73 research outputs found

    Mitigating Wear on Surfaces Utilizing Self-Assembled Wear Passivating Films

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    Controlling tribological interactions, such as friction and adhesion between contacting interfaces is critical for the advancement of technologies such as microelectromechanical systems (MEMS) devices. The challenge in MEMS device lubrication lies in the inherent nature of the material’s surface at the nanoscale as well as the nature of the surfaces typically used during experimentation. Device surfaces often display nanoscale roughness with surface asperities dictating the tribological properties between interfaces, yet the vast majority of past research has focused predominately on nanotribological studies of thin films on flat silicon substrates to model the behavior of these self-assembled wear-reducing coatings. New model surfaces have been manufactured and integrated into experiments in which surfaces with controlled asperity sizes act as more realistic models of MEMS surfaces. As friction and adhesion between real surfaces in sliding contact are dominated by the interactions of nanoscaled surface asperities, this research is an extension of previous work, moving beyond smooth surfaces by manufacturing and implementing new experimental platforms possessing controlled asperity sizes. The influence of asperity size on the tribological properties of these contacts is being studied for both native oxide and organosilane derivatized surfaces. These studies more readily mimic the conditions found at true asperity-asperity contacts. This research has aimed to develop new lubricant thin films that can effectively protect MEMS device surfaces during use with the long term goal of bringing MEMS devices out of the laboratory and into wide scale commercial use. This work investigates how self-assembled monolayers (SAMs) on curved surfaces can be utilized in manners that their analogs on flat surfaces cannot. SAMs on curved asperities can be used to trap short chain alcohols, which during contact may be released to function as an additional lubricant layer on the surface. Both atomic force microscopy and Fourier transform infrared spectroscopy have been employed to evaluate how chain disorder influences the protective function of these molecular lubricant layers on asperities. It was found that functionalized surfaces resisted wear and were able to operate under continuous scanning for longer time frames than unfunctionalized surfaces and that multicomponent films improved upon the performance of their base, single component analogs

    Study of Role of Meniscus and Viscous Forces During Liquid-Mediated Contacts Separation

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    Menisci may form between two solid surfaces with the presence of an ultra-thin liquid film. When the separation operation is needed, meniscus and viscous forces contribute to an adhesion leading stiction, high friction, possibly high wear and potential failure of the contact systems, for instance microdevices, magnetic head disks and diesel fuel injectors. The situation may become more pronounced when the contacting surfaces are ultra-smooth and the normal load is small. Various design parameters, such as contact angle, initial separation height, surface tension and liquid viscosity, have been investigated during liquid-mediated contact separation. However, how the involved forces will change roles for various liquid is of interest and is necessary to be studied. In this study, meniscus and viscous forces due to water and liquid lubricants during separation of two flat surfaces are studied. Previously established mathematical model for meniscus and viscous forces during flat on flat contact separation is simulated. The effect of meniscus and viscous force on critical meniscus area at which those forces change role is studied with different liquid properties for flat on flat contact surfaces. The roles of the involved forces at various meniscus areas are analyzed. Experiments are done in concerns to studying the effect of surface roughness on contact angle. The impact of liquid properties, initial separation heights and contact angle on critical meniscus area for different liquid properties are analyzed. The study provides a fundamental understanding of the forces of the separation process and its value for the design of interfaces. The effect of surface roughness and liquid properties on contact angle are studied

    Structural forces in ionic liquids: the role of ionic size asymmetry

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    Ionic liquids (ILs) are charged fluids composed of anions and cations of different size and shape. The ordering of charge and density in ILs confined between charged interfaces underlies numerous applications of IL electrolytes. Here, we analyze the screening behavior and the resulting structural forces of a representative IL confined between two charge-varied plates. Using both molecular dynamics simulations and a continuum theory, we contrast the screening features of a more-realistic asymmetric system and a less-realistic symmetric one. The ionic size asymmetry plays a nontrivial role in charge screening, affecting both the ionic density profiles and the disjoining pressure distance dependence. Ionic systems with size asymmetry are stronger coupled systems, and this manifests itself both in their response to the electrode polarization and spontaneous structure formation at the interface. Analytical expressions for decay lengths of the disjoining pressure are obtained in agreement with the pressure profiles computed from molecular dynamics simulations

    Theory and simulations of ionic liquids in nanoconfinement.

