Loss of superhydrophobicity of hydrophobic micro/nano structures during condensation

Single Condensation Droplet Self-Ejection from Divergent Structures with Uniform Wettability

Coalescence-induced condensation droplet jumping has been extensively studied for anti-icing, condensation heat transfer, water harvesting, and self-cleaning. Another phenomenon that is gaining attention for potential enhancements is the self-ejection of individual droplets. However, the mechanism underlying this process remains elusive due to cases in which the abrupt detachment of an interface establishes an initial Laplace pressure difference. In this study, we investigate the self-ejection of individual droplets from uniformly hydrophobic microstructures with divergent geometries. We design, fabricate, and test arrays of truncated, nanostructured, and hydrophobic microcones arranged in a square pattern. High-speed microscopy reveals the dynamics of a single condensation droplet between four cones: after cycles of growth and stopped self-propulsion, the suspended droplet self-ejects without abrupt detachments. Through analytical modeling of the droplet in a conical pore as an approximation, we describe the slow isopressure growth phases and the rapid transients driven by surface energy release once a dynamic configuration is reached. Microcones with uniform wettability, in addition to being easier to fabricate, have the potential to enable the self-ejection of all nucleated droplets with a designed size, promising significant improvements in the aforementioned applications and others.

Dropwise Condensate Comb for Enhanced Heat Transfer

Dropwise condensation on superhydrophobic surfaces could potentially enhance heat transfer by droplet spontaneous departure via coalescence-induced jumping. However, an uncontrolled droplet size could lead to a significant reduction of heat transfer by condensation, due to large droplets that resulted in a flooding phenomenon on the surface. Here, we introduced a dropwise condensate comb, which consisted of U-shaped protruding hydrophilic stripes and hierarchical micro-nanostructured superhydrophobic background, for a better control of condensation droplet size and departure processes. The dropwise condensate comb with a wettability-contrast surface structure induced droplet removal by flank contact rather than three-phase line contact. We showed that dropwise condensation in this structure could be controlled by designing the width of the superhydrophobic region and height of the protruding hydrophilic stripes. In comparison with a superhydrophobic surface, the average droplet radius was decreased to 12 μm, and the maximum droplet departure radius was decreased to 189 μm by a dropwise condensate comb with 500 μm width of a superhydrophobic region and 258 μm height of a protruding hydrophilic stripe. By controlling the droplet size and departure on hierarchical micro-nanostructured superhydrophobic surfaces, it was experimentally demonstrated that both the heat transfer coefficient and heat flux could be enhanced significantly. Moreover, the dropwise condensate comb showed a maximum heat transfer coefficient of 379 kW m^-2 K^-1 at a low subcooling temperature, which was 85% higher than that of a superhydrophobic surface, and it showed 113% improvement of high heat flux or heat transfer coefficient when it was compared with that of the hierarchical micro-nanostructured superhydrophobic surface at a high subcooling temperature of ∼10.6 K. This work could potentially transform the design and fabrication space for high-performance heat transfer devices by spatial control of condensation droplet size and departure processes.

Electron microscopy and calorimetry of proteins in supercooled water

Some of the best nucleating agents in nature are ice-nucleating proteins, which boost ice growth better than any other material. They can induce immersion freezing of supercooled water only a few degrees below 0 °C. An open question is whether this ability also extends to the deposition mode, i.e., to water vapor. In this work, we used three proteins, apoferritin, InaZ (ice nucleation active protein Z), and myoglobin, of which the first two are classified as ice-nucleating proteins for the immersion freezing mode. We studied the ice nucleation ability of these proteins by differential scanning calorimetry (immersion freezing) and by environmental scanning electron microscopy (deposition freezing). Our data show that InaZ crystallizes water directly from the vapor phase, while apoferritin first condenses water in the supercooled state, and subsequently crystallizes it, just as myoglobin, which is unable to nucleate ice.

