observation is the vertical asymmetry of the interferogram, which suggests the presence of multiphoton absorption processes under the tip-enhancement and femtosecond IR lasers. Our method demonstrates the feasibility of time-domain detection of the AFM-IR signal in the mid-IR regime and paves the way toward multiphoton vibrational spectroscopy at the nanoscale below the diffraction limit.

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Atomic Force Microscopes (AFM) with 10 nm tip is employed to estimate work of adhesion at nano-scale. The AFM tip is pressed against the surface with forces around a few nano-Newtons and retracted back until it breaks from the surface. Thus estimating the work of adhesion due to this technique can be termed as "hard probing" of the surface. Whereas, we propose another configuration in which a spherical particle is trapped near the surface using a linearly polarized light and the particle attaches to the surface by work of adhesion. Here, by moving the surface in tangential direction, the particle is forced into a rolling motion. This motion can be used to estimate work of adhesion and this technique can be called "soft probing". We used the soft probing configuration to estimate rolling work of adhesion of a birefringent 3 m particle on a glass surface. Further, we have studied the effects of PolydimethylSiloxane (PDMS) which is a hydrophobic surface. This technique is used to probe the rolling work of adhesion of 500 nm nanodiamond bearing Nitrogen-vacancy centers which are birefri... Read More

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So far, the presence of nanobubbles on lubricant-infused surfaces (LIS) has been overlooked, because of the difficulty in detecting them in such a complex system. We recently showed that anomalously large interfacial slip measured on LIS is explained by the presence of nanobubbles [Vega-Snchez, Peppou-Chapman, Zhu and Neto, Nat. Commun., 2022 13, 351]. Crucial to drawing this conclusion was the use of atomic force microscopy (AFM) forcedistance spectroscopy (meniscus force measurements) to directly image nanobubbles on LIS. This technique provided vital direct evidence of the spontaneous nucleation of nanobubbles on lubricant-infused hydrophobic surfaces. In this paper, we describe in detail the data collection and analysis of AFM meniscus force measurements on LIS and show how these powerful measurements can quantify both the thickness and distribution of multiple coexisting fluid layers (i.e., gas and oil) over a nanostructured surface. Using this technique, thousands of force curves were automatically analyzed. The results show that the interfacial tension of the nanobubbles is ... Read More

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Microscopy techniques are essential for understanding the structure of materials of interest in agriculture, food, and the environment. These techniques can be classified according to their operating principles, such as fluorescence, electron, and probe scanning. Their complementary techniques provide specific advantages in the characterization of materials in the above mentioned fields. These approaches facilitate the characterization of the structure and morphology at nanometric and atomic scales of different materials through high-resolution images, as well as the analysis of important characteristics related to the composition and distribution of specific components. In this work, detailed descriptions are given of the operation principles of light microscopy (LM), confocal laser scanning microscopy (CLSM), superresolution microscopy (SRM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A compilation of operating principles is presented along with examples obtained with advanced microscopy techniques applied to the a... Read More

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Understanding the mechanism of antiwear (AW) tribofilm formation and how to tune surface chemistry to control functionality is essential for the development of the next generation of oil lubricants. In particular, understanding and optimizing early AW tribofilm formation can increase the energy efficiency of mechanical systems. However, the mechanism for how these films form is not well understood. The majority of prior work has focused on analyzing only end-of-test surfaces long after the film has formed. In this work, we develop an in situ multimodal chemical imaging methodology to directly visualize the early formation of AW films on steel surfaces. We investigate an oil formulation containing a phosphorus-based additive commonly used to protect surfaces from wear and fatigue processes in machine elements, such as gears, bearings, and sliding contacts. Using nanoscale multimodal chemical imaging on combined platforms of atomic force microscopy (AFM) coupled directly with in situ nano-infrared (nano-IR) spectroscopy, and further combined ex situ with time-of-flight secondary ion ma... Read More

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This paper presents an investigation of atomic force microscopy (AFM) in the force-distance mode for self-assembled monolayers (SAM) by means of molecular dynamics modeling methods. A model system of SAM of biphenyldithiol (BPDT) on a gold substrate and a gold tip with a diameter of 10 nm were used. This work has reproduced the general behavior of forcedistance curves in similar systems observed in experimental studies. The impact of the structural order (i.e., standing up ordered, disordered and striped phases) on the attractive and repulsive molecular interactions and conformations were revealed. The ordered SAM in standing up phase has the highest adhesive force and the striped phase has the lowest. The adhesive forces as a function of the tip-surface distance exhibit some unique and distinct features. An AFM induced ordering for both disordered and striped phases is reported.

