Quasi-Liquid Surfaces for Sustainable High-Performance Steam Condensation

Ultra-Slippery Hydrophilic Surfaces by Hybrid Monolayers

Slippery solid surfaces with low droplet contact angle hysteresis (CAH) are crucial for applications in thermal management, energy harvesting, and environmental remediation. Traditionally, reducing CAH has been achieved by enhancing surface homogeneity. This work challenges this conventional approach by developing slippery yet hydrophilic surfaces through hybrid monolayers composed of hydrophilic polyethylene glycol (PEG)-silane and hydrophobic alkyl-silane molecules. These hybrid surfaces exhibited exceptionally low CAH (<2°), outperforming well-established homogeneous slippery surfaces. Molecular structural analyses suggested that the remarkable slipperiness is due to a unique spatially staggered molecular configuration, where longer PEG chains shield shorter alkyl chains, thus creating additional free volume while ensuring surface coverage. This was supported by the observation of decreased CAH with increasing temperature, highlighting the role of grafted chain mobility in enhancing slipperiness by self-smoothing and fluid-like behaviors. Furthermore, condensation experiments demonstrated the exceptional performance of the hydrophilic slippery surfaces in dew harvesting due to superior condensation nucleation, droplet coalescence, and self-sweeping efficiency. These findings offer a novel paradigm for designing advanced slippery surfaces and provide valuable insights into the molecular mechanisms governing dynamic wetting.

Ultrahigh Subcooling Dropwise Condensation Heat Transfer on Slippery Liquid-like Monolayer Grafted Surfaces

Rapid and continuous droplet shedding is crucial for many applications, including thermal management, water harvesting, and microfluidics, among others. Superhydrophobic surfaces, though effective, suffer from droplet pinning at high subcooling temperature (Tsub). Conversely, slippery liquid-like surfaces covalently bonded with flexible hydrophobic molecules show high stability and low droplet adhesion attributed to their dense and ultrasmooth water repellent polymer chains, enhancing dropwise condensation and rapid shedding. In this work, linear poly(dimethylsiloxane) chains of various viscosities are covalently bonded onto silicon substrates to form thin and smooth monolayer coated surfaces. The formation of the monolayer is characterized by cryogenic transmission electron microscopy. On these surfaces a very low contact angle hysteresis is reported within wide surface temperature ranges as well as continuous dropwise condensation at ultrahigh Tsub of 60 K. In particular, one of the highest condensation heat fluxes of 1392.60 kW·m-2 and a heat transfer coefficient of 23.21 kW·m-2·K-1 at ultrahigh Tsub of 60 K is reported. The experimental heat transfer performance is further compared to the theoretical heat transfer via the individual droplets with the droplet distribution elucidated via both macroscopic observations as well as environmental scanning electron microscopy. Finally, only a mild decrease in the heat transfer coefficient of 20.3% after 100 h of condensation test at Tsub of 60 K is reported. Slippery liquid-like surfaces promote droplet shedding and sustain dropwise condensation at high Tsub without flooding empowered by the lower frictional forces, addressing challenges in heat transfer performance and durability.

Comparative Molecular Dynamics Simulation of Wetting on Liquid-like Surfaces

We utilize molecular dynamics simulations to comparably investigate the wetting and motion behavior of droplets on liquid-like surfaces (LLS) with varying grafting conditions. Polydimethylsiloxane (PDMS) and perfluoropolyether (PFPE) have been considered to be flexible molecules versus rigid molecules of trichloro(octadecyl) silane (OTS) and trichloro(1H,1H,2H,2H-perfluorooctyl) silane (PFOS), respectively. Our findings reveal that droplets on surfaces tethered with either PDMS or PFPE brushes can generate indentations and wetting ridges, providing microscopic evidence of their liquid-like nature. The grafting density of mobile chains exerts a dominant influence on the wetting properties compared to the molecular weight. A parameter map is created to pinpoint the precise range of grafting densities essential for the optimal construction of LLS at predetermined molecular weights. Furthermore, the investigation of droplet motion dynamics on LLS demonstrates that droplets consistently exhibit a rolling state, regardless of the intensity of the applied lateral force. The movement pattern of the droplet shifts only under conditions where the grafting density is significantly reduced and the substrate exhibits hydrophilic tendencies. These findings and the developed model are anticipated to offer valuable guidelines for optimal designs of LLS.

