Understanding the Role of Dynamic Wettability for Condensate Microdrop Self-Propelling Based on Designed Superhydrophobic TiO2 Nanostructures

Superhydrophobic Solar-to-Thermal Materials Toward Cutting-Edge Applications

Solar-to-thermal conversion is a direct and effective way to absorb sunlight for heat via the rational design and control of photothermal materials. However, when exposed to water-existed conditions, the conventional solar-to-thermal performance may experience severe degradation owing to the high specific heat capacity of water. To tackle with the challenge, the water-repellent function is introduced to construct superhydrophobic solar-to-thermal materials (SSTMs) for achieving stable heating, and even, for creating new application possibilities under water droplets, sweat, seawater, and ice environments. An in-depth review of cutting-edge research of SSTMs is given, focusing on synergetic functions, typical construction methods, and cutting-edge potentials based on water medium. Moreover, the current challenges and future prospects based on SSTMs are also carefully discussed.

Dual-Coating of Fluorinated Polydimethylsiloxane/Fluorinated SiO2 Nanoparticles for Superhydrophobic and High-Efficiency Bacteriostatic Surface

A simple two-step spray method is used to prepare superhydrophobic and bacteriostatic surfaces, involving dual-coating with polydimethylsiloxane-normal-fluorine (PDMS-NF) or branched-fluorine (PDMS-BF) in combination with fluorinated silica nanoparticles (FSiO2 -NPs) using a spray technique. This approach has the potential to create surfaces with both water-repellent and antimicrobial properties, which could be useful in a variety of applications. It is noteworthy that the dual-coating on cotton fabric exhibited an impressive dual-scale roughness and achieved superhydrophobicity with a water contact angle of 158° and a hysteresis of less than 3°. Additionally, the coating was subjected to an ultra-high concentration of bacteria (109 CFU/mL) and was still able to inhibit more than 80 % of attachment, demonstrating its effectiveness as a bacteriostatic surface.

Enhancement of Dropwise Condensation Heat Transfer through a Sprayable Superhydrophobic Coating

Improving the shedding rate of condensed droplets has many applications in industries and daily problems, including increasing heat transfer and self-cleaning properties. One way to achieve this goal is by enhancement of the wetting properties of surfaces. In this research, the hierarchical superhydrophobic coating over aluminum has been applied using a relatively cost-effective method, spraying, which is also applicable to any metal surface used as a condenser. According to the results obtained from the experimental tests, the fabricated surface is highly superhydrophobic, with a contact angle of 158° and contact angle hysteresis of less than 5°. The results show that the presented surface increases the heat transfer coefficient by 20.6% at the subcooling temperature of 25.5 °C when the surface temperature and relative humidity are 70 °C and 98%, respectively. In addition, this coated surface showed great potential at lower surface temperatures by increasing the water condensation rate as much as 50.5% at the subcooling temperature of 12 °C, when the surface temperature and relative humidity are 11.25 °C and 70%, respectively. Therefore, it is found that for the fabricated superhydrophobic paint in the present study, the effectiveness of the dropwise condensation mode profoundly depends on surface temperatures besides subcooling temperatures. In other words, a surface with lower temperatures shows better performance for the same subcooling temperatures. In addition, various types of durability tests are carried out. The results reveal that this coating has good durability against high surface temperatures, submerged conditions for 30 days, imposing hot steam for 150 h, corrosion, and organic solvents. Hence, it is suitable for industrial applications.

The Interrelation of Synthesis Conditions and Wettability Properties of the Porous Anodic Alumina Membranes

The results of studies on the wettability properties and preparation of porous anodic alumina (PAA) membranes with a 3.3 ± 0.2 μm thickness and a variety of pore sizes are presented in this article. The wettability feature results, as well as the fabrication processing characteristics and morphology, are presented. The microstructure effect of these surfaces on wettability properties is analyzed in comparison to outer PAA surfaces. The interfacial contact angle was measured for amorphous PAA membranes as-fabricated and after a modification technique (pore widening), with pore sizes ranging from 20 to 130 nm. Different surface morphologies of such alumina can be obtained by adjusting synthesis conditions, which allows the surface properties to change from hydrophilic (contact angle is approximately 13°) to hydrophobic (contact angle is 100°). This research could propose a new method for designing functional surfaces with tunable wettability. The potential applications of ordinary alumina as multifunctional films are demonstrated.

