Low Surface Energy Coatings Research Articles

Bioinspired Surfaces with Superwettability for Anti-Icing and Ice-Phobic Application: Concept, Mechanism, and Design.

Ice accumulation poses a series of severe issues in daily life. Inspired by the nature, superwettability surfaces have attracted great interests from fundamental research to anti-icing and ice-phobic applications. Here, recently published literature about the mechanism of ice prevention is reviewed, with a focus on the anti-icing and ice-phobic mechanisms, encompassing the behavior of condensate microdrops on the surface, wetting, ice nucleation, and freezing. Then, a detailed account of the innovative fabrication and fundamental research of anti-icing materials with special wettability is summarized with a focus on recent progresses including low-surface energy coatings and liquid-infused layered coatings. Finally, special attention is paid to a discussion about advantages and disadvantages of the technologies, as well as factors that affect the anti-icing and ice-phobic efficiency. Outlooks and the challenges for future development of the anti-icing and ice-phobic technology are presented and discussed.

Improvement of anti‐icing properties of low surface energy coatings by introducing phase‐change microcapsules

Polymers with low surface energy such as silicone and fluoropolymers are widely applied in preparing anti‐icing coatings, but they may have some limitations. To improve the anti‐icing properties of the coatings, composite coatings were developed by introducing phase‐change microcapsules (PCMs). Room temperature vulcanized silicone rubber and a fluorosilicone methacrylate copolymer were examined. Tests involving infrared thermal imaging, icing delay time, and ice shear strength were performed to determine the anti‐icing properties of the coatings. It was found that during cooling the composite coatings containing PCMs could release the latent heat of phase change to delay the icing process of water droplets on its surface. The introduction of PCMs increased the surface roughness, and the ice shear strengths of the composite coatings could remain at a low level and ice on the coatings could be easily removed, indicating that PCMs could be practicably applied in anti‐icing coatings. POLYM. ENG. SCI., 58:973–979, 2018. © 2017 Society of Plastics Engineers

Dynamic characteristics of droplet impacting on prepared hydrophobic/superhydrophobic silicon surfaces

Abstract Micro pillar arrays were fabricated on silicon surfaces based on the reactive ion etching technology. Low surface energy coatings were deposited on textured surfaces in order to prepare hydrophobic/superhydrophobic surfaces. After measuring the static contact angles and roll angles, the dynamic characteristics of droplets impacting on these hydrophobic/superhydrophobic surfaces were investigated in detail. Both the static contact angle and the roll angle increased with the increase of micro pillars' height. With the increase of micro pillars' spacing, the roll angle decreased, but the change of static contact angle was irregular. On superhydrophobic surfaces, the spreading coefficient of droplet was affected by both the static contact angle and the roll angle, and the rebounding coefficient of droplet was highly relevant with the roll angle. In addition, the bigger the incline angle of surface, the smaller the spreading coefficient and the smaller the rebounding coefficient of droplet.

Microstructure-alone induced transition from hydrophilic to hydrophobic wetting state on silicon

Surface hydrophobicity is primarily attained through the use of low surface energy materials. Experimental attempts to turn hydrophilic surfaces to hydrophobic have consisted of coating and thin film deposition. However, in many applications low surface energy materials and coatings are not practical, though hydrophobicity is still desired. In this paper, we demonstrate the transition from hydrophilic to hydrophobic wetting states on an intrinsically hydrophilic surface (contact angle less than 45°) using only surface microstructuring. The surface microstructures consist of re-entrant microcavities which interfere with the complete wetting of the surface, causing a liquid droplet to sit on the surface in a Cassie wetting state. The microstructures were fabricated on a silicon-on-insulator (SOI) wafer through steps of photolithography, etching, and bonding. Contact angle measurements demonstrated the ability of the microfabricated surfaces to sustain large contact angles above 100°, compared to a bare silicon surface which has a contact angle of 40°. Energy-dispersive X-ray spectroscopy showed silicon to be the only chemical element on the surface, while optical observations with an inverted microscope hinted to the existence of a Cassie wetting state.