Droplet Manipulation on Bioinspired Slippery Surfaces: From Design Principle to Biomedical Applications

Droplet manipulation with high efficiency, high flexibility, and programmability, is essential for various applications in biomedical sciences and engineering. Bioinspired liquid-infused slippery surfaces (LIS), with exceptional interfacial properties, have led to expanding research for droplet manipulation. In this review, an overview of actuation principles is presented to illustrate how materials or systems can be designed for droplet manipulation on LIS. Recent progress on new manipulation methods on LIS is also summarized and their prospective applications in anti-biofouling and pathogen control, biosensing, and the development of digital microfluidics are presented. Finally, an outlook is made on the key challenges and opportunities for droplet manipulation on LIS.

Durable, Ultrathin, and Antifouling Polymer Brush Coating for Efficient Condensation Heat Transfer

Heat exchangers are made of metals because of their high heat conductivity and mechanical stability. Metal surfaces are inherently hydrophilic, leading to inefficient filmwise condensation. It is still a challenge to coat these metal surfaces with a durable, robust, and thin hydrophobic layer, which is required for efficient dropwise condensation. Here, we report the nonstructured and ultrathin (∼6 nm) polydimethylsiloxane (PDMS) brushes on copper that sustain high-performing dropwise condensation in high supersaturation. Due to the flexible hydrophobic siloxane polymer chains, the coating has low resistance to drop sliding and excellent chemical stability. The PDMS brushes can sustain dropwise condensation for up to ∼8 h during exposure to 111 °C saturated steam flowing at 3 m·s-1, with a 5-7 times higher heat transfer coefficient compared to filmwise condensation. The surface is self-cleaning and can reduce the level of bacterial attachment by 99%. This low-cost, facile, fluorine-free, and scalable method is suitable for a great variety of heat transfer applications.

Interplay of Drop Shedding Mechanisms on High Wettability Contrast Biphilic Stripe-Patterned Surfaces

To improve the rate of DWC, numerous studies have adjusted the distribution of drops through biphilic surface patterning and wettability gradients to control the nucleation and drop shedding rates on the condensing surface, yet the connection between drop shedding mechanisms and surface wettability patterning remains unclear. Moreover, wettability patterning places geometric bounds on the governing forces (i.e., gravity, capillary, and inertia), which drive the droplet shedding mechanisms. Thus, the subsequent influence of droplet distribution along the DWC regions on the shedding mechanisms may not be known a priori. In this study, the area fraction, ADWC, of the DWC and also the DWC region width, LN, were varied between 10 and 50% and 0.5-1.5 mm, respectively, to probe the dominant droplet shedding mechanisms on a high wettability contrast surface (i.e., the contact angle on the DWC was 159 ± 3.4° and the hysteresis 9 ± 3.6°, whereas the FWC was nearly perfectly wetting). Humid air was introduced inside a custom-built chamber with the upright steady-state condensation imaged by both real-time and high-speed imaging techniques. We found that the droplet shedding mechanisms changed with increasing LN where the sliding drop radii are reduced with LN while the jumping drop radii remained unchanged with LN. The maximum drop size for shedding also decreased by 13%, which we attribute to the secondary droplet inertia, which helps gravity overcome the capillary retention force. Lastly, although many studies have probed DWC enhancements via surface wettability patterning, an optimal combination of ADWC and LN provided in this study significantly aids in the improvement of future DWC-based condensers and water collector applications.

