Artificial Compound Eyes Prepared by a Combination of Air-Assisted Deformation, Modified Laser Swelling, and Controlled Crystal Growth

Measurement Methods for Droplet Adhesion Characteristics and Micrometer-Scale Quantification of Contact Angle on Superhydrophobic Surfaces: Challenges and Opportunities

Inspired by nature, superhydrophobic surfaces have been widely studied. Usually the wettability of a superhydrophobic surface is quantified by the macroscopic contact angle. However, this method has various limitations, especially for precision micro devices with superhydrophobic surfaces, such as biomimetic artificial compound eyes and biomimetic water strider robots. These precision micro devices with superhydrophobic surfaces proposed a higher demand for the quantification of contact angles, requiring contact angle quantification technology to have micrometer-scale measurement capabilities. In this review, it is proposed to achieve micrometer-scale quantification of superhydrophobic surface contact angles through droplet adhesion characteristics (adhesion force and contact radius). Existing contact angle quantification techniques and droplet characteristics' measurement methods were described in detail. The advancement of micrometer-scale quantification technology for the contact angle of superhydrophobic surfaces will enhance our understanding of superhydrophobic surfaces.

Superhydrophobic Non-Metallic Surfaces with Multiscale Nano/Micro-Structure: Fabrication and Application

Multiscale nano/micro-structured surfaces with superhydrophobicity are abundantly observed in nature such as lotus leaves, rose petals and butterfly wings, where microstructures typically reinforce mechanical stability, while nanostructures predominantly govern wettability. To emulate such hierarchical structures in nature, various methods have been widely applied in the past few decades to the manufacture of multiscale structures which can be applied to functionalities ranging from anti-icing and water-oil separation to self-cleaning. In this review, we highlight recent advances in nano/micro-structured superhydrophobic surfaces, with particular focus on non-metallic materials as they are widely used in daily life due to their lightweight, abrasion resistance and ease of processing properties. This review is organized into three sections. First, fabrication methods of multiscale hierarchical structures are introduced with their strengths and weaknesses. Second, four main application areas of anti-icing, water-oil separation, anti-fog and self-cleaning are overviewed by assessing how and why multiscale structures need to be incorporated to carry out their performances. Finally, future directions and challenges for nano/micro-structured surfaces are presented.

Miniature bioinspired artificial compound eyes: microfabrication technologies, photodetection and applications

As an outstanding visual system for insects and crustaceans to cope with the challenges of survival, compound eye has many unique advantages, such as wide field of view, rapid response, infinite depth of field, low aberration and fast motion capture. However, the complex composition of their optical systems also presents significant challenges for manufacturing. With the continuous development of advanced materials, complex 3D manufacturing technologies and flexible electronic detectors, various ingenious and sophisticated compound eye imaging systems have been developed. This paper provides a comprehensive review on the microfabrication technologies, photoelectric detection and functional applications of miniature artificial compound eyes. Firstly, a brief introduction to the types and structural composition of compound eyes in the natural world is provided. Secondly, the 3D forming manufacturing techniques for miniature compound eyes are discussed. Subsequently, some photodetection technologies for miniature curved compound eye imaging are introduced. Lastly, with reference to the existing prototypes of functional applications for miniature compound eyes, the future development of compound eyes is prospected.

Superhydrophobic and easy-to-clean full-packing nanopatterned microlens array with high-quality imaging

The high-quality imaging and easy cleaning property of microlens array (MLA) are two very important factors for its outdoor work. Herein, a superhydrophobic and easy-to-clean full-packing nanopatterned MLA with high-quality imaging is prepared by thermal reflow together with sputter deposition. Scanning electronic microscopy (SEM) images demonstrate that the sputter deposition method can improve 84% packing density of MLA prepared by thermal reflow to 100% and add nanopattern on the surface of microlens. The prepared full-packing nanopatterned MLA (npMLA) possess clear imaging with a significant increase of signal-to-noise ratio and higher transparency compared with the MLA prepared by thermal reflow. Besides for excellent optical properties, the full-packing surface displays a superhydrophobic property with a contact angle of 151.3°. Further, the full-packing contaminated by chalk dust become easier to be cleaned by nitrogen blowing and deionized water. As a result, the prepared full-packing is considered to be potential for various applications in the outdoor.

