Designing Plastrons for Underwater Bubble Capture: From Model Microstructures to Stochastic Nanostructures.

Bubbles and foams are often removed via chemical defoamers and/or mechanical agitation. Designing surfaces that promote chemical-free and energy-passive bubble capture is desirable for numerous industrial processes, including mineral flotation, wastewater treatment, and electrolysis. When immersed, super-liquid-repellent surfaces form plastrons, which are textured solid topographies with interconnected gas domains. Plastrons exhibit the remarkable ability of capturing bubbles through coalescence. However, the two-step mechanics of plastron-induced bubble coalescence, namely, rupture (initiation and location) and subsequent absorption (propagation and drainage) are not well understood. Here, the influence of 1) topographical feature size and 2) gas fraction on bubble capture dynamics is investigated. Smaller feature sizes accelerate rupture while larger gas fractions markedly improve absorption. Rupture is initiated solely on solid domains and is more probable near the edges of solid features. Yet, rupture time becomes longer as solid fraction increases. This counterintuitive behavior represents unexpected complexities. Upon rupture, the bubble's moving liquid-solid contact line influences its absorption rate and equilibrium state. These findings show the importance of rationally minimizing surface feature sizes and contact line interactions for rapid bubble rupture and absorption. This work provides key design principles for plastron-induced bubble coalescence, inspiring future development of industrially-relevant surfaces for underwater bubble capture.

Ultrasonic Healing of Plastrons.

Superhydrophobic surfaces (SHS) exhibit a pronounced ability to resist wetting. When immersed in water, water does not penetrate between the microstructures of the SHS. Instead, a thin layer of trapped gas remains, i.e., plastron. This fractional wetting is also known as the Cassie-Baxter state (CB). Impairment of superhydrophobicity occurs when water penetrates the plastron and, when complete wetting is achieved, a Wenzel state (W) results. Subsequent recovery back to CB state is one of the main challenges in the field of SHS wetting. Current methods for plastron recovery require complex mechanical or chemical integration, are time-consuming or lack spatial control. Here an on-demand, contact-less approach for performing facile transitions between these wetting states at micrometer length scales is proposed. This is achieved by the use of acoustic radiation force (ARF) produced by high-intensity focused ultrasound (HIFU). Switching from CB to W state takes <100 µs, while the local recovery back to CB state takes <45 s. To the best of authors knowledge, this is the first demonstration of ARF-induced manipulation of the plastron enabling facile two-way controlled switching of wetting states.

Wetting on silicone surfaces.

Silicone is frequently used as a model system to investigate and tune wetting on soft materials. Silicone is biocompatible and shows excellent thermal, chemical, and UV stability. Moreover, the mechanical properties of the surface can be easily varied by several orders of magnitude in a controlled manner. Polydimethylsiloxane (PDMS) is a popular choice for coating applications such as lubrication, self-cleaning, and drag reduction, facilitated by low surface energy. Aiming to understand the underlying interactions and forces, motivated numerous and detailed investigations of the static and dynamic wetting behavior of drops on PDMS-based surfaces. Here, we recognize the three most prevalent PDMS surface variants, namely liquid-infused (SLIPS/LIS), elastomeric, and liquid-like (SOCAL) surfaces. To understand, optimize, and tune the wetting properties of these PDMS surfaces, we review and compare their similarities and differences by discussing (i) the chemical and molecular structure, and (ii) the static and dynamic wetting behavior. We also provide (iii) an overview of methods and techniques to characterize PDMS-based surfaces and their wetting behavior. The static and dynamic wetting ridge is given particular attention, as it dominates energy dissipation, adhesion, and friction of sliding drops and influences the durability of the surfaces. We also discuss special features such as cloaking and wetting-induced phase separation. Key challenges and opportunities of these three surface variants are outlined.

Polarity-Induced Reactive Wetting: Spreading and Retracting Sessile Water Drops.