Design of Organic-Free Superhydrophobic TiO2 with Ultraviolet Stability or Ultraviolet-Induced Switchable Wettability

Superhydrophobic TiO₂ with great application potential is mainly obtained by surface modification with low surface energy organics, which is easily degraded under sunlight irradiation, which results in the loss of superhydrophobic properties. Herein, we developed a room-temperature pulsed chemical vapor deposition (pulsed CVD) method to develop amorphous TiO₂-deposited TiO₂ nanoparticles. The ultraviolet stability/ultraviolet-induced reversible wettability switch had been simultaneously realized by different and controllable deposition cycles of amorphous TiO₂. The superhydrophobic properties of the organic-free TiO₂ were determined by the micrometer-nanometer-sub-nanometer multiscale structure, the multiscale pore structure, and the large Young's contact angle resulting from carboxylic acid adsorption. Also, we found that the adsorption rate and adsorption stability of oxygen and water at the surface oxygen vacancies were the key to facilitate the reversible switching between superhydrophilic and superhydrophobic states, which was well demonstrated by experimental characterization and theoretical simulation. In addition, we also found that the resistance of dense amorphous TiO₂ films on the TiO₂ surface to the migration of photogenerated electrons and holes was the key to maintain the stable superhydrophobic properties of superhydrophobic TiO₂ under ultraviolet illumination. The powders were strongly ground and the coating surface was rubbed on the surface of the sandpaper, which still maintained superhydrophobic properties, providing favorable conditions for the application of superhydrophobic TiO₂. This work modulates the ultraviolet stability and dark/ultraviolet-induced switchable superhydrophobicity/superhydrophilicity of coated TiO₂ by simply adjusting the number of deposition times in a pulsed CVD process for the first time, thus contributing to the development of organic-free superhydrophobic TiO₂.

Eco-Friendly Superhydrophobic Composites with Thermostability, UV Resistance, and Coating Transparency

Despite the market demand for biofiber assemblies endowed with superhydrophobicity being huge, the current approaches to their production are complicated, time-consuming, and even pose a serious threat to the environment. Here, we report a simple surface treatment strategy to prepare environmentally friendly superhydrophobic biofiber composites. The obtained samples have certain UV resistance properties, which are mainly determined by the titanium dioxide (TiO₂) dosage. Additionally, the sample has excellent thermal stability, and the contact angle is maintained at 153.26° after heat treatment at 140 °C for 1 h. Quite encouragingly, thermal annealing of samples can transform translucent coatings into transparent structures and increase the tensile strength. The results also showed that this strategy could be integrated into the mass production process of other biofiber components as coating, such as coated paper, pulp boards, cotton gauzes, tissues, and so forth. Due to the facile preparation and environment-friendliness, this sustainable paper-based product can be used in diversified applications: packaging and storage of liquid food, protection of ancient books, UV- and rain-proof materials, and teaching demonstrations relevant to bionics, among others.

Superhydrophobic and Antioxidative Film Based on Edible Materials for Food Packaging

Significant wastage of the food deterioration in the food preserving process and residual liquid in a container has become a major concern for scientists and the whole society. In this study, an edible multifunctional film integrated superhydrophobicity and antioxidant ability is constructed by chitosan, tea polyphenol, carnauba wax material that is food and drug administration (FDA)-approved for food packaging. The formed edible packaging materials that exhibit great antioxidant property and extremely low water-absorbing quality, was thus proven to display excellent fresh beef preservation effect during storage of 14 days. Importantly, the formed edible multifunctional interface was also demonstrated to perform excellent superhydrophobicity due to the carnauba wax and exhibited large contact angles for various liquid foods, which could effectively reduce the liquid residue. Moreover, the formed edible multifunctional packaging materials showed good thermostability and biocompatibility, which has the potential to be applied as a functional packaging material.

A Universal Strategy for the Preparation of Dual Superlyophobic Surfaces in Oil-Water Systems

