Friction is a phenomenon that exists extensively in nature and industry; it has proven necessary in daily life and beneficial in energy scavenging but is also alleged to be the main cause of wear ...failures and energy consumption. Therefore, scientists have long worked on the origin of friction, trying in some instances to verify the possibility of achieving an absolute friction-free state, and rapid progress has been made in recent decades. In this article, we review the research on the friction mechanism and the development of a new burgeoning technique named “superlubricity”, which has been demonstrated as an attractive way to achieve ultralow friction and wear with almost no energy dissipation. It is estimated that the future application of superlubricity could result in an economic benefit of trillions of US dollars annually worldwide, as well as having great energy saving potential.
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•The advancements in friction mechanism are reviewed and an outlook for its future research is proposed.•Recent progress of superlubricity is comprehensively summarized.•Significance in economic and energy saving with the application of superlubricity is highlighted.
Friction drag is a nonnegligible matter when relative motion happens between solid and liquid phase, which brings many inconveniences in ship navigation, fluid transportation, microfluid devices, ...etc. Thereby various methods have been developed focusing on friction drag reduction. In this article, a review of several widely studied drag reduction methods is given, specially, their advantages and limitations in practical applications are discussed. Besides, a comparison of different methods is made and the development prospect of drag reduction is concluded.
This article provides a comprehensive overview on jamming strategies in the physical layer for securing wireless communications. As a complement to traditional cryptography-based approaches, physical ...layer based security provisioning techniques offer a number of promising features that have not been previously available. Among the physical layer security techniques, jamming is an effective way to degrade the channel quality of the eavesdroppers for ensuring security. We start by giving a brief introduction to the fundamental principles of physical layer security and jamming. Then we classify jamming strategies from three different perspectives and explain the major related designs in various scenarios. Finally, we discuss the open issues of jamming that can be helpful to foster future research.
Thin film lubrication (TFL), a lubrication regime that fills the gap between boundary lubrication (BL) and elastohydrodynamic lubrication (EHL) regimes, was proposed 20 years ago. Since it was first ...recorded in the literature, TFL has gained substantial interest and has been advanced in the fields of theoretical and experimental research. Following the revelation of the TFL phenomenon and its central ideas, many studies have been conducted. This paper attempts to systematically review the major developments in terms of both the history and the advances in TFL. It begins with the description and definition of TFL, followed by the state-of-art studies on experimental technologies and their applications. Future prospects of relevant studies and applications are also discussed.
During liquid evaporation, the equations for the vapor concentration in the atmosphere and for the temperature in the liquid are coupled and must be solved in an iterative manner. In the present ...paper, a combined field approach which unifies the coupled fields into one single hybrid field and thus makes the iteration unnecessary is proposed. By using this approach, the influences of the evaporative cooling on the evaporation of pinned sessile droplets are investigated, and its predictions are found in good agreement with the previous theoretical and experimental results. A dimensionless number Ec which can evaluate the strength of the evaporative cooling is then introduced, and the results show that both the evaporation flux along the droplet surface and the total evaporation rate of the droplet decrease as the evaporative cooling number Ec increases. For drying droplets, there exists a critical value EcCrit below which the evaporative cooling effect can be neglected and above which the significance of the effect increases dramatically. The present work may also have more general applications to coupled field problems in which all the fields have the same governing equation.
Marine propellers are important propulsion devices for both surface ships and underwater vehicles. Increasingly severe environmental problems have required further performance enhancement for ...propellers. Nowadays, traditional methods to improve propeller performances through geometrical and structural optimizations have been extensively investigated, while the underlying mechanisms of the effects of surface and interface properties on marine propellers are still far from being fully understood. This paper presented a comprehensive review of recent advances in the effects of surface and interface properties, such as surface roughness and surface wettability, on marine propellers with an emphasis on the significant improvements in both hydrodynamic and cavitation performances, hoping to arouse more in-depth investigations in the field of surface/interface science and technologies on marine propellers, and also promote the state-of-the-art technologies, such as superlubricity technology, into practical applications.
