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.

Source

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

Source

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

Source

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

Source

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

Source

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.

Source

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

Source

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

Source

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.

Source

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.

Source

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.

Source

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

Source

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.

Source

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.

Source

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.

Source

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

Source

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

Source

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

Source

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

Source

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

Source