Fabrication of a dual-focus artificial compound eye with improved imaging based on modified microprinting and air-assisted deformation

Natural compound eyes inspire the development of artificial optical devices that feature a large field of view and fast motion detection. However, the imaging of artificial compound eyes dramatically depends on many microlenses. The single focal length of the microlens array significantly limits the actual applications of artificial optical devices, like distinguishing objects at different distances. In this study, a curved artificial compound eye for a microlens array with different focal lengths was fabricated by inkjet printing and air-assisted deformation. By adjusting the space of the microlens array, secondary microlenses were created between intervals of the primary microlens. The diameter/height of the primary and secondary microlens arrays are 75/25 µm and 30/9 µm, respectively. The planar-distributed microlens array was transformed into a curved configuration using air-assisted deformation. Compared with adjusting the curved base to distinguish objects at different distances, the reported technique features simplicity and is easy to operate. The applied air pressure can be used to tune the field of view of the artificial compound eye. The microlens arrays with different focal lengths could distinguish the objects at different distances without additional components. When the external objects move a small distance, they can be detected by the microlens arrays due to their different focal lengths. It could effectively improve the motion perception of the optical system. Moreover, the focusing and imaging performances of the fabricated artificial compound eye were further tested. The compound eye combines the advantages of monocular eyes and compound eyes, holding great potential for developing advanced optical devices with a large field of view and automatic variable-focus imaging.

Heterogeneous compound eye camera for dual-scale imaging in a large field of view

Multi-scale imaging with large field of view is pivotal for fast motion detection and target identification. However, existing single camera systems are difficult to achieve snapshot multi-scale imaging with large field of view. To solve this problem, we propose a design method for heterogeneous compound eye, and fabricate a prototype of heterogeneous compound eye camera (HeCECam). This prototype which consists of a heterogeneous compound eye array, an optical relay system and a CMOS sensor, is capable of dual-scale imaging in large field of view (360°×141°). The heterogeneous compound eye array is composed of 31 wide-angle (WA) subeyes and 226 high-definition (HD) subeyes. An optical relay system is introduced to re-image the curved focal surface formed by the heterogeneous compound eye array on a CMOS sensor, resulting in a heterogeneous compound eye image containing dual-scale subimages. To verify the imaging characteristics of this prototype, a series of experiments, such as large field of view imaging, imaging performance, and real-world scene imaging, were conducted. The experiment results show that this prototype can achieve dual-scale imaging in large field of view and has excellent imaging performance. This makes the HeCECam has great potential for UAV navigation, wide-area surveillance, and location tracking, and paves the way for the practical use of bio-inspired compound eye cameras.

Tunable and Dynamic Optofluidic Microlens Arrays Based on Droplets

Microlens arrays (MLAs) are acquiring a key role in the micro-optical system, which have been widely applied in the fields of imaging processing, light extraction, biochemical sensing, and display technology. Compared with solid MLAs, liquid MLAs have received extensive attention due to their natural smooth interface and adjustability. However, manufacturing tunable liquid MLAs with ideal structures is still a key challenge for current technologies. In this paper, a novel and simple optofluidic method is demonstrated, enabling the tunable focusing and high-quality imaging of liquid MLAs. Tunable droplets are fabricated and self-assembled into arrays as the MLAs, which can be easily adjusted to focus, form images, and display different focal lengths. Tuning of MLAs' focusing properties (range from 550 to 5370 μm) is demonstrated by changing the refractive index (RI) of the droplets with a fixed size of 200 μm, which can be changed by adjusting the flow rates of the two branch streams. Also, the corresponding numerical apertures of the MLAs range from 0.026 to 0.26. Furthermore, the MLAs' functionality for microparticle imaging applications is also illustrated. Combining the MLAs with a 4× objective, microparticle imaging is magnified two times, and the resolution has also been improved on the original basis. Besides, both the size and RI of the MLAs in an optofluidic chip can be further adjusted to detect samples at different positions. These MLAs have the merits of high optical performance, a simple fabrication procedure, easy integration, and good tunability. Thus, it shows promising opportunities for many applications, such as adaptive imaging and sensing.

Fabrication of a bionic compound eye on a curved surface by using a self-assembly technique

Microlens arrays on curved surfaces are regarded as critical elements of bionic compound eyes (BCEs), which exhibit the comparative advantages of a wide field of view and tracking fast-moving objects. However, the fabrication of a curved microlens array is still challenging. Along these lines, in this work, a straightforward, rapid, and low-cost technique for the fabrication of curved microlens arrays is reported by using the self-assembly technique. A reactive ion etching process treated the surface of the curved polydimethylsiloxane (PDMS) substrate to generate a hydrophobic-hydrophilic pattern. Then, the curved microlens array can be realized by dewetting a liquid glue onto the substrate using the dip-coating method and followed by crosslinking. The proposed BCE structure consists of 2400 microlenses (400 - µm diameter and 440 - µm center distance) arranged in a hexagonal configuration on a curved PDMS surface (34 - mm diameter and 40.4 - mm curvature radius). A field-of-view of 50° was demonstrated, which has potential applications in various fields including imaging sensors, medical diagnostics, machine vision systems, and photodetectors.