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Phospholipids and hyaluronan are two key biomolecules that contribute to the excellent lubrication of articular joints. Phospholipids alone and in combination with hyaluronan have also displayed low friction forces on smooth surfaces in micro- and nanosized tribological contacts. In an effort to develop aqueous-based lubrication systems, it is highly relevant to explore if these types of molecules also are able to provide efficient lubrication of macroscopic tribological contacts involving surfaces with roughness larger than the thickness of the lubricating layer. To this end, we investigated the lubrication performance of hyaluronan, the phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and mixtures of these two components using glass surfaces in a mini-traction machine. We compared our data with those obtained using flat silica surfaces in previous atomic force microscopy studies, and we also highlighted insights on hyaluronanphospholipid interactions gained from recent simulations. Our data demonstrate that hyaluronan alone does not provide any lubricating benefit,... Read More

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We used atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to comprehensively study the growth and the cross-linking of dotriacontane (C32H66) nanofilms that were deposited on a silicon wafer by the spin-coating process. It was found that the molecular structure of the nanofilms changed with C32H66 concentration at the given spin speed, of which a monolayer of oriented C32H66 molecules, formed at lower deposition concentrations, was composed of a perpendicular orientation state with the molecular long-chain axis perpendicular to the substrate surface and a parallel orientation state, while the perpendicular state was essentially dominant when the nanofilm was formed at higher deposition concentrations. The shortening of the first perpendicular layer in AFM topography could be attributed to the mixing of both parallel and perpendicular lamellas in the first layer. XPS analysis indicated that the average thickness of the layer almost linearly increased with the C32H66 concentration. The monolayer of C32H66 film could be cross-linked by a hyperthermal hydrogen-indu... Read More

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Abstract This article discusses the operating principles, advantages, and limitations of scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, and secondary ion mass spectroscopy that are used to analyze the surface chemistry of plastics.

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The integration of atomic force microscopy (AFM) with IR radiation provides a reliable route to bypass the diffraction limit to achieve spatial resolution spectroscopic imaging at a 10-nm scale. As of 2021, two main categories of AFM-based IR microscopy exist based on their detection principles: mechanical detection on the tip-enhanced photothermal response of the sample; optical detection through light scattering from the near field of the AFM tip. This chapter on AFM-based IR microscopy will separately describe the working principles of these two routes, followed by some of their applications in the characterization of energetic materials.

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Lubricant-infused surfaces hold promise to reduce the huge frictional drag that slows down the flow of fluids at microscales. We show that infused Teflon wrinkled surfaces induce an effective slip length 50 times larger than expected based on the presence of the lubricant alone. This effect is particularly striking as it occurs even when the infused lubricant's viscosity is several times higher than that of the flowing liquid. Crucially, the slip length increases with increasing air content in the water but is much higher than expected even in degassed and plain Milli-Q water. Imaging directly the immersed interface using a mapping technique based on atomic force microscopy meniscus force measurements reveals that the mechanism responsible for this huge slip is the nucleation of surface nanobubbles. Using a numerical model and the height and distribution of these surface nanobubbles, we can quantitatively explain the large fluid slip observed in these surfaces.

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This article focuses on the atomic force microscopy-infrared (AFM-IR) technique and its recent technological developments. Based on the detection of the photothermal sample expansion signal, AFM-IR combines the high spatial resolution of atomic force microscopy with the chemical identification capability of infrared spectroscopy to achieve submicrometric physico-chemical analyses. Since the first publication in 2005, technological improvements have dramatically advanced the capabilities of AFM-IR in terms of spatial and spectral resolution, sensitivity, and fields of applications. The goal of this paper is to provide an overview of these developments and ongoing limitations. We summarize recent progress in AFM-IR implementations based on the major AFM contact, tapping, and peak force tapping modes. Additionally, three new trends are presented, namely, AFM-IR applied to mineral samples, in fluid and a novel, purely surface sensitive AFM-IR configuration, to probe top layers. These trends demonstrate the immense potential of the technique and offer a good insight into the scope of AFM-... Read More

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By combining capabilities of atomic force microscopy (AFM) with infrared (IR) spectroscopy, AFM-IR resolves nanoscale compositional details. This tutorial reviews technical breakthroughs, working principles, best practices, and future prospects of AFM-IR.

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In this section, the characterisation tools including atomic force microscopy (AFM) [1], scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM) that are used throughout this thesis are outlined in detail.