Superior wet attachment and friction performance without the need of special external or preloaded normal force, similar to the tree frog's toe pad, is highly essential for biomedical engineering, ...wearable flexible electronics, etc. Although various pillar surfaces are proposed to enhance wet adhesion or friction, their mechanisms remain on micropillar arrays to extrude interfacial liquid via an external force. Here, two‐level micropillar arrays with nanocavities on top are discovered on the toe pads of a tree frog, and they exhibit strong boundary friction ≈20 times higher than dry and wet friction without the need of a special external or preloaded normal force. Microscale in situ observations show that the specific micro–nano hierarchical pillars in turn trigger three‐level liquid adjusting phenomena, including two‐level liquid self‐splitting and liquid self‐sucking effects. Under these effects, uniform nanometer‐thick liquid bridges form spontaneously on all pillars to generate strong boundary friction, which can be ≈2 times higher than for single‐level pillar surfaces and ≈3.5 times higher than for smooth surfaces. Finally, theoretical models of boundary friction in terms of self‐splitting and self‐sucking are built to reveal the importance of liquid behavior induced by micro–nano hierarchical structure.
A strong wet attachment bioinspired surface with hierarchical pillars and nanocavities is introduced based on the unique interfacial liquid adjusting effects on the tree frog toe pad, where robust interfacial capillarity from nanometer‐thick liquid film generates a boundary friction ≈20 times its wet and dry friction. Such bioinspired surfaces demonstrate potential applications in fields including medical devices and wearable sensors.
Controlling friction by light field is a low-cost, low-energy, non-polluting method. By applying ultraviolet light on the surface of photosensitive materials, the properties of the friction pairs or ...lubricant can be influenced, thus achieving the purpose of reducing friction. In this study, TiO2, an inorganic photosensitive material, was selected to investigate the modulating effect of light fields on friction lubrication when using polyalphaolefin (PAO) base oil as a lubricant, and the modulation law of light fields on the friction lubrication behavior was investigated under different loads (1–8 N), different speeds (20–380 mm/s), and different viscosities (10.1–108.6 mPa·s) of PAO base oil. The experimental results showed that light treatment could reduce the friction coefficient of PAO4 base oil lubrication from 0.034 to 0.016, with a reduction of 52.9% under conditions of 3 N-load and 56.5 mm/s-speed, and the best regulation effect could be achieved under the mixed lubrication condition. After TiO2 was treated with ultraviolet light, due to its photocatalytic property, PAO molecules were oxidized and adsorbed on the TiO2 surface to form an adsorption layer, which avoided the direct contact of rough peaks and thus reduced the friction coefficient. This study combines photosensitivity, photocatalysis, and friction, presenting a method to reduce the friction coefficient by applying a light field without changing the friction pairs or lubricants, which provides a new direction for friction modulation and gives new ideas for practical applications.
In water-based boundary lubrication regime, the contact gaps (or boundary lubricated film thickness) and surface pressure distribution must be determined to really understand the boundary lubricated ...contact mechanism. However, the accurate determination of these parameters is limited. In this study, a mechanical model based on boundary lubricated contact involving surface force effects is developed. The surface force distribution characteristics, normal force vs. central film thickness curve, and macroscale water-based boundary lubricated contact are investigated numerically. The results show that hydration directly affects surface force interaction. The accurate boundary lubricated film thickness and surface pressure distribution can be obtained using this model in point contact. Furthermore, the mechanism of macroscale water- based liquid boundary lubricated contact is investigated, in which a water-based boundary lubricated film is formed under appropriate operating conditions based on surface force effects during running-in. This study can reveal the water-base boundary lubricated contact behavior and the carrying capacity of the surface force effect, and provides important design guidance for the surface force effect to achieve liquid superlubricity in water-based boundary lubricated contacts.
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