Super-repellent Surfaces Research Articles

Rational design of robust and transparent superrepellent surfaces for long-term marine antifouling

Rational design of robust and transparent superrepellent surfaces for long-term marine antifouling

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.

Suppression of hollow droplet rebound on super-repellent surfaces

Droplet rebound is ubiquitous on super-repellent surfaces. Conversion between kinetic and surface energies suggests that rebound suppression is unachievable due to negligible energy dissipation. Here, we present an effective approach to suppressing rebounds by incorporating bubbles into droplets, even in super-repellent states. This suppression arises from the counteractive capillary effects within bubble-encapsulated hollow droplets. The capillary flows induced by the deformed inner-bubble surface counterbalance those driven by the outer-droplet surface, resulting in a reduction of the effective take-off momentum. We propose a double-spring system with reduced effective elasticity for hollow droplets, wherein the competing springs offer distinct behavior from the classical single-spring model employed for single-phase droplets. Through experimental, analytical, and numerical validations, we establish a comprehensive and unified understanding of droplet rebound, by which the behavior of single-phase droplets represents the exceptional case of zero bubble volume and can be encompassed within this overarching framework.

Superrepellent Porous Polymer Surfaces by Replication from Wrinkled Polydimethylsiloxane/Parylene F.

Superrepellent surfaces, such as micro/nanostructured surfaces, are of key importance in both academia and industry for emerging applications in areas such as self-cleaning, drag reduction, and oil repellence. Engineering these surfaces is achieved through the combination of the required surface topography, such as porosity, with low-surface-energy materials. The surface topography is crucial for achieving high liquid repellence and low roll-off angles. In general, the combination of micro- and nanostructures is most promising in achieving high repellence. In this work, we report the enhancement of wetting properties of porous polymers by replication from wrinkled Parylene F (PF)-coated polydimethylsiloxane (PDMS). Fluorinated polymer foam "Fluoropor" serves as the low-surface-energy polymer. The wrinkled molds are achieved via the deposition of a thin PF layer onto the soft PDMS substrates. Through consecutive supercritical drying, superrepellent surfaces with a high surface porosity and a high water contact angle (CA) of >165° are achieved. The replicated surfaces show low roll-off angles (ROA) <10° for water and <21° for ethylene glycol. Moreover, the introduction of the micro-wrinkles to Fluoropor not only enhances its liquid repellence for water and ethylene glycol but also for liquids with low surface tension, such as n-hexadecane.

Designing non-textured, all-solid, slippery hydrophilic surfaces

Slippery surfaces are sought after due to their wide range of applications in self-cleaning, drag reduction, fouling-resistance, enhanced condensation, biomedical implants etc. Recently, non-textured, all-solid, slippery surfaces have gained significant attention because of their advantages over super-repellent surfaces and lubricant-infused surfaces. Currently, almost all non-textured, all-solid, slippery surfaces are hydrophobic. In this work, we elucidate the systematic design of non-textured, all-solid, slippery hydrophilic (SLIC) surfaces by covalently grafting polyethylene glycol (PEG) brushes to smooth substrates. Furthermore, we postulate a plateau in slipperiness above a critical grafting density, which occurs when the tethered brush size is equal to the inter-tether distance. Our SLIC surfaces demonstrate exceptional performance in condensation and fouling-resistance compared to non-slippery hydrophilic surfaces and slippery hydrophobic surfaces. Based on these results, SLIC surfaces constitute an emerging class of surfaces with the potential to benefit multiple technological landscapes ranging from thermofluidics to biofluidics.

A universal strategy to prepare hot liquid super-repellent surfaces for anti-scalding and anti-scaling

Durably and fully repel hot liquid (representative by water and oil) is difficult to achieve because of the high-flux evaporation–condensation behavior, which caused a lot of trouble in life and production, such as scalding and scaling. In this work, we report a universal strategy to realize hot liquid super-repellency (HLSR) on various solid surfaces by structuring fluorine-modified micro-nano binary topography (F-MNBT). Cassie-Baxter model is still applying to the F-MNBT surfaces even it contacting to 95℃ water, that because nanostructures (with size below 50 nm) design provides hot liquid repellency and microcavities (with size of over 10 μm) design remains in their non-wetting state. Through this strategy, we demonstrate the unique advantage of the HLSR on various cloths for anti-scalding. In addition, we also develop a F-MNBT composite coating that can durably sustain HLSR after enduring various representative mechanical, thermal and chemical stresses, that can continuously perform self-cleaning and anti-scaling at high temperatures. This work promises new solutions for surface applications related to hot liquid, expecting to serve in heat insulation protection, thermal management system, pipeline transport, marine petroleum exploitation, and so on.