Synthesis and evaluation of polystyrene–polybutadiene–polystyrene–dodecafluoroheptyl methacrylate/polystyrene–polybutadiene–polystyrene hybrid antifouling coating

Copolymers SBS–DFHMA and mesoporous silica SBA-15 were respectively synthesized and SBS–DFHMA were mixed with SBA-15 to prepared hybrid antifouling coatings by a spin-coater. By measuring the surface water contact angle and the attachment of Pseudomonas fluorescens, Chlorella and Diatoms, the antifouling properties of coatings were evaluated. The results shown that the surface of hybrid coatings, the water contact angle arrived 120°, were more hydrophobic than the SBS–DFHMA coatings. In terms of resistance of adhesion, low surface energy coatings of SBS–DFHMA could effectively weaken the adhesion behavior of P.fluorescens and Diatoms, but the role to Chlorella was not obvious. When 0.01g/ml SBA-15 was added, the adhesion of three marine microorganisms all had a very significant decrease to the hybrid coatings. These indicated that the fluorinated low surface energy antifouling coatings had limitation on resisting Chlorella attaching, and the addition of SBA-15 not only enhanced the ability of resistance to adhesion but also widen the applicability to more fouling and narrowed its limitations. This surprising effect was due to micro–nano convex structure of the coatings surface caused by hybrid.

Advanced low-energy durable coatings

The development of novel, durable, low-surface energy coatings with antifouling (ice, insects, biofouling etc.) and low friction properties is believed to be a promising direction for research with benefit to the transportation, construction and power generation sectors. Conventional coatings and surface treatments have not been successful in this area because of a lack of long-term durability and limited repellency of environmental fouling agents. New nanostructured coatings, which can be deposited from the liquid state, offer a potential solution that can be easily industrially adopted. However, there is a lack of standardization regarding their characterization and performance evaluation. The strategy selected by TWI and Brunel University to link the properties and structure of high-performance coatings from the nano to macro scale is described and is illustrated by the presentation of some provisional results. Functionalised nanostructured silica species have been used to reinforce an acrylate matrix leading to retention of the hydrophobicity even after aggressive abrasive wear. Copyright © 2014 John Wiley & Sons, Ltd.

Evaluation of insect residue resistant coatings – Correlation of a screening method with a conventional assessment technique

Laminar flow technologies such as laminar flow control (LFC) and hybrid laminar flow control (HLFC) are drag reduction techniques that allow the stabilization of the boundary layer and therefore permit an extended region of laminar flow to be achieved. The laminar flow region is, however, easily disturbed by surface contamination caused by insect debris, ice or dust particles. This causes a transition of the flow, and results in a turbulent boundary layer. Mitigation of surface contamination through the use of low surface energy coatings is thus of great interest. In this paper, the use of a screening method (utilizing a wet abrasion scrub test) to determine the effectiveness of candidate coatings against insect contamination has been employed. The method was compared to a conventional assessment technique, namely an insect delivery device. It was demonstrated that both approaches have merit; both methods provide useful means of directly quantifying the adhesion of insect debris to aircraft wing surfaces. An analogue, which can be used as a substitute to live insect testing, was also evaluated. The effect of humidity and water on the adhesive properties of insect haemolymph has been investigated.

Influence of surface characteristics on insect residue adhesion to aircraft leading edge surfaces

Leading edge contamination caused by insects is problematic for modern aircraft utilizing laminar flow aerofoils. The residue of crushed insect bodies adhering to aircraft leading edge surfaces can cause transition of the boundary layer, from laminar to turbulent, resulting in a significant increase in drag and therefore causing an increase in fuel consumption. Consequently, current research is focused on the evaluation of novel low surface energy coatings that will reduce or prevent insect adhesion. Insect residue adhesion tests were conducted on a range of surfaces, from superhydrophobic to hydrophilic. Surface free energy of the investigated substrates was obtained from measured dynamic contact angle values and surface roughness was measured using profilometry. Live insect testing with Drosophila melanogaster and Drosophila hydei was conducted using an insect delivery device inserted into a medium-speed wind tunnel. Tests were conducted at speeds ranging from 90 to 100m/s (speeds representative of those on take-off and landing of a commercial passenger aircraft). Topography of insect residues was characterized using scanning electron microscopy and confocal laser scanning microscopy. Results obtained indicate that coatings with high surface roughness values and low wettability exhibit good anti-contamination properties.

