Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials,...

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

Fogging occurs when moisture condensation takes the form of accumulated droplets with diameters larger than 190 nm or half of the shortest wavelength (380 nm) of visible light. This problem may be effectively addressed by changing the affinity of a material’s surface for water, which can be accomplished via two approaches: i) the superhydrophilic approach, with a water contact angle (CA) less than 5°, and ii) the superhydrophobic approach, with a water CA greater than 150°, and extremely low CA hysteresis. To date, all techniques reported belong to the former category, as they are intended for applications in optical transparent coatings. A well-known example is the use of photocatalytic TiO2 nanoparticle coatings that become superhydrophilic under UV irradiation. Very recently, a capillary effect was skillfully adopted to achieve superhydrophilic properties by constructing 3D nanoporous structures from layer-by-layer assembled nanoparticles. The key to these two “wet”-style antifogging strategies is for micrometer-sized fog drops to rapidly spread into a uniform thin film, which can prevent light scattering and reflection from nucleated droplets. Optical transparency is not an intrinsic property of antifogging coatings even though recently developed antifogging coatings are almost transparent, and the transparency could be achieved by further tuning the nanoparticle size and film thickness. To our knowledge, the antifogging coatings may also be applied to many fields that do not require optical transparency, including, for example, paints for inhibiting swelling and peeling issues and metal surfaces for preventing corrosion. These types of issues, which are caused by adsorption of moisture, are hard to solve by the superhydrophilic approach because of its inherently “wet” nature. Thus, a “dry”-style antifogging strategy, which consists of a novel superhydrophobic technique that can prevent moisture or microscale fog drops from nucleating on a surface, is desired. Recent bionic researches have revealed that the self-cleaning ability of lotus leaves and the striking ability of a water-strider’s legs to walk on water can be attributed to the ideal superhydrophobicity of their surfaces, induced by special microand nanostructures. To date, the biomimetic fabrication of superhydrophobic microand/or nanostructures has attracted considerable interest, and these types of materials can be used for such applications as self-cleaning coatings and stain-resistant textiles. Although a superhydrophobic technique inspired by lotus leaves is expected to be able to solve such fogging problems because the water droplets can not remain on the surface, there are no reports of such antifogging coatings. Very recently, researchers from General Motors have reported that the surfaces of lotus leaves become wet with moisture because the size of the fog drops are at the microscale—so small that they can be easily trapped in the interspaces among micropapillae. Thus, lotuslike surface microstructures are unsuitable for superhydrophobic antifogging coatings, and a new inspiration from nature is desired for solving this problem. In this communication, we report a novel, biological, superhydrophobic antifogging strategy. It was found that the compound eyes of the mosquito C. pipiens possess ideal superhydrophobic properties that provide an effective protective mechanism for maintaining clear vision in a humid habitat. Our research indicates that this unique property is attributed to the smart design of elaborate microand nanostructures: hexagonally non-close-packed (ncp) nipples at the nanoscale prevent microscale fog drops from condensing on the ommatidia surface, and hexagonally close-packed (hcp) ommatidia at the microscale could efficiently prevent fog drops from being trapped in the voids between the ommatidia. We also fabricated artificial compound eyes by using soft lithography and investigated the effects of microand nanostructures on the surface hydrophobicity. These findings could be used to develop novel superhydrophobic antifogging coatings in the near future. It is known that mosquitoes possess excellent vision, which they exploit to locate various resources such as mates, hosts, and resting sites in a watery and dim habitat. To better understand such remarkable abilities, we first investigated the interaction between moisture and the eye surface. An ultrasonic humidifier was used to regulate the relative humidity of the atmosphere and mimic a mist composed of numerous tiny water droplets with diameters less than 10 lm. As the fog was C O M M U N IC A IO N

The dry-style antifogging properties of mosquito compound eyes and artificial analogues prepared by soft lithography: a superhydrophobic state with high adhesive force

Black silicon is a synthetic nanomaterial that contains high aspect ratio nanoprotrusions on its surface, produced through a simple reactive-ion etching technique for use in photovoltaic applications Surfaces with high aspect-ratio nanofeatures are also common in the natural world, for example, the wings of the dragonfly Diplacodes bipunctata Here we show that the nanoprotrusions on the surfaces of both black silicon and D bipunctata wings form hierarchical structures through the formation of clusters of adjacent nanoprotrusions These structures generate a mechanical bactericidal effect, independent of chemical composition Both surfaces are highly bactericidal against all tested Gram-negative and Gram-positive bacteria, and endospores, and exhibit estimated average killing rates of up to ∼450,000 cells min -1 cm -2 This represents the first reported physical bactericidal activity of black silicon or indeed for any hydrophilic surface This biomimetic analogue represents an excellent prospect for the development of a new generation of mechano-responsive, antibacterial nanomaterials

https://researchonline.jcu.edu.au/31705/1/31705%20Ivanova%20et%20al%202013.pdf

Bactericidal activity of black silicon

This review gives an overview of recent advances in the potential applications of superhydrophobic materials. Such properties are characterized by extremely high water contact angles and various adhesion properties. The conception of superhydrophobic materials has been possible by studying and mimicking natural surfaces. Now, various applications have emerged such as anti-icing, anti-corrosion and anti-bacterial coatings, microfluidic devices, textiles, oil–water separation, water desalination/purification, optical devices, sensors, batteries and catalysts. At least two parameters were found to be very important for many applications: the presence of air on superhydrophobic materials with self-cleaning properties (Cassie–Baxter state) and the robustness of the superhydrophobic properties (stability of the Cassie–Baxter state). This review will allow researchers to envisage new ideas and industrialists to advance in the commercialization of these materials.

