Liquid Flame Spray Research Articles

Silver nanoparticle coatings with adjustable extinction spectra produced with liquid flame spray, and their role in photocatalytic enhancement of TiO2

Silver nanoparticles deposits were produced with liquid flame spray (LFS) on glass and TiO2 substrates to study their optical response and photocatalytic enhancement. The correlation between extinction spectrum of the nanoparticle coating and the LFS process parameters was studied. The spectra consisted of two partly overlapping peaks: one centered in the UV region and the other in the visible light region. The visible light peak redshifted as either the silver mass concentration in the precursor solution or the precursor solution feed rate was increased, which also correlated with growing primary particle size. However, simultaneous correlation with photocatalytic activity of the decorated TiO2 surfaces was not observed, which was attributed to particle sintering on the surface. Instead, the photocatalytic activity was seen to change as the surface coverage of silver nanoparticles was varied. When the surface coverage was raised from ∼10 % to roughly 30 %, the activity, and then decreased as the loading was further raised. The increase was assumed to originate from plasmonic activation, and the decrease was attributed to the excessive amount of silver either blocking reactive area of the TiO2 or absorbing/scattering too much of the incoming light, which hindered the photocatalytic activity.

Effects of Gas Layer Thickness on Capillary Interactions at Superhydrophobic Surfaces.

Strongly attractive forces act between superhydrophobic surfaces across water due to the formation of a bridging gas capillary. Upon separation, the attraction can range up to tens of micrometers as the gas capillary grows, while gas molecules accumulate in the capillary. We argue that most of these molecules come from the pre-existing gaseous layer found at and within the superhydrophobic coating. In this study, we investigate how the capillary size and the resulting capillary forces are affected by the thickness of the gaseous layer. To this end, we prepared superhydrophobic coatings with different thicknesses by utilizing different numbers of coating cycles of a liquid flame spraying technique. Laser scanning confocal microscopy confirmed an increase in gas layer thickness with an increasing number of coating cycles. Force measurements between such coatings and a hydrophobic colloidal probe revealed attractive forces caused by bridging gas capillaries, and both the capillary size and the range of attraction increased with increasing thickness of the pre-existing gas layer. Hence, our data suggest that the amount of available gas at and in the superhydrophobic coating determines the force range and capillary growth.

Fast-facile synthesis of single-phase nanocrystalline β-Ca2P2O7 powder by an aerosol synthesis method

Recently, β-phase of calcium pyrophosphate has gained wide attention for potential in different applications. In this communication, we report for the first time an ultra-fast, cost-effective, and facile one-step synthesis of single-phase β-CPP powder without post-heat treatment by applying liquid flame spray (LFS). LFS is a scalable, continuous, and adjustable one-step flame-based aerosol synthesis method. The effect of Ca/P ratio on structural properties was investigated. β-Ca2P2O7 could be successfully synthesized in spherical nano-and micron-sized particles, as well as plate-like particles. The synthesized single-phase powder exhibited high thermal stability and relatively high specific surface area with different (nano)porous structures.

A facile strategy for fine-tuning the drug release efficacy of poly-L-lactic acid-polycaprolactone coatings by liquid flame spray

The diversity of medical application scenarios demands fine-tuning of drug release profiles for better patient care. Here we report a facile strategy to construct composite coatings as universal drug delivery systems with tunable release efficacy. A poly-L-lactic acid (PLLA)-polycaprolactone (PCL) coating was fabricated by liquid flame spray and its performance was verified by loading chloramphenicol (CAM) as bactericidal component. Physiochemical analyses revealed the homogeneity of chemistry and integrity of functional groups of the composite coatings. Release of CAM was enhanced by the increase of the PCL content in the composite coatings, showing a controllable manner. Kinetics analysis suggested the release mechanism of Korsmeyer-Peppas model for both the PLLA-CAM and the PCL-CAM coatings, and the release regime for the PLLA-PCL composite coatings transited from the Higuchi model to the Korsmeyer-Peppas model when the PCL content increased from 30 % to 70 %. Further antibacterial testing revealed tailorable antibacterial activity of the coatings against both Escherichia coli and Staphylococcus aureus. Regulating the release of the drug CAM through altering PCL content in the PLLA-PCL coating would give inspiring insight into green fabrication of polymeric coatings with tunable drug release for versatile applications.

Ultrafast Bubble Bursting by Superamphiphobic Coatings.

