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Silicones are usually considered to be inert and, thus, not reactive with surfaces. Here we show that the most common silicone, methyl-terminated polydimethylsiloxane, spontaneously and stably bonds on glass-and any other material with silicon oxide surface chemistry-even at room temperature. As a result, a 2-5 nm thick and transparent coating, which shows extraordinary nonstick properties toward polar and nonpolar liquids, ice, and even super glue, is formed. Ten microliter drops of various liquids slide off a coated glass when the sample is inclined by less than 10°. Ice adhesion strength on a coated glass is only 2.7 ± 0.6 kPa, that is, more than 98% less than ice adhesion on an uncoated glass. The mechanically stable coating can be easily applied by painting, spraying, or roll-coating. Notably, the reaction does not require any excess energy or solvents, nor does it induce hazardous byproducts, which makes it an ideal option for environmentally sustainable surface modification in a myriad of technological applications.

An atmospheric pressure aerosol-based wet thin film coating technique called the nFOG is characterized and applied in polymer film coatings. In the nFOG, a fog of droplets is formed by two air-assist atomizers oriented toward each other inside a deposition chamber. The droplets settle gravitationally and deposit on a substrate, forming a wet film. In this study, the continuous deposition mode of the nFOG is explored. We determined the size distribution of water droplets inside the chamber in a wide side range of 0.1–100 µm and on the substrate using aerosol measurement instruments and optical microscopy, respectively. The droplet size distribution was found to be bimodal with droplets of approximately 30–50 µm contributing the most to the mass of the formed wet film. The complementary measurement methods allow us to estimate the role of different droplet deposition mechanisms. The obtained results suggest that the deposition velocity of the droplets is lower than the calculated terminal settling velocity, likely due to the flow fields inside the chamber. Furthermore, the mass flux of the droplets onto the substrate is determined to be in the order of 1 g/m3s, corresponding to a wet film growth rate of 1 µm/s. Finally, the nFOG technique is demonstrated by preparing polymer films with thicknesses in the range of approximately 0.1–20 µm.

Atomic layer deposited (ALD) inorganic thin film coatings can produce high barrier properties against moisture and gas. However, if an ALD film is exposed to mechanical stress, it may lose these properties. In this study, a 40-nm thick aluminum oxide (Al 2 O 3 ) ALD barrier layer turned out fragile on a low density polyethylene (LDPE) coated paper substrate during web handling. In such handling, the processing steps, such as bending, roll contacts, and friction, weakened the structure of the barrier. However, when the Al 2 O 3 layer was deposited on polyethylene terephthalate coated paper, no loss of barrier properties was observed after web handling. We show here that an Al 2 O 3 ALD barrier layer can be protected on an LDPE substrate against mechanical stress by adding a shielding polyethylene layer on top of the ALD coating. After this addition, oxygen permeation through the protected surface remained unchanged during web handling. The results of this study demonstrate that the protective layer was necessary to maintain the excellent barrier properties and intactness of the ALD layer on an LDPE substrate.

A superamphiphobic coating on a biobased material shows extreme liquid repellency with static contact angles (CA) greater than 150° and roll-off angles less than 10° against water, ethylene glycol, diiodomethane and olive oil, and a CA for hexadecane greater than 130°. The coating consisting of titania nanoparticles deposited by liquid flame spray (LFS) and hydrophobized using plasma-polymerized perfluorohexane was applied to a birch hardwood. Scanning electron microscopy (SEM) imaging after sample preparation by UV laser ablation of coated areas revealed that capped structures were formed and this, together with the geometrically homogeneous wood structure, fulfilled the criteria for overhang structures to occur. The coating showed high hydrophobic durability by still being non-wetted after 500 000 water drop impacts, and this is discussed in relation to geometrical factors and wetting forces. The coating was semi-transparent with no significant coloration. A self-cleaning effect was demonstrated with both water and oil droplets. A self-cleanable, durable and highly transparent superamphiphobic coating based on a capped overhang structure has a great potential for commercial feasibility in a variety of applications, here exemplified for a biobased material.