There are some methods to prepare superwetting surfaces with underwater superoleophobicity (UWSOB) or underoil superhydrophobicity (UOSHB), but it is still thorny to put forward a universal strategy for constructing dual superlyophobic surfaces in oil-water systems due to a thermodynamic contradiction. Herein, a universal strategy was proposed to prepare the dual superlyophobic surfaces in oil-water systems only via delicately controlling surface chemistry, that is, adjusting the ratios of superhydrophilic and superhydrophobic counterparts in the spray solution. Three types of materials, attapulgite (APT), TiO₂, and loess, were chosen to prepare a diverse series of mixed coatings (mass gradient of superhydrophobic counterparts from 0 to 100 wt %). With the proportion of each superhydrophobic counterpart increasing, the underwater oil contact angle (θ_o/w^*) of each mixed coating slightly decreased but still was more than 150°, that is, UWSOB. In contrast, the underoil water contact angle (θ_w/o^*) was significantly improved, realizing the transformation from UOHL (or UOHB) to UOSHB. More importantly, the respective mass ratios of superhydrophobic counterparts in the resulting mixed coatings of APT, TiO₂, and loess were finally determined to be 0.3, 0.4, and 0.2, respectively. Taking APT as a model, a train of mixed APT coatings with different superhydrophobic components were systematically characterized and analyzed. Finally, the prepared superlyophobic separation mesh in oil-water systems was applied to the separation of various surfactant-stabilized oil-water emulsions. We envision that this universal strategy we proposed will show a significant application potential in addressing scientific and technological challenges in the field of interfacial chemistry such as oil-water separation, microfluidics, microdroplet manipulation, antifogging/icing, cell engineering, drag reduction, and so forth.

Coalescence-Induced Jumping Droplets on Nanostructured Biphilic Surfaces with Contact Electrification Effects

Benefitting from the coalescence-induced droplet jumping on superhydrophobic surfaces, the condensing droplets on heat exchangers can be removed efficiently, significantly improving the condensation heat-transfer performance of various thermal applications. However, the enhancement of droplet jumping height and self-removal to further improve the condensation heat-transfer performance of the thermal applications remains a challenge due to considerable interfacial adhesion caused by the inevitable partial-Wenzel state condensing droplets on superhydrophobic surfaces. In this study, a biphilic nanostructure is developed to effectively improve the droplet jumping height by decreasing the interfacial adhesion with the formation of Cassie-like droplets. Under atmospheric conditions, ∼28% improvement of droplet jumping height is achieved on a biphilic surface compared to that of a superhydrophobic surface. Additionally, the droplet contact electrification on biphilic surfaces discovered in this work allows the droplets to jump ∼137% higher compared with that under atmospheric conditions. Furthermore, the droplet jumping and electrification mechanisms on the biphilic surface are revealed by building a theoretical model that can predict the experimental results well. Apart from being a milestone for the droplet jumping physics development on biphilic nanostructures, this work also provides new insights into the micro-droplet discipline.

Effect of a Superhydrophobic Surface Structure on Droplet Jumping Velocity

The coalescence-induced droplet jumping on superhydrophobic surfaces is fundamentally significant from an academic or practical viewpoint. However, approaches to enhance droplet jumping velocity are very limited. In this work, the effect of structural parameters of the triangular prism on droplet jumping is studied systematically. The results indicate that droplet jumping velocity can be greatly increased by exploiting structure effects, which is a promising reinforcement method. When the height and apex angle of the triangular prism are fixed, the droplet jumping velocity increases with the length of the triangular prism until a plateau is reached. The ratio of translational kinetic energy to released surface energy during droplet jumping is determined by the apex angle and the height of the triangular prism, which is more effective with a smaller apex angle and a larger height. The results are supposed to provide guidelines for optimization of superhydrophobic surfaces.

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

New insights into unusual droplets: from mediating the wettability to manipulating the locomotion modes

The ability to manipulate droplets can be utilized to develop various smart sensors or actuators, endowing them with fascinating applications for drug delivery, detection of target analytes, environmental monitoring, intelligent control, and so on. However, the stimuli-responsive superhydrophobic/superhydrophilic materials for normal water droplets cannot satisfy the requirements from some certain circumstances, i.e., liquid lenses and biosensors (detection of various additives in water/blood droplets). Stimuli-responsive wetting/dewetting behaviors of exceptional droplets are open issues and are attracting much attention from across the world. In this perspective article, the unconventional droplets are divided into three categories: ionic or surfactant additives in water droplets, oil droplets, and bubble droplets. We first introduce several classical wettability models of droplets and some methods to achieve wettability transition. The unusual droplet motion is also introduced in detail. There are four main types of locomotion modes, which are vertical rebound motion, lateral motion, self-propulsion motion, and anisotropic wettability controlled sliding behavior. The driving mechanism for the droplet motion is briefly introduced as well. Some approaches to achieve this manipulation goal, such as light irradiation, electronic, magnetic, acid-base, thermal, and mechanical ways will be taken into consideration. Finally, the current researches on unconventional droplets extending to polymer droplets and liquid metal droplets on the surface of special wettability materials are summarized and the prospect of unconventional droplet research directions in the field of on-demand transport application will be proposed.