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A fluorescence microscope applies high-intensity light to illuminate the substance that emits fluorescence light. The present chapter describes the basic principle and applications of fluorescence microscopy. There are two types of Fluorescence microscopes: transmitted fluorescent microscope and incident fluorescent microscope. The working principles of both these types of microscopes are described. Confocal microscopy (CFM) provides three-dimensional optical resolution. In CFM, at one time we see the image of the particular depth of the object at a small point. All the out of focus light is eliminated by passing the light through the pinhole. Multiple images at different depths are accumulated and then reconstructed to provide a three-dimensional image. The working principle and applications of CFM are discussed in the present chapter. In addition, the chapter also discusses the various other advanced microscopic techniques such as Scanning tunnelling microscope and atomic force microscopy.

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Abstract Mechanical vibration, as an alternative of application of solid/liquid lubricants, has been an effective means to modulate friction at the macroscale. Recently, atomic force microscopy (AFM) experiments and model simulations also suggest a similar vibration-induced friction reduction effect for nanoscale contact interfaces, although an additional external vibration source is typically needed to excite the system. Here, by introducing a piezoelectric thin film along the contact interface, we demonstrate that friction measured by a conductive AFM probe can be significantly reduced (more than 70%) when an alternating current (AC) voltage is applied. Such real-time friction modulation is achieved owing to the localized nanoscale vibration originating from the intrinsic inverse piezoelectric effect, and is applicable for various material combinations. Assisted by analysis with the PrandtlTomlinson (PT) friction model, our experimental results suggest that there exists an approximately linear correlation between the vibrational amplitude and the relative factor for perturbation ... Read More

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Atomic force microscopy (AFM) with a gold colloid probe modeled as the electrode surface is employed to directly capture the contact resonance frequency of two phosphonium-based ionic liquids (ILs) containing a common anion [BScB]- and differently lengthened cations ([P6,6,6,14]+ and [P4,4,4,8]+). The comparative interfacial studies are performed by creating IL films on the surface of gold, followed by measuring the wettability, thickness of the films, adhesion forces, surface morphology and AFM-probed contact resonance frequency. In addition, the cyclic voltammetry and impedance spectroscopy measurements of the neat ILs are measured on the surface of the gold electrode. The IL with longer cation alkyl chains exhibits a well-defined thin film on the electrode surface and enhanced the capacitance than the shorter chain IL. The AFM contact resonance frequency and force curves reveal that the longer IL prefers to form stiffer ion layers at the gold electrode surface, suggesting the "anion-anion-cation-cation" bilayer structure, in contrast, the shorter-chain IL forms the softer cation... Read More

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Atomic force microscopy (AFM) is widely used for quantifying the mechanical properties of soft materials such as cells. AFM force-indentation curves are conventionally fitted with a Hertzian model to extract elastic properties. These properties solely are, however, insufficient to describe the mechanical properties of cells. Here, we expand the analysis capabilities to describe the viscoelastic behavior while using the same force-indentation curves. Our model gives an explicit relation of force and indentation and extracts physically meaningful mechanical parameters. We first validated the model on simulated force-indentation curves. Then, we applied the fitting model to the force-indentation curves of two hydrogels with different crosslinking mechanisms. Finally, we characterized HeLa cells in two cell cycle phases, interphase and mitosis, and showed that mitotic cells have a higher apparent elasticity and a lower apparent viscosity. Our study provides a simple method, which can be directly integrated into the standard AFM framework for extracting the viscoelastic properties of mate... Read More

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The dynamics of the self-assembled liquid nanostructure of the ionic liquids (ILs) near a mica surface can be determined from video-rate atomic force microscopy (AFM) data.Video-rate AFM has been used to record the nanostructure dynamics of two most widely studied ILs, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM TFSI) and ethylammonium nitrate (EAN), as well as EAN-water mixtures, above a model anode, mica. Diffusion coefficients were extracted from the AFM videos using dynamic differential microscopy and direct tracking.Video rate AFM is able to record the movement of the IL nanostructure. This is the first time that any liquid has been directly visualized at a scale of 10 nm 10 nm in real-time. Diffusion coefficients determined from AFM videos reveal IL nanostructures near surfaces diffuse orders of magnitude more slowly than individual ions in the bulk. Thus, rather than free-flowing liquid, the near-surface nanostructure is better conceptualized as self-assembled aggregates of IL ions diffusing slowly over the cation-rich Stern layer, akin to adsorbed su... Read More