Nanostructure Explains the Behavior of Slippery Covalently Attached Liquid Surfaces

Slippery covalently-attached liquid surfaces (SCALS) with low contact angle hysteresis (CAH, <5°) and nanoscale thickness display impressive anti-adhesive properties, similar to lubricant-infused surfaces. Their efficacy is generally attributed to the liquid-like mobility of the constituent tethered chains. However, the precise physico-chemical properties that facilitate this mobility are unknown, hindering rational design. This work quantifies the chain length, grafting density, and microviscosity of a range of polydimethylsiloxane (PDMS) SCALS, elucidating the nanostructure responsible for their properties. Three prominent methods are used to produce SCALS, with characterization carried out via single-molecule force measurements, neutron reflectometry, and fluorescence correlation spectroscopy. CO2 snow-jet cleaning was also shown to reduce the CAH of SCALS via a modification of their grafting density. SCALS behavior can be predicted by reduced grafting density, Σ, with the lowest water CAH achieved at Σ≈2. This study provides the first direct examination of SCALS grafting density, chain length, and microviscosity and supports the hypothesis that SCALS properties stem from a balance of layer uniformity and mobility.

Advances and challenges in slippery covalently-attached liquid surfaces

Over the past decade, a new class of slippery, anti-adhesive surfaces known as slippery covalently-attached liquid surfaces (SCALS) has emerged, characterized by low values of contact angle hysteresis (CAH, less than 5°) with water and most solvents. Despite their nanoscale thickness (1 to 5 nm), SCALS exhibit behavior similar to lubricant-infused surfaces, including high droplet mobility and the ability to prevent icing, scaling, and fouling. To date, SCALS have primarily been obtained using grafted polydimethylsiloxane (PDMS), though there are also examples of polyethylene oxide (PEO), perfluorinated polyether (PFPE), and short-chain alkane SCALS. Importantly, the precise physico-chemical characteristics that enable ultra-low CAH are unknown, making rational design of these systems impossible. In this review, we conduct a quantitative and comparative analysis of reported values of CAH, molecular weight, grafting density, and layer thickness for a range of SCALS. We find that CAH does not scale monotonically with any reported parameter; instead, the CAH minimum is found at intermediate values. For PDMS, optimal behavior is observed at advancing contact angle of 106°, molecular weight between 2 and 10 kg mol^-1, and grafting density of around 0.5 nm^-2. CAH on SCALS is lowest for layers created from end-grafted chains and increases with the number of binding sites, and can generally be improved by increasing the chemical homogeneity of the surface through the capping of residual silanols. We review the existing literature on SCALS, including both synthetic and functional aspects of current preparative methods. The properties of reported SCALS are quantitatively analyzed, revealing trends in the existing data and highlighting areas for future experimental study.

Omniphobic liquid-like surfaces

Liquid-repellent surfaces, especially smooth solid surfaces with covalently grafted flexible polymer brushes or alkyl monolayers, are the focus of an expanding research area. Surface-tethered flexible species are highly mobile at room temperature, giving solid surfaces a unique liquid-like quality and unprecedented dynamical repellency towards various liquids regardless of their surface tension. Omniphobic liquid-like surfaces (LLSs) are a promising alternative to air-mediated superhydrophobic or superoleophobic surfaces and lubricant-mediated slippery surfaces, avoiding fabrication complexity and air/lubricant loss issues. More importantly, the liquid-like molecular layer controls many important interface properties, such as slip, friction and adhesion, which may enable novel functions and applications that are inaccessible with conventional solid coatings. In this Review, we introduce LLSs and their inherent dynamic omniphobic mechanisms. Particular emphasis is given to the fundamental principles of surface design and the consequences of the liquid-like nature for task-specific applications. We also provide an overview of the key challenges and opportunities for omniphobic LLSs.

Electrically Manipulated Vapor Condensation on the Dimpled Surface: Insights from Molecular Dynamics Simulations

Random vapor nucleation leads to flooding condensation with degraded heat-transfer efficiency. Since an external electric field has a significant effect on manipulating droplets' motion, it is possible to be one of the effective methods to hinder flooding phenomena and improve the heat-transfer rate by applying the external electric field during condensation. However, the motion of nanodroplets is more sensitive to the electric field owing to the scale effect on the nanoscale. The effect of the electric field on growth has not explicitly been comprehended. This work studied the condensation processes on a nanodimpled surface under an electric field with various strengths and directions. The results showed that condensed droplets' growth under the electric field depends on the competition between the electric field force and solid-liquid interactions. Increased vertical electric field strength, the higher torsion by the electric field hindered the motion of vapor, decreased the collision frequency for water molecules with the cooled surface, and elongated the cluster when the electric field force dominates, thus deteriorating the condensation performance. While applying the horizontal electric field, the greater electric field strength leads to better condensation performance by the larger contacting area for heat exchange. A wetting transition induced by the electric field was observed when the electric field strength increased to a certain extent (E > 5.2 × 10⁸ V/m in this study). When the V-shaped surface replaced the dimpled surface as the condensed substrate, the same wetting transition phenomena occurred under a more significant horizontal electric field strength, showing that this method is universal. Besides, different electric field frequencies influenced both the growth and the nucleation, thus exhibiting various condensation performances.