Flexible superhydrophobic surfaces with condensate microdrop self-propelling functionality based on carbon nanotube films

With the development of flexible electronics and wearable devices, there is strong demand for flexible, superhydrophobic, and multifunctional coatings. Motivated by the promise of attractive multifaceted functionality, various techniques have been developed to fabricate flexible surfaces with non-wetting properties. However, until now, there have been few reports on superhydrophobic surfaces with condensate microdrop self-propelling (CMDSP) functionality on a carbon nanotube film. Here, we used a facile electrodeposition method to develop for the first time a new type of flexible superhydrophobic surface with CMDSP functionality based on carbon nanotube films. These flexible CMDSP surfaces are robust after multiple cycles of bending of the film-coated substrate, i.e., without impacting the surface superhydrophobicity and CMDSP performance. The proposed light and flexible surface, combined with CMDSP, will support a novel generation of coatings that are multifunctional, flexible, smart, and energy saving. This new type of functional flexible interface not only opens new avenues in research into the fundamental structure-property relationships of materials, but also exhibits significant application potential for advanced technologies.

Density Maximization of One-Step Electrodeposited Copper Nanocones and Dropwise Condensation Heat-Transfer Performance Evaluation

Currently, it is still a great challenge to obtain copper-based high-efficient dropwise condensation heat transfer (CHT) interfaces via template-free electrodepositing technologies. Here, we report that the density of template-free electrodeposited copper nanocones can maximally reach 1.5 × 10⁶/mm² by the synergistic control of substrate surface roughness, poly(ethylene glycol) (PEG) molecular weight, and PEG concentration. After thiol modification, the densely packed copper nanocone samples can present low-adhesive superhydrophobicity and condensate microdrop self-jumping function at ambient environment. Condensation heat and mass transfer characterizations show that the CHT coefficient of copper surfaces can maximally enhance 98% for 20 °C vapor and 51% for 40 °C vapor by in situ growth of superhydrophobic densely packed copper nanocones. Although the dropwise condensation mode can change from the jumping mode to the mixed jumping and sweeping mode and the shedding-off mode along with the increase of surface subcooling and vapor temperature, the CHT performance of the nanosample is still superior to that of the contrast flat hydrophobic surface during the whole testing range of surface subcooling. As vapor temperature increases to 80 °C, the CHT performance of the nanosample is inferior to that of the contrast sample. The CHT enhancement under low-temperature vapor should be ascribed to the enhancement of small-drop mass transfer ability caused by low-adhesive superhydrophobicity nature of nanosample surfaces. Their performance degradation mainly results from increased drop-drop drag force along with the increase of surface subcooling and vapor temperature. In sharp contrast, the CHT deterioration under high-temperature vapor should be ascribed to larger drop-surface adhesion and drop-drop drag force. The former is caused by vapor penetration, whereas the latter is caused by the dramatically increased nucleation density and growth rate of condensates. These findings would help design and develop copper-based high-efficiency condensation heat transfer interfaces.

Highly Durable Superhydrophobic Polydimethylsiloxane/Silica Nanocomposite Surfaces with Good Self-Cleaning Ability

In this work, we report that superhydrophobic coatings can be prepared by a simple spray-coating technique using readily available materials such as polydimethylsiloxane (PDMS) and hydrophilic and hydrophobic SiO2 nanoparticles. PDMS can combine with the two kinds of SiO2 nanoparticles to form a rough structure, which results in superhydrophobicity of the coatings. The prepared superhydrophobic coating has a water contact angle of 156.4° and a sliding angle of less than 5°. Moreover, the coatings can be applied to various substrates such as glass, paper, and plastic. In addition, the coatings show excellent stability and still remain superhydrophobic after ultraviolet radiation, sand abrasion and water impact, tape peeling, and treatment with a strong alkali/acid solution. Furthermore, the superhydrophobic surfaces are proven to be suitable for antifouling and self-cleaning.

Morphology-controllable synthesis and application of TiO2 nanotube arrays with “photocatalysis and self-cleaning” synergism

The integration of photocatalytic materials and self-cleaning superhydrophobic materials provides a possibility of combining decontamination and antifouling.

