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With the growth of deep drilling and the complexity of the well profile, the requirements for a more complete and efficient exploitation of productive formations increase, which increases the risk of various complications. Currently, reagents based on modified natural polymers (which are naturally occurring compounds) and synthetic polymers (SPs) which are polymeric compounds created industrially, are widely used to prevent emerging complications in the drilling process. However, compared to modified natural polymers, SPs form a family of high-molecular-weight compounds that are fully synthesized by undergoing chemical polymerization reactions. SPs provide substantial flexibility in their design. Moreover, their size and chemical composition can be adjusted to provide properties for nearly all the functional objectives of drilling fluids. They can be classified based on chemical ingredients, type of reaction, and their responses to heating. However, some of SPs, due to their structural characteristics, have a high cost, a poor temperature and salt resistance in drilling fluids, and degradation begins when the temperature reaches 130 °C. These drawbacks prevent SP use in some medium and deep wells. Thus, this review addresses the historical development, the characteristics, manufacturing methods, classification, and the applications of SPs in drilling fluids. The contributions of SPs as additives to drilling fluids to enhance rheology, filtrate generation, carrying of cuttings, fluid lubricity, and clay/shale stability are explained in detail. The mechanisms, impacts, and advances achieved when SPs are added to drilling fluids are also described. The typical challenges encountered by SPs when deployed in drilling fluids and their advantages and drawbacks are also discussed. Economic issues also impact the applications of SPs in drilling fluids. Consequently, the cost of the most relevant SPs, and the monomers used in their synthesis, are assessed. Environmental impacts of SPs when deployed in drilling fluids, and their manufacturing processes are identified, together with advances in SP-treatment methods aimed at reducing those impacts. Recommendations for required future research addressing SP property and performance gaps are provided.

Graphite represents a promising material for solid lubrication of highly loaded tribological contacts under extreme environmental conditions. At low loads, graphite’s lubricity depends on humidity. The adsorption model explains this by molecular water films on graphite leading to defect passivation and easy sliding of counter bodies. To explore the humidity dependence and validate the adsorption model for high loads, a commercial graphite solid lubricant is studied using microtribometry. Even at 1 GPa contact pressure, a high and low friction regime is observed - depending on humidity. Transmission electron microscopy reveals transformation of the polycrystalline graphite lubricant into turbostratic carbon after high and even after low load (50 MPa) sliding. Quantum molecular dynamics simulations relate high friction and wear to cold welding and shear-induced formation of turbostratic carbon, while low friction originates in molecular water films on surfaces. In this work, a generalized adsorption model including turbostratic carbon formation is suggested. Graphite, one of the oldest known dry lubricants, loses its friction-reducing properties in dry environments. Here, the authors show that this effect is associated with both chemical modifications of the surfaces and a structural transformation of the graphite to turbostratic carbon.

Polyelectrolyte brushes consist of charged polymer chains attached to a common backbone or surface. They provide excellent lubrication between two surfaces for both engineered and physiological materials. The packing of the brushes is sensitive to pH, temperature, or added salts. Yu et al. show that the presence of multivalent ions can cause brush collapse, similarly to monovalent ions (see the Perspective by Ballauff). Critically—and not observed with the addition of monovalent ions—very low concentrations of multivalent ions cause bridging between the brushes and increase friction between the surfaces to the extent that their value for biomedical devices is limited. Science , this issue p. 1434 ; see also p. 1399 Low concentrations of multivalent ions dramatically increase the friction between polymer brushes. Polyelectrolyte brushes provide wear protection and lubrication in many technical, medical, physiological, and biological applications. Wear resistance and low friction are attributed to counterion osmotic pressure and the hydration layer surrounding the charged polymer segments. However, the presence of multivalent counterions in solution can strongly affect the interchain interactions and structural properties of brush layers. We evaluated the lubrication properties of polystyrene sulfonate brush layers sliding against each other in aqueous solutions containing increasing concentrations of counterions. The presence of multivalent ions (Y 3+ , Ca 2+ , Ba 2+ ), even at minute concentrations, markedly increases the friction forces between brush layers owing to electrostatic bridging and brush collapse. Our results suggest that the lubricating properties of polyelectrolyte brushes in multivalent solution are hindered relative to those in monovalent solution.

Over the recent decades there has been tremendous progress in understanding and controlling friction between surfaces in relative motion. However the complex nature of the involved processes has forced most of this work to be of rather empirical nature. Two very distinctive physical systems, hard two-dimensional layered materials and soft microscopic systems, such as optically or topographically trapped colloids, have recently opened novel rationally designed lines of research in the field of tribology, leading to a number of new discoveries. Here, we provide an overview of these emerging directions of research, and discuss how the interplay between hard and soft matter promotes our understanding of frictional phenomena. Structural lubricity is one of the most interesting concepts in modern tribology, which promises to achieve ultra-low friction over a wide range of length-scales. Here the authors highlight novel research lines in this area achievable by combining theoretical and experimental efforts on hard two-dimensional materials and soft colloidal and cold ion systems.

Biodiesel is a renewable, clean-burning diesel replacement that can be used in existing diesel engines without modification. Biodiesel is among the nation’s first domestically developed and economically usable advanced biofuels. Throughout the field of biodiesel including FAME/FAGE diesel variants, the concentrations of close to around 20% conform to every requirement out from the existing fuel content guidelines. Larger blending ratios are essential for hydrotreated vegetable oil blends to lubricity enhancers. Of organic biobutanol blends, the suggested blending ratio is restricted to 10% or less to prevent high water content and low cetane content. Here, the presented survey intends to make a review of 65 papers that concerns with biodiesel blends. Accordingly, systematic analyses of the adopted techniques are carried out and presented briefly. In addition, the performances and related maximum achievements of each contribution are also portrayed in this survey. Moreover, the chronological assessment and various blends of biodiesel in the considered papers are reviewed in this work. Finally, the survey portrays numerous research problems and weaknesses that may be helpful for researchers to introduce prospective studies on biodiesel blends.