Recent advances in the potential applications of bioinspired superhydrophobic materials

The metal oxides/graphene composites are one of the most promising supercapacitors (SCs) electrode materials. However, rational synthesis of such electrode materials with controllable conductivity and electrochemical activity is the topical challenge for high-performance SCs. Here, the Co_3O_4/graphene composite is taken as a typical example and develops a novel/universal one-step laser irradiation method that overcomes all these challenges and obtains the oxygen-vacancy abundant ultrafine Co_3O_4 nanoparticles/graphene (UCNG) composites with high SCs performance. First-principles calculations show that the surface oxygen vacancies can facilitate the electrochemical charge transfer by creating midgap electronic states. The specific capacitance of the UCNG electrode reaches 978.1 F g^(−1) (135.8 mA h g^(−1)) at the current densities of 1 A g^(−1) and retains a high capacitance retention of 916.5 F g^(−1) (127.3 mA h g^(−1)) even at current density up to 10 A g^(−1), showing remarkable rate capability (more than 93.7% capacitance retention). Additionally, 99.3% of the initial capacitance is maintained after consecutive 20 000 cycles, demonstrating enhanced cycling stability. Moreover, this proposed laser-assisted growth strategy is demonstrated to be universal for other metal oxide/graphene composites with tuned electrical conductivity and electrochemical activity.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700659

Oxygen-Vacancy Abundant Ultrafine Co3O4/Graphene Composites for High-Rate Supercapacitor Electrodes.

Bioinspired special wettibilities including superhydrophobicity and tunable adhesive force have drawn considerable attention because of their significant potential for fundamental research and practical applications. This review summarizes recent progress in the development of bioinspired wetting surfaces via laser microfabrication, with a focus on controllable, biomimetic, and switchable wetting surfaces, as well as their applications in biology, microfluidic, and paper-based devices, all of which demonstrate the ability of laser microfabrication in producing various multiscale structures and its adaptation in a great variety of materials. In particular, compared to other techniques, laser microfabrication can realize special modulation ranging from superhydrophilic to superhydrophobic without the assistance of fluorination, allowing much more freedom to achieve complex multiple-wettability integration. The current challenges and future research prospects of this rapidly developing field are also being d...

Bioinspired Wetting Surface via Laser Microfabrication

The superhydrophobicity, controllable water adhesion, anisotropic sliding, and anisotropic wetting, which are four typical aspects of the wettability of solid surfaces, have attracted much interest in fundamental research and practical applications. However, how to use a simple and effective method to realize all those properties is still a huge challenge. Here, we present a method to realize periodic line-patterned polydimethylsiloxane (PDMS) surfaces by a femtosecond laser simply and efficiently. By adjusting the period (D) or average distance of adjacent microgrooves, the as-prepared surfaces can exhibit superhydrophobicity, controllable water adhesion, anisotropic sliding, and anisotropic wetting. We believe that these multifunctional surfaces have enormous potential applications in novel microfluidic devices, microdroplet manipulation, liquid microdroplet directional transfer, and lab-on-chips.

A simple way to achieve superhydrophobicity, controllable water adhesion, anisotropic sliding, and anisotropic wetting based on femtosecond-laser-induced line-patterned surfaces

Femtosecond laser microfabrication has been recently utilized in interface science to modify the liquid wettability of solid surfaces. In this paper, a silicon surface with hierarchical micro/nanostructure is fabricated by a femtosecond laser. Similar to fish scales, the laser-induced surface shows superhydrophilicity in air and superoleophobicity underwater. The oil contact angles can reach up to 159.4 ± 1° and 150.3 ± 2°, respectively, for 1,2-dichloroethane and chloroform droplets in water. In addition, the surface exhibits ultralow oil-adhesion. In the oil–water–solid three-phase system, water can be trapped in the hierarchical rough structure and form a repulsive oil layer according to the extended Cassie's theory. The contact area between the as-prepared surface and oil droplets is significantly reduced, resulting in superoleophobicity and ultralow oil-adhesion in water. In addition, as a potential application, the working principle diagram of preventing blockage ability of underwater superoleophobic pipes is propounded.

Bioinspired underwater superoleophobic surface with ultralow oil-adhesion achieved by femtosecond laser microfabrication

We present a rapid, facile, and simple method to realize superhydrophobic patterned polydimethylsiloxane (PDMS) surfaces with tunable adhesion by a femtosecond laser. These surfaces are composed of superhydrophobic laser-induced structures and hydrophobic unstructured square array. The femtosecond laser structured domain shows superhydrophobicity with ultralow water adhesion, while the nonstructured flat PDMS shows ordinary hydrophobicity with ultrahigh water adhesion. By adjusting the relative area fraction of laser structured and nonstructured domains, the as-prepared superhydrophobic surfaces show tunable water adhesion that ranges from ultralow to ultrahigh, on which the sliding angle can be controlled from 1° to 90° (a water droplet cannot slide down even when the as-prepared surface is vertical or turned upside down). The tunable adhesive superhydrophobic surfaces achieved by femtosecond laser microfabrication may be potentially used in microfluidic systems to modulate the mobility of liquid droplets.

Dongshi Zhang

Papers

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

Bioinspired Wetting Surface via Laser Microfabrication

A simple way to achieve superhydrophobicity, controllable water adhesion, anisotropic sliding, and anisotropic wetting based on femtosecond-laser-induced line-patterned surfaces

Bioinspired underwater superoleophobic surface with ultralow oil-adhesion achieved by femtosecond laser microfabrication

Femtosecond Laser Weaving Superhydrophobic Patterned PDMS Surfaces with Tunable Adhesion