Wetting is typically defined by the relative liquid to solid surface tension/energy, which are composed of polar and nonpolar subcontributions. Current studies often assume that they remain invariant, that is, surfaces are wetting-inert. Complex wetting scenarios, such as adaptive or reactive wetting processes, may involve time-dependent variations in interfacial energies. To maximize differences in energetic states, we employ low-energy perfluoroalkyls integrated with high-energy silica-based polar moieties grown on low-energy polydimethylsiloxane. To this end, we tune the hydrophilic-like wettability on these perfluoroalkyl-silica-polydimethylsiloxane surfaces. Drop contact behaviors range from invariantly hydrophobic at ca. 110° to rapidly spreading at ca. 0° within 5 s. Unintuitively, these vapor-grown surfaces transit toward greater hydrophilicity with increasing perfluoroalkyl deposition. Notably, this occurs as sequential silica-and-perfluoroalkyl deposition also leaves behind embedded polar moieties. We highlight how surfaces having such chemical heterogeneity are inherently wetting-reactive. By creating an abrupt wetting transition composed of reactive and inert domains, we introduce spatial dependency. Drops contacting the transition spread before retracting, occurring over the time scale of a few seconds. This phenomenon contradicts current understanding, exhibiting a uniquely (1) decreasing advancing contact angle and (2) increasing receding contact angle. To explain the behavior, we model such time- and space- dependent reactive wetting using first order kinetics. In doing so, we explore how reactive and recovery mechanisms govern the characteristic time scales of spreading and retracting sessile drops.

Design of Fluoro-Free Surfaces Super-Repellent to Low-Surface-Tension Liquids.

Super-liquid-repellent surfaces feature high liquid contact angles and low sliding angles find key applications in anti-fouling and self-cleaning. While repellency for water is easily achieved with hydrocarbon functionalities, repellency for many low-surface-tension liquids (down to 30 mN m-1 ) still requires perfluoroalkyls (a persistent environmental pollutant and bioaccumulation hazard). Here, the scalable room-temperature synthesis of stochastic nanoparticle surfaces with fluoro-free moieties is investigated. Silicone (dimethyl and monomethyl) and hydrocarbon surface chemistries are benchmarked against perfluoroalkyls, assessed using model low-surface-tension liquids (ethanol-water mixtures). It is discovered that both hydrocarbon- and dimethyl-silicone-based functionalization can achieve super-liquid-repellency down to 40-41 mN m-1 and 32-33 mN m-1 , respectively (vs 27-32 mN m-1 for perfluoroalkyls). The dimethyl silicone variant demonstrates superior fluoro-free liquid repellency likely due to its denser dimethyl molecular configuration. It is shown that perfluoroalkyls are not necessary for many real-world scenarios requiring super-liquid-repellency. Effective super-repellency of different surface chemistries against different liquids can be adequately predicted using empirically verified phase diagrams. These findings encourage a liquid-centric design, i.e., tailoring surfaces for target liquid properties. Herein, key guidelines are provided for achieving functional yet sustainably designed super-liquid-repellency.

Tuning the Charge of Sliding Water Drops.

When a water drop slides over a hydrophobic surface, it usually acquires a positive charge and deposits the negative countercharge on the surface. Although the electrification of solid surfaces induced after contact with a liquid is intensively studied, the actual mechanisms of charge separation, so-termed slide electrification, are still unclear. Here, slide electrification is studied by measuring the charge of a series of water drops sliding down inclined glass plates. The glass was coated with hydrophobic (hydrocarbon/fluorocarbon) and amine-terminated silanes. On hydrophobic surfaces, drops charge positively while the surfaces charge negatively. Hydrophobic surfaces coated with a mono-amine (3-aminopropyltriethyoxysilane) lead to negatively charged drops and positively charged surfaces. When coated with a multiamine (N-(3-trimethoxysilylpropyl)diethylenetriamine), a gradual transition from positively to negatively charged drops is observed. We attribute this tunable drop charging to surface-directed ion transfer. Some of the protons accepted by the amine-functionalized surfaces (-NH2 with H+ acceptor) remain on the surface even after drop departure. These findings demonstrate the facile tunability of surface-controlled slide electrification.

Cryofouling avoidance in the Antarctic scallop Adamussium colbecki.

