Repellent Surfaces Research Articles

Unconventional Dually-Mobile Superrepellent Surfaces.

The ability of water droplets to move freely on superrepellent surfaces is a crucial feature that enables effective liquid repellency. Common superrepellent surfaces allow free motion of droplets in the Cassie state, with the liquid resting on the surface textures. However, liquid impalement into the textures generally leads to a wetting transition to the Wenzel state and droplet immobilization on the surface, thereby destroying the liquid repellency. This study reports the creation of a novel type of superrepellent surface through rational structural control combined with liquid-like surface chemistry, which allows for the free movement of water droplets and effective repellency in both the Cassie and Wenzel states. Theoretical guidelines for designing such surfaces are provided, and experimental results are consistent with theoretical analysis. Furthermore, this work demonstrates the enhanced ice resistance of the dually-mobile superrepellent surfaces, along with their distinctive self-cleaning capability to eliminate internal contaminants. This study expands the understanding of superrepellency and offers new possibilities for the development of repellent surfaces with exceptional anti-wetting properties.

Surveying the Directional Transport of Water Droplets upon Impact on Metallic Topographic Gradients.

Drop impact phenomena on raw, polished, and topography-altered gradient surfaces are investigated and presented. The main aim of this study is to demonstrate that in using a one-step industrial patterning process, it is possible to obtain metal topographical wetting gradients that can produce various desired outcomes after droplet impact. The findings could be applied to improving wind or steam turbine blades. The ranges of Weber (We) and Reynolds (Re) numbers in the study are 3-300 and 650-6500, respectively. It is demonstrated that for a fixed We, the droplet transport outcomes change from bouncing-off to side-flipping to deposition depending on the impact location and the gradient strength. The effect of We in combination with the gradient strength was also considered to demonstrate droplet behavior similar to that observed on a uniform water repellent surface and on biphilic systems. In addition, full bouncing-off and directional control have been demonstrated. For the condition We = 95 ± 3, it was possible to achieve a maximum droplet recoil height of ∼6 mm and a side motion of almost 8 mm. A combination of different outcomes (e.g., splashing on one side of a droplet and passive horizontal translation on another) was observed on the studied gradients at We > 200 due to different wetting regimes across the droplet's three-phase line.

Toward vanishing droplet friction on repellent surfaces

Superhydrophobic surfaces are often seen as frictionless materials, on which water is highly mobile. Understanding the nature of friction for such water-repellent systems is central to further minimize resistance to motion and energy loss in applications. For slowly moving drops, contact-line friction has been generally considered dominant on slippery superhydrophobic surfaces. Here, we show that this general rule applies only at very low speed. Using a micropipette force sensor in an oscillating mode, we measure the friction of water drops approaching or even equaling zero contact-line friction. We evidence that dissipation then mainly stems from the viscous shearing of the air film (plastron) trapped under the liquid. Because this force is velocity dependent, it can become a serious drag on surfaces that look highly slippery from quasi-static tests. The plastron thickness is found to be the key parameter that enables the control of this special friction, which is useful information for designing the next generation of ultraslippery water-repellent coatings.

Reactive mesoporous organosoluble multifunctional hyperbranched polymer nanoparticles: Synthesis, post-polymerization reactions and clickable layer-by-layer coating

Synthesis of reactive organosoluble porous polymers, which can be useful for covalent layer-by-layer coatings, is synthetically challenging and not well known. In this work, we demonstrated a new approach to synthesize indole-decorated mesoporous hyperbranched polymer nanoparticles, which can be easily post-modified via triazolinedione (TAD)-indole click reaction. Importantly, further crosslinking of the polymer nanoparticles in solution leads to formation of insoluble mesoporous polymers, where porosity and surface area of the hyperbranched polymer nanoparticles further improved by ∼40%. Consequently, the nanoparticles are useful in layer-by-layer porous coatings and post-surface modifications. While studying their water repellent properties it was noted that nanoparticles having higher surface area/porosity results better water repellent surface which can be further enhanced via suitable surface functionalization using different long chain alkyl functionalities.