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Abstract Dynamic single asperity contact, using Atomic Force Microscope (AFM) probe tips, have been recently shown to be a powerful tool for investigating the tribological properties of surfaces. Here, we demonstrate a novel in-situ approach that produces a multilayer, ziggurat-structured, tribofilm developed over time. On a single sample produced during one uninterrupted experiment, each distinct layer uniquely represents a different stage of tribofilm development arising from decomposition of zinc dialkyl-dithiophosphate (ZDDP). We were able to successfully analyse the structure of the most developed inner layers, which reached a thickness of more than 130 nm, and the less developed outer regions, which were just a few nm thick, as well as intermediate layers. Thus, selected area X-ray photoelectron spectroscopy, measured a shift in the composition of zinc, sulphur and phosphorus, and a decrease in the sulphide:sulphate and ZnS:ZnO ratios as the film developed. Furthermore, AFM-IR, used in this application for the first time, directly identified S = O groups in the early stage of t... Read More

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Wear of multi-asperity interfaces remains difficult to predict from first principles, in part because improvements are required in our ability to quantify and track wear across the micro-to nanometer scale. In this work, we developed a 6 of freedom topographical difference method based on large atomic force microscopy (AFM) measurements, up to 90 90 m2 in size. We detect wear volumes as small as 1.6 1011 3.7 1012 mm3 (0.016 m3), beyond the sensitivity of many existing techniques for the quantification of wear at multi-asperity interfaces. We show that our wear detection technique can be combined with 100 mN normal force ball-on-flat friction experiments to track nanoscale wear across the entire area of apparent contact.

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Numerous hydrocarbon and fluorine-based hydrophobic surfaces have been widely applied in various engineering and bioengineering fields. It is hypothesized that the hydrophobic interactions of hydrocarbon and fluorinated surfaces in aqueous media would show some differences.The hydrophobic interactions of hydrocarbon and fluorinated surfaces with air bubbles in aqueous solutions have been systematically and quantitatively measured using a bubble probe atomic force microscopy (AFM) technique. Ethanol was introduced to water for modulating the solution polarity. The experimental force profiles were analyzed using a theoretical model combining the Reynolds lubrication theory and augmented Young-Laplace equation by including disjoining pressure arisen from the Derjarguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO interactions (i.e., hydrophobic interactions).The experiment results show that the hydrophobic interactions were firstly weakened and then strengthened by increasing ethanol content in the aqueous media, mainly due to the variation in interfacial hydrogen bonding network. The flu... Read More

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Method and system for performing subsurface atomic force microscopy measurements by monitoring the dynamic response of an AFM cantilever and tip in contact with a sample while delivering ultrasound through the sample's bottom surface. The system includes a transducer, signal source, and measurement device to control and measure the vibrational waves interacting with subsurface features, enabling improved control over measurement parameters and reduced dependence on changing measurement conditions.

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We present here a fundamental study on the miscibility between a prototype amphiphilic dye and alkylic and arylic Langmuir monolayers. Embedding dyes in such matrices is crucial for utilizing dyes in any photo-energy conversion process if the involved dyes form aggregates that provide thermal deactivation channels. Because miscibility in Langmuir matrices depends on the blending ratio between the dye and matrix and on the Langmuir film density, as characterized via the surface pressure and the mean molecular area, we employ Langmuir miscibility studies to identify ideal miscibility parameters for each matrix. Atomic force microscopy (AFM) results support miscibility between the dye and both matrix materials at low surface pressures, where smooth and homogeneous films are obtained. AFM and photo-induced force microscopy (PiFM) reveal phase separation if the Langmuir monolayers are deposited at surface pressures above 8 mN/m at which reorientation of the chromophores has been reported. The nanoscale chemical fingerprint mapping enabled by PiFM enables assigning segregated spots to smal... Read More

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We report an atomic force microscopy (AFM) based nanoshaving approach on graphite, covalently functionalized using diazonium chemistry. Upon removal of the covalently bound layer on top of graphite, two different types of breakdown products are observed under ambient conditions, depending on the diazonium salt used. Due to the nanoshaving procedure, the strained graphite lattice is restored to its pristine sp2 nature, as confirmed by Raman microscopy. A general strategy for nanoshaving is provided under ambient and liquid conditions, optimizing the key parameters, and aimed toward retaining the structural integrity of the AFM probe, allowing subsequent imaging. Finally, the self-assembly of n-pentacontane was studied inside such in situ nanoshaved areas to illustrate the potential of AFM based nanoshaving to investigate self-assembly processes occurring under 2D lateral confinement.

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Interfacial water is closely related to many core scientific and technological issues, covering a broad range of fields, such as material science, geochemistry, electrochemistry and biology. The understanding of the structure and dynamics of interfacial water is the basis of dealing with a series of issues in science and technology. In recent years, atomic force microscopy (AFM) with ultrahigh resolution has become a very powerful option for the understanding of the complex structural and dynamic properties of interfacial water on solid surfaces. In this perspective, we provide an overview of the application of AFM in the study of two dimensional (2D) or three dimensional (3D) interfacial water, and present the prospect and challenges of the AFM-related techniques in experiments and simulations, in order to gain a better understanding of the physicochemical properties of interfacial water.