Bio-inspired hemocompatible surface modifications for biomedical applications

When blood first encounters the artificial surface of a medical device, a complex series of biochemical reactions is triggered, potentially resulting in clinical complications such as embolism/occlusion, inflammation, or device failure. Preventing thrombus formation on the surface of blood-contacting devices is crucial for maintaining device functionality and patient safety. As the number of patients reliant on blood-contacting devices continues to grow, minimizing the risk associated with these devices is vital towards lowering healthcare-associated morbidity and mortality. The current standard clinical practice primarily requires the systemic administration of anticoagulants such as heparin, which can result in serious complications such as post-operative bleeding and heparin-induced thrombocytopenia (HIT). Due to these complications, the administration of antithrombotic agents remains one of the leading causes of clinical drug-related deaths. To reduce the side effects spurred by systemic anticoagulation, researchers have been inspired by the hemocompatibility exhibited by natural phenomena, and thus have begun developing medical-grade surfaces which aim to exhibit total hemocompatibility via biomimicry. This review paper aims to address different bio-inspired surface modifications that increase hemocompatibility, discuss the limitations of each method, and explore the future direction for hemocompatible surface research.

Robust Micro-Nanostructured Superhydrophobic Surfaces for Long-Term Dropwise Condensation

Design of hierarchical micromorphology represents an important strategy for developing functional surfaces but has yet to be achieved for promising long-term dropwise condensation. Herein, micropapillaes overlaid with nanograss were created to enhance dropwise condensation. By analyzing the nucleation and evolution of the condensate droplets, we elucidated that these hierarchical micro-nanostructures topologized tapered gaps, which produced upward pressure, to achieve spontaneous dislodging of condensate microdroplet out of gaps, and then to trigger microdroplet navigation before finally departing from the surface by coalescence-induced jumping. The high mobility of condensate delayed flooding and contributed to a very high heat transfer coefficient of 218 kW·m^-2·K^-1. Moreover, these micropapillaes served as forts that protected the nanograss from being destroyed, resulting in improved mechanical and chemical robustness. Our work proposed new examples of topology creation for long-term dropwise condensation heat transfer and shed light on application integration of such promising functional surfaces.

Effect of Steam Flow Rate and Storage Period of Superhydrophobic-Coated Surfaces on Condensation Heat Flux and Wettability

The jumping-droplet phenomenon occurring on superhydrophobic (SHPhob) surfaces under special conditions may be beneficial for numerous systems using condensation, due to the reported increased heat transfer coefficients. One technique to create a SHPhob surface is coating, which can be applied to larger areas of existing elements. However, challenges are associated with coating stability and the realization of continuous dropwise condensation. This research examined the condensation of steam at different flow rates (2, 4 and 6 g/min) and its influence on heat flux and water contact angles on the SHPhob spray-coated aluminum samples. Special emphasis on the impact of time was addressed through a series of one and five-hour condensation experiments on the samples with different storage periods (coated either one year ago or shortly before testing). Over the experimental series at a higher steam flow rate (6 g/min), heat flux decreased by 20% through the old-coated samples and water contact angles transferred from the superhydrophobic (147°) to hydrophobic (125°) region. This can be attributed to the joint effects of the partial coating washout and the adsorption of the condensed water within the porous structures of the coating during steam condensation. The new-coated samples could sustain more than fifty hours of condensation, keeping the same heat fluxes and SHPhob characteristics.

Ultrathin Lubricant-Infused Vertical Graphene Nanoscaffolds for High-Performance Dropwise Condensation

Lubricant-infused surfaces (LIS) are highly efficient in repelling water and constitute a very promising family of materials for condensation processes occurring in a broad range of energy applications. However, the performance of LIS in such processes is limited by the inherent thermal resistance imposed by the thickness of the lubricant and supporting surface structure, as well as by the gradual depletion of the lubricant over time. Here, we present an ultrathin (∼70 nm) and conductive LIS architecture, obtained by infusing lubricant into a vertically grown graphene nanoscaffold on copper. The ultrathin nature of the scaffold, combined with the high in-plane thermal conductivity of graphene, drastically minimize earlier limitations, effectively doubling the heat transfer performance compared to a state-of-the-art CuO LIS surface. We show that the effect of the thermal resistance to the heat transfer performance of a LIS surface, although often overlooked, can be so detrimental that a simple nanostructured CuO surface can outperform a CuO LIS surface, despite filmwise condensation on the former. The present vertical graphene LIS is also found to be resistant to lubricant depletion, maintaining stable dropwise condensation for at least 24 h with no significant change of advancing contact angle and contact angle hysteresis. The lubricant consumed by the vertical graphene LIS is 52.6% less than that of the existing state-of-the-art CuO LIS, also making the fabrication process more economical.

