Sustaining dry surfaces under water

Recent Advances in Superhydrophobic Materials Development for Maritime Applications

Underwater superhydrophobic surfaces stand as a promising frontier in materials science, holding immense potential for applications in underwater infrastructure, vehicles, pipelines, robots, and sensors. Despite this potential, widespread commercial adoption of these surfaces faces limitations, primarily rooted in challenges related to material durability and the stability of the air plastron during prolonged submersion. Factors such as pressure, flow, and temperature further complicate the operational viability of underwater superhydrophobic technology. This comprehensive review navigates the evolving landscape of underwater superhydrophobic technology, providing a deep dive into the introduction, advancements, and innovations in design, fabrication, and testing techniques. Recent breakthroughs in nanotechnology, magnetic-responsive coatings, additive manufacturing, and machine learning are highlighted, showcasing the diverse avenues of progress. Notable research endeavors concentrate on enhancing the longevity of plastrons, the fundamental element governing superhydrophobic behavior. The review explores the multifaceted applications of superhydrophobic coatings in the underwater environment, encompassing areas such as drag reduction, anti-biofouling, and corrosion resistance. A critical examination of commercial offerings in the superhydrophobic coating landscape offers a current perspective on available solutions. In conclusion, the review provides valuable insights and forward-looking recommendations to propel the field of underwater superhydrophobicity toward new dimensions of innovation and practical utility.

Fabrication of Salvinia-inspired surfaces for hydrodynamic drag reduction by capillary-force-induced clustering

For decades, bioinspired functional materials have been attracting the interest of many researchers for their remarkable characteristics. In particular, some plant leaves are well known for their inherent superhydrophobic nature. Salvinia molesta, a free-floating aquatic fern, has egg-beater-shaped hierarchical trichomes on its surface of leaves. Due to the unique structure and complex wettability of the hairs, this plant has the ability to maintain a stable thick air layer upon the structure when it is submerged underwater. Often referred to as the “Salvinia Effect,” this property is expected to be suitable for use in hydrodynamic drag reduction. However, due to the complex shape of the trichome, currently applied fabrication methods are using a three-dimensional printing system, which is not applicable to mass production because of its severely limited productivity. In this work, artificial Salvinia leaf inspired by S. molesta was fabricated using a conventional soft lithography method assisted with capillary-force-induced clustering of micropillar array. The fabrication method suggested in this work proposes a promising strategy for the manufacturing of Salvinia-inspired hydrodynamic drag reduction surfaces.

Salvinia molesta plant has the ability to maintain a stable air layer when submerged underwater due to its specific form. The authors propose here a soft lithography fabrication method of artificial Salvinia leaf assisted with capillary-force induced clustering of micropillar array, for hydrodynamic drag reduction.

An Analysis of Bubble Migration in Horizontal Thermo-Capillarity Using the VOF Modeling

Due to various engineering applications, spontaneous bubble movement on the heated surface has brought huge attention. This work numerically studied the bubble migration driven by the thermo-capillary force under the temperature gradients perpendicular to the gravity direction. This problem is constructed in a two-dimensional domain, and the volume of fluid (VOF) method is adopted to capture the properties of the bubble interface between the vapor and the liquid. One still vapor bubble is initially positioned at the center of the liquid domain, and the temperature gradient is applied to two side walls. The results show that the bubble with a size greater than the capillary length can only oscillate near the initial position even with a larger temperature gradient. The deformation of the bubble such as spheroid and spherical cap can be found around this regime. However, the movement of the bubble with a size smaller than the capillary length is significant under a higher temperature gradient, and it remains a spherical shape. The coefficient of thermo-capillary force (CTh) is defined within this work, and it is found that a larger Weber number (We) accomplishes a larger CTh. This work may provide more precise guidance for smart bubble manipulation and critical heat flux estimation for future nuclear reactor design.

