Non-wetting Characteristics Research Articles

Fluorine-Free and Transparent Superhydrophobic Coating with Enhanced Anti-Icing and Anti-Frosting Performance by Using D26 and KH560 as Coupling Agents

Superhydrophobic surfaces with non-wetting characteristics have been considered to be potential candidates for ice/frost prevention. In this study, a transparent superhydrophobic coating was created by using a simple method that employed (3-glycidoxypropyl) trimethoxysilane (KH560) and 1,2-Bis (trimethoxysilyl) ethane (D26) as coupling agents and epoxy resin (E51) as an adhesive. The synergy between KH560 and D26 significantly improves the long-term outdoor durability, anti-icing, and anti-frosting performance of the superhydrophobic coating. The coating also has good acid and alkali resistance, UV resistance, and durability. The obtained SiO2@E51@KH560@D26 can delay the freezing time of water by 1974 s, much longer than bare glass (345 s) and also longer than the coatings with only D26 (932 s) or with only KH560 (1087 s). Moreover, the SiO2@E51@KH560@D26 showed an improved anti-frosting capability compared with the other three samples and better maintained its superhydrophobic properties at low temperatures. Our study proposes a potential method to fabricate a superhydrophobic coating with both anti-icing and anti-frosting properties.

Spontaneous Transport Mechanics of Water Droplets under a Synergistic Action of Designed Pattern and Non-Wetting Gradient.

The controllable spontaneous transport of water droplets on solid surfaces has a broad application background in daily life. Herein, a patterned surface with two different non-wetting characteristics was developed to control the droplet transport behavior. Consequently, the patterned surface exhibited great water-repellant properties in the superhydrophobic region, and the water contact angle reached 160° ± 0.2°. Meanwhile, the water contact angle on the wedge-shaped hydrophilic region dropped to 22° after UV irradiation treatment. On this basis, the maximum transport distance of water droplets could be observed on the sample surface with a small wedge angle of 5° (10.62 mm), and the maximum average transport velocity of droplets was obtained on the sample surface with a large wedge angle of 10° (218.01 mm/s). In terms of spontaneous droplet transport on an inclined surface (4°), both the 8 μL droplet and 50 μL droplet could move upward against gravity, which showed that the sample surface possessed an obvious driving force for droplet transport. Surface non-wetting gradient and the wedge-shaped pattern provided unbalanced surface tension to produce the driving forces in the process of droplet transport, and the Laplace pressure as well is produced inside the water droplet during this process. This work provides a new strategy to develop a patterned superhydrophobic surface for droplet transport.

Mineral scaling on brass and aluminum surfaces with a range of wettability

Crystallization fouling, or mineral scaling, is fundamental to various surfaces in energy and environmental applications. This paper considers, for the first time, a systematic investigation of scaling of nonwetting aluminum and brass surfaces relative to their bare counterparts. Metallic surfaces with a range of nonwetting characteristics from hydrophilic to superhydrophobic and lubricant infused surfaces were fabricated using a facile etching method. Systematic dynamic flow scaling experiments are conducted on the six different surface types at three flow levels and two temperature levels. The interactive effects of the different parameters are analyzed relative to their correspondence to the nucleation theory. The study reveals that nonwetting metallic superhydrophobic or hydrophobic surfaces reduce fouling by over 50% compared to bare counterparts. Lubricant infused metallic surfaces demonstrate superior anti-scaling performance by over 60% and up to 90% fouling reduction. The study is the first to present fundamental insights into the engineering and anti-scaling performance of nonwetting brass and aluminum surfaces.

The effect of superhydrophobic coating composition on the topography and ice adhesion

Ice accumulation on solid surfaces adversely affects many applications. Among diverse surface modification methods to reduce ice adhesion strength, superhydrophobic (SHP) surfaces have been considered due to their non-wetting characteristics, which minimize the solid-liquid interface, reducing the actual solid-ice contact area. In this work, the shear adhesion strength required for ice removal was studied for a series of thermoplastic-based SHP coatings with a wide particle content range. It was found that particle content plays a complex role in the shear adhesion strength of the coating, which changes the locus of failure for ice removal. The presence of cracks at higher particle loading (above 40 wt%) significantly changed the behavior of the coating, increasing the shear stress for removal. This was attributed to the ice anchoring effect caused by crack formation and penetration of the freezing water into the cracks. To minimize the ice-SHP coating interface shear adhesion, crack formation should be eliminated incorporating the lowest possible NPs content, preserving the Cassie-Baxter wettability state. In the case of the PU nanocomposite coating, 35 and 40 wt% silica NPs coatings demonstrate the optimal result with very low ice adhesion. As crack formation is a common phenomenon in high content NPs nanocomposite coatings, the composition of nanocomposite coatings should be optimized for each material system to minimized crack presence with appropriate surface energetics.

