Superaerophobicity: repellence of air bubbles from submerged, surface-engineered silicon substrates

Locomotion behavior of air bubbles on solid surfaces

Air bubbles are a common occurrence in both natural and industrial settings and are a significant topic in the fields of physics, chemistry, engineering, and medicine. The physical phenomena of the contact between bubbles and submerged solid surfaces, as well as the locomotion behavior of bubbles, are worth exploring. Bubbles are generated in an unbounded liquid environment and rise due to unbalanced external forces. Bubbles of different diameters follow different ascending paths, after which they approach, touch, collide, bounce, and finally adsorb to the solid surface, forming a stable three-phase contact line (TPCL). The bubbles are in an unstable state due to the unbalanced external forces on the solid surface and the effects generated by the two-phase contact surface, resulting in different locomotion behaviors on the solid surface. Studying the formation, transport, aggregation, and rupture behaviors of bubbles on solid surfaces can enable the controllable operation of bubbles. This, in turn, can effectively reduce the loss of mechanical apparatus in agro-industrial production activities and improve corresponding production efficiency. Recent research has shown that the degree of bubble wetting on a solid surface is a crucial factor in the locomotion behavior of bubbles on that surface. This has led to significant progress in the study of bubble wetting, which has in turn greatly advanced our understanding of bubble behavior. Based on this, exploring the manipulation process of the directional motion of bubbles is a promising research direction. The locomotion behavior of bubbles on solid surfaces can be controlled by changing external conditions, leading to the integration of bubble behavior in various scientific and technological fields. Studying the dynamics of bubbles in liquids with infinite boundaries is worthwhile. Additionally, the manipulation process and mode of these bubbles is a popular research direction.

Self-healable, Tolerant Superaerophobic Coating for Improving Electrochemical Hydrogen Production

Gas-evolving electrodes often suffer from the blocking of catalytic active sites-due to unwanted and unavoidable adhesion of generated gas bubbles, which elevates the overpotential for the electrochemical hydrogen evolution reaction (HER)- by raising the resistance of the electrode. Here, a catalyst-free and self-healable superaerophobic coating having ultra-low bubble adhesion is introduced for achieving significantly depleted overpotentials of 209 and 506 mV at both low (50 mA cm-2) and high (500 mA cm-2) current densities, respectively, compared to a bare nickel-foam electrode. The optimized coating ensured an early detachment of the generated tiny (0.8 ± 0.1 mm) gas bubble-and thus, prevented the undesired rise in resistance of the coated electrode. The systematic association of physical (i.e., ionic interactions, H-bonding, etc.) cross-linkage, β-amino ester type covalent cross-linkage and reinforced halloysite nano clay enables the design of such functional material embedded with essential characteristics-including improved mechanical (toughness of 63.7 kJ m-3, and tensile modulus of 26 kPa) property and chemical (extremes of pH (1 and 14), salinity, etc.) stability, rapid (<10 min) self-healing ability (even at alkaline condition) and desired bubble-wettability (bubble contact angle of 158.2 ± 0.2°) with ultralow force (4.2 ± 0.4 µN) of bubble adhesion.

Bioinspired Gas Manipulation for Regulating Multiphase Interactions in Electrochemistry

The manipulation of gas in multiphase interactions plays a crucial role in various electrochemical processes. Inspired by nature, researchers have explored bioinspired strategies for regulating these interactions, leading to remarkable advancements in design, mechanism, and applications. This paper provides a comprehensive overview of bioinspired gas manipulation in electrochemistry. It traces the evolution of gas manipulation in gas-involving electrochemical reactions, highlighting the key milestones and breakthroughs achieved thus far. The paper then delves into the design principles and underlying mechanisms of superaerophobic and (super)aerophilic electrodes, as well as asymmetric electrodes. Furthermore, the applications of bioinspired gas manipulation in hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO2RR), and other gas-involving electrochemical reactions are summarized. The promising prospects and future directions in advancing multiphase interactions through gas manipulation are also discussed.