Nanorough Is Not Slippery Enough: Implications on Shedding and Heat Transfer.

Lowering droplet-surface interactions via the implementation of lubricant-infused surfaces (LISs) has received important attention in the past years. LISs offer enhanced droplet mobility with low sliding angles and the recently reported slippery Wenzel state, among others, empowered by the presence of the lubricant infused in between the structures, which eventually minimizes the direct interactions between liquid droplets and LISs. Current strategies to increase heat transfer during condensation phase-change relay on minimizing the thickness of the coating as well as enhancing condensate shedding. While further surface structuring may impose an additional heat transfer resistance, the presence of micro-structures eventually reduces the effective condensate-surface intimate interactions with the consequently decreased adhesion and enhanced shedding performance, which is investigated in this work. This is demonstrated by macroscopic and optical microscopy condensation experimental observations paying special attention at the liquid-lubricant and liquid-solid binary interactions at the droplet-LIS interface, which is further supported by a revisited force balance at the droplet triple contact line. Moreover, the occurrence of a condensation-coalescence-shedding regime is quantified for the first time with droplet growth rates one and two orders of magnitude greater than during condensation-coalescence and direct condensation regimes, respectively. Findings presented here are of great importance for the effective design and implementation of LISs via surface structure endowing accurate droplet mobility and control for applications such as anti-icing, self-cleaning, water harvesting, and/or liquid repellent surfaces as well as for condensation heat transfer.

Development of Nanostructured Polymer Surfaces by Self-Organization and Application to Liquid Repellent Surfaces

Development of Nanostructured Polymer Surfaces by Self-Organization and Application to Liquid Repellent Surfaces

One-step fabrication of strengthened breathable composite nanofiber membranes by electrospinning/electrospraying for highly waterproof application

Waterproof breathable membranes (WBMs) have attracted intense research interest due to their wide applications, however, there are still great challenges in effectively preparing WBMs with desirable waterproofness, breathability and elasticity. Here, a facile and scalable fabricating strategy combining electrospinning and electrospraying is proposed to manufacture nanofiber WBMs, which are comprised of both hydrophobic thermoplastic polyurethane (TPU) nanofibers and inter-fiber polyvinyl butyral (PVB) bonding containing a short perfluoroalkyl hydrophobic agent. The obtained composite nanofiber membranes showed durable water/oil repellent properties against ultrasonic washing and abrasion. The interconnected fibrous structure together with liquid repellent surface have endowed the WBMs with comprehensive properties with hydrostatic pressure of 73.7 kPa, water vapor transmission rate of 6.88 kg m−2 d−1, outstanding tensile strength of 25.72 MPa and large elongation at break of 217%, which have exceeded the majority of their nanofiber counterparts. This work could provide a novel approach for the preparation of waterproof and breathable membranes with balanced structural stability, waterproofness and wearing comfort, especially suitable for the personal protective material application.

Design and Fabrication of a New Filament, PLA with PTFE as Fillers, for Water Repellent Surfaces

The use of additive manufacturing by fused deposition is a versatile, cost-effective and simple prototyping and manufacturing technique that is generating and accelerating a revolution in equipment and filaments. However, materials are limited to a small number of polymers. It is a scientific challenge to bring new characteristics and properties to the parts obtained. In this work, a new filament has been designed with the combination of PLA (poly lactic acid) as matrix and PTFE (polytetrafluoroethylene) as filler. This filament has improved the water repellency of the parts obtained. Cleaning, demoulding, anti-adherence, anti-frost, anti-humidity and anti-bacterial applications can be deployed with this new filament. Extruded filaments have been obtained with PLA beads and PTFE micropowder. Flat test tubes have been produced with this filament. The experiments included PTFE fillers (1% to 40% by weight). The surfaces have been characterised by sliding angle (SA) and static contact angle (CA) tests, surface roughness (Sa and Sz), flatness error and % water adsorption. The results indicate, as expected, that the higher the fluoropolymer content, the higher the hydrophobicity, reaching values of 125° for CA and 9° for SA, and the % adsorption decreases. In terms of roughness, the surfaces are less rough when the PTFE load increases. On the other hand, the flatness is a property strongly affected by the % PTFE load and at values higher than 15% it produces intense warping and deformation of the specimens. Finally, the PTFE loading thresholds in the PLA matrix have been obtained below which the wettability and dimensional reproduction properties are balanced and optimal.