Fabrication of superhydrophobic surfaces in large scale has been in high interest for several years, also titanium oxide nanostructures having been applied for the purpose. Optimizing the amount and structure of the TiO2 material in the coating will play a key role when considering upscaling. Here, we take a look at fabricating the superhydrophobic surface in a one-step roll-to-roll pilot scale process by depositing TiO2 nanoparticles from a Liquid Flame Spray onto a moving paperboard substrate. In order to find the minimum amount of nanomaterial still sufficient for creating superhydrophobicity, we varied nanoparticle production rate, flame distance from the substrate and line speed. Since the deposited amount of material sideways from the flame path was seen to decrease gradually, spatial analysis enabled us to consistently determine the minimum amount of TiO2 nanoparticles on the substrate needed to achieve superhydrophobicity. Amount as low as 20-30 mg m-2 of TiO2 nanoparticles was observed to be sufficient. The scanning electron microscopy revealed that at this amount, the surface was covered with nanoparticles only partially, but still sufficiently to create a hierarchical structure to affect wetting significantly. Based on XPS analysis, it became apparent that TiO2 gathers hydrocarbons on the surface to develop the surface chemistry towards hydrophobic, but below the critical amount of TiO2 nanoparticles, the chemistry could not enable superhydrophobicity anymore. While varying the deposited amount of TiO2, besides the local spatial variance of the coating amount, also the overall yield was studied. Within the text matrix, a yield up to 44% was achieved. In conclusion, superhydrophobicity was achieved at all tested line speeds (50 to 300 m min-1), even if the amount of TiO2 varied significantly (20 to 230 mg m-2). acceptedVersion

Titanium dioxide (TiO2) is a photoactive material with various interesting and useful properties. One of those is the perfect wettability of TiO2 surface after ultraviolet (UV) illumination. Wettability of a solid surface plays an important role in the field of printing, coating, and adhesion among others. Here we report on a superhydrophobic and photoactive liquid flame spray (LFS) generated TiO2 nanoparticle coating that can be applied on web-like materials such as paper and board in one-step roll-to-roll process. The LFS TiO2 nanoparticle coated paper and board were superhydrophobic instantly after the coating procedure because of spontaneously accumulated carbonaceous overlayer on TiO2, and thus there was no need for any type of separate hydrophobization treatment. The highly photoactive LFS TiO2 nanoparticle coating could be converted steplessly from superhydrophobic to superhydrophilic by UV-illumination, and the coating gave strong response to natural daylight illumination even in the shade. The superhydrophobic LFS TiO2 coated surface can be used as an intelligent substrate, where photo-generated hydrophilic patterns guide the fluid setting and figure formation. Our study reveals that the wettability changes on the LFS TiO2 surface were primarily caused by the photocatalytic removal of the carbonaceous material from TiO2 during the UV-illumination and spontaneous accumulation of the carbonaceous material on the surface of the metal oxide during storage in the dark. The latter mechanism was found to be a temperature activated process which could be significantly speeded up by heat treatment. If other mechanisms such as surface oxidization, increment of hydroxyl groups, or charge separation played a role in the wetting phenomena on TiO2, their effect was rather secondary as the removal and accumulation of the carbonaceous material dominated the wettability changes on the surface. Our study gives valuable information on the complex issue of photo-induced wettability changes on TiO2.

Surface chemical analysis of photocatalytic wettability conversion of the TiO 2 nanoparticle coating is performed by the water contact angle measurement and X-ray photoelectron spectroscopy (XPS). It is observed that the UVA irradiation and heat treatment strongly affect on the water contact angle. Initially superhydrophobic TiO 2 nanoparticle coated paperboard surfaces can undergo several transition loops between hydrophobic and hydrophilic states: the surface hydrophilicity can be induced by ultraviolet A (UVA) radiation whereas high temperature oven treatment results in superhydrophobic surface. The XPS spectra shows that UVA irradiation induces photocatalytic oxidation of the TiO 2 nanocoated surface whereas the oven treatment decreases the amount of oxygen related to the hydroxyl groups. The wettability changes are related to the degree of hydroxyl groups or aliphatic chains present on the TiO 2 surface.