A comparative study of mechanical and chemical durability of non-wetting superhydrophobic and lubricant-infused surfaces

Fabrication of bioinspired non-wetting superhydrophobic surfaces (SHS) and lubricant-infused surfaces (LIS) has been studied extensively on a variety of materials. In contrast, durability of the surfaces exposed to harsh mechanical and chemical environments has been the subject of little attention. This study considers the mechanical and chemical durability of SHS and LIS copper surfaces fabricated via facile electrodeposition and chemical etching methods. The as-fabricated surfaces demonstrate excellent non-wetting characteristics with water contact angle of 160° and sliding angle below 5°. The surfaces are subject to mechanical wear through scratch test and water jet impingement at 15 psi and 20 psi as well as accelerated corrosion following the ASTM-E407 standard. The performance of the electrodeposited and etched non-wetting surfaces is systematically assessed in terms of contact and sliding angles and corrosion rate in a simulated marine environment. All surfaces are shown to be robust to mechanical wear after scratch test, with excellent stability of contact and sliding angles, and up to two orders of magnitude reduced corrosion rate compared to bare copper surface. SHS retained steadfast non-wetting characteristics under high-pressure water jet tests compared to the other surfaces while LIS, regardless of texturing method, showed one to two orders of magnitude reduced corrosion rate compared to bare copper surface throughout water jet impingement and chemical durability tests. The study presents for the first time a systematic comparison of durability of SHS and LIS through a common set of fabrication and testing protocol and helps identify appropriate non-wetting surfaces and fabrication methods based on the use environment.

Temperature-dependent dynamic fouling on superhydrophobic and slippery nonwetting copper surfaces

• Presents, for the first time, temperature-dependent dynamic fouling on nonwetting surfaces. • Elucidates, for the first time, effects of texturing method and surface type on fouling behavior. • Nonwetting surfaces show up to 75% less scaling compared to regular surfaces. • Electrodeposited surfaces offer better scaling resistance compared to etched surfaces. • Presents Hill-Langmuir model, for the first time, to describe the time evolution of scaling. Bioinspired, superhydrophobic and slippery liquid infused surfaces that offer nonwetting characteristics have been explored in recent years for fouling mitigation. However, most of the studies are in the context of biofouling or under static immersion at ambient temperature conditions that are not reflective of the dynamic flow environment in practice. This article presents, for the first time, a systematic study of dynamic fouling of superhydrophobic (SHS) and slippery lubricant-infused surfaces (LIS) over a range of flow and temperature conditions. Copper metallic surfaces were textured via electrodeposition or etching and further functionalized to achieve SHS and, additionally, infiltrated with a lubricant to fabricate LIS. The nonwetting surfaces were studied for their fouling behavior in a rotating Couette flow of a supersaturated calcium sulfate solution at different rotational speed and temperature. Fouling mineral mass accumulation on the different surfaces was measured as a function of time over a period of days using inductively coupled plasma mass spectroscopy and the fouled surfaces were investigated using scanning electron microscopy. Both SHS and LIS showed superior anti-scaling performance at all ranges of variables. An analytical Hill-Langmuir model is presented, for the first time, to describe the time evolution of scaling within 20% accuracy over the range of parameters studied. The study is the first to juxtapose two surface texturing methods, electrodeposition and etching, and two nonwetting surface types, SHS and LIS, subject to a common suite of experiments to elucidate fundamental understanding of mineral fouling on nonwetting surfaces.

Cicada-inspired self-cleaning superhydrophobic surfaces

Abstract Water-based heat transfer surfaces in HVAC&R (Heating, Ventilation, Air Conditioning and Refrigeration) systems have biofouling or microbial growth which might not only impose adverse effects to public health but also deteriorate the heat transfer performance of surfaces. In nature, a living creature is surviving while being exposed to formidable condition such as bacteria and fungi. Cicada wing has known to have multi-functional wings exhibiting nonwetting, and anti-microbial due to nanoscale pillars and biochemicals to achieve non-wetting characteristics. We conducted experiments on cicada wings and cicada-inspired engineered superhydrophobic samples to show that droplets jumping from superhydrophobic wings could carry away individual particles (~100 µm) or cluster of particles. This study was investigated on self-cleaning mechanism using jumping droplet condensation and elucidate further antifouling characteristics of non-wetting heat transfer surfaces.