Green Superlubricity Enabled by Only One Water Droplet on Plant Oil-Infused Surfaces

The increase in energy loss due to friction and the use of large amounts of lubricants to improve it are major challenges we face from a global environmental perspective. Pitcher-plant-inspired liquid-infused surfaces (LISs) are emerging super-repellent surfaces against liquids. However, their coefficient of friction (CoF) against solids is higher than that of conventional lubricant surfaces. Herein, we demonstrate superlubricity with a single water droplet placed on a LIS holding oleic acid, a component of plant oil. When a water droplet is placed on the fluid layer, the CoF under reciprocating and rotating friction is 0.012 and 0.0098, respectively. A force in the direction opposite to the loading due to the Laplace pressure on the droplet and an autonomous positional movement of the water accompanied by the optimization of surface energy maintain the fluid-lubrication state and prevent direct contact between the surface and the friction material, resulting in a decrease of the dependence of the CoF on the friction velocity. The key technology here will not only present a novel strategy for preparing LISs against solids but also serve as a step toward a sustainable green strategy for friction reduction and lubrication, which would greatly reduce energy loss and environmental degradation.

Durable Super-repellent Surfaces: From Solid–Liquid Interaction to Applications

ConspectusA super-repellent surface is a type of liquid-repellency material that allows for various liquid drops to bead up, roll off, or even bounce back. Known for its ability to remain dry, perform self-cleaning, and have a low adhesion, a super-repellent surface presents great advantages in a number of applications. These include antifogging, anti-icing, oil/water separation, and fluid drag reduction. To fend off the liquids or drops, super-repellent surfaces combine the merits of surface chemistry and physical structure. By taking advantage of a low surface energy to prevent liquid from spreading, the super-repellent surfaces utilize the micronano structure to provide a framework that confines the solid–liquid interactions. Compared to beading up the drop of water, the repellence of liquid with low surface tension requires the subtle design of surface structure to resist the wetting of liquids. However, the inherent instabilities of the fragile micronano structure of super-repellent surfaces and solid–liquid interactions further make the fabrication of super-repellent surfaces complex to withstand dynamic environments (friction or wear) during application. In addition, the transparency and thermal stability of super-repellent surfaces are also the restrictive factors in some special application scenarios. To solve these challenges, durable super-repellent surfaces that can repel various liquids, possess robust mechanical and thermal stability, and show high transparency have been explored extensively in recent years.In this Account, we systematically review our recent efforts to promote the super-repellent surfaces for real-world applications. Super-repellent surfaces that exhibit excellent resistance to various liquids, including liquids with low surface tension or high viscosity, were subtly designed and fabricated in some manner. Considering the stability of the wetting state at the solid–liquid interface, we established an evaluation system that includes highly curved surfaces and high Laplace-pressure conditions. To further perfect the wetting mechanism at the solid–liquid interactions, the dynamic wettability of super-repellent surfaces regulated by surface charge enrichment that was generated from solid–liquid interface separation was investigated. To resolve the bottleneck problem of the mechanical stability of super-repellent surfaces in real-world applications, a new decoupling material design mechanism was proposed, with a nanostructure that maintains water repellency and a microstructure providing durability. On the basis of the performance of the liquid-repellency, transparency, and mechanical and thermal stability of the super-repellent surfaces, a series of applications were demonstrated, such as microsphere synthesis, drop transportation and manipulation, and self-cleaning solar panels. Finally, a concise summary of this Account, including challenges and opportunities in super-repellent materials, has been provided. This research provides important guidance on solid–liquid interactions for the design of functional super-repellent surfaces and plays an important role in promoting large-scale industrial applications.

Transparent Super-Repellent Surfaces with Low Haze and High Jet Impact Resistance.

Transparent superhydrophobic surfaces are of vital significance for rising applications in optoelectronics, outdoor displays, building windows, and so on. However, facile fabrication of surfaces combining stable superhydrophobicity and high transparency with particularly low haze remains a challenge. Here, we demonstrate a nonfluorinated hierarchical surface, simply prepared by sequential spraying of a primer of poly(ethylene-co-acrylic acid) (EAA) and silica nanoparticles (SiO2). The resultant surface shows remarkable liquid repellency (e.g., an apparent contact angle of >160° and a sliding angle of <2° for honey) and high transparency (a transmittance of ∼91% and a haze of ∼6%). Especially, flexible EAA adhesive enables the surface to resist water impinging (up to ∼15.0 m s-1, higher than the terminal velocities of raindrops) and mechanical damaging. This super-repellent surface also presents excellent UV and chemical stability, sustaining a superhydrophobic state upon UVA exposure for 60 days and acidic corrosion or oil contamination for 7 days. With multirobustness and scalability, our coatings show great potential in related fields.

