Nanostructured Superhydrophobic Surfaces Research Articles

Understanding condensation halo growth on superhydrophobic surfaces: Insights from a vapor diffusive model

Anti-icing technologies are vital across various sectors, from transportation to energy systems. In this study, we investigate the formation of condensation halos during the process of condensation–freezing on superhydrophobic surfaces. Experimental tests were conducted on metallic nanostructured superhydrophobic surfaces, where droplet icing was induced and observed under controlled conditions. The formation of condensation halos, characterized by the sudden appearance and subsequent vanishing of microdroplets around the freezing droplets, was captured and analyzed. A vapor diffusive model coupled with heterogeneous nucleation theory was developed to understand and quantify the growth of condensation halos. The model considers vapor diffusion around the icing droplet and the critical vapor pressure required for nucleation. Experimental observations and theoretical predictions demonstrated a strong dependence of condensation halo size on the icing droplet radius. The study sheds light on the mechanisms underlying condensation halo formation and provides insights into the intricate interplay between droplet size, surface properties, and environmental conditions in condensation–freezing phenomena, offering valuable perspectives for the development of effective anti-icing strategies.

Freezing-Melting Mediated Dewetting Transition for Droplets on Superhydrophobic Surfaces with Condensation.

The water-repellence properties of superhydrophobic surfaces make them promising for many applications. However, in some extreme environments, such as high humidities and low temperatures, condensation on the surface is inevitable, which induces the loss of surface superhydrophobicity. In this study, we propose a freezing-melting strategy to achieve the dewetting transition from the Wenzel state to the Cassie-Baxter state. It requires freezing the droplet by reducing the substrate temperature and then melting the droplet by heating the substrate. The condensation-induced wetting transition from the Cassie-Baxter state to the Wenzel state is analyzed first. Two kinds of superhydrophobic surfaces, i.e., single-scale nanostructured superhydrophobic surface and hierarchical-scale micronanostructured superhydrophobic surface, are compared and their effects on the static contact states and impact processes of droplets are analyzed. The mechanism for the dewetting transition is analyzed by exploring the differences in the micro/nanostructures of the surfaces, and it is attributed to the unique structure and strength of the superhydrophobic surface. These findings will enrich our understanding of the droplet-surface interaction involving phase changes and have great application prospects for the design of superhydrophobic surfaces.

Successive rebounds of obliquely impinging water droplets on nanostructured superhydrophobic surfaces

The impact and rebound of water droplets on superhydrophobic surfaces frequently happen in nature and also in a number of industrial processes, which has thus stimulated strenuous efforts to explore the underlying hydrodynamics. Despite that massive achievements have been made over the past decades, existing works are mostly focusing on the short-time bouncing dynamics after a single impact; however, the long-term, successive droplet rebounds, which are practically more important, only received very limited attention. In this work, we perform an experimental investigation on the impact of water droplets on inclined nanostructured superhydrophobic surfaces at low Weber numbers, where massive complete rebounds arise. It was found that an obliquely impinging droplet would undergo many impacts on the superhydrophobic surface, accompanying with sliding on the surface, jumping in air, and complex shape evolutions. Based on the kinematic analyses, we demonstrate that the droplet motion on the surface can be decomposed into a perpendicular impact, which is dominated by the capillary and inertial forces, and a translational motion under the drive of gravity. By contrast, the jumping motion after droplet rebound is solely governed by the gravitational force, yet relevant droplet characteristics are affected by the energy loss during the impact on superhydrophobic surface, which sets the maximum height that the droplet rebounds to. In addition, three distinct shape evolution modes–namely, oscillation, rotation and their combination–were identified on jumping droplets, and the direction of a rotational droplet can be altered via the following impingement on the superhydrophobic surface.

Anti-Icing Property of Superhydrophobic Nanostructured Brass via Deposition of Silica Nanoparticles and Nanolaser Treatment.