Salivary Pellicle Formed on Dental Composites Evaluated by Mass Spectrometry-An In Situ Study.

(1) Background: In the oral environment, sound enamel and dental restorative materials are immediately covered by a pellicle layer, which enables bacteria to attach. For the development of new materials with repellent surface functions, information on the formation and maturation of salivary pellicles is crucial. Therefore, the present in situ study aimed to investigate the proteomic profile of salivary pellicles formed on different dental composites. (2) Methods: Light-cured composite and bovine enamel samples (controls) were exposed to the oral cavity for 30, 90, and 120 min. All samples were subjected to optical and mechanical profilometry, as well as SEM surface evaluation. Acquired pellicles and unstimulated whole saliva samples were analyzed by SELDI-TOF-MS. The significance was determined by the generalized estimation equation and the post-hoc bonferroni adjustment. (3) Results: SEM revealed the formation of homogeneous pellicles on all test and control surfaces. Profilometry showed that composite surfaces tend to be of higher roughness compared to enamel. SELDI-TOF-MS detected up to 102 different proteins in the saliva samples and up to 46 proteins in the pellicle. Significant differences among 14 pellicle proteins were found between the composite materials and the controls. (4) Conclusions: Pellicle formation was material- and time-dependent. Proteins differed among the composites and to the control.

Water–soil interactions: Unravelling the processes and stages involved in the wetting of water repellent soils

The water repellent behaviour of soils is a widely studied phenomenon given its implications for infiltration, runoff, erosion and preferential flow. However, the principles underlying the eventual penetration of water into affected soils remain poorly understood. Theoretical considerations of the energetics and kinetics involved as a water drop makes contact with a water repellent soil surface and eventually penetrates into the soil suggest three distinct stages in the overall process. These stages are 1) adhesional wetting as soil and water first make contact, followed by 2) a kinetic barrier transitional stage in which molecular reorganisation of organics on soil reduces the water-soil contact angle to allow the water drop to sit deeper over soil particles of initial contact such that there is contact with particles in directly underlying soil layers, and finally 3) branching interstitial wetting as water penetrates into the bulk soil. Studies presented here of optical microscopy, mass of soil initially wetted, penetration time through layers of soil of different thicknesses, and time-dependent measurements of contact angle, volume of water penetrated, and mass of soil wetted, all give results consistent with this model. However, only for highly water repellent soils can distinct stages in wetting be clearly resolved experimentally, presumably because only these soils have a high enough kinetic barrier in the transitional stage for good separation between stages. For less water repellent soils, while the general time dependent behaviour remains consistent with the model, the distinction between the three stages is not so easy to resolve experimentally. The roles of contact angle, particle size distribution and drop size in determining the rates of these stages is considered, and the implications of the model for understanding soil water repellency are discussed.

Cassie's Law Reformulated: Composite Surfaces from Superspreading to Superhydrophobic.

