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

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

In most animal species, vision is mediated by compound eyes, which offer lower resolution than vertebrate single-lens eyes, but significantly larger fields of view with negligible distortion and spherical aberration, as well as high temporal resolution in a tiny package. Compound eyes are ideally suited for fast panoramic motion perception. Engineering a miniature artificial compound eye is challenging because it requires accurate alignment of photoreceptive and optical components on a curved surface. Here, we describe a unique design method for biomimetic compound eyes featuring a panoramic, undistorted field of view in a very thin package. The design consists of three planar layers of separately produced arrays, namely, a microlens array, a neuromorphic photodetector array, and a flexible printed circuit board that are stacked, cut, and curved to produce a mechanically flexible imager. Following this method, we have prototyped and characterized an artificial compound eye bearing a hemispherical field of view with embedded and programmable low-power signal processing, high temporal resolution, and local adaptation to illumination. The prototyped artificial compound eye possesses several characteristics similar to the eye of the fruit fly Drosophila and other arthropod species. This design method opens up additional vistas for a broad range of applications in which wide field motion detection is at a premium, such as collision-free navigation of terrestrial and aerospace vehicles, and for the experimental testing of insect vision theories.

https://www.pnas.org/content/pnas/110/23/9267.full.pdf

Miniature curved artificial compound eyes

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

A fast and single-step process is developed for the fabrication of low-cost, high-quality, and large-area concave microlens arrays (MLAs) by the high-speed line-scanning of femtosecond laser pulses. Each concave microlens can be generated by a single laser pulse, and over 2.78 million microlenses were fabricated on a 2 × 2 cm2 polydimethylsiloxane (PDMS) sheet within 50 min, which greatly enhances the processing efficiency compared to the classical laser direct writing method. The mechanical pressure induced by the expansion of the laser-induced plasmas as well as a long resolidifing time is the reason for the formation of smooth concave spherical microstructures. We show that uniform microlenses with different diameters and depths can be controlled by adjusting the power of laser pulses. Their high-quality optical performance is also demonstrated in this work.

Rapid fabrication of large-area concave microlens arrays on PDMS by a femtosecond laser.

A biologically inspired compound-eye structure, which composes of ~5,867 honeycomb-patterned microlenses, was fabricated on a hemispherical shell. The fabrication process was simple and low-cost, which involves a femtosecond laser-enhanced wet etching and casting process followed by a thermomechanical process to convert the film into a hemispherical surface. By optimizing the parameters of thermomechanical process to form the curvilinear surface, the experimental result shows that the microlenses are omnidirectionally aligned on the dome with lens diameters of ~85 µm and the angle between two lens of ~2°, and the individual microlenses have rudimentary focusing and imaging properties. The artificial compound-eye structure fabricated by this method has great potential applications in scale-invariant processing, robot vision, and fast motion detection.

A simple route to fabricate artificial compound eye structures.

The invention discloses a method for preparing a microlens array beam shaper through femtosecond laser reinforced chemical etching. The method comprises the steps of: selecting quartz glass a sample material, carrying out surface processing through femtosecond laser to obtain a microlens array dead hole; and placing a hydrogen fluoride solution in the processed microlens array dead hole for carrying out auxiliary etching to complete the preparation of the microlens array beam shaper. A microlens array is prepared by using femtosecond laser reinforced chemical etching, and the advantage of controllable morphology of the prepared microlens array is obtained, wherein the morphology of a microlens can be controlled through reasonable arrangement of laser pulse acting points, and the focus length of the microlens can be controlled through a laser parameter. The method can realize high-efficiency processing of a high packing ratio and large-area microlens array; and the prepared microlens array can simply and conveniently realize space light intensity uniform distribution of a gauss beam.

Method for preparing microlens array beam shaper through femtosecond laser reinforced chemical etching

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A simple route to fabricate artificial compound eye structures.

Method for preparing microlens array beam shaper through femtosecond laser reinforced chemical etching