Ice accumulation on brass surfaces can lead to heat transfer inefficiency, equipment degradation, and potential accidents. To address this issue, superhydrophobic surface technology is utilized. This work aims to develop superhydrophobic nanostructured brass surfaces using the combination of nanolaser ablation and the deposition of silica nanoparticles to achieve the anti-icing property. Four distinct types of brass surfaces namely, the bare surface (BS), the lasered surface (LS), the coated surface (CS), and the coated-lasered surface (CLS) were prepared. The anti-icing performances of the fabricated samples including the effects of the surface structure, the droplet size, and the surface temperature were investigated and evaluated. The results showed that the delayed icing time increased with the increases in the apparent contact angle, the droplet size, and the surface temperature. When the apparent contact angle increased, the contact area between the droplet and the cooling substrate reduced, leading to the longer delayed icing time. With the deposition of silica nanoparticles and nanolaser treatment, CLS achieved the greatest apparent contact angle of 164.5°, resulting in the longest delayed icing time under all experimental conditions. The longest delayed icing time on CLS recorded in this study was 2584 s, which was 575%, 356%, and 27% greater than those on BS, LS, and CS, respectively. The study also revealed that the surface structure played a more crucial role in achieving the anti-icing property when compared to the surface temperature or the droplet size. The shortest delayed icing time on CLS at the lowest surface temperature and at the smallest droplet size was longer than those on BS and LS at all conditions. The results were also discussed in relation to a heat transfer model. The findings of this research can serve as an avenue for advancing knowledge on heat transfer enhancement and energy efficiency.

Inhibition of condensation-induced droplet wetting by nano-hierarchical surfaces

Superhydrophobic nanostructured surfaces can enhance water condensation efficiency by facilitating droplet departure via coalescence-induced jumping. However, condensed droplets tend to transit from a mobile jumping mode to a highly pinned state at high condensation heat flux because excessive water nucleates within the nanostructures and anchors the condensed droplets. The large pinned droplets act as a thermal barrier and insulate the cooling surface, thus severely degrading its heat transfer efficiency. This work developed a nano-hierarchical structured surface by growing branched TiO2 nanorod arrays to prevent condensation-induced droplet pinning. After hydrophobization, the nano-hierarchical structure can spontaneously push the water out of nanostructures with an outward Laplace capillary pressure gradient when the droplet size is only at the nanoscale level. This effective de-wetting process maintains the high droplet mobility on the nano-hierarchical surface over a wide subcooling range, resulting in an up to ∼ 90 % increase in heat transfer coefficient at a high heat flux of 132 kW∙m−2 compared to the single-tier nanorod surface. Our investigation of how the nano-hierarchical structures fundamentally suppress the condensation-induced wetting on superhydrophobic surfaces represents a significant advance in understanding multiphase wetting phenomena and paves the way for the rational design of cooling surfaces.

Hierarchical microporous superhydrophobic surfaces with nanostructures enhancing vapor condensation heat transfer

A new hierarchical superhydrophobic surface with micropores and nanoblades (MN SHS) is proposed to strengthen the jumping ability of droplets and maintain the high-efficiency droplet jumping condensation at the larger subcoolings. The results demonstrate that the jumping frequency and the proportion of multidroplets merging (n ≥ 8) on the MN SHS are higher than those on the nanostructured superhydrophobic surfaces (Nano SHS). Moreover, the maximum diameter of condensate droplets is only 150 μm, which is approximately 50% smaller compared to the Nano SHS. Spontaneous jumping of condensate droplets is observed on the 40PPI MN SHS (PPI: the average number of pores per inch.) at a subcooling of 16 K; while the droplet jumping condensation occurs at a maximum of 5 K on the Nano SHS. As the subcooling ranges from 5 K to 16 K, the heat transfer coefficient of MN SHS is increased by at least 26.4% compared to the Nano SHS. In addition, lattice Boltzmann model is adopted to simulate the self-propelled jumping behaviors on different structured surfaces. The energy conversion efficiency of drops on the MN SHS is higher compared to that on the Nano SHS. This work will contribute to the potential development of dropwise condensation in energy utilization, petrochemical engineering, thermoelectric cooling and aerospace thermal management systems.

