Broadband High-Performance Infrared Antireflection Nanowires Facilely Grown on Ultrafast Laser Structured Cu Surface

Infrared antireflection is an essential issue in many fields such as thermal imaging, sensors, thermoelectrics, and stealth. However, a limited antireflection capability, narrow effective band, and complexity as well as high cost in implementation represent the main unconquered problems, especially on metal surfaces. By introducing precursor micro/nano structures via ultrafast laser beforehand, we present a novel approach for facile and uniform growth of high-quality oxide semiconductor nanowires on a Cu surface via thermal oxidation. Through the enhanced optical phonon dissipation of the nanowires, assisted by light trapping in the micro structures, ultralow total reflectance of 0.6% is achieved at the infrared wavelength around 17 μm and keeps steadily below 3% over a broad band of 14–18 μm. The precursor structures and the nanowires can be flexibly tuned by controlling the laser processing procedure to achieve desired antireflection performance. The presented approach possesses the advantages of material simplicity, structure reconfigurability, and cost-effectiveness for mass production. It opens a new path to realize unique functions by integrating semiconductor nanowires onto metal surface structures

Crack-Initiated Durable Low-Adhesion Trilayer Icephobic Surfaces with Microcone-Array Anchored Porous Sponges and Polydimethylsiloxane Cover

Reducing unfavorable ice accretion on surfaces exposed in cold environment requires effective passive anti-icing/deicing techniques. Icephobic surfaces are widely applied on various infrastructures due to their low ice adhesion strength and flexibility, whereas their poor mechanical durability, common liquid infusion, weak resistance to contamination, and low bonding strength to substrates are the major remaining challenges. According to the fracture mechanics of ice layer, initiating cracks at the ice-solid interfaces via the proper design of internal structures of icephobic materials is a promising way to icephobicity. Herein, a crack initiating icephobic surface with porous PDMS sponges sandwiched between a protective, dense PDMS layer and a textured metal microstructure was proposed and fabricated. The combination of high- and low- stiffness PDMS layers anchored by the structured metal surface give the sandwich-like structure excellent icephobicity with both high durability and low ice adhesion (5.3 kPa in the icing–deicing cycles). The porosity and the elastic modulus of the PDMS sponges and the periodicity of the metal surface structures can both be tailored to realize enhanced icephobicity. The sandwich-like icephobic surface remained insignificantly changed under solid particle impacting and the durability characterized via linear abrasion tests was elevated compared with PDMS coating on flat metal surfaces. Additionally, the trilayer icephobic surface possesses durability, low ice adhesion strength, and improved resistance to contamination and is applicable on various surfaces

Cassie-State Stability of Metallic Superhydrophobic Surfaces with Various Micro/Nanostructures Produced by a Femtosecond Laser

The Cassie-state stability plays a vital role in the applications of metallic superhydrophobic surfaces. Although a large number of papers have reported the superhydrophobic performance of various surface micro/nanostructures, the knowledge of which kind of micro/nanostructure contributes significantly to the Cassie-state stability especially under low temperature and pressure is still very limited. In this article, we fabricated six kinds of typical micro/nanostructures with different topography features on metal surfaces by a femtosecond laser, and these surfaces were modified by fluoro­alkyl­silane to generate super­hydro­phobicity. We then systematically studied the Cassie-state stability of these surfaces by means of condensation and evaporation experiments. The results show that some superhydrophobic surfaces, even with high contact angles and low sliding angles under normal conditions, are unstable under low temperature or external pressure. The Cassie state readily transits to a metastable state or even a Wenzel state under these conditions, which deteriorates their superhydrophobicity. Among the six micro/nanostructures, the densely distributed nanoscale structure is important for a stable Cassie state, and the closely packed micrometer-scale structure can further improve the stability. The dependence of the Cassie-state stability on the fabricated micro/nanostructures and the laser-processing parameters is also discussed. This article clarifies optimized micro/nanostructures for stable and thus more practical metallic superhydrophobic surfaces

Superhydrophobic Surfaces Fabricated by Femtosecond Laser with Tunable Water Adhesion: From Lotus Leaf to Rose Petal

Superhydrophobic surfaces with tunable water adhesion have attracted much interest in fundamental research and practical applications. In this paper, we used a simple method to fabricate superhydrophobic surfaces with tunable water adhesion. Periodic microstructures with different topographies were fabricated on copper surface via femtosecond (fs) laser irradiation. The topography of these microstructures can be controlled by simply changing the scanning speed of the laser beam. After surface chemical modification, these as-prepared surfaces showed superhydrophobicity combined with different adhesion to water. Surfaces with deep microstructures showed self-cleaning properties with extremely low water adhesion, and the water adhesion increased when the surface microstructures became flat. The changes in surface water adhesion are attributed to the transition from Cassie state to Wenzel state. We also demonstrated that these superhydrophobic surfaces with different adhesion can be used for transferring small water droplets without any loss. We demonstrate that our approach provides a novel but simple way to tune the surface adhesion of superhydrophobic metallic surfaces for good potential applications in related areas