Emerging Separation Applications of Surface Superwettability

Human beings are facing severe global environmental problems and sustainable development problems. Effective separation technology plays an essential role in solving these challenges. In the past decades, superwettability (e.g., superhydrophobicity and underwater superoleophobicity) has succeeded in achieving oil/water separation. The mixture of oil and water is just the tip of the iceberg of the mixtures that need to be separated, so the wettability-based separation strategy should be extended to treat other kinds of liquid/liquid or liquid/gas mixtures. This review aims at generalizing the approach of the well-developed oil/water separation to separate various multiphase mixtures based on the surface superwettability. Superhydrophobic and even superoleophobic surface microstructures have liquid-repellent properties, making different liquids keep away from them. Inspired by the process of oil/water separation, liquid polymers can be separated from water by using underwater superpolymphobic materials. Meanwhile, the underwater superaerophobic and superaerophilic porous materials are successfully used to collect or remove gas bubbles in a liquid, thus achieving liquid/gas separation. We believe that the diversified wettability-based separation methods can be potentially applied in industrial manufacture, energy use, environmental protection, agricultural production, and so on.

Precursor film formation on catalyst–electrolyte–gas boundaries during CO2 electroreduction with gas diffusion electrodes

Thin and long layers of catholyte precursor films spread near triple-phase boundaries on composite catalysts containing hydrophobic materials. Dissolved CO2 molecules in the precursor films reduce on the composite catalyst surface without depletion.

Revisiting the supplementary relationship of dynamic contact angles measured by sessile-droplet and captive-bubble methods: Role of surface roughness

HYPOTHESIS

Quantitative characterization of surface wettability through contact angle (CA) measurement using the sessile droplet (SD) or captive bubble (CB) methods is often limited by the intrinsic wetting properties of the substrate. Situations may arise when an extreme surface wettability may preclude using one of the two methods for predicting the behaviors of droplets or bubbles on the surface. This warrants a relationship between the dynamic CAs measured via the SD and CB methods. While the two dynamic CAs (e.g., the advancing CA of SD and receding CA of CB) add up to 180° on a smooth surface, the simple geometric supplementary principle may not apply for rough surfaces.

EXPERIMENTS

We perform a systematic wettability characterization of solid substrates with varying degrees of roughness using the sessile-droplet and captive-bubble methods, and interpret the experimental observations using a theoretical model.

FINDINGS

The dynamic contact angles measured by the sessile-droplet and captive-bubble methods deviate from the supplementary principle as the surface roughness is increased. We present a theoretical explanation for this disparity and predict the values of the contact angles using prevalent thermodynamic models of wetting and contact-angle hysteresis on rough substrates. The theoretical prediction is in good agreement with the experimental observations.

Wettability-Engineered Meshes for Gas Microvolume Precision Handling in Liquids

The interaction of rising gas bubbles with submerged air-repelling or air-attracting surfaces is relevant to various technological applications that rely on gas-microvolume handling or removal. This work demonstrates how submerged metal meshes with super air-attracting/repelling properties can be employed to manipulate microvolumes of air, rising buoyantly in the form of bubbles in water. Superaerophobic meshes are observed to selectively allow the passage of air bubbles depending on the mesh pore size, the bubble volume-equivalent diameter, and the bubble impact velocity on the mesh. On the other hand, superaerophilic meshes reduce or amplify the volume captured from a train of incoming bubbles. Finally, a spatial wettability pattern on the mesh is used to control the size of the outgoing bubble, and an empirical relation is formulated to predict the released gas volume. The study demonstrates how porous materials with controlled wettability can be used to precisely modulate and control the outcome of bubble/mesh interactions.