Effect of Surface Structure Complexity on Interfacial Droplet Behavior of Superhydrophobic Titanium Surfaces for Robust Dropwise Condensation

In general, the dropwise condensation supported by superhydrophobic surfaces results in enhanced heat transfer relative to condensation on normal surfaces. However, in supersaturated environments that exceed a certain supersaturation threshold, moisture penetrates the surface structures and results in attached condensation, which reduces the condensation heat transfer efficiency. Therefore, when designing superhydrophobic surfaces for condensers, the surface structure must be resistant to attached condensation in supersaturated conditions. The gap size and complexity of the micro/nanoscale surface structure are the main factors that can be controlled to maintain water repellency in supersaturated environments. In this study, the condensation heat exchange performance was characterized for three different superhydrophobic titanium surface structures via droplet behavior (DB) mapping to evaluate their suitability for power plant condensers. In addition, it was demonstrated that increasing the surface structure complexity increases the versatility of the titanium surfaces by extending the window for improved heat exchange performance. This study demonstrates the usefulness of DB mapping for evaluating the performance of superhydrophobic surfaces regarding their applicability for industrial condenser systems.

Vapor Flux on Bumpy Surfaces: Condensation and Transpiration on Leaves

Drop condensation and evaporation as a result of the gradient in vapor concentration are important in both engineering and natural systems. One of the interesting natural examples is transpiration on plant leaves. Most of the water in the inner space of the leaves escapes through stomata, whose rate depends on the surface topography and a difference in vapor concentrations inside and just outside of the leaves. Previous research on the vapor flux on various surfaces has focused on numerically solving the vapor diffusion equation or using scaling arguments based on a simple solution with a flat surface. In this present work, we present and discuss simple analytical solutions on various 2D surface shapes (e.g., semicylinder, semiellipse, hair). The method of solving the diffusion equation is to use the complex potential theory, which provides analytical solutions for vapor concentration and flux. We find that a high mass flux of vapor is formed near the top of the microstructures while a low mass flux is developed near the stomata at the leaf surface. Such a low vapor flux near the stomata may affect transpiration in two ways. First, condensed droplets on the stomata will not grow due to a low mass flux of vapor, which will not inhibit the gas exchange through the stomatal opening. Second, the low mass flux from the atmosphere will facilitate the release of highly concentrated vapor from the substomatal space.

Discussion on Water Condensation in Membrane Pores during CO2 Absorption at High Temperature

Water condensation is a possible cause of membrane wetting in the operation of membrane contactors, especially under high-temperature conditions. In this study, water condensation in pores of polytetrafluoroethylene (PTFE) hollow fiber membranes was investigated during high-pressure CO₂ absorption around 70 °C. It was found that the liquid accumulation rate in the treated gas knock-out drum was constant during continuous operation for 24 h when all experimental conditions were fixed, indicating a stable degree of membrane wetting. However, as the operating parameters were changed, the equilibrium vapor pressure of water within membrane pores could change, which may result in a condensation-conducive environment. Water condensation in membrane pores was detected and proven indirectly through the increase in liquid accumulation rate in the treated gas knock-out drum. The Hagen-Poiseuille equation was used to correlate the liquid accumulation rate with the degree of membrane wetting. The degree of membrane wetting increased significantly from 1.8 × 10^-15 m³ to 3.9 × 10^-15 m³ when the feed gas flow rate was reduced from 1.45 kg/h to 0.40 kg/h in this study due to water condensation in membrane pores. The results of this study provide insights into potential operational limitations of membrane contactor for CO₂ absorption under high-temperature conditions.

A Study of Droplet-Behavior Transition on Superhydrophobic Surfaces for Efficiency Enhancement of Condensation Heat Transfer