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    Room-temperature ionic liquids (RTILs) have exciting properties such as nonvolatility, large electrochemical windows, and remarkable variety, drawing much interest in energy storage, gating, electrocatalysis, tunable lubrication, and other applications. Confined RTILs appear in various situations, for instance, in pores of nanostructured electrodes of supercapacitors and batteries, as such electrodes increase the contact area with RTILs and enhance the total capacitance and stored energy, between crossed cylinders in surface force balance experiments, between a tip and a sample in atomic force microscopy, and between sliding surfaces in tribology experiments, where RTILs act as lubricants. The properties and functioning of RTILs in confinement, especially nanoconfinement, result in fascinating structural and dynamic phenomena, including layering, overscreening and crowding, nanoscale capillary freezing, quantized and electrotunable friction, and superionic state. This review offers a comprehensive analysis of the fundamental physical phenomena controlling the properties of such systems and the current state-of-the-art theoretical and simulation approaches developed for their description. We discuss these approaches sequentially by increasing atomistic complexity, paying particular attention to new physical phenomena emerging in nanoscale confinement. This review covers theoretical models, most of which are based on mapping the problems on pertinent statistical mechanics models with exact analytical solutions, allowing systematic analysis and new physical insights to develop more easily. We also describe a classical density functional theory, which offers a reliable and computationally inexpensive tool to account for some microscopic details and correlations that simplified models often fail to consider. Molecular simulations play a vital role in studying confined ionic liquids, enabling deep microscopic insights otherwise unavailable to researchers. We describe the basics of various simulation approaches and discuss their challenges and applicability to specific problems, focusing on RTIL structure in cylindrical and slit confinement and how it relates to friction and capacitive and dynamic properties of confined ions

    Use of Self-Assembled Monolayers to Tailor Surface Properties: From Lubrication to Neuronal Development

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    The subsequent work describes advances in modifying the chemical properties of various substrates to tailor the surface properties for specific applications. This is achieved by making use of a molecular assembly known as self-assembled monolayers, or SAMs. SAMs are composed of tightly packed organic molecules that form a well-ordered structure on a substrate. Typically, the head group of the monomer is covalently anchored to the substrate, and monolayer order and self-assembly is achieved through van der Waals interactions between the long alkyl chains of the monomer\u27s tail group. Monolayers containing head groups consisting of thiols, siloxanes, and phosphonates have been demonstrated on gold, glass, and metal oxides, respectively. We have expanded upon existing monolayer technology and designed monolayers with either new head group or new tail group functionalities. The resulting surfaces have been characterized by a variety of techniques including infrared spectroscopy, contact angle analysis, quartz crystal microbalance analysis, surface plasmon resonance imaging, and atomic force microscopy. We have also explored applications for these functionalized surfaces in areas ranging from microelectromechanical systems: MEMS) lubrication to platforms for studying neuronal development in vitro. In the area of MEMS lubrication, the development of new surface coatings is critical for combating wear and increasing the device lifetime. We reported a class of arsonic acid SAMs that form readily on oxide substrates including silicon oxide, borosilicate glass, and titanium oxide. The monolayers are easily prepared using a straightforward soaking technique, which is amenable to large-scale commercial applications. We have characterized monolayer formation on borosilicate glass and titanium oxide using infrared spectroscopy. Monolayers on borosilicate glass, native silicon oxide and titanium oxide were also evaluated with contact angle measurements, and as wear measurements using nanoscratching experiments. On titanium oxide and borosilicate glass, monolayers prepared from hexadecylarsonic acid provide significantly greater surface protection than surfaces reacted under similar conditions with hexadecylphosphonic acid, a common modifying agent for oxide substrates. To develop a platform for in vitro studies of neuronal development, we have utilized mixed-monolayers incorporating low densities of cell-adhesive peptides. The monomers feature a tetraethylene glycol moiety in the tail group to prevent the non-specific adsorption of proteins, and a low density of monomers were terminated with an azide moiety to specifically attach a laminin-derived peptide: IKVAV) terminated with an alkyne group via the copper-mediated azide-alkyne cycloaddition: CuAAC) reaction. To achieve this, a pentynoic acid molecule was appended to the N-terminus of the peptide during solid phase synthesis. Surfaces containing 0.01% and 0.1% azide-coupled peptide were determined to be resistant to the non-specific adsorption of proteins. Hippocampal neurons dissected from embryonic mice were cultured on these surfaces and the effects of the peptides on neurite outgrowth were observed. Similar neurite numbers per cell were observed on both substrates, but longer neurites were measured on the 0.1% azide-coupled peptide substrate. Unfortunately, further studies revealed that aldehyde fixation methods for immunohistochemistry did not successfully attach neuronal cells to the surface due to limited attachment points on the surface. Many developmental cell biology experiments require downstream immunohistochemical analysis. As such, to overcome this limitation and to simplify the surface preparation, a protein-resistant intermolecular zwitterionic monolayer, which supports cell fixation, was utilized. We have shown that the intermolecular zwitterionic monolayer has well-defined, non-receptor mediated cellular attachment provided by cell-surface sugar interactions. Exploiting these properties, we have developed a monolayer stripe assay, where the interactions between neurons: cell bodies and neurites) and extracellular matrix: ECM) proteins or guidance cues can be observed and quantified. This system goes beyond current technologies and is capable of evaluating neuronal response to the extracellular matrix protein, laminin, which has previously been considered a control molecule in neuronal stripe assays. Taken together, this work highlights advancements in the field of self-assembled monolayer chemistry with practical applications. In particular, we have focused on the functionalization of glass and oxides surfaces for applications in device lubrication. As well, we have developed two alkanethiol self-assembled monolayer approaches for generating surfaces that are both protein resistant and cell permissive, advancing the tools available for studying neuronal development in vitro