In situ tunable droplet adhesion on a super-repellent surface via electrostatic induction effect

SummaryIn this paper, we report a finding that substrate affects the adhesion of charged super-repellent surfaces. Water droplet impacting on a super-repellent surface produces surface charge, whose expression depends on the substrate. The charged super-repellent surface is sticky to droplets for a suspended substrate made of dielectric materials, while it has low adhesion for a conducting substrate or stage attached at the bottom because of electrostatic induction. Theoretical analysis and simulation are conducted to elucidate the mechanism of substrate effect on surface adhesion. Finally, we develop a new approach to reversibly tune the adhesion of super-repellent surface by combining surface-charge-induced adhesion increase and electrostatic-induction-regulated express of net surface charge. As a proof-of-concept experiment, we demonstrate that droplet sorting and manipulations can be realized by using this controllable surface adhesion tuning approach, which has potential applications in advanced lab-on-a-drop platform.

Anti-wettability of Chemically and Physically Modified Glass Surfaces

The demand for new advanced functional materials has driven scientific work over the past decades. Nature has been inspiring in the creation of different types of self-cleaning and super repellent surfaces mimicking those of plants (lotus leaves), animals (shark skin) or insects (butterfly wings, water strider). To produce and maintain super repellent materials, chemical modification of the surface by using low surface energy materials such as fluoropolymers and/or siloxanes is necessary. Also, physical modification of surface roughness enhances super-repellency against various liquids. The surface roughness can be achieved e.g., by the deposition of nano particles (NPs) using Liquid Flame Spray (LFS). Industrial applications like paper coatings, oil-water separation, and microfluidic devices have benefited from the fabrication of super-hydrophobic surfaces by LFS. In our work, glass substrates were fluorinated by chemical vapor deposition (CVD) method, and others were additionally pre-coated with silica NPs by LFS. The coated glass surfaces were characterized for their anti-wettability by measuring the contact angles of water and compare that to bare glass. The influence of the produced coatings on the wettability of surface with different liquids was examined through measuring advancing/receding contact angles as well as the roll off angle. Results showed that compared to bare glass only fluorination of glass increased the water static contact angle from 18 to almost 112 . This is indicative of hydrophobic behaviour. Coating glass with silica NPs by LFS before fluorination, enhanced the water anti-wetting property for super hydrophobicity. LFS coating provided good oleophobic characteristic.

Droplet Manipulations: Nonspecular Reflection of Droplets (Small 3/2021)

Dimples can break all symmetries in the law of reflection and make the bouncing of droplets on super-repellent surfaces non-specular, reported by Liqiu Wang and co-workers (article number 2006695), and thus functioning as a “magic” hand to enable various on-demand droplet manipulations including trapping, shedding, antigravitational transporting, targeted positioning, and coalescing.

Nonspecular Reflection of Droplets.

The bouncing of droplets on super-repellent surfaces normally resembles specular reflection that obeys the law of reflection. Here, the nonspecular reflection of droplet impingement onto solid surfaces with a dimple for energy-efficient, omnidirectional droplet transport is reported. With the dimple of the radius being comparable to that of the droplet, all the symmetries in the law of reflection can be broken down so that the droplet is endowed with a translational velocity finely tunable in both its direction and magnitude simply by varying the radii of the droplet and the dimple, the impinging position, and droplet Weber number. Tailoring the initial and translational velocity of impinging droplets would steer their reflected trajectories at will, thus enabling versatile droplet manipulation including trapping, shedding, antigravity transport, targeted positioning, and on-demand coalescence of droplets.

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

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

Water-Repellent Surfaces Consisting of Nanowires on Micropyramidal Structures

Super-repellent surfaces are relevant for several practical applications, such as water collection and self-cleaning and anti-icing surfaces. However, designing surfaces that can maintain their sup...

Mapping micrometer-scale wetting properties of superhydrophobic surfaces

There is a huge interest in developing superrepellent surfaces for antifouling and heat-transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be inferred indirectly using Furmidge's relation. While easy to implement, contact angle measurements are semiquantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an atomic force microscope cantilever to directly measure adhesion and friction forces with nanonewton force resolutions. We spatially map the micrometer-scale wetting properties of superhydrophobic surfaces and observe the time-resolved pinning-depinning dynamics as the droplet detaches from or moves across the surface.

Ricocheting Droplets Moving on Super-Repellent Surfaces.