Rapid Fabrication of Smooth Micro-Optical Components on Glass by Etching-Assisted Femtosecond Laser Modification

Femtosecond laser (fs-laser) is unfavorable in applications for the fabrication of micro-optical devices on hard materials owing to the problems of low fabrication efficiency and high surface roughness. Herein, a hybrid method combining fs-laser scanning, subsequent etching, and annealing was proposed to realize micro-optical devices with low roughness on glass. Compared to traditional laser ablation, the fabrication efficiency in this work was improved by one order of magnitude, and the surface roughness was decreased to 15 nm. Using this method, aspherical convex microlenses and spherical concave microlenses that possess excellent focusing and imaging properties are realized on photosensitive glass. The diameter and height of the microlenses were controlled by adjusting the fabrication parameters. These results indicate that the fs-laser-based hybrid method will open new opportunities for fabricating micro-optical components on hard materials.

Digital optofluidic compound eyes with natural structures and zooming capability for large-area fluorescence sensing

Compound eyes are ubiquitous natural biosensors that possess high temporal resolution and large fields of view (FOVs). While for solid materials based artificial imaging systems, flexible zooming ability while keeping the constant FOV is still challenging, as well as the low-cost fabrication. Herein, liquid compound eyes with natural structures are presented that synthesize optofluidics and bionics in a non-trivial manner, which enables the deformation-free zooming and flexible cell fluorescence sensing. Experimental results indicate that the innovatively manufactured bionic template possesses low roughness and uniform lens configuration with more than two thousands units, which endows the eyes with high-quality and low aberration imaging ability. Besides, digital controlled miscible liquids switching enables the focus of ommatidia simultaneously be adjusted from 150 μm to 5 mm with 100° view angle, and without bending the microlens curvature, to avoid FOV changing and image aberration. Due to large FOV and tunable ability, large-area cell fluorescence signal arrays and dynamic cell motion are imaged using this liquid compound eyes. This work presents novel strategy for compound lens manufacture at low-cost, and proposes deformation-free and continuous focus-tuning strategy, offering potentials for numerous applications, including biomedical sensing and adaptive imaging with large FOV.

Hierarchical Artificial Compound Eyes with Wide Field-of-View and Antireflection Properties Prepared by Nanotip-Focused Electrohydrodynamic Jet Printing

Artificial compound eyes (ACEs) endowed with durable superhydrophobicity, wide field-of-view (FOV), and antireflection properties are extremely appealing in advanced micro-optical systems. However, the simple and high-efficiency fabrication of ACEs with these functions is still a major challenge. Herein, inspired by moth eyes, ACEs with hierarchical macro/micro/nano structures were fabricated using the combination of nanotip-focused electrohydrodynamic jet (NFEJ) printing and air-assisted deformation processes. The NFEJ printing enables the direct and maskless fabrication of hierarchical micro/nanolens arrays (M/NLAs) without intermediate steps. The introduction of M/NLAs on the eye surface significantly improves the water hydrophobic performance with a water contact angle of 161.1° and contact angle hysteresis (CAH) of 4.2° and generally decreases the reflectance by 51% in the wavelength range of 350-1600 nm in comparison to the macroeye without any structures. The contact angle remains almost unchanged, and the CAH slightly increases from 4.2° to 8.7° after water jet impact for 20 min, indicating a durable superhydrophobicity. Moreover, the results confirm that the durable superhydrophobic ACEs with antireflection properties exhibit excellent imaging quality and a large FOV of up to 160° without distortion.

Superhydrophobic materials used for anti-icing Theory, application, and development

The accumulation of ice will reduce the performance of the base material and lead to all kinds of damage, even a threat to people's life safety. Recent increasing studies suggest that superhydrophobic surfaces (SHSs) originating from nature can remove impacting and condensing droplets from the surface before freezing to subzero temperatures, and it can be seen that hydrophobic/SH coating has good freezing cold resistance. But such anti-icing performances and developments in practical applications are restricted by various factors. In this paper, the mechanism and process of surface icing phenomenon are introduced, as well as how to prevent surface icing on SHS. The development of SH materials in the aspect of anti-icing in recent years is described, and the existing problems in the aspect of anti-icing are analyzed, hoping to provide new research ideas and methods for the research of anti-icing materials.