Tuning nanotubular structures by templateless electropolymerization with thieno[3,4-b]thiophene-based monomers with different substituents and water content

Here, templateless electropolymerization is employed to produce nanotubular structures from various thieno[3,4-b]thiophene-based monomers that differ in substituent structure and size, as well as the linker connecting the thieno[3,4-b]thiophene core and substituent. The formation of densely packed vertically aligned are obtained from monomers with a pyrene substituent and when a significant amount of water (CH₂Cl₂ + H₂O) is included in the solvent. The geometrical parameters of the nanotubes are highly dependent on the electopolymerization method. A significant amount of air is trapped within the structure of the densely packed open nanotubes obtained with Qs = 100 mC cm^-2 causing an increase in water contact angle (θ_w) up to 82.6° (intermediate state between the Wenzel and the Cassie-Baxter state), and θ_w can become even more hydrophobic by further modifying the deposition method or the electrolyte.

Designing a Superhydrophobic Surface for Enhanced Atmospheric Corrosion Resistance Based on Coalescence-Induced Droplet Jumping Behavior

Coalescence-induced droplet jumping behavior of superhydrophobic surfaces has attracted increasing attention for condensation heat transfer, antifrosting, self-cleaning, and electrostatic energy harvesting applications. The potential of applying such functionalized behavior for atmospheric corrosion protection, however, is unknown. Herein, we experimentally demonstrate, for the first time, the feasibility of applying coalescence-induced droplet jumping behavior of a superhydrophobic surface for atmospheric corrosion protection. Based on the rational fabrication of two kinds of superhydrophobic surfaces that are advantageous and not advantageous for coalescence-induced droplet jumping behavior, we reveal a novel atmospheric corrosion protection mechanism by studying the correlations of the surface structure, droplet jumping behavior, and atmospheric corrosion resistance of the two surfaces. Our results demonstrate that the superhydrophobic surface with coalescence-induced droplet jumping behavior presents a better atmospheric corrosion resistance than the superhydrophobic surface without coalescence-induced droplet jumping behavior. This is because coalescence-induced droplet jumping behavior of the superhydrophobic surface offers a possible mechanism to switch the droplets from a partial wetting state to the mobile Cassie state, and this switch is critical for facilitating the recovery of the air film trapped in the microstructure of a surface. In particular, the recovered air film enhances the atmospheric corrosion resistance of a superhydrophobic surface due to its barrier-like character. The insights gained from this work not only open a new avenue for designing first-rank anticorrosion materials but also offer new opportunities for understanding the physics of jumping droplets in other promising applications.

Beetle-Inspired Hierarchical Antibacterial Interface for Reliable Fog Harvesting

The microdroplets in fog flow have been considered as an important resource for supplying fresh drinking water. Most of the reported works of fog collection focus on the water-collecting ability rather than the environmental reliability of selected materials. In this work, a beetle-inspired hierarchical fog-collecting interface based on the antibacterial needle-array (ABN) and hydrophilic/hydrophobic cooperative structure is displayed. The hydrophilic ABN is coated with zwitterionic carboxybetaine (CB) brushes that endow the fog collector with a long-term cleaning in harsh environment. Due to its strong affinity to water molecules, the tilted needles with a CB coating can facilitate the capture of fog and the rapid delivery of condensed water driven by gravity. After being transported to the connected hydrophobic sheet, the collected droplets can be rapidly detached and stored in the container, achieving a high fog-harvesting rate. Furthermore, CB-patterned channels are integrated on the hydrophobic sheet for the pathway-controlled water delivery. The CB coating is able to efficiently resist bacterial adhesion and contamination during fog harvesting, protecting the device from microbiological corrosion. The current design provides a promising method to incorporate antibacterial ability into fog collectors, which offer great opportunity to develop water harvesters for real-world applications.

Study on the Properties of Vertical Carbon Nanotube Films Grown on Stainless Steel Bipolar Plates

Research on the conductivity and corrosion resistance of stainless steel bipolar plates in a proton exchange membrane fuel cell (PEMFC) is commonly performed in a normal-temperature environment (about 20 °C). However, these fuel cells must function in low-temperature environments (lower than 0 °C) in some conditions, such as in vehicle fuel cells and in portable power supplies that operate during the winter in northern China. Stainless steel bipolar plates have higher requirements in terms of their hydrophobic and anti-icing properties, in addition to needing high conductivity and corrosion resistance. In this study, carbon nanotubes (CNTs) are grown on the surface of 304 stainless steel (304 SS) without a catalyst coating by plasma-enhanced chemical vapor deposition (PECVD), which is a simple and cheap method that allows stainless steel to be used as bipolar plates in low-temperature environments. The Raman spectroscopy and scanning electron microscopy (SEM) results show that the CNTs grown on the surface of 304 SS have different morphologies. The stainless steel samples with different CNT morphologies are tested by hydrophobicity and in situ icing experiments to prove that vertical CNTs can achieve a superhydrophobic state and have good anti-icing properties. The interfacial contact resistance (ICR) of the bare 304 SS and the 304 SS with vertical CNTs is compared by voltammetry, and then the corrosion resistances of both types is compared in a simulated PEMFC environment via a three-electrode system. Consequently, the ICR of the 304 SS with vertical CNTs was lower than the bare 304 SS. The corrosion potential was positive, and the corrosion current density was greatly reduced for the stainless steel with vertical CNTs grown directly on its surface when compared with the bare 304 SS. The experimental results show that vertical CNTs have good application prospects as bipolar plates for PEMFCs in low-temperature environments.