In 1948, Cassie provided an equation describing the wetting of a smooth, heterogeneous surface. He proposed that the cosine of the contact angle, θc, for a droplet on a composite surface could be predicted from a weighted average using the fractional surface areas, fi, of the cosines of contact angles of a droplet on the individual component surfaces, i.e., cos θc = f1 cos θ1 + f2 cos θ2. This was a generalization of an earlier equation for porous materials, which has recently proven fundamental to underpinning the theoretical description of wetting of superhydrophobic and superoleophobic surfaces. However, there has been little attention paid to what happens when a liquid exhibits complete wetting on one of the surface components. Here, we show that Cassie's equation can be reformulated using spreading coefficients. This reformulated equation is capable of describing composite surfaces where the individual surface components have negative (droplet state/partial wetting) or positive (film-forming/complete wetting) spreading coefficients. The original Cassie equation is then a special case when the combination of interfacial tensions results in a droplet state on the composite surface for which a contact angle can be defined. In the case of a composite surface created from a partial wetting (droplet state) surface and a complete wetting (film-forming) surface, there is a threshold surface area fraction at which a liquid on the composite surface transitions from a droplet to a film state. The applicability of this equation is demonstrated from literature data including data on mixed self-assembled monolayers on copper, silver, and gold surfaces that was regarded as definitive in establishing the validity of the Cassie equation. Finally, we discuss the implications of these ideas for super-liquid repellent surfaces.

Ice-fouling on superhydrophobic and slippery surfaces textured by 3D printing: revealing key limiting factors

Solid surfaces, subjected to subzero temperatures, experience ice-fouling that poses a substantial economic burden to various industries (e.g. energy, transportation, aviation, building). Employing repellent surface chemistry on engineered surfaces (e.g. pillar, tubular, porous) delays icing and reduce ice adhesion. However, the limiting parameters that fail the icephobic properties of "slippery" and "superhydrophobic (SH)" surfaces are not well explored. Herein, we introduce anodized and chemically modified "slippery", and "SH" surfaces textured by 3D-printing to justify their ice-phobic properties considering the influencing factors, namely surface energy and roughness, temperature and humidity gradient, and droplet-to-surface contact area. Although "slippery" and "SH" surfaces can resist ice nucleation for a few hours at moderate subzero temperatures, they fail when the temperature drops to -12°C. Despite posing ∼10 times lower adhesion strength (i.e. shear stress) than the control, "SH" surface with moisture-induced micro ice-crystals exhibited twice the ice-adhesion measured without the presence of micro ice-crystals on it. In contrast, the "slippery" sample, due to antifreeze properties, did not get affected by the presence of moisture-induced micro ice-crystals, exhibiting ∼17 times lower adhesion strength than the control substrate. The comparative aspects and explored limiting factors of icing on "slippery" and "SH" surfaces are significant for many industries that suffer from ice-fouling.

Reforesting degraded hillslopes with exotic pines in Trinidad and Tobago: Infiltration, repellency and implications for runoff and recharge

In the Caribbean, planting resilient, fast growing exotic pine trees have commonly been used to reforest degraded landscapes. However, little is currently understood about how these practices alter soil water movement and subsequently runoff and recharge. We monitored the surface soil water repellency, hydraulic conductivity, and subsurface soil moisture response to individual storm events in an exotic pine plantation and native forest in the dry and wet seasons. During the wet season, neither forest had a repellent soil surface, and the hydraulic conductivity in the exotic pines (48 mm h−1) were not significantly higher than the native forest (36 mm h−1). The speed and magnitude of the soil moisture response did not differ substantially between both forests in the wet season. However, during the mid dry season, a threshold soil moisture level was reached in the pines which triggered the development of repellent/hydrophobic conditions across all locations. Repellency reduced the surface hydraulic conductivity to 1.6 mm h−1, which in turn severely reduced the soil moisture response to rainfall. Our findings illustrate that throughout most of the year, the exotic pine plantation does not have a substantial negative impact on runoff or recharge. Only under the driest conditions do they negatively affect these processes. Our work also highlights the importance of characterising soil water repellency, hydraulic conductivity, and subsurface soil water responses over longer time periods to account for temporal variability.

Can Superhydrophobic PET Surfaces Prevent Bacterial Adhesion?