Effect of Protein Adsorption on Air Plastron Behavior of a Superhydrophobic Surface

Protein fouling on critical biointerfaces causes significant public health and clinical ramifications. Multiple strategies, including superhydrophobic (SHP) surfaces and coatings, have been explored to mitigate protein adsorption on solid surfaces. SHP materials with underwater air plastron (AP) layers hold great promise by physically reducing the contact area between a substrate and protein molecules. However, sustaining AP stability or lifetime is crucial in determining the durability and long-term applications of SHP materials. This work investigated the effect of protein on the AP stability using model SHP substrates, which were prepared from a mixture of silica nanoparticles and epoxy. The AP stability was determined using a submersion test with real-time visualization. The results showed that AP stability was significantly weakened by protein solutions compared to water, which could be attributed to the surface tension of protein solutions and protein adsorption on SHP substrates. The results were further examined to reveal the correlation between protein fouling and accelerated AP dissipation on SHP materials by confocal fluorescent imaging, surface energy measurement, and surface robustness modeling of the Cassie-Baxter to Wenzel transition. The study reveals fundamental protein adsorption mechanisms on SHP materials, which could guide future SHP material design to better mitigate protein fouling on critical biointerfaces.

How Multilayered Feathers Enhance Underwater Superhydrophobicity

Inspired by ducks, we demonstrate that air pockets within stacked layers of porous superhydrophobic feathers can withstand up to five times more water pressure compared to a single feather. In addition to natural duck feathers, this "layer effect" was replicated with synthetic feathers created by laser cutting micrometric slots into aluminum foil and imparting a superhydrophobic nanostructure. It was revealed that adding layers promotes an increasingly redundant pathway for water impalement, which serves to pressurize the enclosed air pockets. This was validated by creating a probabilistic pore impalement model and also by filling the feathers with an incompressible oil, rather than air, to suppress the layer effect. In addition to revealing a utility of natural duck feathers, our findings suggest that multilayered engineered surfaces can maintain air pockets at high pressures, useful for reducing the drag and fouling of marine structures or enhancing desalination membranes.

Novel Omniphobic Platform for Multicellular Spheroid Generation, Drug Screening, and On-Plate Analysis

Multicellular spheroids are superior to other culture geometries in reproducing critical physiological conditions of tumors, such as the diffusion of oxygen, nutrients, waste, and drugs, leading to a more precise model of in vivo drug sensitivity and resistance. Previously reported spheroid culture platforms are often difficult to use, expensive, single-use, or mechanically unstable. Here, we report a facile, mechanically stable, high-throughput spheroid culture platform based on hierarchically textured omniphobic surfaces. The developed omniphobic surfaces display very high contact angles with a range of different liquids, including the cell-laden culture media, thereby minimizing the cell surface contact area. Additionally, these surfaces maintain these high contact angles for extended periods of time to ensure cell aggregation. Using this novel platform, we demonstrate the generation and maintenance of robust multicellular spheroids, as well as heterogeneous, multicell-type spheroids. The platform is extremely robust, resistant to mechanical shock, allows for on-plate imaging, and is also the first-ever spheroid generation platform that can be reused repeatedly. Finally, the platform is suitable for on-plate drug screening and enables the first-ever, on-plate immunofluorescence staining and imaging of spheroids.

Synthesis and characterization of MWCNT-covered stainless steel mesh with Janus-type wetting properties

Various multi-step methods to fabricate Janus membranes have been reported in literature. However, no article so far reports the durability of the Janus membranes when exposed to liquids. We report on a novel method to fabricate a Janus-type multi-walled carbon nanotubes (MWCNT)-covered stainless steel (SS) mesh, which retains dual-wetting properties even after exposure to water for 540 d. The MWCNTs are grown directly on stainless steel mesh coupons by chemical vapor deposition using acetylene as the carbon source, and are then plasma functionalized using an ammonia-ethylene gas mixture to achieve dual-wettability. We found by x-ray photoelectron spectroscopy that the MWCNTs on the top face of the novel Janus MWCNT-SS mesh, which was directly exposed to the plasma, are coated by a plasma polymer rich in nitrogen-containing functional groups, while the MWCNTs on the bottom face are almost devoid of the plasma polymer coating. Atomic force microscopy studies confirmed that the surface roughness of the bottom face of the mesh is lower than the minimum roughness that allows the capillary ingress of water to sustain its superhydrophobic behavior. In addition, scanning electron microscopy studies also confirmed that the MWCNTs on the bottom face of the treated MWCNT mesh are vertically aligned compared to the MWCNTs on the top face of the mesh. The vertically aligned dense MWCNT forest on the bottom face attributes to its superhydrophobic nature.