Memristor-based biomimetic compound eye for real-time collision detection

The lobula giant movement detector (LGMD) is the movement-sensitive, wide-field visual neuron positioned in the third visual neuropile of lobula. LGMD neuron can anticipate collision and trigger avoidance efficiently owing to the earlier occurring firing peak before collision. Vision chips inspired by the LGMD have been successfully implemented in very-large-scale-integration (VLSI) system. However, transistor-based chips and single devices to simulate LGMD neurons make them bulky, energy-inefficient and complicated. The devices with relatively compact structure and simple operation mode to mimic the escape response of LGMD neuron have not been realized yet. Here, the artificial LGMD visual neuron is implemented using light-mediated threshold switching memristor. The non-monotonic response to light flow field originated from the formation and break of Ag conductive filaments is analogue to the escape response of LGMD neuron. Furthermore, robot navigation with obstacle avoidance capability and biomimetic compound eyes with wide field-of-view (FoV) detection capability are demonstrated.

Stable Nonwetting Artificial Compound Eye with Low Adhesion

Microlens arrays (MLAs) are the key components of miniaturized optical systems. To meet the stringent requirements for their application in humid environments, achieving waterproof properties in these objects is an urgent task. It is noteworthy that conventional methods of microlens production usually sacrifice optical performance for stable superhydrophobicity by increasing the surface roughness of the microlens. In this paper, a large area artificial compound eye (ACE) is efficiently fabricated by combining photolithography and inkjet printing. The added micropillars separated the outside droplet from the microlens, and the water droplet was afterward suspended on the top of micropillars. Furthermore, the micropillars enabled superhydrophobicity (at a contact angle above 150°) and low surface adhesion (at a sliding angle of ∼2.8°) of the microlens without affecting its optical performance. Furthermore, when released from the height of 1 and 2 cm, the droplets were fully detached from the surface without sticking. The surface of the ACE was shown to have relatively stable nonwettability due to a small spacing between the micropillars. This means that tuning the morphology and spacing between micropillars allows one to noticeably improve the surface nonwettability stability. Finally, the performance of the fabricated optical system was demonstrated in a water washing experiment. Therefore, the findings of present study may open up the prospects for significant advancement in superhydrophobicity of the optical systems without affecting their imaging performance for real outdoor applications.

In Situ Electrohydrodynamic Jet Printing-Based Fabrication of Tunable Microlens Arrays

Tunable microlens arrays (MLAs) with controllable focal lengths have been extensively used in optical sensors, biochips, and electronic devices. The commonly used method is electrowetting on dielectric (EWOD) that controls the contact angle of the microlens to adjust the focal length. However, the fabrication of tunable MLAs at the microscale remains a challenge because the size of MLAs is limited by the external electrodes of EWOD. In this study, a highly integrated planar annular microelectrode array was proposed to achieve an electrowetting tunable MLA. The planar microelectrode was fabricated by electrohydrodynamic jet (E-jet) printing and the liquid microlens was then deposited in situ on the microelectrode. This method could realize 36 tunable liquid microlenses with an average diameter of 24 μm in a 320 × 320 μm² plane. The fabricated tunable MLAs with higher integration levels and smaller sizes can be beneficial for cell imaging, optofluidic systems, and microfluidic chips.

Superhydrophobic Artificial Compound Eye with High Transparency

Natural compound eyes have inspired the development of self-cleaning, waterproof, and antifog optical devices. However, the traditional methods generally sacrifice the transparency of optical units to introduce hydrophobicity, which significantly limits the practical applications of state-of-the-art hydrophobic technologies. This work aims to fabricate a microimaging system by combining photolithography, inkjet printing, and chemical growth. Herein, an artificial compound eye (ACE) is endowed with stable superhydrophobicity and high transparency without affecting its optical performance. The obtained ACE system possesses good static and dynamic dewetting properties along with excellent optical performance. Its static contact angle exceeds 160°, whereas the sliding angle and contact angle hysteresis values are ∼5.5° and ∼3.8°, respectively. Furthermore, the contact time is found to be 11.97 s for the Weber number of 12. The droplet undergoes a reversible process during compressing and stretching, and the ACE exhibits no adhesion under a pressure load of 4 mN. This proves that the introduction of nonwetting nanohairs on the sidewalls of the microcone arrays significantly improves the dynamic dewetting of the system. More importantly, the properly designed position of nanohairs ensures that the optical performance of ACE is maintained at a level of ∼95% compared to that of the bare glass. The superhydrophobic ACE exhibits low adhesion and great transparency. This rationally designed ACE may provide useful guidelines for fabrication of superhydrophobic optical devices with high transparency and enable potential applications in military, medical, and some outdoor activity fields.