Atomic layer deposition of solid lubricating coatings on particles

Atomic layer deposition (ALD) has been used to deposit ceramic films on the surfaces of particles to improve the slurry and dry powder rheology of bulk powders. An overview of several studies is presented here, which demonstrates the extent and limitations for this ceramic coating platform technology to improve the tribological and/or flow properties of microfine and ultrafine particles. Direct evidence of the effect of the nanoscale ceramic coatings is shown via a significant improvement in flow properties of dry particles, including dynamic, bulk and shear properties obtained using a FT4 Powder Rheometer, and a marked reduction in slurry viscosity at high solids loadings. Microfine zinc powders, similar to those used in alkaline batteries, have been coated using boron nitride ALD films of sub-nanometer thickness, or about 0.1wt.%. The low surface energy coatings reduced the cohesion of 1–5μm particles by 52%. A highly-loaded slurry of the same material in concentrated KOH showed a 10–30% reduction in slurry viscosity over a range of shear rates, with a shear-thinning effect at high shear rates. Boron nitride platelets were coated using Al2O3 and SiO2 films to change the surface properties from hydrophobic to hydrophilic. The coated and uncoated powders were dispersed into an epoxy to evaluate the solids loading to viscosity ratio. The ALD films improved the particle–resin adhesion and decreased the viscosity of equivalently-loaded slurry of uncoated powder. Coated microfine nickel, aluminum and iron powders were also dispersed into epoxies, and lower viscosities and yield stresses were observed due to ceramic–epoxy interactions being more favorable than metallic–epoxy interactions. The ALD platform can be used to modify surfaces of primary particles in order to change the interparticle and particle–liquid forces, which provides a lubricating effect without detracting from the bulk properties of the core particles themselves.

Nanostructured Superhydrophobic Silver Surfaces

Inspired by “lotus-effect”, a superhydrophobic surface, in general, is prepared via two steps: (i) creating a surface roughness and then (ii) lowering the surface energy via a self-assembly of organic molecules or via low surface energy coatings. Superhydrophobicity cannot result if one of these two essential factors does not coexist. In the present work, it has been shown that superhydrophobic properties can be achieved on silver surfaces both via two-steps and a novel and simple one-step process. In the two step-processes a fractal-structured silver film deposited on copper surface by galvanic exchange reactions was passivated using stearic acid organic molecules to reduce the surface energy resulting in the superhydrophobicity. In the one-step process, however, the copper substrates were simply immersed in the silver nitrate solution containing fluoroalkylsilane (FAS-17) molecules resulting in superhydrophobicity. The silver films prepared both via two-steps and one-step processes were found to be highly water repellant with the water drops rolling off those surfaces. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were utilized to understand the morphology, molecular bonding, and chemical properties of the superhydrophobic silver surfaces.

Poly(2‐oxazoline)s based on fatty acids

AbstractPoly(2‐oxazoline)s are an interesting class of synthetic poly(amide)s that can be prepared by living cationic ring‐opening polymerization. The properties of poly(2‐oxazoline)s can be tuned by variation of the 2‐substituent of the 2‐oxazoline monomer. In this contribution the utilization of fatty acids as side chains for poly(2‐oxazoline)s is highlighted. The incorporation of such long aliphatic side‐chains results in hydrophobic crystalline poly(2‐oxazoline)s that are often applied for the preparation of amphiphilic structures as well as for low surface energy and low adhesive coatings. The most popular fatty‐acid based poly(2‐oxazoline)s are the saturated poly(2‐nonyl‐2‐oxazoline) and poly(2‐undecyl‐2‐oxazoline) as well as the unsaturated poly(2‐decenyl‐2‐oxazoline) and poly(2‐“soy alkyl”‐2‐oxazoline).

One-step fabrication process of superhydrophobic green coatings

In general, creation of superhydrophobic surfaces is composed of two steps: (i) creation of a rough surface and (ii) passivation of the surface with the low surface energy molecules or coatings. Superhydrophobic properties cannot be achieved on a surface without these two essential factors fulfilled. In the present work we have demonstrated that superhydrophobic silver films on copper (Cu) substrates can be created in just a one-step process via galvanic reactions by immersing the Cu substrates in silver nitrate solution containing benzoic acid, simplifying the complexity of two different steps involved in the former method. Silver films were also fabricated using similar process without benzoic acid for comparative studies. The X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) confirmed the formation of benzoic acid incorporated silver films. Scanning electron microscopy (SEM) images showed micro-nano structured leaf-like and flower-like morphological features in the films prepared without and with benzoic acid, respectively. Benzoic acid incorporated flower-like silver films demonstrated water repellency as the water drops rolled off those surfaces whereas complete absorption of water drops were encountered on the leaf-like silver surfaces prepared without benzoic acid.