Nature inspired solid-liquid phase amphibious adhesive

Here we report a new class of bio-inspired solid-liquid adhesive, obtained by simple mechanical dispersion of PVDF (polyvinylidene fluoride) (solid spheres) into PDMS (polydimethylsiloxane) (liquid). The adhesive behavior arises from strong solid-liquid interactions. This is a chemical reaction free adhesive (no curing time) that can be repeatedly used and is capable of instantaneously joining a large number of diverse materials (metals, ceramic, and polymer) in air and underwater. The current work is a significant advance in the development of amphibious multifunctional adhesives and presents potential applications in a range of sealing applications, including medical ones.

Anisotropic Spreading of Bubbles on Superaerophilic Straight Trajectories beneath a Slide in Water

Although the bubble contacting a uniformly superaerophilic surface has caused concern due to its application potential in various engineering equipment, such as mineral flotation, very little is known about the mechanism of how the bubble spreads on a surface with anisotropic superaerophilicity. To unveil this mystery, we experimentally studied the anisotropic behavior of a bubble (2 mm in diameter) spreading on the superaerophilic straight trajectories (SALTs) of different widths (0.5 mm–5 mm) in water using a high-speed shadowgraphy system. The 1–3 bounces mostly happened as the bubble approached the SALTs before its spreading. It is first observed that the bubble would be split into two highly symmetrical sub-bubbles with similar migration velocity in opposite directions during the anisotropic spreading. Two essential mechanisms are found to be responsible for the anisotropic spreading on the narrow SALTs (W ≤ 2 mm with two subregimes) and the wide SALTs (W ≥ 3 mm with four subregimes). Considering the combined effect of the surface tension effect of SALT and Laplace pressure, a novel model has been developed to predict the contact size r(t) as a function of time. The nice agreement between this model and our experiments reconfirms that the surface tension effect and Laplace pressure prevail over the hydrostatic pressure.

Role of Scale Wettability on Rain-Harvesting Behavior in a Desert-Dwelling Rattlesnake

During storms in the southwestern United States, several rattlesnake species have been observed drinking rain droplets collected on their dorsal scales. This process often includes coiling and flattening of the snake's body, presumably to enhance water collection. Here, we explored this rain-harvesting behavior of the Western Diamond-backed Rattlesnake (Crotalus atrox) from the perspective of surface science. Specifically, we compared surface wettability and texture, as well as droplet impact and evaporation dynamics on the rattlesnake epidermis with those of two unrelated (control) sympatric snake species (Desert Kingsnake, Lampropeltis splendida, and Sonoran Gopher Snake, Pituophis catenifer). These two control species are not known to show rain-harvesting behavior. Our results show that the dorsal scales of the rattlesnake aid in water collection by providing a highly sticky, hydrophobic surface, which pins the impacting water droplets. We show that this high pinning characteristic stems from surface nanotexture made of shallow, labyrinth-like channels.

Superhydrophobic and superaerophilic hierarchical Pt@MIL-101/PVDF composite for hydrogen water isotope exchange reactions

A hierarchical porous composite of Pt@MIL-101/ployvinylidene fluoride (Pt@MIL-101/PVDF) was successfully prepared through a solution-processed method. This composite possesses advanced superhydrophobic and superaerophilic performance which makes it a promising catalyst facilitating liquid phase catalytic exchange techniques (LPCE) in hydrogen-water isotope exchange process. Its superhydrophobic property results in the repellence of water drops from flooding the catalytic surface with a relatively large contact angle in the exchange reaction, and its superaerophilic surface broke hydrogen bubbles into thin film so as to reach higher catalytic reactive efficiency. High reactivity and long-term stability in the reaction process can also be achieved by the configuration of mesoporous cages of MIL-101 confining Pt nanoparticles and preventing them from sintering.