Corona, flame, atmospheric plasma, and liquid flame spray (LFS) techniques were used to create highly hydrophilic surfaces for pigment-coated paper and board and machine-glossed paper. All the surface modification techniques were performed continuously in ambient atmosphere. The physical changes on the surfaces were characterized by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy and Parker Print-Surf surface roughness. The chemical changes were analysed by X-ray photoelectron spectroscopy. The superhydrophilic surfaces, i.e. contact angle of water (CAW)

Nanostructured coatings have been prepared on a flexible, moving paperboard using deposition of ca. 40-nm-sized titanium dioxide nanoparticles generated by a liquid flame spray process, directly above the paperboard, to achieve improved functional properties for the material. Properties such as surface wettability can be extensively improved by a thin layer of nanoparticles on the substrate. Owing to the vulnerability to heat, the substrate needs to be moved rapidly through the flame. This, on the other hand, generates a setting for a roll-to-roll coating process, which favors upscaling of the method. In this article, we characterize the flame process for nanoparticle coating and quantify the operational window for this method. The amount of deposited material as a function of substrate speed through the flame is discussed. Although the thermophoretic flux of nanoparticles is estimated to be very high from the hot flame onto the cold substrate, other factors were observed to limit the deposited amount of ...

This paper introduces a new method for generating nanoscale coatings in a continuous roll-to-roll process at normal pressure. Nanostructured and transparent coating, based on titanium dioxide nanoparticles, was successfully deposited on-line at atmospheric conditions on pigment coated paperboard using a thermal spray method called the Liquid Flame Spray (LFS). The LFS coating process is described and the influences of process parameters on coating quality are discussed. Nanocoating was investigated by a field emission gun scanning electron microscope (FEG-SEM), an atomic force microscope (AFM), an X-ray photoelectron spectroscopy (XPS) and a water contact angle measurement. The highest measured water contact angles on the nanocoated paperboard surface were over 160°. Falling water droplets were able to bounce off the surface, which is illustrated by high speed video system images. Regardless of the high hydrophobicity, the coating showed sticky nature, creating a high adhesion to water droplets immediately as the motion of the droplets stopped. Nanocoating with full coverage of the substrate was produced at line speeds up to 150 m/min. Therefore, the LFS coating has scale up potential to industrial level as an affordable and efficient method for coating large volumes at high line speeds.

Nanostructured coatings have been prepared on a flexible, moving paperboard using deposition of ca. 10-50-nm-sized titanium dioxide and silicon dioxide nanoparticles generated by a liquid flame spray process, directly above the paperboard, to achieve improved functional properties for the material. With moderately high production rate (∼ g/min), the method is applicable for thin aerosol coating of large area surfaces. LFS-made nanocoating can be synthesized e.g. on paper, board or polymer film in roll-to-roll process. The degree of particle agglomeration is governed by both physicochemical properties of the particle material and residence time in aerosol phase prior to deposition. By adjusting the speed of the substrate, even heat sensitive materials can be coated. In this study, nanoparticles were deposited directly on a moving paperboard with line speeds 50-300 m/min. Functional properties of the nanocoating can be varied by changing nanoparticle material; e.g. TiO2and SiO2are used for changing the surface wetting properties. If the liquid precursors are dissolved in one solution, synthesis of multi component nanoparticle coatings is possible in a one phase process. Here, we present analysis of the properties of LFS-fabricated nanocoatings on paperboard. The thermophoretic flux of nanoparticles is estimated to be very high from the hot flame onto the cold substrate. A highly hydrophobic coating was obtained by a mass loading in the order of 50–100 mg/m2of titanium dioxide on the paperboard.

