Thermodynamics of Trapping Gases for Underwater Superhydrophobicity

Plasma-Induced Superhydrophobicity as a Green Technology for Enhanced Air Gap Membrane Distillation

Superhydrophobicity has only recently become a requirement in membrane fabrication and modification. Superhydrophobic membranes have shown improved flux performance and scaling resistance in long-term membrane distillation (MD) operations compared to simply hydrophobic membranes. Here, we introduce plasma micro- and nanotexturing followed by plasma deposition as a novel, dry, and green method for superhydrophobic membrane fabrication. Using plasma micro- and nanotexturing, commercial membranes, both hydrophobic and hydrophilic, are transformed to superhydrophobic featuring water static contact angles (WSCA) greater than 150° and contact angle hysteresis lower than 10°. To this direction, hydrophobic polytetrafluoroethylene (PTFE) and hydrophilic cellulose acetate (CA) membranes are transformed to superhydrophobic. The superhydrophobic PTFE membranes showed enhanced water flux in standard air gap membrane distillation and more stable performance compared to the commercial ones for at least 48 h continuous operation, with salt rejection >99.99%. Additionally, their performance and high salt rejection remained stable, when low surface tension solutions containing sodium dodecyl sulfate (SDS) and NaCl (down to 35 mN/m) were used, showcasing their antiwetting properties. The improved performance is attributed to superhydrophobicity and increased pore size after plasma micro- and nanotexturing. More importantly, CA membranes, which are initially unsuitable for MD due to their hydrophilic nature (WSCA ≈ 40°), showed excellent performance with stable flux and salt rejection >99.2% again for at least 48 h, demonstrating the effectiveness of the proposed method for wetting control in membranes regardless of their initial wetting properties.

Second-Level Microgroove Convexity is Critical for Air Plastron Restoration on Immersed Hierarchical Superhydrophobic Surfaces

Air plastrons trapped on the surfaces of underwater superhydrophobic surfaces are critical for their function. Fibrillar morphologies offer a natural pathway, yet they are limited to a narrow range of liquid-surface systems and are vulnerable to pressure fluctuations that irreversibly destroy the air layer plastron. Inspired by the convexly grooved bases of water fern (Salvinia) leaves that support their fibrous outgrowths, we focus on the effect of such second-level grooved structures or microgrooves on the plastron restoration on immersed three-dimensional (3D)-printed hierarchical surfaces. Elliptical, interconnected microgrooves are fabricated with varying surface curvatures to study the effect of their morphology. Immersion experiments reveal that the convex groove curvature stabilizes a seed gas layer (SGL) that facilitates plastron restoration for all immersed hydrophobic surfaces. Theoretical calculations and atomic-scale computations reveal that the SGL storage capacity that sets the SGL robustness follows from the liquid menisci adaption to the groove geometry and pressure, from micro- to nanoscales, and it can be further tuned using separated grooves. Our study highlights groove convexity as a key morphological feature for the design of second-level architectures for underwater air plastron restoration on hierarchical superhydrophobic surfaces.

Design and applications of surfaces that control the accretion of matter

Surfaces that provide control over liquid, solid, or vapor accretion provide an evolutionary advantage to numerous plants, insects, and animals. Synthetic surfaces inspired by these natural surfaces can have a substantial impact on diverse commercial applications. Engineered liquid and solid repellent surfaces are often designed to impart control over a single state of matter, phase, or fouling length scale. However, surfaces used in diverse real-world applications need to effectively control the accrual of matter across multiple phases and fouling length scales. We discuss the surface design strategies aimed at controlling the accretion of different states of matter, particularly those that work across multiple length scales and different foulants. We also highlight notable applications, as well as challenges associated with these designer surfaces' scale-up and commercialization.

The challenges, achievements and applications of submersible superhydrophobic materials

Superhydrophobic materials have been widely reported throughout the scientific literature. Their properties originate from a highly rough morphology and inherently water repellent surface chemistry. Despite promising an array of functionalities, these materials have seen limited commercial development. This could be attributed to many factors, like material compatibility, low physical resilience, scaling-up complications, etc. In applications where persistent water contact is required, another limitation arises as a major concern, which is the stability of the air layer trapped at the surface when submerged or impacted by water. This review is aimed at examining the diverse array of research focused on monitoring/improving air layer stability, and highlighting the most successful approaches. The reported complexity of monitoring and enhancing air layer stability, in conjunction with the variety of approaches adopted, results in an assortment of suggested routes to achieving success. The review is addressing the challenge of finding a balance between maximising water repulsion and incorporating structures that protect air pockets from removal, along with challenges related to the variant approaches to testing air-layer stability across the research field, and the gap between the achieved progress and the required performance in real-life applications.

Temperature-regulated adhesion of impacting drops on nano/microtextured monostable superrepellent surfaces

The monostable Cassie state is a favorable wetting state for superhydrophobic materials, in which water drops can automatically transfer from the Wenzel wetting state to the Cassie wetting state, such that as a consequence the water repellency can be maintained. Drop impact phenomena are ubiquitous in nature and of critical importance in industry, and previous works show that the efficiency of self-cleaning and dropwise condensation could benefit from drop impact on monostable surfaces. However, whether such a feature is sufficiently robust remains unclear when the temperature of the surface is taken into consideration. Here, we report that there exists a lower bound of the temperature of the surface, under which a transition from the Cassie wetting state to the Wenzel wetting state arises. By varying the temperature of the surface, it is found that the solid-liquid wetting region could be regulated. Based on thermodynamics, we propose a model to predict the controllable wetting region, and we show that the gradual transition of the wetting state is a result of the accumulation of droplets on the nanoscale. Connections between the dynamics occurring at the solid-liquid interfaces on the microscale and the condensation occurring in the nanotextures are constructed. These results deepen our understanding of the breakdown of superhydrophobicity under dynamic impinging in high humidity. Moreover, this study will shed new light on the applications for controllable liquid deposition and surface decoration, such as catalysts on the superhydrophobic surfaces.