Bubble Formation in a Finite Cone: More Pieces to the Puzzle

We investigate the stability of bubble formation, starting with a convex or a concave meniscus, from a liquid solution (of water and a dissolved gas) inside a finite cone at constant temperature and constant liquid pressure (above the saturation pressure of the pure solvent). It is assumed that the dissolved gas (nitrogen) forms a dilute solution at equilibrium, which can be described by Henry's law. The number and nature of equilibrium states are determined with Gibbsian composite-system thermodynamics, both from the intersection of the equilibrium Kelvin radius with the geometry radius and from the extrema in the plot of free energy of the system versus size of the new phase. Bubble stability is studied along the whole growth path, as the bubble grows inside, gets pinned, and grows further outside the finite cone. The changes in the concentration of the liquid bulk phase and the vapor phase during the growth of the bubble are carefully incorporated in the equations. The effects of various parameters, including cone apex angle, cone half mouth radius, contact angle, total number of moles, and initial degree of saturation, on the stability of the bubble are also investigated. Stability of bubble formation from a liquid solution inside a confined geometry such as a finite cone is of interest in areas such as restoring underwater superhydrophobicity and adhesion of particles to the roughness of synthetic biomaterials.

Hemocompatibility of Super-Repellent surfaces: Current and Future

Virtually all blood-contacting medical implants and devices initiate immunological events in the form of thrombosis and inflammation. Typically, patients receiving such implants are also given large doses of anticoagulants, which pose a high risk and a high cost to the patient. Thus, the design and development of surfaces with improved hemocompatibility and reduced dependence on anticoagulation treatments is paramount for the success of blood-contacting medical implants and devices. In the past decade, the hemocompatibility of super-repellent surfaces (i.e., surfaces that are extremely repellent to liquids) has been extensively investigated because such surfaces greatly reduce the blood-material contact area, which in turn reduces the area available for protein adsorption and blood cell or platelet adhesion, thereby offering the potential for improved hemocompatibility. In this review, we critically examine the progress made in characterizing the hemocompatibility of super-repellent surfaces, identify the unresolved challenges and highlight the opportunities for future research on developing medical implants and devices with super-repellent surfaces.

Interaction of blood plasma proteins with superhemophobic titania nanotube surfaces

The need to improve blood biocompatibility of medical devices is urgent. As soon as blood encounters a biomaterial implant, proteins adsorb on its surfaces, often leading to several complications such as thrombosis and failure of the device. Therefore, controlling protein adsorption plays a major role in developing hemocompatible materials. In this study, the interaction of key blood plasma proteins with superhemophobic titania nanotube substrates and the blood clotting responses was investigated. The substrate stability was evaluated and fibrinogen adsorption and thrombin formation from plasma were assessed using ELISA. Whole blood clotting kinetics was also investigated, and Factor XII activation on the substrates was characterized by an in vitro plasma coagulation time assay. The results show that superhemophobic titania nanotubes are stable and considerably decrease surface protein adsorption/Factor XII activation as well as delay the whole blood clotting, and thus can be a promising approach for designing blood contacting medical devices.

Plastron Regeneration on Submerged Superhydrophobic Surfaces Using In Situ Gas Generation by Chemical Reaction

Superhydrophobic surfaces submerged under water appear shiny due to total internal reflection of light from a thin layer of air (plastron) trapped in their surface texture. This entrapped air is advantageous for frictional drag reduction in various applications ranging from microfluidic channels to marine vessels. However, these aerophilic textures are prone to impregnation by water due to turbulent pressure fluctuations from external flows and dissolution of the trapped gas into the water. We demonstrate a novel chemical method to replenish the plastron in situ by using the decomposition reaction of hydrogen peroxide on superhydrophobic surfaces prepared with a catalytic coating. We also provide a thermodynamic framework for designing superhydrophobic surfaces with optimal texture and chemistry for underwater plastron regeneration. We finally demonstrate the practical utility of this method by fabricating periodic microtextures on aluminum surfaces that incorporate a cheap catalyst, manganese dioxide. We perform drag-reduction experiments under turbulent flow conditions in a Taylor-Couette cell (TC cell), which show that more than half of the drag increase ensuing from plastron collapse can be recovered spontaneously by injection of dilute H₂O₂ into the TC cell. Thus, we present a low-cost, scalable method to enable in situ plastron regeneration on large surfaces for marine applications.

