Bioinspired Topological Surface for Directional Oil Lubrication

Regulation of Liquid Self-Transport Through Architectural-Thermal Coupling: Transitioning From Free Surfaces to Open Channels

In this work, the regulation of liquid self-transport is achieved through architectural and thermal coupling, transitioning from free surfaces to open channels. Hierarchical structures inspired by the skin of a Texas horned lizard are designed, with the primary structure of wedged grooves and the secondary structure of capillary crura. This design enables advantages including long-distance self-transport, liquid storage and active reflux management on free surfaces, directional transportation, synthesis and detection of reagents in confined spaces, as well as controllable motion and enhanced heat dissipation in open channels. The regulation capacity can be precisely controlled by adjusting the secondary capillary crura and external thermal gradients. The regulation mechanism is further elucidated through microscopic flow observation and a deduced theoretical model. The proposed structures are expected to introduce a new concept for designing lubrication systems, microfluidic chips, methods for chemical synthesis, and heat transfer in the future.

Advances in small droplets manipulation on bio-inspired slippery surfaces: chances and challenges

The manipulation of droplets with non-destructive, efficient, and high-precision features is of great importance in several fields, including microfluidics and biomedicine. The lubrication layer of bioinspired slippery surfaces demonstrates remarkable stability and self-restoration capabilities when subjected to external perturbations. Consequently, research into the manipulation of droplets on slippery surfaces has continued to make progress. This paper presents a review of the methods of droplet manipulation on bioinspired slippery surfaces. It begins by outlining the basic theory of slippery surfaces and the mechanism of droplet motion on slippery surfaces. Furthermore, droplet manipulation methods on slippery surfaces are classified into active and passive approaches based on the presence of external stimuli (e.g., heat, light, electricity, and magnetism). Finally, an outlook is provided on the current challenges facing droplet manipulation on slippery surfaces, and potential solution ideas are presented.

Stretch-Controlled Branch Shape Microstructures for Switchable Unidirectional Self-Driven Spreading of Oil Droplets

Unidirectional transport of liquids has attracted the attention of researchers in recent years for its wide application foreground. However, it is still a challenge to control the spreading of liquid, especially for oils with relatively high viscosity. In this paper, a flexible surface textured with branch-shaped microstructures is proposed. These asymmetric microstructures exhibit excellent unidirectional spreading behaviors for various oils. By suitably stretching the flexible surface to different stretch ratios, the spreading length of the oil droplets can be controlled. Moreover, the ongoing forward spreading of oil droplets can be suspended dynamically when the surface is stretched to 40%. Corresponding mechanism analysis demonstrates that surface stretching can narrow and close the microvalves between adjacent branches, which restrain the flow of the precursor film and the primary droplet. The switchable unidirectional spreading behavior enables the surface with such microstructures to be used for oil transportation, oil-water separation, and controllable lubrication.

Fundamental, mechanism and development of hydration lubrication: From bio-inspiration to artificial manufacturing

Friction and lubrication are ubiquitous in all kinds of movements and play a vital role in the smooth operation of production machinery. Water is indispensable both in the lubrication systems of natural organisms and in hydration lubrication systems. There exists a high degree of similarity between these systems, which has driven the development of hydration lubrication from biomimetic to artificial manufacturing. In particular, significant advancements have been made in the understanding of the mechanisms of hydration lubrication over the past 30 years. This enhanced understanding has further stimulated the exploration of biomimetic inspiration from natural hydration lubrication systems, to develop novel artificial hydration lubrication systems that are cost-effective, easily transportable, and possess excellent capability. This review summarizes the recent experimental and theoretical advances in the understanding of hydration-lubrication processes. The entire paper is divided into three parts. Firstly, surface interactions relevant to hydration lubrication are discussed, encompassing topics such as hydrogen bonding, hydration layer, electric double layer force, hydration force, and Stribeck curve. The second part begins with an introduction to articular cartilage in biomaterial lubrication, discussing its compositional structure and lubrication mechanisms. Subsequently, three major categories of bio-inspired artificial manufacturing lubricating material systems are presented, including hydrogels, polymer brushes (e.g., neutral, positive, negative and zwitterionic brushes), hydration lubricant additives (e.g., nano-particles, polymers, ionic liquids), and their related lubrication mechanism is also described. Finally, the challenges and perspectives for hydration lubrication research and materials development are presented.