Robust Superhydrophobic Membrane for Membrane Distillation with Excellent Scaling Resistance

We report in this study a scalable and controllable approach for fabricating robust and high-performance superhydrophobic membranes for membrane distillation (MD). This novel approach combines electro-co-spinning/spraying (ES²) with chemical vapor welding and enables the formation of robust superhydrophobic (r-SH) membranes that are mechanically strong, highly porous, and robustly superhydrophobic. Compared with superhydrophobic membranes obtained using surface deposition of fluorinated nanoparticles, the r-SH membranes have more robust wetting properties and higher vapor permeability in MD. MD scaling experiments with sodium chloride and gypsum show that the r-SH membrane is highly effective in mitigating mineral scaling. Finally, we also discuss the mechanism of scaling resistance enabled by superhydrophobic membranes with a highlight on the roles of the surface-bound air layer in reducing the crystal-membrane contact area, nucleation propensity, and ion-membrane contact time.

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.

Tuning the Wettability of a Thin Polymer Film by Gradually Changing the Geometry of Nanoscale Pore Edges

Controlling wetting of solids by liquids attracts attention because of its scientific and technological importance. In this paper, the wettability of a highly uniform porous poly(methyl methacrylate) film on a silicon substrate containing a high density of randomly distributed self-similar pores was gradually tuned by changing the shape of nanometric crownlike structures around the pores. Fine-tuning the topography of these thin films was performed by isothermal annealing. The equilibrium contact angle of a water droplet placed on the surface of the films could be varied from 72 to 102°. The contact angle changes were assumed to be a consequence of changes in surface topography in the nanoscale. A simple method of a quantitative description of the change of the topography of these films was developed. Critical dimensions of these films were determined in horizontal and vertical directions relative to the surface plane. The slope coefficient (SC) describing how sharp the structures are, is defined as the ratio between the critical dimensions: the root-mean-square roughness σ and the autocorrelation length ξ. For SC > 0.08, the contact angle increased proportionally to the value of SC, whereas for SC < 0.08, the contact angle proportionally decreased. At the highest SC values, the contact angles were 6-10% higher than those predicted for flat porous surfaces using the Cassie-Baxter equation. We suggest that this discrepancy is due to the capillary tension caused by the submicron-scale undulation of the triple line, which was found to be proportional to the height of the crownlike pore edges and the value of SC. The same effect is responsible for the linear dependence of the contact angle on the SC value.

High-speed X-ray imaging of the Leidenfrost collapse

The Leidenfrost layer is characterized by an insulating vapor film between a heated surface and an ambient liquid. The collapse of this film has been canonically theorized to occur from an interfacial instability between the liquid and vapor phases. The interfacial instability alone, however, is insufficient to explain the known influence of the surface on the film collapse process. In this work, we provide visual evidence for two key mechanisms governing the film collapse: the interfacial instability, and the nucleation of vapor upon multiple non-terminal liquid-solid contacts. These results were obtained by implementing high-speed X-ray imaging of the film collapse on a heated sphere submerged in liquid-water. The X-ray images were synchronized with a second high-speed visible light camera and two thermocouples to provide insight into the film formation and film collapse processes. Lastly, the dynamic film thickness was quantified by analysis of the X-ray images. This helped assess the influence of surface roughness on the disruption of the film. The results of this work encourage further investigation into non-linear stability theory to consolidate the role of the surface on the liquid-vapor interface during the film collapse process.

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.

Ultimate Stable Underwater Superhydrophobic State

Underwater metastability hinders the durable application of superhydrophobic surfaces. In this work, through thermodynamic analysis, we theoretically demonstrate the existence of an ultimate stable state on underwater superhydrophobic surfaces. Such a state is achieved by the synergy of mechanical balance and chemical diffusion equilibrium across the entrapped liquid-air interfaces. By using confocal microscopy, we in situ examine the ultimate stable states on structured hydrophobic surfaces patterned with cylindrical micropores in different pressure and flow conditions. The equilibrium morphology of the meniscus is tuned by the dissolved gas saturation degree within a critical range at a given liquid pressure. Moreover, with fresh lotus leaves, we prove that the ultimate stable state can also be realized on randomly rough superhydrophobic surfaces. The finding here paves the way for applying superhydrophobic surfaces in environments with different liquid pressure and flow conditions.

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.

Vibrations and Spatial Patterns Change Effective Wetting Properties of Superhydrophobic and Regular Membranes

Small-amplitude fast vibrations and small surface micropatterns affect properties of various systems involving wetting, such as superhydrophobic surfaces and membranes. We review a mathematical method of averaging the effect of small spatial and temporal patterns. For small fast vibrations, this method is known as the method of separation of motions. The vibrations are substituted by effective force or energy terms, leading to vibration-induced phase control. A similar averaging method can be applied to surface micropatterns leading surface texture-induced phase control. We argue that the method provides a framework that allows studying such effects typical to biomimetic surfaces, such as superhydrophobicity, membrane penetration and others. Patterns and vibration can effectively jam holes and pores in vessels with liquid, separate multi-phase flow, change membrane properties, result in propulsion, and lead to many other multiscale, non-linear effects. Here, we discuss the potential application of these effects to novel superhydrophobic membranes.