The presence of supercooled water in polar regions causes anchor ice to grow on submerged objects, generating costly problems for engineered materials and life-endangering risks for benthic communities. The factors driving underwater ice accretion are poorly understood, and passive prevention mechanisms remain unknown. Here we report that the Antarctic scallop Adamussium colbecki appears to remain ice-free in shallow Antarctic marine environments where underwater ice growth is prevalent. In contrast, scallops colonized by bush sponges in the same microhabitat grow ice and are removed from the population. Characterization of the Antarctic scallop shells revealed a hierarchical micro-ridge structure with sub-micron nano-ridges which promotes directed icing. This concentrates the formation of ice on the growth rings while leaving the regions in between free of ice, and appears to reduce ice-to-shell adhesion when compared to temperate species that do not possess highly ordered surface structures. The ability to control the formation of ice may enable passive underwater anti-icing protection, with the removal of ice possibly facilitated by ocean currents or scallop movements. We term this behavior cryofouling avoidance. We posit that the evolution of natural anti-icing structures is a key trait for the survival of Antarctic scallops in anchor ice zones.

Frost spreading and pattern formation on microstructured surfaces.

Frost is found in nature as a symphony of nucleation and heat and mass transport, cascading from angstroms to several meters. Here, we use laser-induced fluorescence microscopy to investigate the pattern formation of frost growth in experiments which tune the mesoscopic length scale by using microstructured pillar arrays as a frost condenser surface. By controlling the degree of surface supercooling and the amount of condensate, different modes of frost patterning are uncovered, ranging from complete surface coverage to fractal-looking and limited-coverage structures of spiky appearance.

Super liquid repellent surfaces for anti-foaming and froth management.

Wet and dry foams are prevalent in many industries, ranging from the food processing and commercial cosmetic sectors to industries such as chemical and oil-refining. Uncontrolled foaming results in product losses, equipment downtime or damage and cleanup costs. To speed up defoaming or enable anti-foaming, liquid oil or hydrophobic particles are usually added. However, such additives may need to be later separated and removed for environmental reasons and product quality. Here, we show that passive defoaming or active anti-foaming is possible simply by the interaction of foam with chemically or morphologically modified surfaces, of which the superamphiphobic variant exhibits superior performance. They significantly improve retraction of highly stable wet foams and prevention of growing dry foams, as quantified for beer and aqueous soap solution as model systems. Microscopic imaging reveals that amphiphobic nano-protrusions directly destabilize contacting foam bubbles, which can favorably vent through air gaps warranted by a Cassie wetting state. This mode of interfacial destabilization offers untapped potential for developing efficient, low-power and sustainable foam and froth management.

Wetting-regulated gas-involving (photo)electrocatalysis: biomimetics in energy conversion.

(Photo)electrolysis of water or gases with water to species serving as industrial feedstocks and energy carriers, such as hydrogen, ammonia, ethylene, propanol, etc., has drawn tremendous attention. Moreover, these processes can often be driven by renewable energy under ambient conditions as a sustainable alternative to traditional high-temperature and high-pressure synthesis methods. In addition to the extensive studies on catalyst development, increasing attention has been paid to the regulation of gas transport/diffusion behaviors during gas-involving (photo)electrocatalytic reactions towards the goal of creating industrially viable catalytic systems with high reaction rates, excellent long-term stabilities and near-unity selectivities. Biomimetic surfaces and systems with special wetting capabilities and structural advantages can shed light on the future design of (photo)electrodes and address long-standing challenges. This article is dedicated to bridging the fields of wetting and catalysis by reviewing the cutting-edge design methodologies of both gas-evolving and gas-consuming (photo)electrocatalytic systems. We first introduce the fundamentals of various in-air/underwater wetting states and their corresponding bioinspired structural properties. The relationship amongst the bubble transport behavior, wettability, and porosity/tortuosity is also discussed. Next, the latest implementations of wetting-related design principles for gas-evolving reactions (i.e. the hydrogen evolution reaction and oxygen evolution reaction) and gas-consuming reactions (i.e. the oxygen reduction reaction and CO2 reduction reaction) are summarized. For photoelectrode designs, additional factors are taken into account, such as light absorption and the separation, transport and recombination of photoinduced electrons and holes. The influences of wettability and 3D structuring of (photo)electrodes on the catalytic activity, stability and selectivity are analyzed to reveal the underlying mechanisms. Finally, remaining questions and related future perspectives are outlined.