Temperature-responsive peristome-structured smart surface for the unidirectional controllable motion of large droplets

The manipulation of fast, unidirectional motion for large droplets shows important applications in the fields of fog collection and biochemical reactions. However, driving large droplets (>5 μL) to move directionally and quickly remains challenging due to the nonnegligible volume force. Herein, we fabricated a scalable, bionic peristome substrate with a microcavity width of 180 μm using a 3D printing method, which could unidirectionally drive a large water droplet (~8 μL) at a speed reaching 12.5 mm/s by temperature-responsive wettability. The substrate surface was grafted with PNIPAAm, which could reversibly change its wettability in response to temperature, thereby enabling a temperature-responsive smart surface that could regulate droplet movement in real-time by changing the temperature. A series of temperature-responsive smart patterns were designed to induce water transport along specific paths to further realize controllable droplet motion with the antibacterial treatment of predesignated areas. The ability to achieve temperature-responsive unidirectional motion and dynamic control of droplet movement could allow programmable fluidic biosensors and precision medical devices. A temperature-responsive smart surface was produced to control the unidirectional motion of large droplets between spreading and pinning movement by changing the surface wettability.

Natural Cilia and Pine Needles Combinedly Inspired Asymmetric Pillar Actuators for All-Space Liquid Transport and Self-Regulated Robotic Locomotion

Natural structures and motion behaviors open new avenues for effective small-scale transport, such as the plant-inspired energy-free liquid transport surfaces and cilia-inspired propulsion systems. However, they are restricted by either the fixed structure or nonself-regulating beating modes, making many complex tasks remain challenging, e.g., the controllable multidirectional liquid transport and flexible propulsion. Herein, inspired by pine needles and natural cilia, we report an asymmetric-structured intelligent magnetic pillar actuator (AI-MPA) with both the "passive" and "active" transport features. Under the control of the magnetic field, the AI-MPA shows an all-space liquid transport ability toward arbitrary directions. Moreover, benefiting from the material's magnetoelasticity and asymmetric-structured design, the AI-MPA enables self-regulation of two-dimensional (2D)/three-dimensional (3D) cilia-like beating modes and can be further developed for robotic crawling and self-rotatable motion. The AI-MPA integrates the superiority of static and dynamic systems in nature and exhibits intelligent self-regulation that could not be achieved before. Confirmed theoretically and demonstrated experimentally, this work provides insights into increasingly functional and intelligent miniature biomimetic systems, with applications from directional liquid transport to robotic locomotion.

Bioinspired Functional Structures for Lubricant Control at Surfaces and Interfaces: Wedged-Groove with Oriented Capillary Patterns

In this work, a design concept of bioinspired functional surfaces is proposed for lubricant control at surfaces and interfaces subjected to external thermal gradients. Inspired by the conical structures of cactus and the motion configuration of Centipedes, a bioinspired surface of wedged-groove with an oriented capillary pattern is constructed. The effect of geometrical parameters on the directional lubricant manipulation capacity and sliding anisotropy is discussed. It is found that by regulating the orientation of the capillary pattern, a controllable lubricant self-transport capacity can be achieved for varying conditions from surfaces to interfaces, with or without thermal gradients. The lubricant self-transport process is captured, and the mechanism is revealed. The design philosophy of the proposed bioinspired functional surface is believed to have potential applications for lubricant control in modern machinery and complex liquid control in lab-on-a-chip and microfluidics devices.

Review of Tribological Failure Analysis and Lubrication Technology Research of Wind Power Bearings

Wind power, being a recyclable and renewable resource, makes for a sizable portion of the new energy generation sector. Nonetheless, the wind energy industry is experiencing early failure of important components of wind turbines, with the majority of these issues also involving wind power bearings. Bearing dependability is directly tied to the transmission efficiency and work performance of wind turbines as one of its major components. The majority of wind turbine failures are due to bearings, and the vast majority of bearing failures are due to lubrication. The topic of improving the accuracy and life of wind power bearing motion is becoming increasingly essential as the wind power industry develops rapidly. This study examines the various constructions and types of wind turbines, as well as their bearings. We also examined the most typical causes of friction and lubrication failure. Furthermore, contemporary research on wind turbine bearings has been compiled, which mostly comprises the study and development of lubrication technology and other areas. Finally, a conclusion and outlook on current challenges, as well as future research directions, are offered.

Asymmetric Mass Transport through Dense Heterogeneous Polymer Membranes: Fundamental Principles, Lessons from Nature, and Artificial Systems

Many organisms rely on directional water transport schemes for the purpose of water retention and collection. Directional transport of water and other fluids is also technologically relevant, for example to harvest water, in separation processes, packaging solutions, functional clothing, and many other applications. One strategy to promote mass transport along a preferential direction is to create compositionally asymmetric, multi-layered, or compositionally graded architectures. In recent years, the investigation of natural and artificial membranes based on this design has attracted growing interest and allowed researchers to develop a good understanding of how the properties of such membranes can be tailored to meet the demands of particular applications. Here a summary of theoretical works on mass transport through dense asymmetric membranes, comprehensive reviews of biological and artificial membranes featuring this design, and a discussion of applications, remaining questions, and opportunities are provided.