    Atomic simulations of kinetic friction and its velocity dependence at Al/Al and alpha-Al_2O_3/alpha-Al_2O_3 interfaces

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    Kinetic friction during dry sliding along atomistic-scale Al(001)/Al(001) and alpha-Al2O3(0001)/alpha-Al2O3(0001) interfaces has been investigated using molecular dynamics (MD) with recently developed Reactive Force Fields (ReaxFF). It is of interest to determine if kinetic friction variations predicted with MD follow the macroscopic-scale friction laws known as Coulomb's law (for dry sliding) and Stokes' friction law (for lubricated sliding) over a wide range of sliding velocities. The effects of interfacial commensuration and roughness on kinetic friction have been studied. It is found that kinetic friction during sliding at commensurate alpha-Al2O3(0001)/alpha-Al2O3(0001) interfaces exceeds that due to sliding at an incommensurate alpha-Al2O3(0001)/alpha-Al2O3(0001) interface. For both interfaces, kinetic friction at lower sliding velocities deviates minimally from Coulombic friction, whereas at higher sliding velocities, kinetic friction follows a viscous behavior with sliding damped by thermal phonons. For atomically smooth Al(001)/Al(001), only viscous friction is observed. Surface roughness tends to increase kinetic friction, and adhesive transfer causes kinetic friction to increase more rapidly at higher sliding velocities

    Ionic Liquids as High-Performance Lubricants and Lubricant Additives

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    Taking into account the environmental awareness and ever-growing restrictive regulations over contamination, the study of new lubricants or lubricant additives with high performance and low toxicity over the traditional lubes to reduce the negative impact on the environment is needed. In this chapter, the current literature on the use of ionic liquids, particularly protic ionic liquids, as high-performance lubricants and lubricant additives to different types of base lubricants are reviewed and described. The relation between ionic liquids structures and their physicochemical properties, such as viscosity, thermal stability, corrosion behavior, biodegradability, and toxicity, is elaborated. Friction reduction and wear protection mechanisms of the ionic liquids are discussed with relation to their molecular structures and physicochemical properties

    Molecular Tribology

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    Surface-active agents play an important role in lubrication technology and are often used as additives in liquid lubricants films to reduce the friction and wear. Under high loads, in asperity contacts or when there is no relative motion between two surfaces, fluid lubrication breaks down and boundary lubricants are essential to prevent wear and seizure. In other applications where the use of ‘thick’ films of liquid lubricants is not possible or desirable (i.e. micromotors, hard disk drives), lubrication between contacting surfaces is exclusively in the boundary regime. Despite the use of boundary lubricants in engineering applications for centuries, our understanding of how boundary lubricants work at the molecular level remains unclear. My thesis describes the use of total internal reflection (TIR) Raman scattering to characterise model boundary lubricants both ex situ and in situ, under realistic conditions of pressure and shear. The model systems comprise either Langmuir-Blodgett (LB) monolayers of long chain fatty acids (e.g. Zn arachidate) and phospholipids (e.g. DPPC) deposited on silica and SF10 glass, or phospholipid bilayers (e.g. DMPC) fused to silica and SF10 glass surfaces in water. TIR Raman scattering is a form of vibrational spectroscopy with sub-nanometer sensitivity and spatial resolution of a few microns. Control of the polarisation of the incoming and scattered light allows us to probe the orientation of adsorbed molecules and how that orientation changes under pressure and shear. The resonant frequency and intensity of different molecular vibrations is also sensitive to the packing and conformational order in the lubricant film. LB monolayers of Zn arachidate and DPPC are first characterised ex situ and then subjected to increasing load (upto ~750 MPa) in a contact between a fused silica ball and the flat surface of an SF10 hemisphere. A better packing or a higher orderliness of the molecules are observed at higher pressure without the monolayers being squeezed out. In contrast, application of load to the DMPC bilayer appears to squeeze some of the lipid materials out of contact. The designs of two Raman tribometers are described that allow Raman measurements in a sheared contact with simultaneous measurements of friction and load. Elasto-hydrodynamic and boundary lubrication regimes are studied with the tribometers overcoming the engineering difficulties up to a significant extent
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