Crack-Initiated Durable Low-Adhesion Trilayer Icephobic Surfaces with Microcone-Array Anchored Porous Sponges and Polydimethylsiloxane Cover

Reducing unfavorable ice accretion on surfaces exposed in cold environment requires effective passive anti-icing/deicing techniques. Icephobic surfaces are widely applied on various infrastructures due to their low ice adhesion strength and flexibility, whereas their poor mechanical durability, common liquid infusion, weak resistance to contamination, and low bonding strength to substrates are the major remaining challenges. According to the fracture mechanics of ice layer, initiating cracks at the ice-solid interfaces via the proper design of internal structures of icephobic materials is a promising way to icephobicity. Herein, a crack initiating icephobic surface with porous PDMS sponges sandwiched between a protective, dense PDMS layer and a textured metal microstructure was proposed and fabricated. The combination of high- and low- stiffness PDMS layers anchored by the structured metal surface give the sandwich-like structure excellent icephobicity with both high durability and low ice adhesion (5.3 kPa in the icing–deicing cycles). The porosity and the elastic modulus of the PDMS sponges and the periodicity of the metal surface structures can both be tailored to realize enhanced icephobicity. The sandwich-like icephobic surface remained insignificantly changed under solid particle impacting and the durability characterized via linear abrasion tests was elevated compared with PDMS coating on flat metal surfaces. Additionally, the trilayer icephobic surface possesses durability, low ice adhesion strength, and improved resistance to contamination and is applicable on various surfaces

Crack-Initiated Durable Low-Adhesion Trilayer Icephobic Surfaces with Microcone-Array Anchored Porous Sponges and Polydimethylsiloxane Cover

Reducing unfavorable ice accretion on surfaces exposed in cold environment requires effective passive anti-icing/deicing techniques. Icephobic surfaces are widely applied on various infrastructures due to their low ice adhesion strength and flexibility, whereas their poor mechanical durability, common liquid infusion, weak resistance to contamination, and low bonding strength to substrates are the major remaining challenges. According to the fracture mechanics of ice layer, initiating cracks at the ice-solid interfaces via the proper design of internal structures of icephobic materials is a promising way to icephobicity. Herein, a crack initiating icephobic surface with porous PDMS sponges sandwiched between a protective, dense PDMS layer and a textured metal microstructure was proposed and fabricated. The combination of high- and low- stiffness PDMS layers anchored by the structured metal surface give the sandwich-like structure excellent icephobicity with both high durability and low ice adhesion (5.3 kPa in the icing–deicing cycles). The porosity and the elastic modulus of the PDMS sponges and the periodicity of the metal surface structures can both be tailored to realize enhanced icephobicity. The sandwich-like icephobic surface remained insignificantly changed under solid particle impacting and the durability characterized via linear abrasion tests was elevated compared with PDMS coating on flat metal surfaces. Additionally, the trilayer icephobic surface possesses durability, low ice adhesion strength, and improved resistance to contamination and is applicable on various surfaces

Passive Anti-Icing Performances of the Same Superhydrophobic Surfaces under Static Freezing, Dynamic Supercooled-Droplet Impinging, and Icing Wind Tunnel Tests

Overcoming ice accretion on external aircraft wing surfaces plays a crucial role in aviation, and developing environmentally friendly passive anti-icing surfaces is considered to be a promising strategy. Superhydrophobic surfaces (SHSs) have attracted increasing attention due to their potential advantages of keeping the airframe dry without causing additional aerodynamic losses. However, the passive anti-icing performances of SHSs reported to date varied a lot under different icing test conditions. Therefore, a systematic investigation is necessary to elucidate the icing conditions where SHSs can remain effective and pave the way for SHSs toward practical anti-icing applications. Herein, we designed and fabricated a typical type of SHS featuring dual-scale hierarchical structures with arrayed micromountains (with both spacings and heights of tens of micrometers) covered by single-scale sandy-corrugation-like periodic structures (with both spacings and heights of only several micrometers) (termed SS1). Its anti-icing performances under three representative icing conditions, including static water freezing, dynamic supercooled-droplet impinging, and icing wind tunnel conditions, were comparatively investigated. The SS1 SHS maintained a lower static ice-adhesion strength (<60 kPa even after 50 deicing cycles at temperatures as low as −25 °C), which was attributed to a cumulative cracking effect facilitating the ice detachment. Within the laboratory dynamic icing tests, the SS1 SHSs with micromountain heights of 20–30 μm performed optimally in the antiadhesion of supercooled droplets (at an impinging velocity of 3.4 m/s and temperatures of −5 to −25 °C). In spite of the significant anti-icing performances of the SS1 SHSs in both static and dynamic laboratory tests, they could hardly sustain reliable passive anti-icing performances in harsher icing wind tunnel tests with supercooled droplets impinging their surfaces at velocities of up to 50 m/s at a temperature of −5 °C for 10 min. This study can inspire the development of improved SHSs for achieving satisfactory anti-icing performances in real-aviation conditions