Droplet bouncing on repellent solid surfaces (e.g., the lotus leaf effect) is a common phenomenon that has aroused interest in various fields. However, the scenario of a droplet bouncing off another droplet (either identical or distinct chemical composition) while moving on a solid material (i.e., ricocheting droplets, droplet billiards) is scarcely investigated, despite it having fundamental implications in applications including self‐cleaning, fluid transport, and heat and mass transfer. Here, the dynamics of bouncing collisions between liquid droplets are investigated using a friction‐free platform that ensures ultrahigh locomotion for a wide range of probing liquids. A general prediction on bouncing droplet–droplet contact time is elucidated and bouncing droplet–droplet collision is demonstrated to be an extreme case of droplet bouncing on surfaces. Moreover, the maximum deformation and contact time are highly dependent on the position where the collision occurs (i.e., head‐on or off‐center collisions), which can now be predicted using parameters (i.e., effective velocity, effective diameter) through the concept of an effective interaction region. The results have potential applications in fields ranging from microfluidics to repellent coatings.

Topography-Directed Hot-Water Super-Repellent Surfaces.

Natural and artificial super‐repellent surfaces are frequently textured with pillar‐based discrete structures rather than hole‐based continuous ones because the former exhibits lower adhesion from the reduced length of the three‐phase contact line. Counterintuitively, here, the unusual topographic effects are discovered on hot‐water super‐repellency where the continuous microcavity surface outperforms the discrete microneedle/micropillar surface. This anomaly arises from the different dependencies of liquid‐repellency stability on the surface structure and water temperature in the two topographies. The unexpected wetting dynamics are interpreted by determining timescales for droplet evaporation, vapor condensation, and droplet bouncing. The associated heat transfer process is unique to the wetting states and remarkably distinct from each other in the two topographies. It is envisioned that hot‐water super‐repellent microcavity surfaces will be advantageous for a variety of applications, especially when both self‐cleaning and thermal insulation are imperative, such as clothing for scald protection and digital microfluidics for exothermic reactions.

Conical micro-structures as a route for achieving super-repellency in surfaces with intrinsic hydrophobic properties

Hydrophobic surfaces like Lotus leaves show amazing self-cleaning properties with the apparent water contact angle above 150° and contact angle hysteresis below 10°. Thus, at low inclination angles, millimeter drops can roll-off easily. This effect can be a consequence of the air trapped below the drop, which allows the droplet to reach a superhydrophobic Cassie-Baxter state. However, the superhydrophobic state can be accompanied by very different adhesive properties due to the pinning of the droplet to the microstructures, implying that even in a hydrophobic or superhydrophobic state, the droplet might not roll-off easily. A superhydrophobic state with minimum adhesion to the surface has been the pursuit in many applications where a super-repellent state is highly desired. Many microstructures have been shown to be able to reach a superhydrophobic state, but only a few have been shown to be capable of achieving a super-repellent state without the help of more complex hierarchical structures. Here, we show that conical structures provide a template for designing super-repellent surfaces where the wetting characteristics look to be invariant in the microscale range. The conical structures can maintain a super-repellent state for all intrinsic contact angles larger than 90°, and the transition from the Cassie-Baxter to the Wenzel state is controlled by the apex angle of the conical structures. This finding advances the understanding of why conical structures can show a superhydrophobic state, which will be beneficial for the design of super-repellent surfaces with a wider intrinsic contact angle range.

Dropwise Condensation on Multiscale Bioinspired Metallic Surfaces with Nanofeatures.

Nonwetting surfaces engineered from intrinsically hydrophilic metallic materials are promising for self-cleaning, anti-icing, or condensation heat transfer applications where the durability of commonly applied hydrophobic coatings is an issue. In this work, we fabricate and study the wetting behavior and the condensation performance on two metallic nonwetting surfaces with varying number and size of roughness tiers without the need for further hydrophobic coating procedure. On one hand, the surface resembling a rose petal exhibits a sticky nonwetting behavior as drops wet the microscopic roughness features with consequent enhanced drop adhesion, which leads to filmwise condensation. On the other hand, the surface resembling a lotus leaf provides super-repellent nonwetting behavior prompting the continuous nucleation, growth, and departure of spherical drops in a dropwise condensation fashion. On a lotus leaf surface, the third nanoscale roughness tier (created by chemical oxidation) combined with ambience exposure prompts the growth of drops in the Cassie state with the benefit of minimal condensate adhesion. The two different condensation behaviors reported are well supported by a drop surface energy analysis, which accounts for the different wetting performance and the surface structure underneath the condensing drops. Further, we coated the above-mentioned surfaces with polydimethylsiloxane, which resulted in filmwise condensation due to the smoothening of the different roughness tiers. Continuous dropwise condensation on a hierarchical bioinspired lotus leaf metallic surface without the need for a conformal hydrophobic coating is hence demonstrated, which offers a novel path for the design and manufacture of noncoated metallic super-repellent surfaces for condensation phase change applications, among others.