Droplet detachment force and its relation to Young-Dupre adhesion.

Droplets adhere to surfaces due to their surface tension γ and understanding the vertical force Fd required to detach the droplet is key to many technologies (e.g., inkjet printing, optimal paint formulations). Here, we predicted Fd on different surfaces by numerically solving the Young-Laplace equation. Our numerical results are consistent with previously reported results for a wide range of experimental conditions: droplets subjected to surface vs. body forces with |Fd| ranging from nano- to milli-newtons, droplet radii R ranging from tens of microns to several millimetres, and for various surfaces (micro-/nano-structured superhydrophobic vs. lubricated surfaces). Finally, we derive an analytic solution for Fd on highly hydrophobic surfaces and further show that for receding contact angle θr > 140°, the normalized Fd/πR is equivalent to the Young-Dupre work of adhesion γ(1 + cos θr).

Erythrocyte interaction with titanium nanostructured surfaces

Titanium and its alloys are used to make different medical devices such as stents, artificial heart valves, and catheters for cardiovascular diseases due to their superior biocompatibility. Thrombus formation begins on the surface of these devices as soon as they encounter blood. This leads to the formation of blood clots, which obstructs the flow of blood that leads to severe complications. Recent advancements in nanoscale fabrication and superhydrophobic surface modification techniques have demonstrated that these surfaces have antiadhesive properties and the ability to reduce thrombosis. In this study, the interaction of erythrocytes and whole blood clotting kinetics on superhydrophobic titanium nanostructured surfaces was investigated. These surfaces were characterized for their wettability (contact angle), surface morphology and topography (scanning electron microscopy (SEM)), and crystallinity (glancing angled X-ray diffraction (GAXRD)). Erythrocyte morphology on different surfaces was characterized using SEM, and overall cell viability was demonstrated through fluorescence microscopy. The hemocompatibility of these surfaces was characterized using commercially available assays: thrombin generation assay thrombin generation, hemolytic assay hemolysis, and complement convertase assay complement activity. The results indicate that superhydrophobic titanium nanostructured surfaces had lower erythrocyte adhesion, less morphological changes in adhered cells, lower thrombin generation, lower complement activation, and were less cytotoxic compared to control surfaces. Thus, superhydrophobic titanium nanostructured surfaces may be a promising approach to prevent thrombosis for several medical devices.

Nanostructure-based wettability modification of TiAl6V4 alloy surface for modulating biofilm production: Superhydrophilic, superhydrophobic, and slippery surfaces

In recent decades, with the rapid development of nanosurface technology, superhydrophobic surfaces and slippery liquid-infused porous surfaces (SLIPS) have been researched for industrialization because of their superior antibiofilm performance. The antibiofilm effect of the surface can influence the recovery of an affected area in the human body, making it an important research topic in the medical community. Although several application-based studies have been conducted using surfaces with various wetting properties, such surface modification studies have rarely been applied to TiAl6V4, the most representative metal inserted into the human body. In this study, we present various wettability modification methods, including the formation of superhydrophilic/hydrophobic surfaces and SLIPS, for TiAl6V4. Moreover, the antibiofilm performance of each fabricated surface was evaluated and compared. Each surface in the process was carefully analyzed physically and chemically. Modified TiAl6V4 surfaces were used to evaluate the antibiofouling effect that prevents surgical infection using Pseudomonas aeruginosa and Staphylococcus aureus, which have been reported in several clinical cases of periprosthetic infection. Based on crystal violet staining of the biofilm, it was confirmed that the TiAl6V4 plate perfectly suppressed biofilm generation when exposed to bacteria when it had a micro-nanostructured or nanostructured superhydrophobic surface.

Contact line friction and dynamic contact angles of a capillary bridge between superhydrophobic nanostructured surfaces.