Enhancement in heat-transfer performance via dropwise condensation on superhydrophobic surfaces is much greater than that realized via generic condensation on a regular surface. However, if the supersaturation level during condensation increases above a specific value, water may seep to greater depths between structures. This may lead to attached condensation, which reduces condensation heat-transfer efficiency below that of ordinary surfaces. Therefore, it is critical to avoid the occurrence of supersaturation when superhydrophobic surfaces are employed in condenser design. The proposed study presents a simple method for regulating supersaturation on the laboratory scale. Experiments concerning droplet behavior on a superhydrophobic plate were performed to investigate droplet detachment and attachment in accordance with the surface and droplet temperatures. Results obtained have been represented as a ″droplet-behavior map″, which clearly depicts boundaries dividing the detachment and attachment regions. The supersaturation threshold obtained from the said map has been compared against results obtained from condensation heat-transfer experiments performed in an actual condenser environment. As observed, the two results demonstrate excellent agreement. Although superhydrophobicity of surfaces remains unchanged at room temperature, changes may occur in the extent of the supersaturation section, which improves condensation heat-transfer performance, depending on the surface-structure complexity. Therefore, droplet-behavior mapping has been used in this study to determine the available supersaturation section in accordance with the variation in surface roughness. Results confirm that the available supersaturation region increases with increasing surface roughness and structural complexity. Therefore, prior to applying superhydrophobicity to condensers, droplet-behavior mapping must be performed to avoid operation under the supersaturation conditions, which causes attached condensation.

Evaporation-Crystallization Method to Promote Coalescence-Induced Jumping on Superhydrophobic Surfaces

Biphilic surfaces exhibit outstanding condensation efficiency compared to surfaces having homogeneous wettability. Especially, hydrophilic patterns on a superhydrophobic substrate significantly promote the coalescence-induced jumping of condensed droplets by increasing the nucleation rate of condensation, thus enhancing the condensation efficiency drastically. However, the application of biphilic surfaces in practical industries remains challenging because controlling the size and spacing of the hydrophilic spots on large and complex surfaces is difficult. In this study, we have achieved heterogeneous wettability using the evaporation-crystallization method, which can be applied to various surfaces as required by industries. The crystals generated using the evaporation-crystallization process drastically increased the number density of condensed droplets on a superhydrophobic surface (SHS), so the developed biphilic surface increased the cumulative volume of jumping droplets by up to 63% compared to that on a conventional superhydrophobic surface. Furthermore, the condensation dynamics on the biphilic surface were analyzed with the classical nucleation theory and the Ohnesorge number. The analysis results indicated that the generated hydrophilic crystals can reduce the nucleation energy barrier and decrease the available excessive surface energy of coalesced droplets on the biphilic surface; this implies that the size distribution of the crystals determines the condensation dynamics. In sum, this study not only introduced an effective surface tailoring approach for enhancing condensation but also provided insights into the design of optimum biphilic surfaces for various conditions, creating new opportunities to widen the applicability of biphilic surfaces in practical industries that exploit condensation.

Nanograssed Zigzag Structures To Promote Coalescence-Induced Droplet Jumping

To increase the efficiency of jumping-droplet condensation, this study proposes a hierarchical superhydrophobic surface that promotes coalescence-induced jumping. Inspired by the phenomenon in which a growing droplet moves spontaneously within a superhydrophobic V structure, we fabricated nanograssed zigzag structures on the surface to induce the spontaneous motion of condensed droplets. The direction of the motion was parallel to the surface, so the condensed droplets easily coalesced on it. Compared with a conventional nanograssed superhydrophobic surface, the proposed surface increased the frequency of coalescence-induced jumping by ≥17 times and increased the cumulative volume of jumping droplets by ∼1.8 times. The proposed surface has great potential to increase the efficiency of applications such as water- and energy-harvesting and cooling systems that exploit jumping-droplet condensation.

Is dropwise condensation feasible? A review on surface modifications for continuous dropwise condensation and a profitability analysis

The interest in surface treatments promoting dropwise condensation has grown exponentially in the past decades. Savings in the operating and maintenance costs of steam processes involving phase changes are promised. Numerous surface preparation methods allow the formation of droplets during condensation. However, stable dropwise condensation has been hardly realized in industrial applications. This review aims to highlight the surface preparation techniques that promote dropwise condensation. It emphasizes on their durability and the resulting stability of dropwise condensation. Furthermore, the possibilities of implementation at an industrial level are discussed, apart from evaluating the economic feasibility through a case study. Despite years of research and numerous surface design possibilities, dropwise condensation cannot be maintained: coating deterioration and fluctuating process conditions commonly lead to surface flooding within hours or weeks. A more profound understanding of the mechanisms of dropwise condensation and innovative design concepts for self-renewing heat transfer surfaces may diminish encountered challenges.