Ultrafast Bubble Bursting by Superamphiphobic Coatings.

Controlling bubble motion or passively bursting bubbles using solid interfaces is advantageous in numerous industrial applications including flotation, catalysis, electrochemical processes, and microfluidics. Current research has explored the formation, dissolution, pinning, and rupturing of bubbles on different surfaces. However, the ability to tune and control the rate of bubble bursting is not yet achieved. Scaling down surface-induced bubble bursting to just a few milliseconds is important for any application. In this work, the hierarchical structure of superamphiphobic surfaces is tuned in order to rapidly rupture contacting bubbles. Surfaces prepared using liquid flame spray show ultrafast bubble bursting (down to 2 ms) and superior durability. The coatings demonstrate excellent mechanical and chemical stability even in the presence of surface-active species. Air from the ruptured bubble is absorbed into the aerophilic Cassie-state. Long-term applicability is demonstrated by preventing the accumulation of air in the plastron via a connection of the plastron to the environment. The times recorded for bubble rupture and complete reorganization of air are reduced by approximately a factor of 3 compared to previously reported values. The concept is utilized to passively control surfactant-rich foam in froth flotation. Material collection efficiency increased by more than 60 times compared to controls.

The Effect of Surface Entropy on the Heat of Non-Wetting Liquid Intrusion into Nanopores.

On-demand access to renewable and environmentally friendly energy sources is critical to address current and future energy needs. To achieve this, the development of new mechanisms of efficient thermal energy storage (TES) is important to improve the overall energy storage capacity. Demonstrated here is the ideal concept that the thermal effect of developing a solid-liquid interface between a non-wetting liquid and hydrophobic nanoporous material can store heat to supplement current TES technologies. The fundamental macroscopic property of a liquid's surface entropy and its relationship to its solid surface are one of the keys to predict the magnitude of the thermal effect by the development of the liquid-solid interface in a nanoscale environment-driven through applied pressure. Demonstrated here is this correlation of these properties with the direct measurement of the thermal effect of non-wetting liquids intruding into hydrophobic nanoporous materials. It is shown that the model can resonably predict the heat of intrusion into rigid mesoporous silica and some microporous zeolite when the temperature dependence of the contact angle is applied. Conversely, intrusion into flexible microporous metal-organic frameworks requires further improvement. The reported results with further development have the potential to lead to the development of a new supplementary method and mechanim for TES.

How Frost Forms and Grows on Lubricated Micro- and Nanostructured Surfaces.

Frost is ubiquitously observed in nature whenever warmer and more humid air encounters colder than melting point surfaces (e.g., morning dew frosting). However, frost formation is problematic as it damages infrastructure, roads, crops, and the efficient operation of industrial equipment (i.e., heat exchangers, cooling fins). While lubricant-infused surfaces offer promising antifrosting properties, underlying mechanisms of frost formation and its consequential effect on frost-to-surface dynamics remain elusive. Here, we monitor the dynamics of condensation frosting on micro- and hierarchically structured surfaces (the latter combines micro- with nano- features) infused with lubricant, temporally and spatially resolved using laser scanning confocal microscopy. The growth dynamics of water droplets differs for micro- and hierarchically structured surfaces, by hindered drop coalescence on the hierarchical ones. However, the growth and propagation of frost dendrites follow the same scaling on both surface types. Frost propagation is accompanied by a reorganization of the lubricant thin film. We numerically quantify the experimentally observed flow profile using an asymptotic long-wave model. Our results reveal that lubricant reorganization is governed by two distinct driving mechanisms, namely: (1) frost propagation speed and (2) frost dendrite morphology. These in-depth insights into the coupling between lubricant flow and frost formation/propagation enable an improved control over frosting by adjusting the design and features of the surface.

How a water drop removes a particle from a hydrophobic surface.