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A surface analysis method that enhances the characterization of organic materials on sample surfaces by extracting the breaking length from force curves obtained through scanning probe microscopy. The method involves first-order differentiation of the force curves to generate a differential curve, from which the breaking length is calculated as the distance to the farthest peak. This breaking length serves as a critical parameter for understanding the mechanical properties of the organic material.

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Over the time, atomic force microscopy (AFM) turned out to be besides a powerful technique used to visualize and to measure nanometric surface structures, an innovative technique that can be successfully applied for various nanometer scale modifications of polymer samples. Among the AFM-based lithographic techniques, dynamic plowing lithography (DPL) offers the possibility of surface modeling by indenting it with a vibrating tip using semicontact mode after a predefined pattern with a spacing even of a few nanometers without leading to edge irregularities. The advantage of the method lies in the fact that both the lithography and the imaging of the lithographed surface can be made without changing the scanning tool or the cantilever with no undesirable modifications. Among the polymers that can be patterned at nanoscale with DPL procedure, polyimides represent a special class due to their attractive mechanical, thermal, and electrical properties. A complex characterization of the obtained morphology through the surface texture and functional parameters can reveal the influence of the... Read More

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Lubricant-infused surfaces (LIS) have emerged as an innovative way to combat several modern challenges such as biofouling, ice formation, and surface drag. The favorable properties of LIS are dependent on the presence and distribution of a lubricant layer coating the underlying substrate. Unfortunately, this layer is not indefinitely stable and depletes due to external forces. Here, we study how an air/water interface depletes the lubricant from LIS as a function of lubricant wettability on the substrate by varying the chemistry of both the lubricant and the substrate. The lubricants were chosen to represent some of those most commonly used in the literature (silicone oil, perfluoropolyethers, and mineral oil). We use an optical Wilhelmy plate tensiometer to measure the contact angle of the air/water interface on the LIS in situ as the sample is driven through the air/water interface and contact angle hysteresis as a qualitative measure of lubricant depletion. This data is augmented with ex situ quantitative mapping of lubricant thickness using atomic force microscopy (AFM) meniscus ... Read More

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Abstract Atomic force microscopy (AFM) is central to investigating the piezoelectric potentials of one-dimensional nanomaterials. The AFM probe is used to deflect individual piezoelectric nanorods and to measure the resultant current. However, the torsion data of AFM probes have not been exploited to elucidate the relationship between the applied mechanical force and resultant current. In this study, the effect of the size of ZnO nanorods on the efficiency of conversion of the applied mechanical force into current was investigated by simultaneously acquiring the conductive AFM and lateral force microscopy signals. The conversion efficiency was calculated based on linear regression analysis of the scatter plot of the data. This method is suitable for determining the conversion efficiencies of all types of freestanding piezoelectric nanomaterials grown under different conditions. A pixel-wise comparison of the current and lateral force images elucidated the mechanism of current generation from dense arrays of ZnO nanorods. The current signals generated from the ZnO nanorods by the AFM ... Read More

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Total internal reflection (TIR) infrared spectroscopy is a convenient measurement tool for collecting spectra for chemical identification. However, TIR infrared microscopy lacks high spatial resolution due to the optical diffraction limit and difficulty to preserve a high-quality wave front for focus. In this article, we present the peak force infrared microscopy in the TIR geometry to achieve a 10 nm spatial resolution. Instead of optical detection, photothermal responses of the sample are collected in the peak force tapping mode of atomic force microscopy. We demonstrate the technique on two representative samples: structured polymers for soft matters and a hexagonal boron nitride flake for two-dimensional materials. As an extension of the apparatus, we also demonstrate nanoinfrared imaging with the TIR excitation for photoinduced force microscopy. The combination of TIR geometry with nanoinfrared microscopies simplifies the optical alignment, providing alternative instrument-designing principles for atomic force microscopy-based infrared microscopy.

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The presence of nanobubbles on lubricant-infused surfaces (LIS) has so far been overlooked due to the difficulty in detecting them in such a complex system. We recently showed that anomalously large interfacial slip measured on LIS is explained by the presence of nanobubbles. Crucial to drawing this conclusion was the use of atomic force microscopy (AFM) force-distance spectroscopy to directly image nanobubbles on LIS. This technique provided vital direct evidence of the spontaneous nucleation of nanobubbles on lubricant-infused hydrophobic surfaces. In this paper, we describe in detail the data collection and analysis of AFM meniscus force measurements on LIS and show how these powerful measurements can quantify both the thickness and distribution of multiple coexisting fluid layers (i.e. gas and oil) over a nanostructured surface.