On-demand concentration of an analyte on laser-printed polytetrafluoroethylene

Controllable targeted deposition of an analyte dissolved in a liquid drop evaporating on a superhydrophobic surface has recently emerged as a promising concentrator approach with various applications ranging from ultrasensitive bioidentification to DNA molecule sorting. Here, we demonstrate that surface textures with non-uniform wettability fabricated using direct easy-to-implement femtosecond-pulse filament-assisted ablation of polytetrafluoroethylene substrates can be used to concentrate and deposit an analyte at a designated location out of a water droplet. The proposed surface textures contain a central superhydrophilic trap surrounded by superhydrophobic periodically arranged pillars with a hierarchical roughness. By optimizing the arrangement and geometry of the central trap and the surrounding superhydrophobic textures, the analyte dissolved in a 5 μL water drop was fixed onto a 90 × 90 μm² target. The proposed textures provide a concentration factor of 10³, an order of magnitude higher than those for the previously reported surface textures. This promising ultrasensitive versatile platform allows the detection of fingerprints of the deposited analyte via surface-enhanced spectroscopy techniques (Raman scattering or photoluminescence) at an estimated detection threshold better than 10^-15 mol L^-1.

Effects of wettability on droplet movement in a V-shaped groove

As basic research to understand the behavior of droplets on structured surfaces, we investigated droplet movement in a V-shaped groove while the volume of the droplet changes. We developed a model to explain the mechanism of the droplet movement and the effects of the wettability of the inner walls of the groove on the droplet movement. Furthermore, the model predicted new phenomena and explains the effect of the nonhomogeneous wettability on droplet movement. The predictions of the model match the experimental results well. This research can provide the basic knowledge for manipulating droplets with structured surfaces for various applications.

Double-exclusive liquid repellency (double-ELR): an enabling technology for rare phenotype analysis

Double-exclusive liquid repellency (double-ELR) is an extreme wettability phenomenon in which adjacent regions selectively and completely repel immiscible liquids with different surface chemistries on a non-textured substrate (i.e., a substrate in absence of micro/nano-structures). Under double-ELR conditions, each liquid exhibits no physical contact (contact angle of 180°) with its non-preferred surface chemistry, thus enabling complete partitioning of adjacent fluidic volumes (e.g., between water and oil). This enables a new type of cell culture-based assay, where cell loss from common failure modes (e.g., biofouling from inadvertent cell adhesion, detrimental moisture loss/gain, and liquid handling dead volumes) is significantly mitigated. Importantly, the principles of double-ELR were leveraged to achieve underoil sweep patterning, a no-loss, robust and high-throughput distribution of sub-microliter volumes of aqueous media (and cells). In addition to high-efficiency distribution via sweep patterning, double-ELR can be used to construct "modular" (i.e., easily implemented and/or linked together with spatial and temporal control) higher-order architectures for in vitro imitation of physiologically relevant microenvironments that are of particular interest within the cell assay community, including multi-phenotype cultures with excellent spatial and temporal control, three-dimensional layered multi-phenotype cultures, cultures with selective mechanical cues of extracellular matrix (i.e., collagen fiber alignment), and spheroid cultures. Together, these features of double-ELR uniquely facilitate culture and high content analysis of limited cellular samples (e.g., a few hundred to a few thousand cells).

Rationally 3D-Textured Copper Surfaces for Laplace Pressure Imbalance-Induced Enhancement in Dropwise Condensation

Enhancing the thermal efficiency of a broad range of condenser devices requires means of achieving sustainable dropwise condensation on metallic surfaces, where heat transfer can be further enhanced, by harvesting the advantage of the sweeping action of vapor flow over the surface, facilitating a reduction in the droplet departure diameter. Here, we present a rationally driven, hierarchical texturing process of copper surfaces, guided by fundamental principles of wettability and coalescence, which achieves controlled droplet departure under vapor flow conditions and thus significantly enhances phase change thermal transport. The desired texture is attained by fabricating an array of 3D laser-structured truncated microcones on the surface, covered with papillae-like nanostructures and a hydrolytically stable, low surface energy self-assembled-monolayer coating. Passive droplet departure on this surface is achieved through progressive coalescence of droplets arising from microcavities formed by the microcone array, resulting in depinning and subsequent departure of the depinned condensate drops through vapor shear. The synergistic combination of vapor shear and the sustained dropwise condensation on the hierarchical copper surface results in a nearly 700% increase in heat transfer coefficients as compared to filmwise condensation from identical, standard unstructured surfaces.