The Cassie-Baxter state of wetting explains a large equilibrium contact angle and the slippery dynamics of a water droplet on a superhydrophobic rough surface. It also causes a contact angle hysteresis (CAH) that cannot be fully described by dynamic wetting theories including the molecular kinetic theory (MKT). We analyze the contact line dynamics on a superhydrophobic surface in the framework of the MKT. Multi-body dissipative particle dynamics simulations of a capillary bridge confined between two rough surfaces under steady shear are performed. We find that, in addition to the contact line friction force from the MKT, an additional friction force contribution is needed on rough surfaces. It can be obtained by subtracting from the total friction force the force solely caused by the actual liquid-solid contact area. We find that the additional force is almost constant at all contact line velocities. Thus, it is directly related to the CAH. The CAH originates not only from contact line pinning but also from the shear flow due to the strong friction in the central region of the liquid-solid interface away from the contact line. The analysis of the particle flow inside the capillary bridge shows that liquid particles trapped in the grooves of the surface texture actually move with the same velocity as the surface and exert strong additional friction to other liquid particles. This work extends the MKT to rough surfaces, as well as to elucidate the origin of the CAH of a capillary bridge. The finding would help to better understand other situations of dynamic wetting on superhydrophobic surfaces.

Nanostructured Superhydrophobic Titanium-Based Materials: A Novel Preparation Pathway to Attain Superhydrophobicity on TC4 Alloy.

This study develops the nanostructured superhydrophobic titanium-based materials using a combined preparation method of laser marking step and the subsequent anodizing step. The structural properties were determined using an X-ray diffractometer (XRD) and scanning electron microscope (SEM), while the performance was explored by wear and corrosion tests. The laser marking caused a rough surface with paralleled grooves and protrusions, revealing surface superhydrophobicity after organic modification. The anodizing process further created a titanium oxide (TiO2) nanotube film. Both phase constituent characterization and surface elemental analysis prove the uniform nanofilm. The inert nanosized oxide film offers improved stability and superhydrophobicity. Compared to those samples only with the laser marking process, the TiO2 nanotube film enhances the corrosion resistance and mechanical stability of surface superhydrophobicity. The proposed preparation pathway serves as a novel surface engineering technique to attain a nanostructured superhydrophobic surface with other desirable performance on titanium alloys, contributing to their scale-up applications in diverse fields.

Effect of surface nanostructure on enhanced atmospheric corrosion resistance of a superhydrophobic surface

Coalescence-induced droplet jumping behavior (CIDJB) of superhydrophobic surfaces has a potential application in atmospheric corrosion protection by spontaneously removing the corrosive water film/droplets. However, there are few studies constructed on the zinc substrate studying the CIDJB. In addition, the effects of the complex structure characteristics on the CIDJB and the subsequent atmospheric anti-corrosion performance of the superhydrophobic surface are still lacking systematic understanding. Herein, three superhydrophobic surfaces, namely, the microstructured superhydrophobic surface, the nanostructured superhydrophobic surface, and the complex structured superhydrophobic surface were rationally prepared on the zinc surface. First, the effects of the complex structure characteristics on the CIDJB and the subsequent atmospheric corrosion protection performance of the superhydrophobic surfaces were studied. Second, the corresponding mechanism of atmospheric corrosion protection based on the CIDJB of the superhydrophobic surfaces was revealed. The results suggest that the presence of the nanostructure is an important factor for CIDJB as a result of the reduced solid-liquid contact area and interfacial adhesion. Compared with Micro SS without CIDJB, both the Nano SS and complex SS with CIDJB exhibit a better anti-corrosion performance after the simulated condensation experiments due to the jumping-induced wetting transformation mechanism. This study provides criteria for designing efficient anti-corrosion materials based on the CIDJB.

Self-cleaning of superhydrophobic nanostructured surfaces at low humidity enhanced by vertical electric field.