Biomimetic coating-free surfaces for long-term entrapment of air under wetting liquids

Trapping air at the solid-liquid interface is a promising strategy for reducing frictional drag and desalting water, although it has thus far remained unachievable without perfluorinated coatings. Here, we report on biomimetic microtextures composed of doubly reentrant cavities (DRCs) and reentrant cavities (RCs) that can enable even intrinsically wetting materials to entrap air for long periods upon immersion in liquids. Using SiO2/Si wafers as the model system, we demonstrate that while the air entrapped in simple cylindrical cavities immersed in hexadecane is lost after 0.2 s, the air entrapped in the DRCs remained intact even after 27 days (~106 s). To understand the factors and mechanisms underlying this ten-million-fold enhancement, we compared the behaviors of DRCs, RCs and simple cavities of circular and non-circular shapes on immersion in liquids of low and high vapor pressures through high-speed imaging, confocal microscopy, and pressure cells. Those results might advance the development of coating-free liquid repellent surfaces.

Drag crisis moderation by thin air layers sustained on superhydrophobic spheres falling in water

We investigate the effect of thin air layers naturally sustained on superhydrophobic surfaces on the terminal velocity and drag force of metallic spheres free falling in water. The surface of 20 mm to 60 mm steel or tungsten-carbide spheres is rendered superhydrophobic by a simple coating process that uses a commercially available hydrophobic agent. By comparing the free fall of unmodified spheres and superhydrophobic spheres in a 2.5 meter tall water tank, it is demonstrated that even a very thin air layer (∼1-2 μm) that covers the freshly dipped superhydrophobic sphere can reduce the drag force on the spheres by up to 80%, at Reynolds numbers from 10⁵ to 3 × 10⁵, owing to an early drag crisis transition. This study complements prior investigations on the drag reduction efficiency of model gas layers sustained on heated metal spheres falling in liquid by the Leidenfrost effect. The drag reduction effects are expected to have significant implications for the development of sustainable air-layer-based energy saving technologies.

A numerical investigation of the effect of surface wettability on the boiling curve

Surface wettability is recognized as playing an important role in pool boiling and the corresponding heat transfer curve. In this work, a systematic study of pool boiling heat transfer on smooth surfaces of varying wettability (contact angle range of 5° − 180°) has been conducted and reported. Based on numerical simulations, boiling curves are calculated and boiling dynamics in each regime are studied using a volume-of-fluid method with contact angle model. The calculated trends in critical heat flux and Leidenfrost point as functions of surface wettability are obtained and compared with prior experimental and theoretical predictions, giving good agreement. For the first time, the effect of contact angle on the complete boiling curve is shown. It is demonstrated that the simulation methodology can be used for studying pool boiling and related dynamics and providing more physical insights.

Friction and Wetting Transitions of Magnetic Droplets on Micropillared Superhydrophobic Surfaces

Reliable characterization of wetting properties is essential for the development and optimization of superhydrophobic surfaces. Here, the dynamics of superhydrophobicity is studied including droplet friction and wetting transitions by using droplet oscillations on micropillared surfaces. Analyzing droplet oscillations by high-speed camera makes it possible to obtain energy dissipation parameters such as contact angle hysteresis force and viscous damping coefficients, which indicate pinning and viscous losses, respectively. It is shown that the dissipative forces increase with increasing solid fraction and magnetic force. For 10 µm diameter pillars, the solid fraction range within which droplet oscillations are possible is between 0.97% and 2.18%. Beyond the upper limit, the oscillations become heavily damped due to high friction force. Below the lower limit, the droplet is no longer supported by the pillar tops and undergoes a Cassie-Wenzel transition. This transition is found to occur at lower pressure for a moving droplet than for a static droplet. The findings can help to optimize micropillared surfaces for low-friction droplet transport.