To understand the removal of particles from surfaces by water drops, we used an inverted laser scanning confocal microscope to image the collision between a water drop and a particle on a flat polydimethylsiloxane (PDMS) surface. The dynamic drop-particle contact line was monitored by fixing the drop directly above the objective lens while moving the sample stage at well-defined speeds (10-500 µm s-1). The lateral force acting on the drop during the collision was measured as a function of speed, using a force sensor mounted on the microscope. Depending on the collision speed, the particle either stays attached at the rear of the drop or detaches from it. We propose a criterion to determine whether the particle remains attached to the drop based on the capillary and resistive forces acting on the particle during the collision. The forces measured when the particle crosses the air-water interface are compared to existing models. We adapted these to account for rolling of the particle. By comparing our experimental measurements with an analytical model for the capillary torque acting on a particle rolling at an interface, we provide detailed insights on the origins of the resistive force acting on the particle when it is pushed or pulled by the drop. A low friction force between the surface and the particle increases the likelihood of particle removal.

The challenge of lubricant-replenishment on lubricant-impregnated surfaces.

Lubricant-impregnated surfaces are two-component surface coatings. One component, a fluid called the lubricant, is stabilized at a surface by the second component, the scaffold. The scaffold can either be a rough solid or a polymeric network. Drops immiscible with the lubricant, hardly pin on these surfaces. Lubricant-impregnated surfaces have been proposed as candidates for various applications, such as self-cleaning, anti-fouling, and anti-icing. The proposed applications rely on the presence of enough lubricant within the scaffold. Therefore, the quality and functionality of a surface coating are, to a large degree, given by the extent to which it prevents lubricant-depletion. This review summarizes the current findings on lubricant-depletion, lubricant-replenishment, and the resulting understanding of both processes. A multitude of different mechanisms can cause the depletion of lubricant. Lubricant can be taken along by single drops or be sheared off by liquid flowing across. Nano-interstices and scaffolds showing good chemical compatibility with the lubricant can greatly delay lubricant depletion. Often, depletion of lubricant cannot be avoided under dynamic conditions, which warrants lubricant-replenishment strategies. The strategies to replenish lubricant are presented and range from spraying or stimuli-responsive release to built-in reservoirs.

Capillary Balancing: Designing Frost-Resistant Lubricant-Infused Surfaces.

Slippery lubricant-infused surfaces (SLIPS) have shown great promise for anti-frosting and anti-icing. However, small length scales associated with frost dendrites exert immense capillary suction pressure on the lubricant. This pressure depletes the lubricant film and is detrimental to the functionality of SLIPS. To prevent lubricant depletion, we demonstrate that interstitial spacing in SLIPS needs to be kept below those found in frost dendrites. Densely packed nanoparticles create the optimally sized nanointerstitial features in SLIPS (Nano-SLIPS). The capillary pressure stabilizing the lubricant in Nano-SLIPS balances or exceeds the capillary suction pressure by frost dendrites. We term this concept capillary balancing. Three-dimensional spatial analysis via confocal microscopy reveals that lubricants in optimally structured Nano-SLIPS are not affected throughout condensation (0 °C), extreme frosting (-20 °C to -100 °C), and traverse ice-shearing (-10 °C) tests. These surfaces preserve low ice adhesion (10-30 kPa) over 50 icing cycles, demonstrating a design principle for next-generation anti-icing surfaces.

Adaptive Wetting of Polydimethylsiloxane.

To better understand the wetting of cross-linked polydimethylsiloxane (PDMS), we measured advancing and receding contact angles of sessile water drops on cross-linked PDMS as a function of contact line velocity (up to 100 µm/s). Three types of samples were investigated: pristine PDMS, PDMS where oligomers were removed by toluene treatment, and PDMS with an enriched concentration of oligomers. Depending on the velocity of advancing contact lines and the contact time with water, different modes of wetting were observed: one with a relatively low contact angle hysteresis (Δθ ≈ 10°) and one with a larger hysteresis. We attribute the low hysteresis state, called the lubricated state, to the enrichment of free oligomers at the water-PDMS interface. The enrichment of oligomers is induced by drop contact. The kinetics of the transition to the lubricated state can be described by adaptation theory. PDMS adapts to the presence of water by an enrichment of free oligomers at the interface and a correlated reduction in interfacial tension.