Self-cleaning is the key factor that makes superhydrophobic nanostructured materials have wide applications. The self-cleaning effect, however, strongly depends on formations and movement of water droplets on superhydrophobic nanostructured surfaces, which is greatly restricted at low humidity (< 7.6 g·kg−1). Therefore, we propose a self-cleaning method at low humidity in which the pollution is electro-aggregated and driven in the electric field to achieve the aggregation and cleaning large areas. The cleaning efficiency of this method is much higher than that of water droplet roll-off, and will not produce “pollution bands”. A simplified numerical model describing pollution movements is presented. Simulation results are consistent with experimental results. The proposed method realizes the self-cleaning of superhydrophobic nanostructured surfaces above dew point curve for the first time, which extends applications of superhydrophobic nanostructured materials in low humidity, and is expected to solve self-cleaning problems of outdoor objects in low humidity areas (< 5.0 g·kg−1).Electronic Supplementary MaterialSupplementary material (experimental procedures, computational details, modeling process, supplementary figures, tables, and videos) is available in the online version of this article at 10.1007/s12274-022-4093-0.

Droplet behavior analysis on inclined, highly sticky, or slippery superhydrophobic nanostructured surfaces by observation and SPH simulation

Although the motion of water droplets on a superhydrophobic surface is important for industrial processes, the characteristics of the slippery/sticky contact line are not fully understood at the macroscopic continuum fluid scale. In this study, we tracked the dynamic contact angle of the solid–liquid–gas phase when droplets moved on a superhydrophobic nanostructured surface. High-speed observations revealed the pinning/unpinning behavior of the droplets’ receding contact part. The droplets were modeled by introducing momentum attenuation as a friction model in the smoothed particle hydrodynamics (SPH) framework, which assumes a macroscopically smooth surface with different slippery properties. From a macroscopic perspective, droplet pinning requires both horizontal movement and the suppression of the rotational motion acting on the solid–liquid interface. A sticky solid surface was successfully modeled using this simple model; thus, it can be applied to more practical problems such as prediction of the motion of melts on coke.

Effervescence‐Inspired Self‐Healing Plastrons for Long‐Term Immersion Stability

AbstractThe use of superhydrophobic/superamphiphobic surfaces demands the presence of a stable plastron, i.e., a film of air between micro‐ and nanostructures. Without actively replenishing the plastron with gases, it eventually disappears during immersion. The air diffuses into the immersion liquid, i.e., water. Current methods for sustaining the plastron under immersion remain limited to techniques such as electrocatalysis, electrolysis, boiling, and air‐refilling. These methods are difficult to implement at scale, are either energy‐consuming, or require continuous monitoring of the plastron (and subsequent intervention). Here, the concept of passive on‐demand recovery of the plastron via the use of a chemical reaction (effervescence) is presented. A superhydrophobic nanostructured surface is layered onto a wetting‐reactive, gas‐forming (effervescent) sublayer. During extended exposure to moisture, the effervescent layer must be protected by a moisture‐absorbent, water‐soluble polymer. Under prolonged immersion, partial collapse of the Cassie‐state induces contact of water with the effervescent layer. This induces the local formation of gases from effervescence, which restores the Cassie‐state. These facile and scalable design principles offer a new route toward intervention‐free and immersion‐durable superhydrophobic/superamphiphobic surfaces.

Gradient mixed wettability surfaces for enhancing heat transfer in dropwise flow condensation

Steam condensation plays a pivotal role in a broad range of industrial applications. Dropwise condensation, which occurs on hydrophobic and superhydrophobic surfaces, has been proven to have a larger heat transfer coefficient than filmwise condensation because of enhanced droplet detachment. However, hydrophilic surfaces possess a better droplet nucleation rate compared to hydrophobic and superhydrophobic surfaces. These two characteristics (high droplet nucleation rate and better droplet removal) require counteracting properties. Many studies increased the nucleation rate by creating nanostructured surfaces. Wettability contrast mechanism is another approach, which could achieve both high nucleation rate and droplet removal, and have attracted much attention in enhancing a better heat transfer performance. In this study, gradient mixed wettability surfaces were developed on copper surfaces to improve heat transfer during steam flow condensation inside a minichannel. Circular hydrophobic islands were fabricated on a nanostructured superhydrophobic background surface. Various configurations of different island and pitch sizes are considered. Real-time images captured by a high-speed camera are used to examine the droplet dynamics and the influence of wettability on droplet size distribution. Thus, a parametric study is conducted to present the optimum conditions at different steam mass fluxes. According to the heat transfer analysis results, the heat transfer coefficient could be enhanced by 37% compared to a plain hydrophobic surface. A higher ratio of hydrophobic area provided better conditions for droplet nucleation and growth, while larger superhydrophobic regions in the downstream section was advantageous for droplet removal and sweeping. As a result, the role of mixed wettability in achieving the best performance was revealed for dropwise flow condensation.