Two component silicone modified epoxy foul release coatings: Effect of modulus, surface energy and surface restructuring on pseudobarnacle and macrofouling behavior

Two component silicone modified epoxy resin based low surface energy and non-toxic foul release coatings were developed. Silicone modified epoxy resin with 15 and 30% silicone content was used as component A and a polyether diamine (Jeffamine-500) was used as the component B. Free standing films were prepared by casting a mixture of components A and B in stoichiometric proportions. The surface composition, surface topography and wetting properties of the coatings were studied by angle resolved X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle goniometry respectively. The mechanical properties of the cured films were evaluated by tensile measurements as well as dynamic mechanical analysis. Pseudobarnacles made of aluminium studs were attached to the coated panels and adhesion tests were carried out by a pseudobarnacle tester. Coated panels were exposed in Mumbai harbor for fouling studies for a period of 90 days. Surface restructuring studies of the coatings upon immersion in seawater were carried out by measuring the changes in advancing and receding contact angles by contact angle goniometry. The effect of surface energy, modulus and surface restructuring of the coatings on the macrofouling and pseudobarnacle adhesion properties has been discussed in detail.

Nanomechanical and nanotribological testing of ultra-thin carbon-based and MoST films for increased MEMS durability

Reliability of MEM (microelectromechanical) devices can be limited by stiction forces that develop in use. It is desirable to alter the mechanical and interfacial behaviour of the silicon surfaces by the application of very thin, low surface energy and low stress coatings. In this publication we report the nanomechanical and nanotribological characterization of a range of 5–150 nm thin films deposited on silicon by filtered cathodic vacuum arc (FCVA) and closed field unbalanced magnetron sputtering. A method of analysing nano-scratch data with spherical indenters is proposed. The method suggests the onset of non-elastic deformation in the nano-scratch test is due to substrate yield rather than film deformation on all but the softest films studied in this publication. The critical load for total film failure is a marked function of indenter radius, the ratio of hardness to modulus and the film thickness. The FCVA films were tested with probes of different radii (1.1, 3.1 and 9.0 µm) and the critical load for film failure was found to vary strongly with probe radius. The deposition of <100 nm amorphous carbon films on Si could be a promising strategy for improving the reliability of Si-based MEMS devices as none of the very thin films tested underwent stress-related delamination failures that occur behind the indenter during the nano-scratch testing of thicker amorphous carbon films.

Nano-textured polymer surfaces with controlled wetting and optical properties using plasma processing

Wettability control is of great importance in many applications, from manufacturing of water repellent surfaces to droplet frictionless motion in microfluidics, and biocompatibility tuning. Wetting or repellent behaviour is governed by both surface chemistry and topography. In particular, super-hydrophobicity is attained by combining high-aspect-ratio (HAR) geometrical characteristics with low surface energy coatings. In this study we continue our research efforts pertaining a novel, simple, generic and fast technique to fabricate stable super-hydrophobic, yet transparent surfaces by means of high-density plasma etching and deposition. A High Density Plasma reactor is used to treat two different kind of polymers; an organic one (PMMA) and a hybrid one (PDMS). Different plasma chemistries specific to each polymer are implemented to etch the surfaces followed by a fluorocarbon deposition to control the surface roughness and the surface chemistry respectively. AFM is used to characterise morphology and water contact angle measurements to characterise wetting properties. We demonstrate high aspect ratio pillars with height ranging from ca. 350 nm to several microns depending on the processing time, and contact angles of 150° with hysteresis lower than 10°. Surfaces with pillars shorter than 400 nm are simultaneously transparent.

Sol-Gel Process Derived Superhydrophobic Silica Thin Films for Antistiction of MEMS Devices

ABSTRACTBased on the theory of superhydrophobicity for low surface energy coatings, we describe a superhydrophobic antistiction silica coating for MEMS devices. The process uses a novel sol-gel process sequence with a eutectic liquid as a templating agent. The eutectic liquid displays negligible vapor pressure and very low melting point (12°C at ambient conditions) to reduce solvent loss during the high speed spincoating process. After a fluoroalkyl silane treatment, superhydrophobicity is achieved on the as-prepared silica thin film. The solvent can be extracted after the gelation and aging processes. Spin speed effect, eutectic liquid:TEOS ratio in the solution were studied in order to optimize the surface roughness to ensure excellent super-hydrophobicity. Comparison of the silica thin films with silicon pillar surfaces showed that superhydrophobicity for the traditional sol-gel derived silica films demonstrated significant improvement, especially under humid conditions. The AFM force curve obtained with a tipless probe showed that the interaction force is greatly reduced on a rough silica superhydrophobic surface. This result offers great potential to reduce stiction failures in MEMS devices.