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The data include Atomic Force Microscopy (AFM) measurements of forces acting between two cleaved (104) calcite surfaces collected using a JPK NanoWizard AFM equipped with an Olympus IX71 optical microscope in solutions containing a low or high concentration of dissolved Ca²⁺. Further details on how to reuse these data are included in the attached data_description.txt file.

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Abstract Atomic force microscopy (AFM) is one of the most popular imaging and characterizing methods applicable to a wide range of nanoscale material systems. However, highresolution imaging using AFM generally suffers from a low scanning yield due to its method of raster scanning. Here, a systematic method of data acquisition and preparation combined with a deeplearningbased image superresolution, enabling rapid AFM characterization with accuracy, is proposed. Its application to measuring the geometrical and mechanical properties of structured DNA assemblies reveals that around a tenfold reduction in AFM imaging time can be achieved without significant loss of accuracy. Through a transfer learning strategy, it can be efficiently customized for a specific target sample on demand.

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Wettability effect has long been a concern in various aqueous lubrication systems including biological and industrial applications. The wettability may affect lubrication performance by changing interfacial viscosity or hydration force. The key point to reveal the mechanism is to design an ideal experimental system to exclude other bulk factors other than surface wettability. In this work, silicon surfaces with different treatments were used to study the single factor effect of wettability on aqueous lubrication. The normal and friction forces of these surfaces were quantified by atomic force microscopy (AFM) in water environment. The interfacial viscosity was evaluated according to the probe dynamic approaching process. Macroscale and microscale lubrication experiments of other materials were also conducted as verification and supplement. A semi-quantitative relationship between friction and wettability was revealed and attributed to the competition between the attractive van der Waals interactions and wettability-dependent repulsive hydration interaction, which determined the stren... Read More

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Atomic force microscopy (AFM) is a kind of high-precision nanoscale instrument to measure the surface morphology of various samples. Nevertheless, the standard AFM scanning process takes a very long time to obtain high-resolution images. Compressive sensing (CS) can be used to achieve fast AFM imaging. But, the traditional CS-AFM imaging is difficult to balance the image quality of each local area, resulting in poor quality in the object area at low sampling rate. Therefore, a novel imaging scheme of adaptive CS-AFM is proposed. The fast scanning is first used to generate a low resolution image in a short time, and then bicubic interpolation is performed to obtain a high resolution image. Afterwards, an advanced detection algorithm is used to realize the accurate detection and positioning of the objects. Furthermore, the supplementary scanning is carried out to achieve adaptive sampling on the objects. After sampling, the measurement matrix corresponding to the measurement points is constructed. Finally, Total Variation Minimization by Augmented Lagrangian and Alternating Direction A... Read More

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Several industrial applications require bearings to work under slow oscillating motions and very high contact pressures (aircraft actuators, wind turbine, robotic arms, etc.). Hence, a boundary lubrication regime predominates. However, grease provides the lubrication essential to assure bearing integrity. In this study, the mechanisms involved in protecting the contact surfaces are investigated. High loaded oscillating movements have been applied on a commercial greased deep groove ball bearing. The morphology of its contact was observed using Scanning Electron Microscopy (SEM), revealing superficial transformations. Further, with an extreme surface X-Ray Photoelectron Spectroscopy (XPS) analysis, three cross-sections made by a nanomachining process (FIB) were investigated using Transmission Electron Microscopy (TEM). The analyses revealed a modified layer at the contacts generated by grease interactions.

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This chapter discusses the morphology of the polymeric coatings. The factors such as concentration of the polymers, types of blends, processing, and annealing time that affect the surface and composition of the polymeric coatings are also discussed in this chapter. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical microscopy have successfully been used to characterize the morphology of the polymeric coatings.

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Abstract Conventional atomic force microscopy (AFM) tips have remained largely unchanged in nanomachining processes, constituent materials, and microstructural constructions for decades, which limits the measurement performance based on force-sensing feedbacks. In order to save the scanning images from distortions due to excessive mechanical interactions in the intermittent shear-mode contact between scanning tips and sample, we propose the application of controlled microstructural architectured material to construct AFM tips by exploiting material-related energy-absorbing behavior in response to the tipsample impact, leading to visual promotions of imaging quality. Evidenced by numerical analysis of compressive responses and practical scanning tests on various samples, the essential scanning functionality and the unique contribution of the cellular buffer layer to imaging optimization are strongly proved. This approach opens new avenues towards the specific applications of cellular solids in the energy-absorption field and sheds light on novel AFM studies based on 3D-printed tips p... Read More