Impact dynamics of a polystyrene suspension droplet on nonwetting surfaces measured using a quartz crystal microresonator and a high-speed camera

We investigated the impact dynamics of droplets containing various sizes and concentrations of polystyrene microparticles (PSM) on superhydrophobic surfaces using a high-speed camera and a quartz crystal microresonator (QCM). Gold nanoflake structures were synthesized on QCM surfaces and treated with perfluorooctane ethylthiol molecules to obtain superhydrophobic characteristics. Upon the collision onto the superhydrophobic surface, the droplet spread, retracted, and bounced off the surface due to its nonwetting characteristics. Within the experimental range of PSM size and PSM concentration, the high-speed camera detected only negligible changes in the impact dynamics; however, abnormally large changes in the resonance frequency were observed with the QCM when the concentration or sizes of the PSM exceeded a certain threshold. The abnormal change in frequency was attributed to the formation of a shear-induced PSM structure.

Reusable dry adhesives based on ethylene vinyl acetate copolymer with strong adhesion

ABSTRACTThis work proposed the design and fabrication method for T‐shaped dry adhesives based on stretchable ethylene‐vinyl acetate (EVA) copolymer, with strong normal adhesion strength up to 70 N cm−2. As fabricated adhesives based on the inexpensive soft replication process have comprehensive advantages including easy demolding applicable for various curable materials; self‐cleaning characteristic based on excellent nonwetting characteristics to both water and oil; reversible adhere/detach cycling for reusability. The flexible and transparent EVA dry adhesive films were readily fabricated to be novel dry‐adhesive coatings decorated on various substrates and shapes. We also explored the adhere and detach mechanism for the significantly enhanced adhesion of T‐shaped EVA microstructures, which indicated the increased contact area and the friction dissipation between the undercut and the substrate contribute to the adhesion enhancement, while the excellent elastic deformation of EVA may play an important role for adhesion enhance during the pulling‐off process. These results showed the great potential of high‐performance T‐shaped EVA‐based polymer dry adhesives for practical applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47296.

Soil water repellency: A molecular-level perspective of a global environmental phenomenon

Soil water repellency affects vast regions of agricultural and native land across the globe. Sandy soils are most likely to develop non-wetting characteristics with susceptibility decreasing with clay content. This study uses molecular dynamics simulations combined with laboratory scale experiments to study the interactions and assembly of saturated C16 straight chain alcohol and carboxylic acid molecules on a range of soil surfaces. The development of severe water repellency on sand at low loading levels of palmitic acid (0.75 × 10−6 mol g−1) is explained in terms of favourable H-bonding with silica surfaces and the development of 2–3 layers of lateral aligned organic molecules on the mineral surface. The efficacy of kaolinite in lowering soil water repellency is attributed to the higher surface area of this mineral and favourable surface chemistry, which hinders formation of persistent organic multilayers.

High-performance polymer dry adhesives based on ethylene vinyl acetate copolymer and high-adhesion mechanism.

We have proposed a new soft replication fabrication method to fabricate mushroom-shaped dry adhesives with excellent high adhesion, and analyzed its performance and mechanism by taking into consideration the microstructure deformation. The large Young's modulus of ethylene-vinyl acetate (EVA) copolymer (48.0 MPa) together with rational structure design contribute to easy demolding as well as strong adhesion. The fabricated EVA dry adhesives exhibited strong normal adhesion up to 70 N/cm $^{\mathbf {2}}$, which is one of the best records reported for polymer-based dry adhesives by far. Flexible and transparent EVA dry adhesive films were readily microfabricated through our soft replication method in an inexpensive and high-throughput manner, which enables low-cost adhesive coatings for various substrates and shapes.. In addition, the non-wetting characteristics to repel water and oil makes it promising for robust self-cleaning and reusability. These results may shed new lights for the mass-production of dry adhesives, and to understand the reversible adhesion/detachment mechanism.