Bioinspired Hierarchical Surfaces Fabricated by Femtosecond Laser and Hydrothermal Method for Water Harvesting

The world is facing a global issue of water scarcity where two-thirds of the population does not have access to safe drinking water. Water harvesting from the ambient environment has a potential equivalent to ∼10% of the fresh water available on the earth's surface, but its efficiency requires a special control of surface morphology. We report a novel facile physicochemical hybrid method that combines femtosecond laser structuring with hydrothermal treatment to create a surface with a well-arranged hierarchical nanoneedle structures. Polydimethylsiloxane treatment of the thus-produced hierarchical structures nurtured superhydrophobic functionality with a very low water sliding angle (∼3°) and a high water adhesion ability. About 2.2 times higher water-collection efficiency was achieved using hierarchical structures over untreated flat Ti surfaces of the same area under a given experimental condition. The comparison of water-collection behavior with other samples showed that the improved efficiency is due to the structure, and wettability induced superior water attraction and removal ability. Moreover, a uniform water condensation under low humidity (28%) is achieved, which has potential applications in harvesting water from arid environments and in high-precision drop control.

Parahydrophobic and Nanostructured Poly(3,4-ethylenedioxypyrrole) and Poly(3,4-propylenedioxypyrrole) Films with Hyperbranched Alkyl Chains

Here, we control the surface hydrophobicity and the adhesion of water droplets by electrodeposition of poly(3,4-ethylenedioxypyrrole) (PEDOP) and poly(3,4-propylenedioxypyrrole) (PProDOP) with branched alkyl chains placed preferentially on the bridge to favor the formation of nanofibers. Branched alkyl chains of various sizes from very short (C₃) to hyperbranched (C₁₈) are studied because they have lower surface hydrophobicity than long alkyl or fluoroalkyl chains (preferable for parahydrophobic properties). The electrodeposition is much more favored with the PEDOP derivatives because the ProDOP films are more soluble. However, the formation of nanoparticles is favored with the PEDOP polymers in contrast to the formation of fibers, resembling the wax nanoclusters observed on lotus leaves, with the ProDOP polymers. With both these PEDOP and PProDOP derivatives, it is possible to reach parahydrophobic properties characterized by a sticking behavior toward water droplets. This kind of surfaces could be used in the future in water harvesting systems, for example.

Effects of NH4F and distilled water on structure of pores in TiO2 nanotube arrays

In this study, we report the influences of distilled water and ammonium fluoride (NH4F) on morphology of pores in honeycomb-like titanium dioxide (TiO2) nanotube arrays. We observed the structure and arrangement of pores in the TiO2 nanotube arrays based on scanning electron microscopy images and analyzed the spatial distribution of the pores using fast Fourier transform and Voronoi diagram. We studied the individual pore properties including pore diameter, wall thickness, and interpore distance and found that locally connected ordering defects decreased with increasing distilled water concentration. Furthermore, we found that the optimum conditions of well-ordered hexagonal pore arrangement were 2 and 10 vol% distilled water with 0.2 and 0.4 wt% NH4F, respectively. Throughout this study, we provide a better understanding about the roles of distilled water and NH4F in forming well-ordered nanoscale pore structure with less ordering defects in the honeycomb-like TiO2 nanotube arrays.

Surface Nanostructuration and Wettability of Electrodeposited Poly(3,4-ethylenedioxypyrrole) and Poly(3,4-propylenedioxypyrrole) Films Substituted by Aromatic Groups

In the aim to obtain parahydrophobic materials (both high contact angles and high hysteresis) for possible applications in water harvesting systems, we report the synthesis of novel 3,4-ethylenedioxypyrrole (EDOP) and 3,4-propylenedioxypyrrole (ProDOP) monomers with aromatic rings on the 3,4-alkylenedioxy bridge and the resulting conducting polymer films were prepared by electropolymerization. We show that the surface properties can be tuned by the nature of the aromatic ring (phenyl, biphenyl, diphenyl, naphthalene, fluorene, and pyrene) and the polymerizable core (EDOP or ProDOP). The best results are obtained with both EDOP and diphenyl, with which extremely high hydrophobic properties (up to 116°) are obtained, even if the polymers are intrinsically hydrophilic. These surfaces could be applied in the future, for example, in water harvesting systems or in water/oil separation membranes. The synthesis strategy is extremely interesting, and many other molecules will be envisaged in the future.