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Atomic Force Microscopy (AFM), in conjunction with Peak Force Kelvin Probe Force Microscopy (PF-KPFM) and Peak Force Scanning Spreading Resistance Microscopy (PF-SSRM), was used to assess changes on thin metal films that underwent accelerated aging. The AFM technique provides a relatively easy, non-destructive methodology that does not require high-vacuum facilities to obtain nanometer-scale spatial resolution of surface chemistry changes. Surface morphology, roughness, contact potential difference, and spreading resistance were monitored to qualitatively identify effects of aging-morphology changes and oxidation of Au, Al, Cu thin film standards as well as diffusion of CuAu and AlAu thin film stacks at 65C under dried nitrogen flow conditions. AFM PF-KPFM and PF-SSRM modes have been exercised, refined and have proven to be viable and necessary early aging detection tools.

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Graphene has attracted tremendous attention as a promising additive in lubricants due to its unique lamellar structure and excellent mechanical strength. Yet, unlike its use in oil and water lubricants, the amount of graphene additive should be considered when it is introduced into the grease which is a two-phase colloid. In this work, graphene was added into the lithium grease in different concentrations, and lubrication behaviors were investigated using a four-ball testing method under various operating conditions. Prior to the four-ball friction tests, graphene and grease materials were characterized by scanning (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and Raman spectroscopy. Friction test results demonstrate that the graphene concentration in grease varies at different tribological contact conditions to reach the optimum lubrication behavior. On the basis of the results from friction tests and worn scar morphology analysis, a lubrication mechanism was proposed to better understand the interactions among grease eleme... Read More

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Herein, an in situ atomic force microscopy (AFM) study of tribological properties of the AZ91 Mg alloy under lubricant containing acrylamide additive is reported. The in situ nanoscale study unravels the significance of microstructure (matrix vs second phase), stress, and temperature on the friction, wear, and tribofilm growth within the localized regions of an AZ91 Mg alloy. The results suggest that at elevated temperature (110 C), the nanometer thick tribofilm forms on precipitate and matrix surfaces; however, there was no evidence of its formation at ambient temperature. The tribofilm growth in both regions was strongly dependent on the contact stress. Moreover, there exists an apparent stress value at which the thickness of the tribofilm is maximum. The friction force with sliding time shows similar variation over precipitate and matrix during the tribofilm growth. However, the magnitude of the friction force is strongly dependent on the thickness of the tribofilm. Finally, the results suggest acrylamide as a potential alternative to zinc dialkyldithiophosphate (ZDDP) for effect... Read More

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Ionic liquids (ILs) are promising candidates for the next-generation nanometer-thick lubricants in hard disc drives (HDDs) because of their excellent physiochemical properties and low monolayer (ML) thicknesses. However, the commercially available ILs have higher surface tension and higher friction than the state-of-the-art perfluoropolyether (PFPE) lubricant. In the current study, a fluorinated IL (FIL) lubricant, which contains cations with highly fluorinated alkyl chains, has been successfully synthesized. The surface tension of FIL is comparable to that of PFPE, which is ideal for enhancing the tribological performance of the FIL lubricant. The thermogravimetric analysis results showed that FIL has higher thermal stability than PFPE Ztetraol. Atomic force microscopy revealed that because of the intrinsically smaller molecular size of FIL, the ML thickness is only 50% of that of Ztetraol, which is expected to induce a lower lubricant thickness and higher areal density. Compared to the commercially available ILs, reduced friction of the nanometer-thick FIL lubricant on carbon over... Read More

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Sustaining the current trend toward lower viscosity lubricants requires the development of improved friction modifier additives to compensate for the increased prevalence of boundary lubrication. In this work, we demonstrate that functionalizing oil-soluble olefin copolymers (OCP) with a random distribution of a small amount (around 1.4 mol %) of polar-aromatic groups provides an additive with potential for enhanced boundary lubrication. Extensional relaxation time measurements show that the functionalized olefin copolymer (FOCP) can form transient, associative bonds between polymer chains, while quartz crystal microbalance with dissipation monitoring (QCM-D) demonstrates that the amphiphilic nature of FOCP drives adsorption at the solidliquid interface. Colloid probe atomic force microscope (AFM) measurements show that the adsorbed layer forms a steric barrier that reduces friction in the boundary regime. Comparing interactions between preadsorbed FOCP layers in bulk solution with and without FOCP chains reveals that FOCP chains in the solution interact with the adsorbed polymer to... Read More