Prevention of bacterial adhesion is a way to reduce and/or avoid biofilm formation, thus restraining its associated infections. The development of repellent anti-adhesive surfaces, such as superhydrophobic surfaces, can be a strategy to avoid bacterial adhesion. In this study, a polyethylene terephthalate (PET) film was modified by in situ growth of silica nanoparticles (NPs) to create a rough surface. The surface was further modified with fluorinated carbon chains to increase its hydrophobicity. The modified PET surfaces presented a pronounced superhydrophobic character, showing a water contact angle of 156° and a roughness of 104 nm (a considerable increase comparing with the 69° and 4.8 nm obtained for the untreated PET). Scanning Electron Microscopy was used to evaluate the modified surfaces morphology, further confirming its successful modification with nanoparticles. Additionally, a bacterial adhesion assay using an Escherichia coli expressing YadA, an adhesive protein from Yersinia so-called Yersinia adhesin A, was used to assess the anti-adhesive potential of the modified PET. Contrarily to what was expected, adhesion of E. coli YadA was found to increase on the modified PET surfaces, exhibiting a clear preference for the crevices. This study highlights the role of material micro topography as an important attribute when considering bacterial adhesion.

Advanced polymer processing technologies for micro‐ and nanostructured surfaces: A review

AbstractFunctional surfaces with attractive properties, such as superhydrophobicity and omniphobicity often rely on the synergy between intrinsic material properties and dual scale micro‐ and nano‐hierarchical structures for achieving desired wettability. Historically, engineered liquid repellent surfaces have attracted a great deal of interest from academia and industry due to their broad application prospects. Hence, for several years, there have been significant scientific efforts by researchers exploring state of the art manufacturing technologies that are efficient and yet cost effective to produce functional liquid repellent surfaces at an industrial scale. This technical review summarizes the various advanced and state of the art polymer processing technologies employed to fabricate the micro‐ and nanostructured polymer surfaces, with a special focus given to superhydrophobic and omniphobic applications. Here, we discuss the merits and limitations of each fabrication methods available for micro‐ and nanostructuring of polymer‐based surfaces. In addition, an attempt has been made to provide insight into the relationship between geometry of micro/nanostructures (size and shape) and intrinsic wettability on liquid repellency. A special section has been devoted to feature and document all commercialization efforts, including various commercially available products that were developed in the past decade. Finally, outlook and the development trend in the polymer micro‐ and nanostructured surfaces are highlighted to lead future research.

Highly Stable Hierarchically Structured All-Polymeric Lubricant-Infused Films Prevent Thrombosis and Repel Multidrug-Resistant Pathogens.

Thrombus formation and infections caused by bacterial adhesion are the most common causes of failure in blood-contacting medical devices. Reducing the interaction of pathogens using repellent surfaces has proven to be a successful strategy in preventing device failure. However, designing scale-up methodologies to create large-scale repellent surfaces remains challenging. To address this need, we have created an all-polymeric lubricant-infused system using an industrially viable swelling-coagulation solvent (S-C) method. This induces hierarchically structured micro/nano features onto the surface, enabling improved lubricant infusion. Poly(3,3,3-trifluoropropylmethylsiloxane) (PTFS) was used as the lubricant of choice, a previously unexplored omniphobic nonvolatile silicone oil. This resulted in all-polymeric liquid-infused surfaces that are transparent and flexible with long-term stability. Repellent properties have been demonstrated using human whole blood and methicillin-resistant Staphylococcus aureus (MRSA) bacteria matrices, with lubricated surfaces showing 93% reduction in blood stains and 96.7% reduction in bacterial adherence. The developed material has the potential to prevent blood and pathogenic contamination for a range biomedical devices within healthcare settings.

How to Select Phase Change Materials for Tuning Condensation and Frosting?

AbstractSynthetic surfaces engineered to regulate phase transitions of matter and exercise control over its undesired accrual (liquid or solid) play a pivotal role in diverse industrial applications. Over the years, the design of repellant surfaces has transitioned from solely modifying the surface texture and chemistry to identifying novel material systems. In this study, selection criteria are established to identify bio‐friendly phase change materials (PCMs) from an extensive library of vegetable‐based/organic/essential oils that can thermally respond by harnessing the latent heat released during condensation and thereby delaying ice/frost formation in the very frigid ambient that is detrimental to its functionality. Concurrently, a comprehensive investigation is conducted to elucidate the relation between microscale heat transport phenomena during condensation and the resulting macroscopic effects (e.g., delayed droplet freezing) on various solidified PCMs as a function of their inherent thermo‐mechanical properties. In addition, to freeze protection, many properties that are responsive to the thermal reflex of the surface, such as the ability to dynamically tune optical transparency, moisture harvesting, ice shedding, and quick in‐field repairability, are achievable, resulting in the development of protective coatings capable of spanning a wide range of functionalities and thereby having a distinctive edge over conventional solutions.