Rational design of materials interface at nanoscale towards intelligent oil-water separation

Oil-water separation is critical for the water treatment of oily wastewater or oil-spill accidents. The oil contamination in water not only induces severe water pollution but also threatens human beings' health and all living species in the ecological system. To address this challenge, different nanoscale fabrication methods have been applied for endowing biomimetic porous materials, which provide a promising solution for oily-water remediation. In this review, we present the state-of-the-art developments in the rational design of materials interface with special wettability for the intelligent separation of immiscible/emulsified oil-water mixtures. A mechanistic understanding of oil-water separation is firstly described, followed by a summary of separation solutions for traditional oil-water mixtures and special oil-water emulsions enabled by self-amplified wettability due to nanostructures. Guided by the basic theory, the rational design of interfaces of various porous materials at nanoscale with special wettability towards superhydrophobicity-superoleophilicity, superhydrophilicity-superoleophobicity, and superhydrophilicity-underwater superoleophobicity is discussed in detail. Although the above nanoscale fabrication strategies are able to address most of the current challenges, intelligent superwetting materials developed to meet special oil-water separation demands and to further promote the separation efficiency are also reviewed for various special application demands. Finally, challenges and future perspectives in the development of more efficient oil-water separation materials and devices by nanoscale control are provided. It is expected that the biomimetic porous materials with nanoscale interface engineering will overcome the current challenges of oil-water emulsion separation, realizing their practical applications in the near future with continuous efforts in this field.

Drop Capturing Based on Patterned Substrate in Space

In this work, we introduced a method for capturing aqueous drop based on a patterned substrate in space. Through the manipulation test of a colloidal drop, it could be verified that this patterned substrate had excellent control ability for aqueous drop in microgravity condition. The confinement mechanism of this substrate was clarified, which showed that drops with different volume could be pinned and attracted at a given area on the substrate. The confinement capability was related to the gravity effect, and the patterned substrate could confine aqueous drops with larger volume under microgravity than in normal gravity. With advantages of simple operation and strong capability to control large drops, this technique exhibited the wide application prospect in the fields of fluid management, biosensing, and pharmacy in microgravity condition in the future.

Bio-inspired Edible Superhydrophobic Interface for Reducing Residual Liquid Food

Significant wastage of residual liquid food, such as milk, yogurt, and honey, in food containers has attracted great attention. In this work, a bio-inspired edible superhydrophobic interface was fabricated using U.S. Food and Drug Administration-approved and edible honeycomb wax, arabic gum, and gelatin by a simple and low-cost method. The bio-inspired edible superhydrophobic interface showed multiscale structures, which were similar to that of a lotus leaf surface. This bio-inspired edible superhydrophobic interface displayed high contact angles for a variety of liquid foods, and the residue of liquid foods could be effectively reduced using the bio-inspired interface. To improve the adhesive force of the superhydrophobic interface, a flexible edible elastic film was fabricated between the interface and substrate material. After repeated folding and flushing for a long time, the interface still maintained excellent superhydrophobic property. The bio-inspired edible superhydrophobic interface showed good biocompatibility, which may have potential applications as a functional packaging interface material.

Recent Progress in Bionic Condensate Microdrop Self-Propelling Surfaces

Bionic condensate microdrop self-propelling (CMDSP) surfaces are attracting increased attention as novel, low-adhesivity superhydrophobic surfaces due to their value in fundamental research and technological innovation, e.g., for enhancing heat transfer, energy-effective antifreezing, and electrostatic energy harvesting. Here, the focus is on recent progress in bionic CMDSP surfaces. Metal-based CMDSP surfaces, which are the most promising in their respective fields, are highlighted for use in future applications. The selected topics are divided into four sections: biological prototypes, mechanism and construction rules, fabrication, and applications of metal-based CMDSP surfaces. Finally, the challenges and future development trends in bionic CMDSP surfaces are envisioned, especially the utilization of potential bionic inspiration in the design of more advanced CMDSP surfaces.

