Large-area fabrication of superhydrophobic micro-conical pillar arrays on various metallic substrates

Bioinspired Superhydrophobic Fibrous Materials

Natural fibers with robust water repellency play an important role in adapting organisms to various environments, which has inspired the development of artificial superhydrophobic fibrous materials with applications in self-cleaning, antifogging, water harvesting, heat exchanging, catalytic reactions, and microrobots. However, these highly textured surfaces (micro/nanotextured) suffer from frequent liquid penetration in high humidity and abrasion-induced destruction of the local environment. Herein, bioinspired superhydrophobic fibrous materials are reviewed from the perspective of the dimension scale of fibers. First, the fibrous dimension characteristics of several representative natural superhydrophobic fibrous systems are summarized, along with the mechanisms involved. Then, artificial superhydrophobic fibers are summarized, along with their various applications. Nanometer-scale fibers enable superhydrophobicity by minimizing the liquid-solid contact area. Micrometer-scale fibers are advantageous for enhancing the mechanical stability of superhydrophobicity. Micrometer-scale conical fibrous structures endow a Laplace force with a particular magnitude for self-removing condensed tiny dewdrops in highly humid air and stably trapping large air pockets underwater. Furthermore, several representative surface modification strategies for constructing superhydrophobic fibers are presented. In addition, several conventional applications of superhydrophobic systems are presented. It is anticipated that the review will inspire the design and fabrication of superhydrophobic fibrous systems.

Oriented bouncing of droplets with a small Weber number on inclined one-dimensional nanoforests

Asymmetric superhydrophobic structures with anisotropic wettability can achieve directional bouncing of droplets and thus can have applications in directional self-cleaning, liquid transportation, and heat transfer. To achieve convenient large-scale preparation of asymmetric superhydrophobic surfaces, inclined nanoforests are prepared in this work using a technique of competitive ablation polymerization, which allows the control of the inclined angles, diameters, and heights of the nanostructures. In this study, such asymmetric structures with the smallest dimension (230 nm diameter) known are achieved by a simple etching method to guide droplet unidirectional bouncing. With such nanoforests, the mechanism of droplet bouncing on their surface is investigated, and controllable droplet bouncing over a long distance is achieved using droplets with a low Weber number. The proposed structure has a promising future in directional self-cleaning, liquid transportation and heat transfer.

Recent progress on the development of bioinspired surfaces with high aspect ratio microarray structures: From fabrication to applications

Surfaces with high aspect ratio microarray structures can implement sophisticated assignment in typical fields including microfluidics, sensor, biomedicine, et al. via regulating their deformation or the material properties. Inspired by natural materials and systems, for example sea cockroaches, water spiders, cacti, lotus leaves, rice leaves, and cedar leaves, many researchers have focused on microneedle functional surface studies. When the surface with high aspect ratio microarray structures is stimulated by the external fields, such as optical, electric, thermal, magnetic, the high aspect ratio microarray structures can undergo hydrophilic and hydrophobic switching or shape change, which may be gifted the surfaces with the ability to perform complex task, including directional liquid/air transport, targeted drug delivery, microfluidic chip sensing. In this review, the fabrication principles of various surfaces with high aspect ratio microarray structures are classified and summarized. Mechanisms of liquid manipulation on hydrophilic/hydrophobic surfaces with high aspect ratio microarray structures are clarified based on Wenzel model, Cassie model, Laplace pressure theories and so on. Then the intelligent control strategies have been demonstrated. The applications in microfluidic, drug delivery, patch sensors have been discussed. Finally, current challenges and new insights of future prospects for dynamic manipulation of liquid/air based on biomimetic surface with high aspect ratio microarray structures are also addressed.

Golden section criterion to achieve droplet trampoline effect on metal-based superhydrophobic surface

Clarifying the consecutive droplet rebound mechanisms can provide scientific inspirations to regulate dynamic wettability of superhydrophobic surface, which facilitates the practical applications on efficient heat control and active anti-icing. Generally, droplet rebound behaviors are directly affected by surface structure and Weber number. Here, we report a novel "golden section" design criterion to regulate the droplet rebound number determined by the structure spacing, subverting conventional knowledge. Especially, the droplet can continuously rebound for 17 times on the metal-based surface, exhibiting an amazing phenomenon of "droplet trampoline". The droplet rebound number has been experimentally revealed to be closely related to Weber number. We propose novel quantitative formulas to predict droplet rebound number and clarify the coupling effect of the structure spacing and the Weber number on the rebound mechanisms, which can be utilized to establish the regulation criteria of rebound numbers and develop novel metal-based superhydrophobic materials.

Passive-Cooling Building Coating with Efficient Cooling Performance and Excellent Superhydrophobicity

Passive-cooling building materials can achieve cooling without external energy consumption, which is an energy-saving and environmentally friendly cooling method. However, the existing passive-cooling building materials have the limitations of high cost, complicated processes, and a toxic organic solvent, which hinders the passive-cooling technology applied in practical building. To overcome these limitations, we developed a facile, high-efficiency, non-toxic, and superhydrophobic passive-cooling building coating (SPCBC) with an efficient cooling capability and excellent durability that was composed of polydimethylsiloxane and SiO2. The fabricated SPCBC demonstrated a high reflectance and a high emittance, showing a superior cooling capability with a 14 °C temperature drop compared with a bare cement surface on a hot summer day. In addition, the SPCBC could not be wetted or contaminated by muddy water, corrosive aqueous solutions, or dust, which presented an excellent anti-fouling and self-cleaning capability. Moreover, the fabricated SPCBC could work outdoors for 30 days, withstand UV irradiation for 30 days, and resist accelerated aging for 100 h without any significant changes in the superhydrophobicity and the cooling capability, meaning that the SPCBC had an outstanding durability. This work provides a new method to facilitate passive-cooling technology to apply in practical building in hot weather regions of the world.

Enhanced water transportation on a superhydrophilic serial cycloid-shaped pattern

Spontaneous and directional water transportation (SDWT) is considered as an ideal water transportation method and has a great prospect in the aerospace and ship fields. Nonetheless, the existing SDWT has the limitation of a slow water transportation velocity because of its geometry structure configuration, which hinders the practical application of the SDWT. To overcome this limitation, we developed a new superhydrophilic serial cycloid-shaped pattern (SSCP) which was inspired by the micro-cavity shape of the Nepenthes. First, we experimentally found that the water transportation velocity on the SSCP was faster than that on the superhydrophilic serial wedge-shaped pattern (SSWP) and analyzed the faster water transportation mechanism. Then, the influence of the SSCP parameters on the transportation velocity was investigated by a single-factor experiment. In addition, the water transportation velocity on the SSCP was enhanced to 289 mm s^-1 by combining the single-factor experiment, orthogonal optimization design, streamline junction transition optimization, and pre-wet pattern, which was the fastest in the SDWT. Moreover, the SSCP demonstrated its superior capability in long-distance water transportation, gravity resistant water transportation, heat transfer, and fog collection. This finding shows remarkable application prospects in the high-performance fluid transportation system.