Durable superhydrophobic surfaces made by intensely connecting a bipolar top layer to the substrate with a middle connecting layer

This study reported a simple fabrication method for a durable superhydrophobic surface. The superhydrophobic top layer of the durable superhydrophobic surface was connected intensely to the substrate through a middle connecting layer. Glycidoxypropyltrimethoxysilane (KH-560) after hydrolysis was used to obtain a hydrophilic middle connecting layer. It could be adhered to the hydrophilic substrate by covalent bonds. Ring-open reaction with octadecylamine let the KH-560 middle layer form a net-like structure. The net-like sturcture would then encompass and station the silica particles that were used to form the coarse micro structures, intensely to increase the durability. The top hydrophobic layer with nano-structures was formed on the KH-560 middle layer. It was obtained by a bipolar nano-silica solution modified by hexamethyldisilazane (HMDS). This layer was connected to the middle layer intensely by the polar Si hydroxy groups, while the non-polar methyl groups on the surface, accompanied by the micro and nano structures, made the surface rather hydrophobic. The covalently interfacial interactions between the substrate and the middle layer, and between the middle layer and the top layer, strengthened the durability of the superhydrophobic surface. The abrasion test results showed that the superhydrophobic surface could bear 180 abrasion cycles on 1200 CW sandpaper under 2 kPa applied pressure.

Tuning Superhydrophobic Nanostructures To Enhance Jumping-Droplet Condensation

It was recently discovered that condensation growing on a nanostructured superhydrophobic surface can spontaneously jump off the surface, triggered by naturally occurring coalescence events. Many reports have observed that droplets must grow to a size of order 10 μm before jumping is enabled upon coalescence; however, it remains unknown how the critical jumping size relates to the topography of the underlying nanostructure. Here, we characterize the dynamic behavior of condensation growing on six different superhydrophobic nanostructures, where the topography of the nanopillars was systematically varied. The critical jumping diameter was observed to be highly dependent upon the height, diameter, and pitch of the nanopillars: tall and slender nanopillars promoted 2 μm jumping droplets, whereas short and stout nanopillars increased the critical size to over 20 μm. The topology of each surface is successfully correlated to the critical jumping diameter by constructing an energetic model that predicts how large a nucleating embryo needs to grow before it can inflate into the air with an apparent contact angle large enough for jumping. By extending our model to consider any possible surface, it is revealed that properly designed nanostructures should enable nanometric jumping droplets, which would further enhance jumping-droplet condensers for heat transfer, antifogging, and antifrosting applications.

Growth Rates and Spontaneous Navigation of Condensate Droplets Through Randomly Structured Textures

Dropwise condensation is a phenomenon of common occurrence in everyday life, the understanding and controlling of which is of great interest to applications ranging from technology to nature. Scalable superhydrophobic textures on metals are of direct relevance in improving phase change heat transport in realistic industrial applications. Here we reveal important facets of individual droplet growth rate and droplet departure during dropwise condensation on randomly structured hierarchical superhydrophobic aluminum textures, that is, surfaces with a microstructure consisting of irregular re-entrant microcavities and an overlaying nanostructure. We demonstrate that precoalescence droplet growth on such a surface can span a broad range of rates even when the condensation conditions are held constant. The fastest growth rates are observed to be more than 4 times faster as compared to the slowest growing droplets. We show that this variation in droplet growth on the hierarchical texture is primarily controlled by droplet growth dynamics on the nanostructure overlaying the microstructure and is caused by condensation-induced localized wetting nonuniformity on the nanostructure. We also show that the droplets nucleating and growing within the microcavities are able to spontaneously navigate the irregular microcavity geometry, climb the microtexture, and finally depart from the surface by coalescence-induced jumping. This self-navigation is realized by a synergistic combination of self-orienting Laplace pressure gradients induced within the droplet as it dislodges itself and moves through the texture, as well as multidroplet coalescence.