Slippery, liquid-infused porous surfaces offer a promising route for producing omniphobic and anti-icing surfaces. Typically, these surfaces are made as a coating with expensive and time consuming assembly methods or with fluorinated films and oils. We report on a route for producing liquid-infused surfaces, which utilizes a liquid precursor fed oxygen-hydrogen flame to produce titania nanoparticles deposited directly on a low-density polyethylene film. This porous nanocoating, with thickness of several hundreds of nanometers, is then filled with silicone oil. The produced surfaces are shown to exhibit excellent anti-icing properties, with an ice adhesion strength of ∼12 kPa, which is an order of magnitude improvement when compared to the plain polyethylene film. The surface was also capable of maintaining this property even after cyclic icing testing. Slippery, liquid-infused porous surfaces (SLIPSs) are nature inspired surfaces that are designed to repel liquid and solid materials. These surfaces have been shown to pose anti-icing properties, which broadens the available end-uses from the chemical industry to arctic transportation and energy production. The method behind repellency of SLIPSs relies on preventing outside liquids from penetrating the surface structure to the Wenzel state. Instead, the slippery liquid within the porous solid supports the Cassie-Baxter state (instead of air, here the porous structure is filled with lubricant), where the reduced area of the porous solid surface is available to interact with the liquid or ice to be repelled. The difference between Wenzel and Cassie-Baxter states is illustrated in Figure 1. This phenomenon is exploited in many superhydrophobic surfaces where an air cushion is entrapped within the porous solid surface. As a result, spherical water drops easily roll off the surface (and have static contact angles larger than 150°). publishedVersion

Paper-based devices provide an alternative technology for simple, low-cost, portable, and disposable diagnostic tools for many applications, including clinical diagnosis, food quality control, and environmental monitoring. In this study we report a two-step fabrication process for creating two-dimensional microfluidic channels to move liquids on a hydrophobized paper surface. A highly hydrophobic surface was created on paper by TiO2 nanoparticle coating using a high-speed, roll-to-roll liquid flame spray technique. The hydrophilic pattern was then generated by UV irradiation through a photomask utilizing the photocatalytic property of TiO2. The flow dynamics of five model liquids with differing surface tensions 48-72 mN·m(-1) and viscosities 1-15 mN·m(-2) was studied. The results show that the liquid front (l) in a channel advances in time (t) according to the power law l=Zt0.5 (Z is an empirical constant which depend on the liquid properties and channel dimensions). The flow dynamics of the liquids with low viscosity show a dependence on the channel width and the droplet volume, while the flow of liquids with high viscosity is mainly controlled by the viscous forces.

Compressibility of liquid flame spray-deposited porous TiO2 nanoparticle coating was studied on paperboard samples using a traditional calendering technique in which the paperboard is compressed between a metal and polymer roll. Surface superhydrophobicity is lost due to a smoothening effect when the number of successive calendering cycles is increased. Field emission scanning electron microscope surface and cross‒sectional images support the atomic force microscope roughness analysis that shows a significant compressibility of the deposited TiO2 nanoparticle coating with decrease in the surface roughness and nanoscale porosity under external pressure. PACS 61.46.-w; 68.08.Bc; 81.07.-b