3D Multiscale Micro-/Nanofolds by Femtosecond Laser Intermittent Ablation and Constrained Heating on a Shape Memory Polymer

Spontaneous wrinkling of films with a thickness gradient offers a new opportunity for constructing various 3D hierarchical surface morphologies. Unfortunately, accurately and facilely controlling the gradient film thickness to yield multiscale and 3D hierarchical micro-/nanostructures is still difficult. Here, a rapid, facile, and highly controllable fabricating strategy for realizing 3D multiscale hierarchical micro-/nanofolds on a shape memory polymer (SMP) surface is reported. First, the nanoparticle film with gradient thickness is rapidly (100 ms to 4 s) and facilely obtained by laser intermittent ablation on the SMP, termed as laser ablation-induced gradient thickness film. Following one-time constrained heating, the 3D micropillars grow out of the substrate based on the "self-growing effect," and the nanoparticle gradient film on its top shrinks into multiscale micro-/nanofolds simultaneously. Significantly, the evolution process and the underlying mechanism of the 3D micro-/nanofolds are systematically investigated. Fundamental basis enables us to accurately regulate the gradient thickness of nanoparticle films and feature size of folds by varying laser scanning times and scanning path. Finally, desirable patterns on micro-/nanofolds can be readily realized by programmable laser cleaning technology, and the tunable adhesion of the water droplet on the multiscale structured surface is demonstrated, which is promising for microdroplet manipulation.

Large-Scale Bio-Inspired Flexible Antireflective Film with Scale-Insensitivity Arrays

Natural creatures can always provide perfect strategies for excellent antireflection (AR), which is valuable for photovoltaic industry, optical devices, and flexible displays. However, limited by precision, it is still difficult to guarantee the consistency between the artificial structures and the original biological structures. Here, a novel large-scale flexible AR film is inspired by the cicada wings and successfully fabricated with a recycled template. On the one hand, the adjustable structures on porous templates make it possible to optimize the design of AR structure parameters toward the practical demand. On the other hand, it breaks the limitation of the biological organism size, accomplishing the replication of AR nanostructure units in a large scale. Interestingly, even if the film is covered by enlarged dome cone arrays, it still maintains almost perfect AR property, achieving excellent scale-insensitivity AR performance. This work numerically and experimentally investigates its scale-insensitivity AR performance in detail. Compared with subwavelength nanocones, enlarged cones change the original optical behaviors, and the proportion of transmitted light is reduced while scattering and absorption increase. Based on this, these bio-inspired scale-insensitivity AR arrays could be used in flexible displays, photothermic conversion, solar cells, and so on.

Fabrication of self-aligning convergent waveguides of microlens arrays to collect and guide light

The optical properties of microlens arrays may be significantly affected by the optical crosstalk effect between adjacent lenses. Recently, this issue has triggered increasing attention in the scientific community. In this study, an integrated microlens array (MLA) consisting of self-aligning convergent waveguides of microlenses was fabricated. The optical crosstalk effect does not influence the performance of such system. Based on the self-focusing effect principle, self-writing of the waveguide array was achieved in a photosensitive polymer. The light collection and guiding performance of the MLA with and without thermal cross-linking treatment was analyzed in depth. The relation between the stray light and the filling rate of the MLA shows that a high filling rate decreases the optical crosstalk. Finally, an integrated MLA with a large area, high uniformity, and excellent optical performance was fabricated.