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There are two types of friction modifiers (FMs) used as lubricant additives: Reaction film FMs (RF-FMs) and adsorption film FMs (AF-FMs). While RF-FMs provide good performance in severe conditions, AF-FMs excel in mild conditions. This empirical evidence leads us to combine these two FMs to cover broader conditions. However, the effects of their combination are highly complicated due to the interaction between these FMs. If the interaction force of AF-FMs with various materials can be evaluated, it would help us to improve tribological performances of lubricants. Although atomic force microscopy seems suitable for this application, we found some obstacles, such as fluid resistance, electrostatic force, and laser positioning of the cantilever, to achieve proper measurements of the adsorption force. In this study, the adsorption force between the polar group and the surface was directly measured in oil with a 1 m silica probe modified with CH3 or COOH. This paper proposed how to eliminate errors included in the adsorption force measurement using AFM and a calibration method for obtain... Read More

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Atomic force microscopy has been used to measure the lubricity of a series of ionic liquids (ILs) at mica surfaces in the boundary friction regime. A previously unreported cation bilayer structure is detected at the ILmica interface due to the formation of H-bonds between the hydroxy-functionalized cations [(c-c) H-bonds], which enhances the ordering of the ions in the boundary layer and improves the lubrication. The strength of the cation bilayer structure is controlled by altering the strength of (c-c) H-bonding via changes in the hydroxyalkyl chain length, the cation charge polarizability, and the coordination strength of the anions. This reveals a new means of controlling IL boundary nanostructure via H-bonding between ions of the same charge, which can impact diverse applications, including surface catalysis, particle stability, electrochemistry, etc.

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Several fabrication technologies for molecular level ultra-thin films, from organic to organicinorganic hybrids, are introduced using the surface imaging technology of scanning probe microscopy (SPM). Electrochemical scanning tunneling microscopy (EC-STM), atomic force microscopy (AFM) and related techniques enable the observation of the surface of 2D materials in near real-time and in real-space. The combination of 2D materials and SPM technology opens up a new avenue of surface and material science, despite its many restrictions. Several fabrication technologies for molecular level ultra-thin films, from organic to organicinorganic hybrids, are introduced using the surface imaging technology of scanning probe microscopy (SPM). Electrochemical scanning tunneling microscopy (EC-STM), atomic force microscopy (AFM) and the related techniques enable the observation of the surface of 2D materials in near real-time and in real-space. The combination of 2D materials and SPM technology opens up a new avenue of surface and material science, despite its many restrictions.

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The stability of particle suspensions, which is important in various industrial processes, is generally dominated by the interaction forces between the particles. Understanding the interaction forces between surfaces in liquids is therefore fundamentally important to evaluate and control how particulate matters, including two phase fluids such as emulsions and bubbles, disperse and aggregate in various systems. The invention of the surface force apparatus SFA enabled the direct measurement of interaction forces between surfaces in liquids with molecular level resolution has led to remarkable progress in understanding surface forces in detail and the application of atomic force microscopy AFM to force measurement has further extended the possibility of force measurements to a broad field of research. This review provides an overview of developments in the investigating of interaction forces between surfaces using SFA and AFM. The properties of various interaction forces are described in detail, particularly focusing on how the nanoscopic structures of various interfaces, including... Read More

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A third of the energy from fuel combustion in passenger car gasoline engines is lost due to friction. Carefully designed engine lubricants can recover some of these losses by reducing friction and wear by forming a nanometer-scale chemico-physico tribofilm between surfaces. Accordingly, attention has focused on developing oil formulations that form low-friction tribofilms. However, analyses of resultant tribofilms are typically conducted after tribo-tests with conventional characterization tools and do not offer insights into tribofilm formation and evolution, precluding information critical to tuning tribofilm properties. In this work, we developed a unique multimodal methodology based on Atomic Force Microscopy (AFM) with local probe heating for in situ tribological studies that activates friction modifiers and simultaneously captures the evolution of friction and surface roughness, with nanometer resolution. As a platform to demonstrate the ability of this methodology to visualize dynamics of tribofilm formation in situ, we apply it to molybdenum-based friction modifiers to distin... Read More

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The atomic force microscopy (AFM) is a versatile instrument that undoubtedly contributes to the development of the nanosciences. This chapter deals with a new technological breakthrough based on the AFM to study nano-tribology: the circular-mode AFM (CM-AFM). It consists in modifying the electronics of the AFM to generate a relative circular displacement of the AFM tip/sample contact. Unlike the traditional AFM modes, the CM-AFM allows performing friction force measurements in a quasi-stationary regime with a high sliding velocity (higher than 1mm/s). Advantages and limitations of the CM-AFM are discussed. Finally, applications of the CM-AFM for investigating friction mechanisms of hydrophilic and viscoelastic surfaces, nano-wear mechanisms and mechanical properties of cell membranes are reported.

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