Super-Slippery Poly(Dimethylsiloxane) Brush Surfaces: From Fabrication to Practical Application

Superwetting surfaces with special slippery performances have been the focus of practical applications and basic research for decades. Compared to superhydrophobic/superoleophobic and slippery liquid-infused porous surfaces (SLIPS), liquid-like covalently attached poly(dimethylsiloxane) (PDMS) brush surfaces have no trouble in constructing the micro/nanostructure and the loss of infused lubricant, meanwhile, it can also provide lots of new advantages, such as smooth, transparent, pressure- and temperature-resistant, and low contact angle hysteresis (CAH) to diverse liquids. This paper focuses on the relationship between the wetting performance and practical functional application of PDMS brush surfaces. Recent progress of the preparation of PDMS brush surfaces and their super-slippery performances, with a special focus on diverse functional applications were summarized. Finally, perspectives on future research directions are also discussed.

Design of Nanostructured Surfaces for Efficient Condensation by Controlling Condensation Modes

To meet the different needs of various industrial fields, it is of great application value to find a feasible method for controlling the condensation mode on the surface. Inspired by biological surfaces, tuning the surface structure and wettability is considered as a potential way to control the surface condensation behavior. Herein, the coupling effect of the geometric parameters and wettability distribution of the surface on the condensation process has been investigated systematically at the nanoscale. The results illustrate that the condensation mode is primarily determined by the nanopillar wettability when the nanopillars are densely distributed, while the substrate wettability dominates the condensation mode when the nanopillars are sparsely distributed. Besides, the effective contact area fraction is proposed, which more accurately reflects the influence of geometric parameters on the condensation rate of the nanopillar surface at the nanoscale. The condensation rate of the nanopillar surface increases with the increase of the effective contact area fraction. Furthermore, three surface design methods are summarized, which can control the condensation mode of water vapor on the surface into the dropwise condensation mode that generates Cassie-Baxter droplets, and this condensation process is very attractive for many practical applications.

Acoustic Tweezer-Modulated Biomimetic Patterned Particle-Polymer Composite for Water Vapor Harvesting

With the recent threat of climate change and global warming, ensuring access to safe drinking water is a great challenge in many areas worldwide. Designing functional materials for capturing water from natural resources like fog and mist has become one of the key research areas to maximize the production of clean water. From this aspect, nature is a great source for designing bioinspired functional materials as some of the plant leaves and animal exoskeletons can harness water and then store it to save themselves from arid, xeric conditions. Inspired by the Stenocara beetle, we have designed a composite surface structure with periodic islands made of aluminum microparticles surrounded by poly(dimethylenesiloxane) (PDMS). An acoustic tweezer-based method was used to fabricate the bioinspired composite structures, where surface acoustic waves at specific frequencies and amplitudes are applied to align the microparticles as islands in the polymer matrix. An oxygen plasma etching step was applied to expose the microparticles on the PDMS surface. The average water harvesting efficiencies for structures made with 120 and 80 kHz acoustic frequencies and 1 hour etching time were found to be 9.41 and 8.84 g cm^-2 h^-1, respectively. The acoustically patterned biomimetic composite surface showed higher water harvesting efficiency compared with completely hydrophobic PDMS and hydrophilic aluminum surfaces, demonstrating the advantages of the bioinspired composite material design and acoustic-assisted manufacturing technique. The biomimetic fog water harvesting material is a promising avenue to fulfill the demand for a cost-effective, sustainable, and energy-efficient solution to safe drinking water.