3D-Printed Topological MoS2/MoSe2 Heterostructures for Macroscale Superlubricity

Superlubricity is a fascinating phenomenon which attracts people to continuously expand ultralow friction and wear from microscale to macroscale. Despite the impressive advances in this field, it is still limited to specific materials and extreme operating conditions. Introducing a heterostructure with intrinsic lattice mismatch into delicate topologies mimicked from nature provides a promising alternative toward macroscopic superlubricity. Herein, 3D-printed MoS₂/MoSe₂ heterostructures with bioinspired circular-cored square/hexagonal honeycomb topologies were developed. Compared to 3D-printed Al₂O₃, all topological structures with both high hardness and excellent flexural strength achieve more than 30% decrease in the friction coefficient. The circular-cored hexagonal honeycomb composite with 30% area density exhibits a stable ultralow friction coefficient of 0.09 and a low wear rate of 2.5 × 10^-5 mm³·N^-1 m^-1 under 5 N. Even under 10 N, a highly desirable coefficient value of 0.08 can be maintained within 370 s. The extraordinary ultralow friction could be attributed to the small contact area, high lubricant mass loading, efficient collection and storage of both abrasive debris and lubricant, and the self-orientation in the lubricating film. This work provides new insights into developing high-efficiency lubrication devices and aids in the industrial application of macroscopic superlubricity in future life.

Bioinspired Geometry-Gradient Metal Slippery Surface by One-Step Laser Ablation for Continuous Liquid Directional Self-Transport

Liquid directional self-transport on the functional surface plays an important role in both industrial and academic fields. Inspired by the natural cactus spine and pitcher plant, we have successfully designed a kind of geometry-gradient slippery surface (GGSS) based on aluminum alloy materials which could actively achieve directional self-movement and also antigravity self-movement of various liquid droplets by topography gradient. The mechanism of liquid directional self-transport was theoretically explored through the mechanical analysis of the triple contact line, which was mainly related to the competition between the driven force induced by Laplace pressure and the adhesive force induced by viscous resistance. The adhesive force between the droplet and the surface was quantitatively measured using a homemade experimental apparatus and the results showed that the lateral adhesive force on the GGSS is much smaller than that on the original surface. Additionally, a series of quantitative experiments were conducted to explore the influence of droplet volume and vertex angle on the transport distance and velocity. Finally, we achieved the antigravity self-transport of the droplet on the inclined GGSS to further verify the self-transport ability of the GGSS. We believe that the proposed GGSS with liquid directional self-transport ability in the present work would provide some potential opportunities in modern tribo-systems to optimize the lubricating qualities, especially the lubrication and friction at the extreme contact interface.

Directional Liquid Transport from the Cold Region to the Hot Region on a Topological Surface

Manifested from the "tears of wine" to the "coffee-ring effect", the directional transport of a liquid governed by the Marangoni effect is highly pervasive in our daily life and has brought a great number of applications. Similar to this surface tension gradient-dominated process, the fluid preferentially flows from the hot region to the cold region. In contrast to this perception, in this study, we report that water liquid deposited on a specially designed topological surface can flow from the low-temperature region to the high-temperature region in a spontaneous, long-range, and unidirectional manner. We show that such a behavior is mainly owing to a strong topological effect that outweighs the thermal gradient imposed along the surface. Moreover, the specific temperature range applied on the topological surface for the occurrence of such a unidirectional liquid transport phenomenon is also identified. Our findings would find important insights for developing next-generation cooling devices where a rapid flow from the condensation region to the evaporation/boiling region is preferred.

Anisotropy-induced directional self-transportation of low surface tension liquids: a review

Inspired by natural surfaces such as butterfly wings, cactus leaves, or the Nepenthes alata plant, synthetic materials may be engineered to directionally transport liquids on their surface without external energy input. This advantageous feature has been adopted for various mechanical and chemical processes, e.g. fog harvesting, lubrication, lossless chemical reactions, etc. Many studies have focused on the manipulation and transport of water or aqueous droplets, but significantly fewer have extended their work to low surface tension (LST) liquids, although these fluids are involved in numerous industrial and everyday processes. LST liquids completely wet most surfaces which makes spontaneous transportation an active challenge. This review focuses on recently developed strategies for passively and directionally transporting LST liquids.