Dual-Energy-Barrier Stable Superhydrophobic Structures for Long Icing Delay

Using superhydrophobic surfaces (SHSs) with the water-repellent Cassie–Baxter (CB) state is widely acknowledged as an effective approach for anti-icing performances. Nonetheless, the CB state is susceptible to diverse physical phenomena (e.g., vapor condensation, gas contraction, etc.) at low temperatures, resulting in the transition to the sticky Wenzel state and the loss of anti-icing capabilities. SHSs with various micronanostructures have been empirically examined for enhancing the CB stability; however, the energy barrier transits from the metastable CB state to the stable Wenzel state and thus the CB stability enhancement is currently not enough to guarantee a well and appliable anti-icing performance at low temperatures. Here, we proposed a dual-energy-barrier design strategy on superhydrophobic micronanostructures. Rather than the typical single energy barrier of the conventional CB-to-Wenzel transition, we introduced two CB states (i.e., CB I and CB II), where the state transition needed to go through CB I and CB II then to Wenzel state, thus significantly improving the entire CB stability. We applied ultrafast laser to fabricate this dual-energy-barrier micronanostructures, established a theoretical framework, and performed a series of experiments. The anti-icing performances were exhibited with long delay icing times (over 27,000 s) and low ice-adhesion strengths (0.9 kPa). The kinetic mechanism underpinning the enhanced CB anti-icing stability was elucidated and attributed to the preferential liquid pinning in the shallow closed structures, enabling the higher CB-Wenzel transition energy barrier to sustain the CB state. Comprehensive durability tests further corroborated the potentials of the designed dual-energy-barrier structures for anti-icing applications

Passive Anti-Icing Performances of the Same Superhydrophobic Surfaces under Static Freezing, Dynamic Supercooled-Droplet Impinging, and Icing Wind Tunnel Tests

Overcoming ice accretion on external aircraft wing surfaces plays a crucial role in aviation, and developing environmentally friendly passive anti-icing surfaces is considered to be a promising strategy. Superhydrophobic surfaces (SHSs) have attracted increasing attention due to their potential advantages of keeping the airframe dry without causing additional aerodynamic losses. However, the passive anti-icing performances of SHSs reported to date varied a lot under different icing test conditions. Therefore, a systematic investigation is necessary to elucidate the icing conditions where SHSs can remain effective and pave the way for SHSs toward practical anti-icing applications. Herein, we designed and fabricated a typical type of SHS featuring dual-scale hierarchical structures with arrayed micromountains (with both spacings and heights of tens of micrometers) covered by single-scale sandy-corrugation-like periodic structures (with both spacings and heights of only several micrometers) (termed SS1). Its anti-icing performances under three representative icing conditions, including static water freezing, dynamic supercooled-droplet impinging, and icing wind tunnel conditions, were comparatively investigated. The SS1 SHS maintained a lower static ice-adhesion strength (<60 kPa even after 50 deicing cycles at temperatures as low as −25 °C), which was attributed to a cumulative cracking effect facilitating the ice detachment. Within the laboratory dynamic icing tests, the SS1 SHSs with micromountain heights of 20–30 μm performed optimally in the antiadhesion of supercooled droplets (at an impinging velocity of 3.4 m/s and temperatures of −5 to −25 °C). In spite of the significant anti-icing performances of the SS1 SHSs in both static and dynamic laboratory tests, they could hardly sustain reliable passive anti-icing performances in harsher icing wind tunnel tests with supercooled droplets impinging their surfaces at velocities of up to 50 m/s at a temperature of −5 °C for 10 min. This study can inspire the development of improved SHSs for achieving satisfactory anti-icing performances in real-aviation conditions

1000 °C High-Temperature Wetting Behaviors of Molten Metals on Laser-Microstructured Metal Surfaces

The melting of metals at high temperatures is common and important in many fields, e.g., metallurgy, refining, casting, welding, brazing, even newly developed batteries, and nuclear fusion, which is thus of great value in modern industrialization. However, the knowledge of the wetting behaviors of molten metals on various substrate surfaces remains insufficient, especially when the temperature is over 1000 °C and with microstructured metal substrate surfaces. Herein, we selected molten cerium (Ce) on a tantalum (Ta) substrate as an example and investigated in detail its wetting at temperatures up to 1000 °C by modulating the microstructures of the substrate surfaces via laser processing. We discovered that the wetting states of molten Ce on Ta surfaces at temperatures over 900 °C could be completely altered by modifying the laser-induced surface microstructures and the surface compositions. The molten Ce turned superlyophilic with its contact angle (CA) below 10° on the only laser-microstructured surfaces, while it exhibited lyophobicity with a CA of about 135° on the laser-microstructured plus oxidized ones, which demonstrated remarkably enhanced resistance against the melt with only tiny adhesion in this circumstance. In contrast, the CA of molten Ce on Ta substrate surfaces only changed from ∼25 to ∼95° after oxidization without laser microstructuring. We proved that modulating the substrate surface microstructures via laser together with oxidization was capable of efficiently controlling various molten metals’ wetting behaviors even at very high temperatures. These findings not only enrich the understanding of molten metal high-temperature wettability but also enable a novel practical approach to control the wetting states for relevant applications