Superaerophobic hydrogels for enhanced electrochemical and photoelectrochemical hydrogen production

The efficient removal of gas bubbles in (photo)electrochemical gas evolution reactions is an important but underexplored issue. Conventionally, researchers have attempted to impart bubble-repellent properties (so-called superaerophobicity) to electrodes by controlling their microstructures. However, conventional approaches have limitations, as they are material specific, difficult to scale up, possibly detrimental to the electrodes' catalytic activity and stability, and incompatible with photoelectrochemical applications. To address these issues, we report a simple strategy for the realization of superaerophobic (photo)electrodes via the deposition of hydrogels on a desired electrode surface. For a proof-of-concept demonstration, we deposited a transparent hydrogel assembled from M13 virus onto (photo)electrodes for a hydrogen evolution reaction. The hydrogel overlayer facilitated the elimination of hydrogen bubbles and substantially improved the (photo)electrodes' performances by maintaining high catalytic activity and minimizing the concentration overpotential. This study can contribute to the practical application of various types of (photo)electrochemical gas evolution reactions.

Thermally Driven Interfacial Switch between Adhesion and Antiadhesion on Gas Bubbles in Aqueous Media

It is greatly important to understand the gas bubble behaviors and realize their reliable manipulation. There still remain many challenges in the capture, transport, and release of gas bubbles at a preferred location intelligently. Herein, we provide a simple approach to manufacture a composite film with poly(N-isopropylacrylamide) (PNIPAAM) and polypropylene that exhibits smart, reversible, and reliable regulation of gas bubble adhesive behaviors (high and low adhesion) by controlling the temperature (above or below the lower critical solution temperature). By adjusting the composite surface temperature, thermally driven interface switching between intermolecular and intramolecular hydrogen bonding of the PNIPAAM chains resulted in low and high adhesion of air bubbles in an aqueous medium. Gas bubbles could be precisely captured, directionally transported, and precisely released at any preferred location.

Wetting transitions on rough surfaces revealed with captive bubble experiments. The role of surface energy

HYPOTHESIS

Wettability of solid surfaces is mostly probed with sessile drops rather than bubbles because this method is readily followed out. This recurrent use may lead to a misleading connection of certain phenomena to the hydrophobicity/hydrophilicity of materials. For instance, the Cassie-Baxter regime and the wicking effect are generally associated only to hydrophobic and hydrophilic surfaces, respectively. However, the same phenomenology should be observed when air bubbles (underwater conditions) in contact with solid surfaces are used instead. In particular, one might expect that rough-hydrophilic surfaces become superaerophobic due to the appearance of a hybrid dewetting regime, like the Cassie-Baxter regime described for rough-hydrophobic surfaces. Otherwise, rough-hydrophobic surfaces might become superaerophilic due to air-wicking.

EXPERIMENTS

To elucidate this issue, in this work, we analyzed the wettability of surfaces with very different intrinsic contact angle and roughness degree. The analysis was performed with both Sessile Drop and Captive Bubble methods.

FINDINGS

Our results with captive bubbles for rough-hydrophilic surfaces revealed phenomena only explained by the occurrence of a transition from the Wenzel regime to an "inverse" Cassie-Baxter regime. In addition, our results with captive bubbles for rough-hydrophobic surfaces showed evidences of air percolation through the interconnected asperities. This effect reminds the wicking effect reproduced on rough-hydrophilic surfaces, responsible for superhydrophilicity.

Underwater Superaerophobic and Superaerophilic Nanoneedles-Structured Meshes for Water/Bubbles Separation: Removing or Collecting Gas Bubbles in Water

Water/bubbles separation has great practical significance, which can avoid the harm caused by underwater bubbles or cleverly collect useful gas bubbles in water. Here, a Cu(OH)2-nanoneedles-structured rough copper mesh is fabricated by a one-step chemical reaction. The original rough mesh shows superhydrophilicity in air and superaerophobicity in water. The underwater superaerophobic mesh has great anti/removing-bubbles ability in water. In contrast, the rough mesh switches to superhydrophobicity in air and superaerophilicity in water after further being modified with fluoroalkylsilane. The underwater superaerophilic mesh can absorb bubbles and allow bubbles to pass through the mesh. Based on the superhydrophilic/superaerophobic mesh and the superhydrophobic/superaerophilic mesh, a strategy to remove gas bubbles from the water pipe is proposed, and an in-water bubbles-collection device is also designed. It is believed that these two kinds of mesh will have more applications in controlling the behavior of underwater bubbles.