Droplet Cloaking Imaging and Characterization

Lubricant infused surfaces (LIS or SLIPS) have potential to enhance a variety of applications due to their non-wetting characteristics. Recently, LIS and SLIPS have been shown to greatly improve dropwise condensation heat transfer performance by enabling ultra-low contact angle hysteresis (< 5°) and droplet-surface adhesion. The low adhesion characteristics arise from the ultra-smooth liquid-liquid interface that depends on the lubricating fluid remaining infused into the surface by capillary forces. A key limiting phenomenon of LIS and SLIPS is cloaking, whereby the infused lubricant spreads and forms a very thin layer (∼100 nm) over liquid droplet residing on the substrate. This can lead to degradation due to drainage of the lubricant layer. Cloaking involves several fluid interfaces between the lubricant, droplet and air, and the resolution of the lubricant-droplet interface is essential to cloaking imaging and characterization. Our work involves goniometric measurements of advancing and receding contact angles, imaging of cloaked water droplets on LIS and SLIPS, and the characterization of cloaking during shear induced flow. The LIS and SLIPS substrates were created by spin coating lubricants of various viscosities (5 – 2712 cSt) onto superhydrophobic nanostructured boehmite (AlO(OH)). Cloaking effects were observed by inhibition of water droplet evaporation (Fig. a). Furthermore, the lubricantdroplet interface for non-cloaking droplets was resolved using lubricant-miscible dyes (Fig. a). Cloaking was also characterized by studying shear induced flow of both condensate and artificially injected droplets, resulting in lubricant drainage as shown in Figures (b) and (c). The results of this work not only provide a basis for understanding the lubricant-droplet interactions on LIS and SLIPS, but also material design guidelines for future LIS and SLIPS coatings.

Assessment of optical transmittance of oil impregnated and non-wetted surfaces in outdoor environment towards solar energy harvesting

Self-cleaning of optically transmitting surfaces is important to improve the performance of solar harvesting devices in harsh environments such as dust and humid ambient. Non-wetting characteristics of surfaces can create self-cleaning effects while lowering efforts required to remove dust from surfaces. Non-wetting, in which the texture has been filled with some lubricant, have robust self-cleaning characteristics than surfaces with air pockets trapped between the textures. Consequently, non-wetting surface in which the textures are impregnated with silicone oil is examined in relation to solar energy harvesting applications. In this case, a hierarchal texture is generated first onto the transparent glasses by the convective assembly of silica particles. The resulting textures are characterized incorporating scanning electron and atomic force microscopes. The textures are then impregnated with silicone oil after functionalization with OTS (Octadecyltrichlorosilane). Non-wetting behavior, film thickness and transmittance of the resulting surface are assessed using contact angle goniometer, ellipsometer, and UV–Visible spectrophotometer. The resulting surfaces are tested in outdoor environments and transmittance reduction of UV–visible spectrum is assessed. It is found that silicon oil impregnation improves water droplet mobility at the surface; however, optical transmittance reduces significantly over the time due to dust particles settlement at the silicon oil film interface.

Multiphase static droplet simulations in hierarchically structured super-hydrophobic surfaces

The surface of first part of study is textured with microscopic pillars of prototypical top geometries as a rectangle. The second one is textured with a hierarchical structure, composed of secondary pillar structures added on the primary texture. The length ratio between two scales of texture is 1:16. We evaluated the non-wetting characteristics of two types of surfaces by measuring CAs as well as the transition from the Wenzel’s to Cassie’s regimes. We measure the Contact angles (CAs), using the Lattice Boltzmann model (LBM), for two different surface configurations. We evaluated the effect of the hierarchical structure; the robustness of the Cassie regime is enhanced and the apparent contact angle is increased by the secondary structures. This is achieved by increasing the energy barrier against the transition between wetting and non-wetting regimes.

An oxidized liquid metal-based microfluidic platform for tunable electronic device applications.

Easy movement of oxidized Galinstan in microfluidic channels is a promising way for the wide application of the non-toxic liquid metal. In this paper, two different surface modification techniques (physical and chemical) are reported, which dramatically improve the non-wetting characteristics of oxidized Galinstan in the microfluidic channel. In the physical technique, normal paper textures are transferred to the inner wall of polydimethylsiloxane (PDMS) channels and four types of nanoparticles are then coated on the surface of the wall for further improvement of the non-wetting characteristics. Highest advancing angle of 167° and receding angle of 151° are achieved on the paper-textured PDMS with titanium oxide (TiO2) nanoparticles. In the chemical technique, three types of inorganic acids are employed to generate dual-scale structures on the PDMS surface. The inner wall surface treated with sulfuric acid (H2SO4) shows the highest contact angle of 167° and a low hysteresis of ~14° in the dynamic measurement. Creating, transporting, separating and merging of oxidized Galinstan droplets are successfully demonstrated in the fabricated PDMS microfluidic channels. After optimization of these modification techniques, the potential application of tunable capacitors and electronic filters is realized by using liquid metal-based microfluidic devices.

The multi-layered protective cuticle of Collembola: a chemical analysis.