Strain-Multiplex Metalens Array for Tunable Focusing and Imaging

Metalenses on a flexible template are engineered metal-dielectric interfaces that improve conventional imaging system and offer dynamic focusing and zooming capabilities by controlling the focal length and bandwidth through a mechanical or external stretch. However, realizing large-scale and cost-effective flexible metalenses with high yields in a strain-multiplex fashion remains as a great challenge. Here, single-pulsed, maskless light interference and imprinting technique is utilized to fabricate reconfigurable, flexible metalenses on a large-scale and demonstrate its strain-multiplex tunable focusing. Experiments, in accordance with the theory, show that applied stretch on the flexible-template reconfigurable diffractive metalenses (FDMLs) accurately mapped focused wavefront, bandwidth, and focal length. The surface relief metastructures consisted of metal-coated hemispherical cavities in a hexagonal close-packed arrangement to enhance tunable focal length, numerical aperture, and fill factor, FF ≈ 100% through normal and angular light illumination with external stretch. The strain-multiplex of FDMLs approach lays the foundation of a new class of large-scale, cost-effective metalens offering tunable light focusing and imaging.

Microtip focused electrohydrodynamic jet printing with nanoscale resolution

Electrohydrodynamic jet (E-Jet) printing is a promising manufacturing technique for micro-/nano-patterned structures with high resolution, high efficiency and high material compatibility. However, further improvement of the necking ratio of the E-Jet is still limited by the focusing principle. Moreover, ink viscosity is limited to values well below 90 mPa s owing to the high probability of nozzle blockage. Here, we propose a microtip focused electrohydrodynamic jet (MFEJ) printing to overcome these limitations. This technique uses a solid microtip with a radius of curvature (ROC) of several micrometers rather than a hollow nozzle, which is very simple and highly efficient to prepare and can effectively avoid nozzle clogging problems even with high-viscosity printing ink. High-resolution patterns in diverse geometries were printed using different inks with a wide range of viscosities (8.4-3500 mPa s). Nanodroplets with an average diameter of 73 nm were achieved. Moreover, nanofibers with a diameter of 30 nm were obtained using a 4 μm ROC microtip and the necking ratio was as high as 266 : 1. To the best of our knowledge, this is the smallest droplet or fiber diameter directly obtained via E-Jet printing to date without further physical or chemical processing. This MFEJ printing technique can improve printing resolution at the nanoscale, significantly enlarge the material applicability and effectively avoid nozzle clogging for the fabrication of nanodevices.

Designable Ultratransparent and Superhydrophobic Surface of Embedded Artificial Compound Eye with Extremely Low Adhesion

Real-world implementation of artificial compound eye (ACE) has been limited by its poor transparency and high requirement for the stable Cassie state. In general, the improvement of surface dewetting performance sacrifices the transparency of ACE. Herein, ACE was obtained by an integrated manufacturing technology that combined photolithography, microprinting, and chemical growth. Through skillful manipulation of the fabrication process, dewetting hairs were fabricated on the top of micropillars and around the microlens. The combination of nanohairs and micropillars resulted in outstanding superhydrophobicity (∼170°), pristine lotus effect with low sliding angle (∼1°), and contact angle hysteresis (∼2°). Moreover, the surface showed almost no adhesion under a preload of 4 mN, exhibiting excellent stable Cassie state and antiadhesion performance. Furthermore, dynamic impact showed that the impacting droplet was quickly detached from the surface (contact time ∼14.1 ms) without sticking for We = 60. The designed transparency resulted in high performance of optical unit (∼99%, bare glass for comparison). Moreover, ACE exhibited better focusing and imaging capability under larger aperture diameter than microlens without nanohairs. We envision that this research presents a significant advancement in imparting superhydrophobicity and transparency to a so-far inapplicable family of optical devices for many practical outdoor applications.

Effects of Surface Wettability on the Dewetting Performance of Hydrophobic Surfaces

We studied the impact dynamics of a droplet on two types of surfaces, i.e., nanostructured/hierarchical (NS/HS) surfaces, with different extents of hydrophobicity. It was found that the contact time is related to wetting hysteresis. It can be concluded that wetting hysteresis plays a significant role in the contact process of bouncing drops based on the work done against resistance produced by contact angle hysteresis (CAH). For similar surface roughness, the work done by CAH dominates, and a lower CAH creates a smaller contact time. Compared with NS surfaces, the energy stored during the Cassie-Baxter/Wenzel state transition because of the more pronounced air pocket formation provides the upward kinetic energy, resulting in rapid detachment of a droplet from HS surfaces. Thus, HS-3 has a smaller contact/elongation time (∼8/2 ms) because of the enhanced air pocket formation and more favorable wettability (larger contact angle (CA) and smaller contact angle hysteresis (CAH)) than other surfaces. In addition, the results show that surface morphology affects the contact time of bouncing drops mainly by influencing the elongation stage. For different Weber numbers (We), the upward energy storage dominates and results in different varying trends of contact time with We for NS-3 and HS-3. For further study, the morphology evolution of bouncing drops with We was also investigated in detail. The results show that a satellite droplet is launched in a certain We range because of high adhesion resulting from the Cassie-Baxter/Wenzel state transition. These findings provide guidelines for the preparation of surfaces for both self-cleaning and anti-icing purposes.