TiO2 nanowire-templated hierarchical nanowire network as water-repelling coating

Extraordinary water-repelling properties of superhydrophobic surfaces make them novel candidates for a great variety of potential applications. A general approach to achieve superhydrophobicity requires low-energy coating on the surface and roughness on nano- and micrometre scale. However, typical construction of superhydrophobic surfaces with micro-nano structure through top-down fabrication is restricted by sophisticated fabrication techniques and limited choices of substrate materials. Micro-nanoscale topographies templated by conventional microparticles through surface coating may produce large variations in roughness and uncontrollable defects, resulting in poorly controlled surface morphology and wettability. In this work, micro-nanoscale hierarchical nanowire network was fabricated to construct self-cleaning coating using one-dimensional TiO2 nanowires as microscale templates. Hierarchical structure with homogeneous morphology was achieved by branching ZnO nanowires on the TiO2 nanowire backbones through hydrothermal reaction. The hierarchical nanowire network displayed homogeneous micro/nano-topography, in contrast to hierarchical structure templated by traditional microparticles. This hierarchical nanowire network film exhibited high repellency to both water and cell culture medium after functionalization with fluorinated organic molecules. The hierarchical structure templated by TiO2 nanowire coating significantly increased the surface superhydrophobicity compared to vertical ZnO nanowires with nanotopography alone. Our results demonstrated a promising strategy of using nanowires as microscale templates for the rational design of hierarchical coatings with desired superhydrophobicity that can also be applied to various substrate materials.

Bioinspired Surfaces with Superwettability for Anti-Icing and Ice-Phobic Application: Concept, Mechanism, and Design

Ice accumulation poses a series of severe issues in daily life. Inspired by the nature, superwettability surfaces have attracted great interests from fundamental research to anti-icing and ice-phobic applications. Here, recently published literature about the mechanism of ice prevention is reviewed, with a focus on the anti-icing and ice-phobic mechanisms, encompassing the behavior of condensate microdrops on the surface, wetting, ice nucleation, and freezing. Then, a detailed account of the innovative fabrication and fundamental research of anti-icing materials with special wettability is summarized with a focus on recent progresses including low-surface energy coatings and liquid-infused layered coatings. Finally, special attention is paid to a discussion about advantages and disadvantages of the technologies, as well as factors that affect the anti-icing and ice-phobic efficiency. Outlooks and the challenges for future development of the anti-icing and ice-phobic technology are presented and discussed.

Immobilization of Pt Nanoparticles via Rapid and Reusable Electropolymerization of Dopamine on TiO2 Nanotube Arrays for Reversible SERS Substrates and Nonenzymatic Glucose Sensors

Inspired by mussel-adhesion phenomena in nature, polydopamine (PDA) coatings are a promising route to multifunctional platforms for decorating various materials. The typical self-polymerization process of dopamine is time-consuming and the coatings of PDA are not reusable. Herein, a reusable and time-saving strategy for the electrochemical polymerization of dopamine (EPD) is reported. The PDA layer is deposited on vertically aligned TiO₂ nanotube arrays (NTAs). Owing to the abundant catechol and amine groups in the PDA layer, uniform Pt nanoparticles (NPs) are deposited onto the TiO₂ NTAs and can effectively prevent the recombination of electron-hole pairs generated from photo-electrocatalysis and transfer the captured electrons to participate in the photo-electrocatalytic reaction process. Compared with pristine TiO₂ NTAs, the as-prepared Pt@TiO₂ NTA composites exhibit surface-enhanced Raman scattering sensitivity for detecting rhodamine 6G and display excellent UV-assisted self-cleaning ability, and also show promise as a nonenzymatic glucose biosensor. Furthermore, the mussel-inspired electropolymerization strategy and the fast EPD-reduced nanoparticle decorating process presented herein can be readily extended to various functional substrates, such as conductive glass, metallic oxides, and semiconductors. It is the adaptation of the established PDA system for a selective, robust, and generalizable sensing system that is the emphasis of this work.

Robust electrochemical metal oxide deposition using an electrode with a superhydrophobic surface

Described herein is the first study of metal oxide deposition on an electrode with a solid/liquid/gas triphase reaction interface. Such an electrode enables the formation of a locally high interface pH value that is independent of the bulk conditions, and allows the deposition of a wide variety of metal oxides in a robust electrolyte. The results reveal that the control of electrode surface wettability is of significant importance for the regulation of the electrode-electrolyte local interface environment, which provides a new set of guidelines and perspective for future electrode preparation and application.