Doubly Reentrant Cavities Prevent Catastrophic Wetting Transitions on Intrinsically Wetting Surfaces

Omniphobic surfaces, that is, which repel all known liquids, have proven of value in applications ranging from membrane distillation to underwater drag reduction. A limitation of currently employed omniphobic surfaces is that they rely on perfluorinated coatings, increasing cost and environmental impact and preventing applications in harsh environments. Thus, there is a keen interest in rendering conventional materials, such as plastics, omniphobic by micro/nanotexturing rather than via chemical makeup, with notable success having been achieved for silica surfaces with doubly reentrant micropillars. However, we found a critical limitation of microtextures comprising pillars that they undergo catastrophic wetting transitions (apparent contact angles, θ_r → 0° from θ_r > 90°) in the presence of localized physical damages/defects or on immersion in wetting liquids. In response, a doubly reentrant cavity microtexture is introduced, which can prevent catastrophic wetting transitions in the presence of localized structural damage/defects or on immersion in wetting liquids. Remarkably, our silica surfaces with doubly reentrant cavities could exhibit apparent contact angles, θ_r ≈ 135° for mineral oil, where the intrinsic contact angle, θ_o ≈ 20°. Further, when immersed in mineral oil or water, doubly reentrant microtextures in silica (θ_o ≈ 40° for water) were not penetrated even after several days of investigation. Thus, microtextures comprising doubly reentrant cavities might enable applications of conventional materials without chemical modifications, especially in scenarios that are prone to localized damages or immersion in wetting liquids, for example, hydrodynamic drag reduction and membrane distillation.

The Thermodynamics of Restoring Underwater Superhydrophobicity

Superhydrophobic surfaces submerged in liquids are susceptible to permanently becoming wet. This is especially true when the ambient liquid is pressurized or undersaturated with air. To gain insight into the thermodynamics of restoring underwater superhydrophobicity, nucleation theory is applied to the design of spontaneously dewetting conical pores. It is found that, for intrinsically hydrophobic materials, there is a geometric constraint for which reversible superhydrophobic behavior may occur. Molecular dynamics simulations are implemented to support the theory, and steered molecular dynamics simulations are used to investigate the energy landscape of the dewetting process. The results of this work have implications for the efficacy of underwater superhydrophobicity and enhanced nucleation sites for boiling heat transfer.

The impact of surface geometry, cavitation, and condensation on wetting transitions: posts and reentrant structures

The fundamental impacts of surface geometry on the stability of wetting states, and the transitions between them are elucidated for square posts and reentrant structures in three dimensions. We identify three principal outcomes of particular importance for future surface design of liquid-repellent surfaces. Firstly, we demonstrate and quantify how capillary condensation and vapour cavitation affect wetting state stabilities. At high contact angles, cavitation is enhanced about wide, closely-spaced square posts, leading to the existence of suspended states without an associated collapsed state. At low contact angles, narrow reentrant pillars suppress condensation and enable the suspension of even highly wetting liquids. Secondly, two distinct collapse mechanisms are observed for 3D reentrant geometries, base contact and pillar contact, which are operative at different pillar heights. As well as morphological differences in the interface of the penetrating liquid, each mechanism is affected differently by changes in the contact angle with the solid. Finally, for highly-wetting liquids, condensates are shown to critically modify the transition pathways in both the base contact and pillar contact modes.

Effect of Flow and Particle-Plastron Collision on the Longevity of Superhydrophobicity

Among diverse methods for drag reduction, superhydrophobicity has shown considerable promise because it can produce a shear-free boundary without energy input. However, the plastron experiences a limited lifetime due to the dissolution of trapped air from surface cavities, into the surrounding water. The underwater longevity of the plastron, as it is influenced by environmental conditions, such as fine particles suspended in the water, must be studied in order to implement superhydrophobicity in practical applications. We present a proof-of-concept study on the kinetics of air loss from a plastron subjected to a canonical laminar boundary layer at Reδ = 1400 and 1800 (based on boundary layer thickness) with and without suspending 2 micron particles with density of 4 Kg/m3. To monitor the air loss kinetics, we developed an in situ non-invasive optical technique based on total internal reflection at the air-water interface. The shear flow at the wall is characterized by high resolution particle image velocimetry technique. Our results demonstrate that the flow-induced particle-plastron collision shortens the lifetime of the plastron by ~50%. The underlying physics are discussed and a theoretical analysis is conducted to further characterize the mass transfer mechanisms.