Controlling bubble motion or passively bursting bubbles using solid interfaces is advantageous in numerous industrial applications including flotation, catalysis, electrochemical processes, and microfluidics. Current research has explored the formation, dissolution, pinning, and rupturing of bubbles on different surfaces. However, the ability to tune and control the rate of bubble bursting is not yet achieved. Scaling down surface-induced bubble bursting to just a few milliseconds is important for any application. In this work, the hierarchical structure of superamphiphobic surfaces is tuned in order to rapidly rupture contacting bubbles. Surfaces prepared using liquid flame spray show ultrafast bubble bursting (down to 2ms) and superior durability. The coatings demonstrate excellent mechanical and chemical stability even in the presence of surface-active species. Air from the ruptured bubble is absorbed into the aerophilic Cassie-state. Long-term applicability is demonstrated by preventing the accumulation of air in the plastron via a connection of the plastron to the environment. The times recorded for bubble rupture and complete reorganization of air are reduced by approximately a factor of 3 compared to previously reported values. The concept is utilized to passively control surfactant-rich foam in froth flotation. Material collection efficiency increased by more than 60times compared to controls.

Crystallographic phase formation of iron oxide particles produced from iron nitrate by liquid flame spray with a dual oxygen flow

AbstractWe fabricated iron oxide particles from iron(III) nitrate in a liquid flame spray synthesis. Unlike in most liquid flame spray studies, we implemented a secondary oxygen flow. The effect of the gas flow setup and two additives to the precursor solution, oxalic acid and citric acid, on the resulting particles was studied, with the focus on crystallographic phase composition. The synthesis yielded either pure maghemite or maghemite/hematite mixed phase powders. For solutions without additives, the maghemite fraction was almost linearly dependent on the equivalence ratio. The specific surface area was highest for the smallest equivalence ratios, then decreased, and increased again for the highest values. Some variation was observed between samples with equal equivalence ratios but the total oxygen flow divided differently between the two oxygen channels, a higher atomization flow promoting larger hematite fraction, and higher specific surface area. Both additives reduced the amount of hematite in the powder samples, citric acid being the more efficient one. Citric acid slightly raised the specific surface area, whereas oxalic acid dropped it in half.

Photocatalytic Activity of Multicompound TiO2/SiO2 Nanoparticles

Multicompound TiO2/SiO2 nanoparticles with a diameter of 50–70 nm were generated using a liquid flame spray (LFS) nanoparticle deposition in a single flame. Here, we study the photocatalytic activity of deposited multicompound nanoparticles in gas-phase via oxidation of acetylene into carbon dioxide that gives new insight about the multicompound nanoparticle morphology. A small addition of SiO2 content of 0.5%, 1.0% and 3.0% significantly suppressed the photocatalytic activity by 33%, 44% and 70%, respectively, whereas 5.0% SiO2 addition completely removed the activity. This may be due to a formation of a thin passivating SiO2 layer on top of the of the TiO2 nanostructures during the LFS nanoparticle deposition. Surface wetting results support this hypothesis with a significant increase in water contact angle as the SiO2 content is increased.

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.

Facile Fabrication of Polyimide-alumina Composite Coatings by Liquid Flame Spray

Polyimide is type of heterocyclic polymer compound characterized by an imide ring structure. The aromatic ring structure in polyimide facilitates good electrical properties, mechanical properties, thermal properties and radiation resistance, etc. It is widely used in the aerospace field, in electronic components and in the semiconductor industry. Traditional methods for the preparation of polyimide films have different limitations. Here, a new one-step method was established to prepare polyimide coatings using liquid flame spray. The effect of different mass fraction on the surface and cross section structure characteristics of polyimide coatings was studied. The formation process of the polyimide-alumina composite coatings was analyzed using single particles and coatings with different amount of alumina. The composite coating shows a flat and dense structure and excellent dielectric properties. The results shed light on construction of polymer-inorganic composite layers for widespread applications.

Paperboard as a substrate for biocompatible slippery liquid-infused porous surfaces

Abstract Slippery liquid-infused porous surfaces or SLIPS were first introduced in 2011 by Wong et al. who reported a bioinspired self-repairing surface with remarkable slippery properties. Generally, production of these surfaces includes fossil-based or expensive materials and processes that are available mainly in laboratory scale. In this study, slippery surfaces with sliding angles of less than 10° are obtained using fibre-based material – paperboard – that is commercially available in large-scale and also cheap compared to substrates generally used in this field. The hierarchical nanostructure that is a necessary condition for appropriate droplet mobility was obtained by the liquid flame spray method. This method is fast, scalable, has a variety of optimization parameters and can be utilized in roll-to-roll technology that is traditional in paper industry. In this work, paperboard serves not only as a substrate, but also as a reservoir for the lubricant, thus it is important to evaluate the affinity of the material for the oils and estimate the capillary movement. Therefore, Cobb and Klemm methods were used when choosing a paperboard material. In addition to synthetic oils, rapeseed oil was also utilized as a lubricant, which potentially leads to eco-friendly and recyclable slippery liquid-infused porous surfaces.