Tailoring surface properties for controlling droplet formation at microsieve membranes

The preparation of water-in-oil emulsions with silicon nitride microfluidic nozzle arrays requires a surface, which is not wetted by the continuous water phase. Hence, we deposited several hydrophobic coatings on silicon nitride and studied the wetting behavior of water in pure and emulsifier containing hexadecane. In pure medias we could demonstrate that an octyl-chain terminated is the optimal hydrophobic coating. It has a high affinity to hexadecane due to its similar chemical nature expressed by a very low solid/liquid interfacial tension and a low affinity to water resulting in the highest measured water in hexadecane contact angle of 164°. Unmodified silicon nitride in contrast has the highest affinity to water and the lowest to hexadecane resulting in a contact angle of 97°. Based on the determination of the solid/liquid interfacial tensions in pure liquids we found for several hydrophobic coatings and for unmodified silicon nitride a deviation from Young's equation, which can be related to negative Hamaker constants of the studied systems. With the soybean-based emulsifier BolecMT we observed for several coatings no negative influence of the adsorbed emulsifier on the hydrophobicity. In fact, the water/hexadecane contact angles increased in time. In case of unmodified silicon nitride the surface was rendered into a more hydrophobic state. For low surface energy coatings, we observed a dramatic time-dependent change of the wetting characteristics. After a certain period of time water drops collapsed and wetted the surface. The time scales of this observed wetting behavior is several magnitudes larger than the drop formation process itself. Therefore, its relevance for the drop formation process could be neglected. With the unmodified silicon nitride surface successful drop formation was not possible and the low surface energy coating performed successfully in membrane emulsification experiments.

Time-of-flight secondary ion mass spectrometry measurements of a fluorocarbon-based self-assembled monolayer on Si

Low surface energy coatings and films are needed to minimize stiction, high friction, and wear of the oxide-terminated silicon-based microelectromechanical systems (MEMSs) to ensure reliable device function. One common approach is to deposit self-assembled monolayer (SAM) films of molecules possessing a reactive silane head group and low energy pendant chain to cover the complex structures used in MEMS devices. The composition of these films is difficult to characterize and quantify. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is an excellent tool for characterizing these types of films because of its ability to detect small quantities of surface species at high spatial resolution and surface sensitivity; however, interpretation of the spectral data generated is complex and challenging. We are using the combination of x-ray Photoelectron Spectrometry and ToF-SIMS to detect and quantify a fluorocarbon-based SAM on Si coupons and on MEMS devices. Unexpected fragmentation caused by the interaction of the primary ion beam with the fluorocarbon chain in contact with the Si surface creates ions whose presence is not intuitive. We can account for these ion fragments and use them to aid in quantifying the film composition. Our methods include the use of coverage-dependent fragmentation signatures along with the application of multivariate statistical techniques to establish the covariance in these signatures.

Structural organisations in polystyrene-based semifluorinated block copolymers for low surface energy coatings

New narrow-polydispersity AB block copolymers were prepared by 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO)-mediated, controlled radical polymerisation. The A block was a poly(styrene) block, and the B block was a semifluorinated block, using either a poly(styrene) or a polyacrylate backbone. The semifluorinated side segments self-assembled into smectic mesophases. The bulk morphology and the interplay between microphase separation and liquid crystalline order were examined using transmission electron microscopy (TEM) and x-ray scattering. Near-edge x-ray absorption fine structure (NEXAFS) studies revealed that the semifluorinated side groups were ordered and titled at the outer surface of polymer films and allowed evaluation of the orientational order parameters. The orientational order parameter of the C-F bonds of the semifluorinated side groups at the surface was correlated with the critical surface tension of the polymer that had been extrapolated from Zisman plots with n-alkanes as the interrogating liquids

The Ship Hull Fouling Penalty

The ship resistance penalties of slime, shell and weed are discussed in turn. Methods to measure the hard paint roughness of antifouling coatings are recapitulated. The determination of a satisfactory roughness parameter from correlations with measured roughness functions is described. This in turn, allows a relationship between ship added friction and roughness height to be found. This recapitulation allows consideration of using the same route for a surface with filamentous fouling. Consideration is given to low surface energy coatings and their roughness idiosyncrasies. The determination of economic penalties is discussed, both for a particular ship and globally.