Room-temperature ionic liquids and their mixtures with organic solvents as lubricants open a route to control lubricity at the nanoscale via electrical polarization of the sliding surfaces. Electronanotribology is an emerging field that has a potential to realize in situ control of friction—that is, turning the friction on and off on demand. However, fulfilling its promise needs more research. Here we provide an overview of this emerging research area, from its birth to the current state, reviewing the main achievements in non-equilibrium molecular dynamics simulations and experiments using atomic force microscopes and surface force apparatus. We also present a discussion of the challenges that need to be solved for future applications of electrotunable friction. This Review discusses the development of electronanotribology, its intersection with room-temperature ionic liquids and how such collaboration can be used to electrically control friction at the nanoscale.

With the resource-intensive meat industry accounting for over 50% of food-linked emissions, plant protein consumption is an inevitable need of the hour. Despite its significance, the key barrier to adoption of plant proteins is their astringent off-sensation, typically associated with high friction and consequently poor lubrication performance. Herein, we demonstrate that by transforming plant proteins into physically cross-linked microgels, it is possible to improve their lubricity remarkably, dependent on their volume fractions, as evidenced by combining tribology using biomimetic tongue-like surface with atomic force microscopy, dynamic light scattering, rheology and adsorption measurements. Experimental findings which are fully supported by numerical modelling reveal that these non-lipidic microgels not only decrease boundary friction by an order of magnitude as compared to native protein but also replicate the lubrication performance of a 20:80 oil/water emulsion. These plant protein microgels offer a much-needed platform to design the next-generation of healthy, palatable and sustainable foods. Demand for plant proteins is increasing, but these often give an astringent sensation due to poor lubrication performance. Here, the authors report the development of plantprotein microgels with improved lubricity with potential in sustainable foods.

We present a computational study of sliding between gold clusters and a highly oriented pyrolytic graphite substrate, a material system that exhibits ultra-low friction due to structural lubricity. By means of molecular dynamics, it is found that clusters may undergo spontaneous rotations during manipulation as a result of elastic instability, leading to attenuated friction due to enhanced interfacial incommensurability. In the case of a free cluster, shear stresses exhibit a non-monotonic dependency on the strength of the tip-cluster interaction, whereby rigid clusters experience nearly constant shear stresses. Finally, it is shown that the suppression of the translational degrees of freedom of a cluster’s outermost-layer can partially annihilate out-of-plane phonon vibrations, which leads to a reduction of energy dissipation that is in compliance with Stokesian damping. It is projected that the physical insight attained by the study presented here will result in enhanced control and interpretation of manipulation experiments at structurally lubric contacts.

Exploring the lubricity characters in van der Waals (vdW) interfaces is a hot topic in nanotribology. We study the friction performances for the Ohmic-contacted lattice-matched SnS 2 /NbTe 2 and lattice-mismatched SnS 2 /graphene vdW systems through DFT simulations. The friction in the lattice-mismatched system is found to be two orders of magnitudes smaller. The weaker interaction between the interface and the smoother condition at the interface in the SnS 2 /graphene system leads to a smoother potential energy fluctuation, which is responsible for the tiny friction in compared to the SnS 2 /NbTe 2 system. Moreover, the friction coefficients are quite low for the lattice-mismatched bilayer with values of 5.16 × 10 –3 –6.46 × 10 –2 under a load of 3.46 × 10 –3  ~ 0.15 nN/atom. Our work suggests the SnS 2 /graphene vdW system is a potential solid lubricant with superlubricity. Graphical Abstract The lattice-mismatched SnS 2 /graphene vdW system guarantees a distinctly low friction feature due to the tiny variation of charge density difference

The development of mechanically robust super-lubrication hydrogel materials with sustained lubricity at high contact pressures is challenging. In this work, inspired by the durable lubricity feature of the earthworm epidermis, a multilevel structural super-lubrication hydrogel (MS-SLH) system, the so-called lubricant self-pumping hydrogel, is developed. The MS-SLH system is manufactured by chemically dissociating a double network hydrogel to generate robust and wrinkled lubrication layer, and then laser etching was used to generate cylindrical texture pores as gland-like pockets for storing lubricants. The surface of MS-SLH system shows ultrafast hydration characteristics and reversible pore-closing and pore-opening behavior. The current MS-SLH system shows excellent SL features, as follows: a very low COF (~0.0079) at high contact pressure condition ( P: 11.32 MPa); a stable and robust SL lifespan (COF: ~0.0028, P : 8.48 MPa, 100k cylces) without surface wear; and a sustained lubricity period (3700 cycles) with limited lubricant volume (5 μL) in air. The robust and sustained lubricity of the MS-SLH system is likely attributed to the synergy from the strong electrostatic repulsion effect at the sliding interface, the robust but compliant modulus of the dissociation lubrication layer, and the self-pumping lubricant release from the gland-like pocket of the texture pores during the dynamic shearing process. The demonstration experiments based on self-built equipments intuitively exhibit durable SL behavior of MS-SLH system. This work provides an easy strategy for the large-scale manufacture of high-performance water-lubrication coatings suitable for high-end medical devices or moving parts. Developing mechanically robust hydrogels with high lubricity is challenging but desirable. Here, the authors report the development of a layered hydrogel with a robust and wrinkled lubrication layer, and pores for storing lubricants, for sustained lubricity.