From Initial Nucleation to Cassie-Baxter State of Condensed Droplets on Nanotextured Superhydrophobic Surfaces

Understanding how droplet condensation happens plays an essential role for our fundamental insights of wetting behaviors in nature and numerous applications. Since there is a lack of study of the initial formation and growing processes of condensed droplets down to nano-/submicroscale, relevant underlying mechanisms remain to be explored. We report an in situ observation of vapor condensation on nano-/microtextured superhydrophobic surfaces using optical microscopy. An interesting picture of the vapor condensation, from the initial appearance of individual small droplets (≤1 μm) to a Cassie-Baxter wetting state (>30 μm), are exhibited. It is found that individual droplets preferentially nucleate at the top and the edge of single micropillars with very high apparent contact angles on the nanotextures. Scenarios of two distinguished growing modes are reported statistically and the underlying mechanisms are discussed in the view of thermodynamics. We particularly reveal that the formation of the Cassie-Baxter wetting state is a result of a continuous coalescence of individual small droplets, in which the nanotexture-enhanced superhydrophobicity plays a crucial role. We envision that these fundamental findings can deepen our understanding of the nucleation and development of condensed droplets in nanoscale, so as to optimize design strategies of superhydrophobic materials for a broad range of water-harvesting and heat-transfer systems.

Sprayable superhydrophobic nano-chains coating with continuous self-jumping of dew and melting frost

Spontaneous movement of condensed matter provides a new insight to efficiently improve condensation heat transfer on superhydrophobic surface. However, very few reports have shown the jumping behaviors on the sprayable superhydrophobic coatings. Here, we developed a sprayable silica nano-porous coating assembled by fluorinated nano-chains to survey the condensates’ dynamics. The dewdrops were continuously removed by self- and/or trigger-propelling motion due to abundant nano-pores from random multilayer stacking of nano-chains. In comparison, the dewdrops just could be slipped under the gravity effect on lack of nano-pores coatings stacked by silica nano-spheres and nano-aggregates. More interestingly, the spontaneous jumping effect also occurred on micro-scale frost crystals under the defrosting process on nano-chains coating surfaces. Different from self-jumping of dewdrops motion, the propelling force of frost crystals were provided by a sudden increase of the pressure under the frost crystal.

Condensed dewdrops self-ejecting on sprayable superhydrophobic CNT/SiO2 composite coating

We report a type of novel condensed dewdrops self-ejecting coating from sprayable paint, which was prepared by a self-assembly process of SiO₂ nano-particles on hydroxylated carbon nano-tubes with subsequent chemical modification.

Effect of Aluminum Substrate Surface Modification on Wettability and Freezing Delay of Water Droplet at Subzero Temperatures

In this study, we have investigated the freezing delay of a water droplet on precooled substrates of an aluminum alloy that is commonly used for heat-exchanger fins. The surfaces of the substrates were modified to obtain surfaces with different hydrophilicity/hydrophobicity and different surface chemistry but without significantly modifying the surface topography. The freezing delays and water contact angles were measured as a function of the substrate temperature and the results were compared to the predictions of the heterogeneous ice nucleation theory. Although the trends for each sample followed the trend in this theory, the differences in the extents of freezing delays were in apparent disagreement with the predictions. Concretely, a slightly hydrophilic substrate modified by (3-aminopropyl) triethoxysilane (APTES) showed longer freezing delays than both more hydrophilic and more hydrophobic substrates. We suggest that this is because this particular surface chemistry prevents ice formation at the interface of the substrate, prior to the deposition of the water droplet. On the basis of our results, we suggest that not only wettability and topography but also the concrete surface chemistry plays a significant role in the kinetics of the ice formation process when a water droplet is placed on a precooled substrate.

Bioinspired Interfacial Materials with Enhanced Drop Mobility: From Fundamentals to Multifunctional Applications

The development of bioinspired interfacial materials with enhanced drop mobility that mimic the innate functionalities of nature will have a significant impact on the energy, environment and global healthcare. Despite extensive progress, state of the art interfacial materials have not reached the level of maturity sufficient for industrial applications in terms of scalability, stability, and reliability. These are complicated by their operating environments and lack of facile approaches to control the local structural texture and chemical composition at multiple length scales. The recent advances in the fundamental understanding are reviewed, as well as practical applications of bioinspired interfacial materials, with an emphasis on the drop bouncing and coalescence-induced jumping behaviors. Perspectives on how to catalyze new discoveries and to foster technological adoption to move this exciting area forward are also suggested.

A novel dynamic pseudo[1]rotaxane based on a mono-biotin-functionalized pillar[5]arene

A stable pillar[5]arene-based pseudo[1]rotaxane P1′ was synthesized by the click reaction, which exhibited a dynamic slow disassembly process upon adding a strong-polar solvent or competitive guest. Moreover, this dynamic behavior might be used as a switch to turn on or off the bioactivity of the biotin moiety in aqueous solution.