Mini-Review on Bioinspired Superwetting Microlens Array and Compound Eye

Microlens arrays (MLAs) and MLA-based artificial compound eyes (ACEs) are the important miniaturized optical components in modern micro-optical systems. However, their optical performance will seriously decline once they are wetted by water droplets (such as fog, dew, and rain droplets) or are polluted by contaminations in a humid environment. In this mini-review, we summarize the research works related to the fabrication of superwetting MLAs and ACEs and show how to integrate superhydrophobic and superoleophobic microstructures with an MLA. The fabrication strategy can be split into two categories. One is the hybrid pattern composed of the MLA domain and the superwetting domain. Another is the direct formation of superwetting nanostructures on the surface of the microlenses. The superhydrophobicity or superoleophobicity endows the MLAs and ACEs with liquid repellence and self-cleaning function besides excellent optical performance. We believe that the superwetting MLAs and ACEs will have significant applications in various optical systems that are often used in the humid or liquid environment.

A Contrastive Investigation on the Anticorrosive Performance of Stearic Acid and Fluoroalkylsilane-Modified Superhydrophobic Surface in Salt, Alkali, and Acid Solution

Stearic acid and fluoroalkylsilane are widely used in chemical modification to fabricate superhydrophobic surfaces in corrosion-resistant exploration. However, extensive works have just explored their anticorrosive performance in salt solution. Very rare work has focused on comparing their corrosion-resistant performance in corrosive solution (salt, alkali, and acid) systematically. In this study, two kinds of superhydrophobic surfaces were obtained on laser-processed rough IN304 surface after the stearic acid and FAS modification processes, respectively. The investigation and comparison of anticorrosion performance in salt, alkali, and acid electrolyte were carried out via potentiodynamic polarization and electrochemical impedance spectroscopy measurements. The promotion mechanism or impairing mechanism was further proposed based on the results of surface wettability, surface morphology, and X-ray photoelectron spectroscopy. Besides, the long-term anticorrosion performance and the stability of surface wettability were also investigated. It is hoped that these research findings could provide an explicit guidance of suitable anticorrosion methods selection for metals in different kinds of corrosive solution (salt, alkali, and acid), which will further raise the promising prospect of functional surfaces for practical applications in industry.

Fabrication of Waterproof Artificial Compound Eyes with Variable Field of View Based on the Bioinspiration from Natural Hierarchical Micro-Nanostructures

Planar and curved microlens arrays (MLAs) are the key components of miniaturized microoptical systems. In order to meet the requirements for advanced and multipurpose applications in microoptical field, a simple manufacturing method is urgently required for fabricating MLAs with unique properties, such as waterproofness and variable field-of-view (FOV) imaging. Such properties are beneficial for the production of advanced artificial compound eyes for the significant applications in complex microcavity environments with high humidity, for instance, miniature medical endoscopy. However, the simple and effective fabrication of advanced artificial compound eyes still presents significant challenges. In this paper, bioinspired by the natural superhydrophobic surface of lotus leaf, we propose a novel method for the fabrication of waterproof artificial compound eyes. Electrohydrodynamic jet printing was used to fabricate hierarchical MLAs and nanolens arrays (NLAs) on polydimethylsiloxane film. The flexible film of MLAs hybridized with NLAs exhibited excellent superhydrophobic property with a water contact angle of 158°. The MLAs film was deformed using a microfluidics chip to create artificial compound eyes with variable FOV, which ranged from 0° to 160°.

Investigation and Prediction on Regulation of Hydrophobicity of Polymethyl Methacrylate (PMMA) Surface by Femtosecond Laser Irradiation

This study presents the contact angle prediction model of a trapezoidal groove structure based on the laser irradiation on polymethyl methacrylate (PMMA). The trapezoidal groove structure was designed and proposed according to the characteristics of a femtosecond laser. First, the complete wetting model and incomplete wetting model which were compatible with the characteristics of the laser mechanism were constructed based on the Gibbs free energy and the structural parameters of the trapezoidal groove structure. Then, based on the contact angle prediction models constructed, the samples were divided into two groups according to the designed structural parameters, and the experimental investigations were carried out. The result demonstrated that the incomplete wetting prediction model was more in line with the actual situation. The convex width and the top edge length of spacing of the trapezoidal groove structure both affected the contact angle prediction results. From both the experimental contact angles and the contact angles predicted by the incomplete wetting model, it could be known that the contact angle reached 138.09° when the ratio of the convex width to the top edge length of spacing was 0.25, indicating that the smaller the ratio of the convex width to the top edge length of spacing, the better the hydrophobicity of PMMA.