Collembola, also known as springtails, are soil-dwelling arthropods that typically respire through the cuticle. To avoid suffocating in wet conditions, Collembola have evolved a complex, hierarchically nanostructured, cuticle surface that repels water with remarkable efficiency. In order to gain a more profound understanding of the cuticle characteristics, the chemical composition and architecture of the cuticle of Tetrodontophora bielanensis was studied. A stepwise removal of the different cuticle layers enabled controlled access to each layer that could be analysed separately by chemical spectrometry methods and electron microscopy. We found a cuticle composition that consisted of three characteristic layers, namely, a chitin-rich lamellar base structure overlaid by protein-rich nanostructures, and a lipid-rich envelope. The specific functions, composition and biological characteristics of each cuticle layer are discussed with respect to adaptations of Collembola to their soil habitat. It was found that the non-wetting characteristics base on a rather typical arthropod cuticle surface chemistry which confirms the decisive role of the cuticle topography.

Directed assembly of nanoparticles to isolated diatom valves using the non-wetting characteristics after pyrolysis

A novel strategy for a directed nanoparticle coupling to isolated Stephanopyxis turris valves is presented. After pyrolysis, the valves exhibit incomplete wetting due to their characteristic T-shaped profiles as a prerequisite for a regioselective coupling reaction. A micromanipulation system allows for precise handling and their immobilization onto an adhesive substrate and manipulation into arrays.

Characterization of Plasma Polymerized Hexamethyldisiloxane Films Prepared by Arc Discharge

Herein, we present a simple method for fabricating plasma polymerized hexamethyldisiloxane films (pp-HMDSO) possessing superhydrophobic characteristics via arc discharge. The pp-HMDSO films were deposited on a soda–lime–silica float glass using HMDSO monomer vapor as a precursor. A detailed surface characterization was performed using scanning electron microscopy and atomic force microscopy. The growth process of the pp-HMDSO films was investigated as a function of deposition time from 30 to 300 s. The non-wetting characteristics of the pp-HMDSO films were evaluated by means of contact angle (CA) measurements and correlated with the morphological characteristics, as obtained from microscopy measurements. The deposited films were found to be nano-structured and exhibited dual-scale roughness with the static CA values close to 170°. Fourier transform infrared spectroscopy analysis was carried out to investigate chemical and functional properties of these films. Methyl groups were identified spectroscopically to be present within the pp-HMDSO films and were proposed to result in the low surface energy of material. The synergy between the dual-scale roughness and low surface energy resulted in the superhydrophobic characteristics of the pp-HMDSO films. A possible mechanism for the pp-HMDSO film formation is proposed.

Model Passivated Carbon Electrodes for Fluorine Generation in Molten KF-2HF Electrolyte

Vulcan carbon electrodes of varying PTFE-content (20 w.% and 35 w.%) were synthesized using a sol-gel process. Electrochemical fluorination was carried out over a range of potentials for fluorine generation (4 to 8 V) in molten, KF·2HF electrolyte at ~90 °C. Electrochemical behaviour of PTFE-embedded carbon electrodes was compared to both passivated and non-passivated graphite and amorphous carbon electrodes. An attempt to correlate the amount of embedded PTFE with the degree of passivation on carbon electrodes was made. This was used to compare the non-wetting characteristics of an electrochemically passivated carbon electrode to those of an artificially “passivated” electrode. PTFE contents in the embedded electrodes were confirmed using TGA. The effects of surface and embedded C-F compounds on the electrical properties of the different electrodes were investigated using AC impedance.

Water droplet behavior on superhydrophobic SiO2 nanocomposite films during icing/deicing cycles

This work investigates water droplet behavior on superhydrophobic (water contact angle value of 162 ± 1°) SiO2 nanocomposite films subjected to repetitive icing/deicing treatments, changes in SiO2 nanocomposite film surface morphology and their non-wetting characteristics. During the experiment, water droplets on SiO2 nanocomposite film surface are subjected to a series of icing and deicing cycles in a humid (~ 70% relative humidity) atmosphere and the resulting morphological changes are monitored and characterized using atomic force microscopy (AFM) and contact angle measurements. Our data show that the formation of the frozen or thawed water droplet, with no further shape change, on superhydrophobic SiO2 nanocomposite film, is obtained faster within each cycle as the number of the icing/deicing cycles increases. After 10 icing and deicing cycles, the superhydrophobic SiO2 nanocomposite film had a water contact angle value of 146 ± 2° which is effectively non-superhydrophobic. AFM analysis showed that the superhydrophobic SiO2 nanocomposite film surface area under the water droplet undergoes gradual mechanical damage during the repetitive icing/deicing cycles. We propose a possible mechanism of the morphological changes to the film surface that take place during the consecutive icing/deicing experiments.