Superwettability of Gas Bubbles and Its Application: From Bioinspiration to Advanced Materials

Gas bubbles in aqueous media are common and inevitable in, for example, agriculture and industrial processes. The behaviors of gas bubbles on solid interfaces, including generation, growth, coalescence, release, transport, and collection, are crucial to gas-bubble-related applications, which are always determined by gas-bubble wettability on solid interfaces. Here, the recent progress regarding the study of interfaces with gas-bubble superwettability in aqueous media, i.e., superaerophilicity and superaerophobicity, is summarized. Some examples illustrate how to design microstructures and chemical compositions to achieve reliable and effective manipulation of gas-bubble wettability on artificial interfaces. These designed interfaces exhibit excellent performance in gas-evolution reactions, gas-adsorption reactions, and directional gas-bubble transportation. Moreover, progress in the theoretical investigation of gas-bubble superwettability is reported. Lastly, some challenges are presented, such as the reliable manipulation of gas-bubble wettability and the establishment of mature theory for exactly and systematically explaining gas-bubble wetting phenomena.

Superaerophilic Carbon-Nanotube-Array Electrode for High-Performance Oxygen Reduction Reaction

A micro-/nanostructured "superaerophilic" electrode constructed by direct growth of cobalt-incorporated and nitrogen-doped carbon-nanotube arrays with subsequent hydrophobic modification is demonstrated for a high-performance oxygen-reduction-reaction electrode, superior to the Pt/C-air electrode. This high performance is attributed to the simultaneously accelerated gas-diffusion and electron-transport processes induced by the unique structural advantages.

High-Performance Water Electrolysis System with Double Nanostructured Superaerophobic Electrodes

Catalysts screening and structural optimization are both essential for pursuing a high-efficient water electrolysis system (WES) with reduced energy supply. This study demonstrates an advanced WES with double superaerophobic electrodes, which are achieved by constructing a nanostructured NiMo alloy and NiFe layered double hydroxide (NiFe-LDH) films for hydrogen evolution and oxygen evolution reactions, respectively. The superaerophobic property gives rise to significantly reduced adhesion forces to gas bubbles and thereby accelerates the hydrogen and oxygen bubble releasing behaviors. Benefited from these metrics and the high intrinsic activities of catalysts, this WES affords an early onset potential (≈1.5 V) for water splitting and ultrafast catalytic current density increase (≈0.83 mA mV(-1) ), resulting in ≈2.69 times higher performance compared to the commercial Pt/C and IrO2 /C catalysts based counterpart under 1.9 V. Moreover, enhanced performance at high temperature as well as prominent stability further demonstrate the practical application of this WES.

Bioinspired TiO₂ nanostructure films with special wettability and adhesion for droplets manipulation and patterning

Patterned surfaces with special wettability and adhesion (sliding, sticky or patterned superoleophobic surface) can be found on many living creatures. They offer a versatile platform for microfluidic management and other biological functions. Inspired by their precise arrangement of structure and chemical component, we described a facile one-step approach to construct large scale pinecone-like anatase TiO₂ particles (ATP) film. The as-prepared ATP film exhibits excellent superamphiphilic property in air, changes to underwater superoleophobicity with good dynamical stability. In addition, erasable and rewritable patterned superamphiphobic ATP films or three-dimensional (3D) Janus surfaces were constructed for a versatile platform for microfluidic management and biomedical applications. In a proof-of-concept study, robust super-antiwetting feet for artificial anti-oil strider at the oil/water interface, novel superamphiphobic surface for repeatable oil/water separation, and multifunctional patterned superamphiphobic ATP template for cell, fluorecent probe and inorganic nanoparticles site-selective immobilization were demonstrated.