Smooth Transparent Omniphobic Coatings with Remarkable Liquid Repellence

AbstractLiquid repellent surfaces are of high interest in various fields like protective coatings and droplet manipulation due to their outstanding properties from self‐cleaning to anti‐fouling. Most reported surfaces rely on the fabrication of micro/‐nanostructured surfaces, the infusion of porous surfaces with immiscible lubricants, or the grafting of monolayers with low surface energy to achieve repellence of various liquids. However, these methods are limited in terms of their durability and long‐term stability. Here, the fabrication of smooth, transparent fluorinated poly (perfluoroalkyl methacrylate) coatings via UV‐initiated radical polymerization in a simple one‐step procedure is reported. The resulting surfaces show very low surface energies of down to 7 mN m−1, low contact angle hysteresis below 5°, very good repellence towards various liquids with different polarities as well as self‐cleaning and anti‐graffiti ability. The fabricated coatings can be applied to various substrates from glass slides to microfiber cloths. This demonstrates that they outperform common commercially available fluorinated coatings, such as, for example, Hyflon AD 40L S, in terms of liquid repellence and their performance in real‐world applications.

Dynamic behavior of impinging drops on water repellent surfaces: Machine learning-assisted approach to predict maximum spreading

The study of drop dynamic undergoing collision with solid surfaces seems quite necessary due to its practical applications ranging from coating industries to anti-icing and self-cleaning surfaces. Therefore, we experimentally studied the dynamic of impinging drop on water-repellent surfaces for a wide range of drop properties and initial velocities in terms of weber number (We). We considered the maximum spreading diameter to quantify the spreading dynamic. We modified one of the existing energy-balance models to analytically predict the observed maximum spreading diameters. We showed that above a critical We number (roughly 60–80), the maximum spreading diameter of superhydrophobic surfaces starts to deviate from those of hydrophobic surfaces. Therefore, we incorporated an adjusting factor into the energy-balance model to consider the transition from hydrophobicity to superhydrophobicity. Moreover, we developed a machine learning approach to predict the maximum spreading diameter as a function of drop properties and surface characteristics. Using the machine learning approach, it was found that beyond a critical contact angle (CAadv ∼ 150°–160°) the maximum spreading diameter does not depend on the contact angle anymore. Moreover, for low We numbers, the maximum spreading diameter decrease with increasing the contact angle, while for high We numbers they are directly proportional.

Liquid-Repellent Surfaces

Surfaces are vibrant sites for various activities with environments, especially as the transfer station for mass and energy exchange. In nature, natural creatures exhibit special wetting and interfacial properties such as water repellency and water affinity to adapt to various environmental challenges by taking advantage of air or liquid infusion media. Inspired by natural surfaces, various engineered liquid-repellent surfaces have been developed with a wide range of applications in both open and closed underwater environments. In particular, underwater conditions are characterized by high viscosity, high pressure, and complex compositions, which pose more challenges for the design of robust and functional repellent surfaces. In this Perspective, we take a parallel approach to introduce two classical liquid-repellent surfaces: an air-infused repellent surface and a lubricated liquid-repellent surface. Then we highlight fundamental challenges and design configurations of robust liquid-repellent surfaces both in air and underwater. We summarize the advantages and drawbacks of two kinds of repellent surfaces and list several applications of liquid-repellent surfaces for use in the ocean, medical care, and energy harvesting. Finally, we provide an outlook of research directions for robust liquid-repellent surfaces.