Experimental investigation of laser engraving quality on paper

Laser engraving technology is a type of laser processing technology, widely used for product coding, marking, and so on. A large amount of research has reported the results of metal surface engraving; however, few research results, to the best of our knowledge, have provided theoretical support for the application of paper packaging laser engraving. In this paper, the quality of paper laser engraving is investigated by experimental methods. First, various phenomena appearing in paper carving were studied, including plant fiber burning, charcoal, and edge marks; second, the main factors affecting the quality of laser engraving are researched, and the influence of laser intensity and the preset width of carving marks on the engraving quality are discussed. The results show that the engraving precision is the best when the laser power is 11 W and the preset width is small (0.26 mm). Finally, the laser engraving precision of UV coated paper is studied, and the effect of UV material melting and secondary crystallization on engraving the quality of paper laser engraving quality is discussed. When the laser power is small, the maximum and minimum values of UV film melting and secondary crystallization engraving trace are relatively small as well; further, when the laser power increases, the maximum width of engraving is basically consistent with the preset width, and the precision of laser engraving is optimal.

Fabrication of Artificial Compound Eye with Controllable Field of View and Improved Imaging

Many arthropods have compound eyes, which are made up of numerous separate visual units (microlenses) or ommatidia. These natural compound eyes have exceptional optical properties such as wide field of view (FOV), low aberration, and fast motion tracking capability. In this paper, a large-scale artificial compound eye (ACE) is fabricated efficiently using a combination of inkjet printing and air-assisted deformation processes. Both size and geometry of the microlens are controlled via superposed drops on the substrate. The simulation results show that the light intensity of the ACE follows a systematic distribution for tilted incident light, which represents a significant improvement, compared to planar distributed microlenses. We then manufacture ACEs with different heights and diameters, and their FOVs are compared with the theoretically predicted results. The measured FOV was 50°-140°. The acceptance angles for the different ACEs are determined, and their relationship with the ratio of height to radius (H/r) of the microlens is investigated in more detail. Furthermore, the imaging properties of the microlenses with different angles of incidences are studied, which suggest a FOV up to 140° and an acceptance angle of about 50°. The microlens captures images even at an angle of incidence of about 60°. The corresponding distortion in both the x and y directions is also investigated. Our findings provide guidelines for the development and fabrication of ACEs with large FOVs and acceptance angles, which may find applications in military, robotics, medical imaging, and astronomy.

Synthesis of Au/SnO2 nanostructures allowing process variable control

Theoretical advances in science are inherently time-consuming to realise in engineering, since their practical application is hindered by the inability to follow the theoretical essence. Herein, we propose a new method to freely control the time, cost, and process variables in the fabrication of a hybrid featuring Au nanoparticles on a pre-formed SnO₂ nanostructure. The above advantages, which were divided into six categories, are proven to be superior to those achieved elsewhere, and the obtained results are found to be applicable to the synthesis and functionalisation of other nanostructures. Furthermore, the reduction of the time-gap between science and engineering is expected to promote the practical applications of numerous scientific theories.

Fabrication of Hierarchical Micro/Nano Compound Eyes

Fabrication of a hierarchical macro-/micro-/nano compound eye is presented in this paper. This bioinspired compound (BIC) eye is obtained by an integrated manufacturing technology that combines (i) nanoimprinting, (ii) picosecond laser swelling, and (iii) air-assisted deformation. The diameter and height of nanopillars, microlens, and macrobase can be controlled precisely by fine-tuning the process parameters. The multifunctional properties of the BIC eye, such as superhydrophobicity, antireflection, and other optical characteristics, are investigated. It is found that the microlens with nanopillars can effectively improve the surface wettability with a contact angle of 152° and contact angle hysteresis of 12°, and enhance transmittance by 2% over the wavelength range of 200-1200 nm. Moreover, the final hierarchical compound eye exhibits the excellent imaging properties and a wide field-of-view of 120° without distortion. These multifunctional properties will enable the widespread application of the compound eye in diverse real-time environmental conditions.