Protective stainless steel micropillars for enhanced photocatalytic activity of TiO2 nanoparticles during wear

Enhanced photocatalytic activity of liquid flame spray (LFS) deposited TiO2 nanoparticles under tribological wear was investigated on metal injection molded (MIM) stainless steel substrates having micropillars as the load carrying support. A combination of LFS with MIM microtextures on the surface is a simple and cost-efficient way for manufacturing efficient photocatalytic substrates. Computer controlled microworking technique in combination with MIM was used to produce stainless steel micropillar surfaces that were functionalized by the LFS deposition of photocatalytically active TiO2 nanoparticles. The photocatalytic activity was measured in gas-phase with an in-house built photoreactor. Our results show that micropillars with controlled spacing we can control the surface area and increased photocatalytic activity of the micropillar substrate was observed compared to flat reference. The wear test confirmed that micropillars not only increase the surface area but they also provide protective support against wear whereas flat reference substrate lost the photocatalytic activity completely during wear.

Controlling the phase of iron oxide nanoparticles fabricated from iron(III) nitrate by liquid flame spray

AbstractIron oxide nanoparticles were synthesized in a liquid flame spray process from iron(III) nitrate. The choice of chemicals and all other process parameters affects the crystallographic phase composition and the quality of the material. Adjustment of the solvent composition and the gas flow rates was used to control the phase composition of the produced particles. All samples consisted of pure maghemite (γ‐Fe2O3) or a mixture of maghemite and hematite (α‐Fe2O3). When using pure alcohols as solvents, the maghemite/hematite phase ratio could be adjusted by changing the equivalence ratio that describes the oxidation conditions in the flame zone. A large residual particle mode formed in the size range of ~20‐700 nm along with a dominant very fine particle mode (2‐8 nm). Both phases seemed to contain large particles. A partial substitution of methanol with carboxylic acids turned the hematite phase into maghemite completely, even though some of particles were possibly not fully crystallized. Residual particles were still present, but their size and number could be decreased by raising the heat of combustion of the precursor solution. 30 vol‐% substitution of methanol with 2‐ethylhexanoic acid was adequate to mostly erase the large particles.

Fabrications and Applications of Slippery Liquid-infused Porous Surfaces Inspired from Nature: A Review

The slippery liquid-infused porous surfaces inspired by the microstructure of carnivorous nepenthes have aroused widespread attention, which show stable liquid repellency, glorious self-repairing powers and effective anti-fouling properties. The surfaces are manufactured via the infusion of lubricant oil into porous structures, a process which allows other fluids to slide off the interfaces readily. However, the practical applications of slippery liquid-infused surfaces are limited to the complicated preparation processes and poor oil lock ability. We aim to, in this review, present the fundamental theories of the slippery liquid-infused porous surface. Some typical natural examples are clarified while representative fabricating methods such as liquid flame spray, layer-by-layer assembly, lithography and so on are listed. The slippery surface can facilitate the manufacture of transparent and multi-functional slippery materials by means of straightforward procedures. The slippery liquid-infused porous surfaces were applied in hot water repellency, anti-fouling, ice-phobic, water condensation, control of underwater bubble transport and drag reduction. This article discusses all these issues along with emerging applications as well as future challenges.

Deposition of hollow sphere In2O3 coatings by liquid flame spray

ABSTRACT Hollow sphere In(OH)3 coatings were deposited by a simple liquid flame spray with hollow sphere In(OH)3 suspension, which was synthesized by improved soft template methods. The morphology of the In(OH)3 was well preserved and oxygen vacancies were produced in the harsh thermomechanical environment of the flame spraying process. The In2O3 coatings and In2O3 powder were obtained by calcinating the In(OH)3 coatings and In(OH)3 precursor, respectively. The thickness of the coatings increased from 12 to 22 μm as the diameter of the feed nozzle increased, and the interconnection between the single hollow spheres In2O3 in the coatings was better than that of the as-synthesized In2O3 powder. The diameter of the single hollow spheres In2O3 in the coatings was about 1.2∼1.4 μm, which is similar to the as-synthesized In2O3 powder. The In2O3 coatings with hollow spheres possessed a very high specific surface area, excellent interconnections, controllable thickness and numerous oxygen defects.

