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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.

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.

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.

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.

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.

The sliding interfaces found in the body—within the eyes, the digestive system, and the articulating joints, for example—are soft and permeable yet extremely robust, possessing low friction. The common elements among these systems are hydrophilic biopolymer networks that provide physical surfaces, elasticity, and fluid permeability. Stiff, impermeable probes are traditionally used to assess the frictional properties of most surfaces, including soft, permeable materials. However, both sides of physiological articulating interfaces are soft and hydrated. Measuring the friction response on just one-half of the cornea–eyelid interface or the cartilage–cartilage interface using a stiff, impermeable probe may not reproduce physiological lubrication. Here, we present lubricity measurements of the interface between two soft, hydrated, and permeable hydrogels. We explore the distinctions between the self-mated “Gemini” hydrogel interface and hydrogels sliding against hard impermeable countersurfaces. A rigid impermeable probe sliding against a soft permeable hydrogel exhibits strong frictional dependence on sliding speed, and a hydrogel probe sliding against flat glass shows a strong friction dependence on time in contact. The twin Gemini interface shows very low friction μ  < 0.06, with little dependence on sliding speed or time in contact. This consistently low-friction Gemini interface emulates the physiological condition of two like permeable surfaces in contact, providing excellent lubricity.

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 lubricity of coatings made from different types of carbon nanoparticles such as carbon onions, carbon nanohorns and carbon nanotubes is investigated on line-patterned AISI 304 stainless-steel substrates using ball-on-disc tribometry over 200,000 sliding cycles. Picosecond direct laser interference patterning is used to create line-patterns on the substrate surfaces which are subsequently coated by electrophoretic deposition. Friction testing is conducted on as-processed surfaces in linear reciprocal mode at a normal load of 100 mN with alumina and 100Cr6 as counter body materials. The resulting wear tracks on the substrates as well as wear scars on the counter bodies are characterized by scanning electron microscopy as well as energy-dispersive X-ray spectroscopy. Tribometry shows that CNTs have the ability to maintain lubricity against both counter body materials. CO and CNH coatings sustain their lubricity against 100Cr6 over the full test duration but fail against alumina. In contrast to alumina, substantial carbon transfer from the substrate surface to 100Cr6 counter body is observed.