Wettability of a solid surface by a liquid plays an important role in several phenomena and applications, for example in adhesion, printing, and self-cleaning. In particular, wetting of rough surfaces has attracted great scientific interest in recent decades. Superhydrophobic surfaces, which possess extraordinary water repelling properties due to their low surface energy and specific nanometer- and micrometer-scale roughness, are of particular interest due to the great variety of potential applications ranging from self-cleaning surfaces to microfluidic devices. In recent years, the potential of superhydrophobic cellulose-based materials in the function of smart devices and functional clothing has been recognized, and in the past few years cellulose-based materials have established themselves among the most frequently used substrates for superhydrophobic coatings. In this Review, over 40 different approaches to fabricate superhydrophobic coatings on cellulose-based materials are discussed in detail. In addition to the anti-wetting properties of the coatings, particular attention is paid to coating durability and other incorporated functionalities such as gas permeability, transparency, UV-shielding, photoactivity, and self-healing properties. Potential applications for the superhydrophobic cellulose-based materials range from water- and stain-repellent, self-cleaning and breathable clothing to cheap and disposable lab-on-a-chip devices made from renewable sources with reduced material consumption.

Paper can be coated with liquid flame spray (LFS) generated nanoparticles to control the wettability of its surface from hydrophilic to superhydrophobic. The adhesion of the nanoparticles on paper is of interest both for understanding the product durability during its lifetime and for product safety issues. Poor particle adhesion influences the desired functional properties and released nanoparticles cause health and environmental concerns. To investigate the wear resistance of LFS-TiO_2 and -SiO_2 coated papers, the nanoparticle surfaces were exposed to rotary abrasion tests. The changes in the samples were analyzed by contact angle measurements and high resolution field-emission scanning electron microscopy (FESEM). After abrasive action with another paperboard surface, only relatively small changes in wettability of superhydrophobic/hydrophilic coatings were found. A more severe abrasive action will remove some of the nanoparticle coating, but the hydrophobic/hydrophilic character of the surface is still maintained to large extent. The results indicate that the wear resistance of LFS nanocoated paper surfaces differs and depends on the nanoparticle material type used for the coating. This is clearly reflected as changes in surface structure shown by FE-SEM and wettability. The results can help understanding which paper-related application areas could be targeted with the LFS-nanoparticle coating process.

Superhydrophobic nanoparticle coating was created on the surface of board using liquid flame spray (LFS). The LFS coating was carried out continuously in ambient conditions without any additional hydrophobization steps. The contact angle of water (CAW) of ZrO2, Al2O3 and TiO2 coating was adjusted reversibly from >150° down to ~10−20° using different stimulation methods. From industrial point of view, the controlled surface wetting has been in focus for a long time because it defines the liquid-solid contact area, and furthermore can enhance the mechanical and chemical bonding on the interface between the liquid and the solid. The used stimulation methods included batch-type methods: artificial daylight illumination and heat treatment and roll-to-roll methods: corona, argon plasma, IR (infra red)- and UV (ultra violet)-treatments. On the contrary to batch-type methods, the adjustment and switching of wetting was done only in seconds or fraction of seconds using roll-to-roll stimulation methods. This is significant in the converting processes of board since they are usually continuous, high volume operations. In addition, the creation of microfluidic patterns on the surface of TiO2 coated board using simple photomasking and surface stimulation was demonstrated. This provides new advantages and possibilities, especially in the field of intelligent printing. Limited durability and poor repellency against low surface tension liquids are presently the main limitations of LFS coatings.

Low roll-off angle, high impalement pressure, and mechanical robustness are key requirements for super-liquid-repellent surfaces to realize their potential in applications ranging from gas exchange membranes to protective and self-cleaning materials. Achieving these properties is still a challenge with superamphiphobic surfaces, which can repel both water and low-surface-tension liquids. In addition, fabrication procedures of superamphiphobic surfaces are typically slow and expensive. Here, by making use of liquid flame spray, a silicon dioxide-titanium dioxide nanostructured coating is fabricated at a high velocity up to 0.8 m s-1 . After fluorosilanization, the coating is superamphiphobic with excellent transparency and an extremely low roll-off angle; 10 µL drops of n-hexadecane roll off the surface at inclination angles even below 1°. Falling drops bounce off when impacting from a height of 50 cm, demonstrating the high impalement pressure of the coating. The extraordinary properties are due to a pronounced hierarchical nanotexture of the coating.