Growth and self-jumping of single condensed droplet on nanostructured surfaces: A molecular dynamics simulation

Our molecular dynamics simulation demonstrates that the condensed nanodroplets can achieve self-jumping off the nanostructured surface driven by the Laplace pressure difference. The growth and self-jumping dynamics of nanodroplets condensed on the superhydrophobic nanostructured surface with local hydrophilic pinning site are systematically investigated. We reveal that a curvature difference between the droplet top and bottom is generated due to the confinement of the groove walls, which leads to a Laplace pressure difference. It increases with growing droplet and the droplet detaches from the pinning site and finally jumps off when it reaches the threshold against the pinning force from the hydrophilic pinning site. We find that the characteristics of the hydrophilic pinning site shows competitive effects on the whole process, which can be generally divided into incubation and burst stages. Increasing the surface wettability and size of the pinning site promotes the droplet growth but blocks the droplet self-jumping in the meantime due to the increased pinning force. We also find that the droplet-jumping highly depends on the confined portion of the droplet after detaching from the pinning site and its decrease will diminish the excessive surface energy stored in the droplet, which makes the converted kinetic energy insufficient to support the jump-off. The present work sheds light on the fundamental understandings of passive method for condensate removal, self-cleaning, thermal management, etc.

Super Wear Resistant Nanostructured Superhydrophobic Surface

Super Wear Resistant Nanostructured Superhydrophobic Surface

Enhancement of vapor condensation heat transfer on the micro- and nano-structured superhydrophobic surfaces

Recently, micro- or nano- structured surfaces haven been developed to enhance condensation heat transfer, water harvesting and self-cleaning. However, at large subcoolings, condensate floods the subcooled substrate, thus deteriorating the heat transfer efficiency. Here, the superhydrophobic surfaces with micropillared and nanopillared structures are proposed to enhance heat transfer at large subcoolings. The influence of micropillar spacing and surface subcooling on the droplet dynamics and heat transfer performance is experimentally investigated using microscopic visualization techniques. In addition, the microscopic modeling of condensation heat transfer on the microstructured surfaces is performed using the mesoscopic lattice Boltzmann method. The results demonstrate that the droplet size distribution on the micropillared surface is significantly smaller over that of the nanostructured surface. The heat transfer coefficient decreases with the increase of micropillar spacing. As the subcooling rises, although the condensate floods the substrate, the heat transfer coefficient of the S10R30 (S10R30 represents the micropillar arrays with s = 10 μm and 2r = 60 μm) surface is enhanced by 26.4% compared to the hydrophobic nanostructured surface. This is because the height of liquid film is the same of order of magnitude as the micropillars, reducing the thermal resistance caused by the liquid layer. Combining environmental scanning electron microscope (ESEM) observations and LB simulation results, it is concluded that the droplets first nucleate at the bottom corner of micropillars. In addition, the condensate droplets merge to form a film, fill the micropillar gaps, and cover the entire micropillars, leading to a sharp decrease in heat flux. These findings provide a theoretical and experimental guidance for the development of condensing surfaces to enhance heat transfer.

Irregular, nanostructured superhydrophobic surfaces: Local wetting and slippage monitored by fluorescence correlation spectroscopy

In applications superhydrophobic surfaces are often irregular and nanostructured. On such a surface, it has been found that only a part of the surface is homogeneously wetted, i.e. wetting and slippage corresponding to that of a regularly structured surface. On the remaining part, large air inclusions occurred that possess a comparatively large slip length. In order to measure the velocity profile at distances below 1\textmu{}m close to the surface, an enhanced evaluation method for fluorescence correlation spectroscopy is developed in this work.