Microdroplet Contaminants: When and Why Superamphiphobic Surfaces Are Not Self-Cleaning.

Superamphiphobic surfaces are commonly associated with superior anticontamination and antifouling properties. Visually, this is justified by their ability to easily shed off drops and contaminants. However, on micropillar arrays, tiny droplets are known to remain on pillars' top faces while the drop advances. This raises the question of whether remnants remain even on nanostructured superamphiphobic surfaces. Are superamphiphobic surfaces really self-cleaning? Here we investigate the presence of microdroplet contaminants on three nanostructured superamphiphobic surfaces. After brief contact with liquids having different volatilities and surface tension (water, ethylene glycol, hexadecane, and an ionic liquid), confocal microscopy reveals a "blanket-like" layer of microdroplets remaining on the surface. It appears that the phenomenon is universal. Notably, when placing subsequent drops onto the contaminated surface, they are still able to roll off. However, adhesion forces can gradually increase by up to 3 times after repeated liquid drop contact. Therefore, we conclude that superamphiphobic surfaces do not warrant self-cleaning and anticontamination capabilities at sub-micrometric length scales.

Surface Chemistry Enhancements for the Tunable Super-Liquid Repellency of Low-Surface-Tension Liquids.

Super-hydrophobic, super-oleo(amphi)phobic, and super-omniphobic materials are universally important in the fields of science and engineering. Despite rapid advancements, gaps of understanding still exist between each distinctive wetting state. The transition of super-hydrophobicity to super-(oleo-, amphi-, and omni-)phobicity typically requires the use of re-entrant features. Today, re-entrant geometry induced super-(amphi- and omni-)phobicity is well-supported by both experiments and theory. However, owing to geometrical complexities, the concept of re-entrant geometry forms a dogma that limits the industrial progress of these unique states of wettability. Moreover, a key fundamental question remains unanswered: are extreme surface chemistry enhancements able to influence super-liquid repellency? Here, this was rigorously tested via an alternative pathway that does not require explicit designer re-entrant features. Highly controllable and tunable vertical network polymerization and functionalization were used to achieve fluoroalkyl densification on nanoparticles. For the first time, relative fluoro-functionalization densities are quantitatively tuned and correlated to super-liquid repellency performance. Step-wise tunable super-amphiphobic nanoparticle films with a Cassie-Baxter state (contact angle of >150° and sliding angle of <10°) against various liquids is demonstrated. This was tested down to very low surface tension liquids to a minimum of ca. 23.8 mN/m. Such findings could eventually lead to the future development of super-(amphi)omniphobic materials that transcend the sole use of re-entrant geometry.

Cassie-Levitated Droplets for Distortion-Free Low-Energy Solid-Liquid Interactions.

Despite the rapid advent of superomniphobic materials, there is a lack of methodologies to accurately investigate the ultralow-energy interactions taking place on these interfaces. For instance, universally employed models such as the pendant droplet often fail to provide representative information on the wetting properties of superomniphobic surfaces. The delicate balance between the forces acting at the droplet-surface and droplet-needle interfaces can easily result in heavily distorted droplet profiles. Here, we introduce a Cassie-levitating droplet model which overcomes the limitations of the pendant droplet model, allowing a distortion-free assessment of the interactions between super(amphi)omniphobic materials and low surface tension liquids. Comparative analysis in wetting of low surface tension fluids such as hexadecane (∼27.47 mN/m) on superamphiphobic surfaces via the Cassie-levitating and pendant droplet models reveals up to 70° (800%) deviations in the estimated contact angle hysteresis. A theoretical framework is developed to assess experimentally observed profile distortions against ideal gravity-induced sagging of droplet shapes during dynamic droplet expansion and contraction cycles. Notably, pendant droplets resulted in up to 50% distortion while the Cassie-levitating ones achieved less than just 10%. We believe that the Cassie-levitating droplet model bears ample potential for the characterization of the rapidly emerging family of superomniphobic materials, setting the basis for their future engineering in numerous emerging applications.