Sustaining Superheated Liquid within Hydrophilic Surface Texture

During pool boiling of water, it is advantageous to keep liquid touching the surface in order to delay the onset of filmwise boiling. This allows water to remain in the nucleate boiling regime, leading to increased heat transfer. In this work, we propose a mechanism to sustain superheated liquid within hydrophilic pores. This mechanism for the design of superwetting hydrophilic surfaces does not rely on the transport of vapor and offers an additional pathway for wetting via the condensation of vapor within the surface texture. We adapt nucleation theory to design the surface geometry and implement molecular dynamics simulations to verify this concept. Simulation results are consistent with theory and demonstrate superheated liquid residing within the surface texture.

Spreading of Bubbles after Contacting the Lower Side of an Aerophilic Slide Immersed in Water

While the dynamics of complete wetting has been widely studied for liquids, the way a gas spreads on a solid is by far less known. We report here the events following the rise of a millimeter-size air bubble towards a textured material immersed in water and covered by a thin plastron of air. Bubbles contact the material either directly at the end of the rise, or after a few rebounds, which affects the initial shape of the bubble and the resulting dynamics of contact. Then, air spreads on the material, owing to surface tension and later buoyance, which tends to flatten further the bubble. The corresponding dynamics are shown to result from the inertial resistance of water, which explains how spreading bubbles reach centimeter sizes in typically 10 ms.

Bioinspired surfaces for turbulent drag reduction

In this review, we discuss how superhydrophobic surfaces (SHSs) can provide friction drag reduction in turbulent flow. Whereas biomimetic SHSs are known to reduce drag in laminar flow, turbulence adds many new challenges. We first provide an overview on designing SHSs, and how these surfaces can cause slip in the laminar regime. We then discuss recent studies evaluating drag on SHSs in turbulent flow, both computationally and experimentally. The effects of streamwise and spanwise slip for canonical, structured surfaces are well characterized by direct numerical simulations, and several experimental studies have validated these results. However, the complex and hierarchical textures of scalable SHSs that can be applied over large areas generate additional complications. Many studies on such surfaces have measured no drag reduction, or even a drag increase in turbulent flow. We discuss how surface wettability, roughness effects and some newly found scaling laws can help explain these varied results. Overall, we discuss how, to effectively reduce drag in turbulent flow, an SHS should have: preferentially streamwise-aligned features to enhance favourable slip, a capillary resistance of the order of megapascals, and a roughness no larger than 0.5, when non-dimensionalized by the viscous length scale.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

Thermodynamics of Trapping Gases for Underwater Superhydrophobicity

Rough surfaces submerged in a liquid can remain almost dry if the liquid does not fully wet the roughness, and gases are sustained in roughness grooves. Such partially dry surfaces can help reduce drag, enhance boiling, and reduce biofouling. Gases sustained in roughness grooves would be composed of air and the vapor phase of the liquid itself. In this work, the thermodynamics of sustaining gases (e.g., air) is considered. Governing equations are presented along with a solution methodology to determine a critical condition to sustain gases. The critical roughness scale to sustain gases is estimated for different degrees of saturation of gases dissolved in the liquid. It is shown that roughness spacings of less than a micron are essential to sustain gases on surfaces submerged in water at atmospheric pressure. This is consistent with prior empirical data.

Low Ice Adhesion on Nano-Textured Superhydrophobic Surfaces under Supersaturated Conditions

Ice adhesion on superhydrophobic surfaces can significantly increase in humid environments because of frost nucleation within the textures. Here, we studied frost formation and ice adhesion on superhydrophobic surfaces with various surface morphologies using direct microscale imaging combined with macroscale adhesion tests. Whereas ice adhesion increases on microtextured surfaces, a 15-fold decrease is observed on nanotextured surfaces. This reduction is because of the inhibition of frost formation within the nanofeatures and the stabilization of vapor pockets. Such "Cassie ice"-promoting textures can be used in the design of anti-icing surfaces.