High-speed production of antibacterial fabrics using liquid flame spray

Healthcare associated infections (HAIs) are known as one of the major problems of the modern healthcare system, which result in additional cost and mortality. It has also been shown that pathogenic bacteria are mostly transferred via surfaces in healthcare settings. Therefore, antibacterial surfaces, which include fabrics and textiles, can be used in a healthcare environment to reduce the transfer of pathogenic bacteria, hence reducing HAIs. Silver nanoparticles have been shown to have broad spectrum antibacterial properties, and therefore they have been incorporated into fabrics to provide antibacterial functionality. Liquid flame spray (LFS) nanoparticle synthesis allows nanoparticles to be produced and deposited on surfaces at speeds up to and beyond 300 m/min. Herein, LFS is used to deposit silver nanoparticles onto two fabrics that are commonly used in the hospital environment with the aim of producing antibacterial fabrics. A thin plasma coating on top of the fabrics after silver deposition is used to improve nanoparticle adhesion. Fabrics coated with silver nanoparticles demonstrated antibacterial properties against Escherichia coli. Nanoparticle imaging and surface chemical characterization are performed using scanning electron microscopy and X-ray photoelectron spectroscopy. The highlights of this research are as follows: • high-speed synthesis and deposition of silver nanoparticles on fabrics; • plasma coating onto fabrics with silver nanoparticles; • antibacterial fabrics for potential use in healthcare environments.

Antibacterial activity of silver and titania nanoparticles on glass surfaces

A liquid flame spray (LFS) nanoparticle deposition process was used to generate glass surfaces with silver (Ag) and titania (TiO2) nanoparticles for antibacterial activity against two common pathogenic bacteria causing community-associated and hospital-acquired infections, gram positive Staphylococcus aureus (S aureus) and gram negative Escherichia coli (E coli). All nanoparticle coatings increased antibacterial activity compared to a reference glass surface. The Ag nanoparticle coatings showed the highest antibacterial activity with E coli. On the contrary, TiO2 nanoparticle coatings were found to have a higher antibacterial activity against S aureus than E coli. No significant differences in antibacterial activity were observed between the two used nanoparticle deposition amounts.

Fabrication of Fiber Filters with Antibacterial Properties for VOC and Particle Removal

ABSTRACT The use of filters to control air quality has been implemented widely in all types of structures. Unfortunately, filters risk becoming platforms for the growth of bacteria, which can then be dispersed further in the air stream. To combat this, antibacterial materials are being incorporated into filter media. In this work, we tested two routes for introducing nanoscale silver into filters containing activated carbon fibers (ACF): first, by adding silver nanofibers directly to the fiber fabrication process and second, by coating a pre-existing filter with silver nanoparticles generated by a liquid flame spray (LFS). The resultant filters were evaluated for methanol adsorption, particle penetration and antibacterial activity. The results show that both methods are suitable for producing antibacterial filters as well as being highly tailorable and scalable for specific needs.

Effect of plasma coating on antibacterial activity of silver nanoparticles

Silver nanoparticles (NPs) are known to provide antimicrobial properties for surfaces. However, there are environmental concerns due to reports of toxicity after exposure to the environment during or after end-use. Immobilizing silver NPs to the surface of substrates could ensure that particles are readily available for antibacterial activity with limited environmental exposure. A plasma coating on top of silver NPs could improve the adhesion of NPs to a substrate, but it could also impede the release of silver NPs completely. Furthermore, silver has been shown to require direct contact to demonstrate antibacterial activity. This study demonstrates immobilization of silver NPs with plasma coating onto a surface while maintaining its antibacterial properties. Silver NPs are simultaneously synthesized and deposited onto a surface with liquid flame spray aerosol technique followed by hexamethyldisiloxane plasma coating to immobilize the NPs. Atomic force microscope scratch testing is used to demonstrate improved nanoparticle adhesion. Antibacterial activity against gram-negative Escherichia coli is maintained even for plasma coating thicknesses of 195 nm. NP adhesion to the surface is significantly improved. Gram-positive Staphylococcus aureus was found be resistant to all the plasma-coated samples. The results show promise of using plasma coating technology for limiting NP exposure to environment.

On the limit of superhydrophobicity: defining the minimum amount of TiO2 nanoparticle coating

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).

Characterization of flame coated nanoparticle surfaces with antibacterial properties and the heat-induced embedding in thermoplastic-coated paper

Silver nanoparticles deposited on surfaces can provide an antibacterial effect with potential uses in, for example, healthcare settings. However, release of nanoparticles and their potential exposure to the environment is of concern. The current work demonstrates a continuous synthesis that simultaneously deposits silver nanoparticles onto plastic coated paper surface by utilizing the liquid flame spray (LFS) aerosol process. Heat from LFS is used to soften the thermoplastic paper surface, which enables partial and full embedding of the nanoparticles, thereby improving adhesion. The embedding is confirmed with atomic force and scanning electron microscopy, and the deposited silver amounts are quantified with X-ray photoelectron spectroscopy. The results suggest that embedding was more effective in PE-coated paper samples due to the lower glass transition temperature when compared to PET-coated paper samples. The antibacterial properties of the surfaces against E. coli and S. aureus were maintained and confirmed with a previously developed ‘Touch-Test Method’. The LFS process has the potential to be used for large-scale manufacturing of antibacterial surfaces with improved nanoparticle adhesion on appropriately chosen thermoplastic surfaces.