Vapor Condensation on Bioinspired Hierarchical Nanostructured Surfaces with Hybrid Wettabilities

Vapor condensation on bioinspired hierarchical nanostructured surfaces with hybrid wettabilities has been investigated using molecular dynamics simulations. A series of hierarchical surfaces consisting of nanocylinder arrays with hydrophilic top and hydrophobic nanopillar arrays are constructed. The results manifest that the condensed nanodroplets undergo three states in the whole water vapor condensation process, and the total condensed atom number on surfaces increases with the increase of nanocylinder diameter (D), which indicates that the introduction of hydrophilic nanocylinders is conducive to improving the condensation performance compared with that on the hydrophobic surface patterned with homogeneous nanopillars. However, the nucleation sites on hierarchical nanostructured surfaces are covered by the condensed nanodroplets at the end of condensation, which suppresses the further enhancement of condensation performance. To solve these problems, we add a collection region close to the edge of the nanostructured surface. It is noticed that the condensed nanodroplets can roll into the collection regions gradually during the condensation process, which keeps the nucleation sites on nanostructured surfaces exposed effectively, especially for the cases of 20 Å ≤ D ≤ 40 Å. Moreover, the cluster number, the total condensed atom number, and the condensation enhancement efficiency on hierarchical nanostructured surfaces with collection regions at 20 Å ≤ D ≤ 40 Å are higher obviously compared with those on surfaces without collection regions. Our study demonstrates that the bioinspired hierarchical nanostructured surface with the collection region is beneficial to boost the vapor condensation performance, which can bring new insights into water vapor condensation.

Hydrophilic reentrant SLIPS enabled flow separation for rapid water harvesting

Water harvesting from air has the potential to alleviate water scarcity in arid regions around the globe. To achieve efficient water harvesting, we prefer rapid vapor condensation and droplet collection simultaneously. Prior techniques are not able to separate the vapor and liquid flow, so the condensed droplets always hinder the vapor condensation. In this work, we report a flow-separation condensation mode on a hydrophilic reentrant slippery liquid-infused porous surface. The slippery reentrant channels absorb the condensed droplets, lock the liquid columns inside, and transport them to the end of each channel. As a result, the sustainable flow separation significantly increases the water harvesting rate.

Water harvesting from air is desired for decentralized water supply wherever water is needed. When water vapor is condensed as droplets on a surface the unremoved droplets act as thermal barriers. A surface that can provide continual droplet-free areas for nucleation is favorable for condensation water harvesting. Here, we report a flow-separation condensation mode on a hydrophilic reentrant slippery liquid-infused porous surface (SLIPS) that rapidly removes droplets with diameters above 50 μm. The slippery reentrant channels lock the liquid columns inside and transport them to the end of each channel. We demonstrate that the liquid columns can harvest the droplets on top of the hydrophilic reentrant SLIPS at a high droplet removal frequency of 130 Hz/mm². The sustainable flow separation without flooding increases the water harvesting rate by 110% compared to the state-of-the-art hydrophilic flat SLIPS. Such a flow-separation condensation approach paves a way for water harvesting.

Durable Liquid- and Solid-Repellent Elastomeric Coatings Infused with Partially Crosslinked Lubricants

Surfaces that are resistant to both liquid fouling and solid fouling are critical for many industrial and biomedical applications. However, surfaces developed to address these challenges thus far have been generally susceptible to mechanical damage. Herein, we report the design and fabrication of robust solid- and liquid-repellent elastomeric coatings that incorporate partially crosslinked lubricating chains within a durable polymer matrix. In particular, we fabricated partially crosslinked omniphobic polyurethane (omni-PU) coatings that can repel a broad range of liquid and solid foulants. The fabricated coatings are an order of magnitude more resistant to cyclic abrasion than current state-of-the-art slippery surfaces. Further through the integration of classic wetting and tribology models, we introduce a new material design parameter (K_AR) for abrasion-resistant polymeric coatings. This combination of mechanical durability and broad antifouling properties enables the implication of such coatings to a wide variety of industrial and medical settings, including biocompatible implants, underwater vehicles, and antifouling robotics.

Robust and durable liquid-repellent surfaces

This review provides a comprehensive summary of characterization, design, fabrication, and application of robust and durable liquid-repellent surfaces.