Mikko Aromaa

Nanostructures increase water droplet adhesion on hierarchically rough superhydrophobic surfaces.

Hierarchical roughness is known to effectively reduce the liquid-solid contact area and water droplet adhesion on superhydrophobic surfaces, which can be seen for example in the combination of submicrometer and micrometer scale structures on the lotus leaf. The submicrometer scale fine structures, which are often referred to as nanostructures in the literature, have an important role in the phenomenon of superhydrophobicity and low water droplet adhesion. Although the fine structures are generally termed as nanostructures, their actual dimensions are often at the submicrometer scale of hundreds of nanometers. Here we demonstrate that small nanometric structures can have very different effect on surface wetting compared to the large submicrometer scale structures. Hierarchically rough superhydrophobic TiO(2) nanoparticle surfaces generated by the liquid flame spray (LFS) on board and paper substrates revealed that the nanoscale surface structures have the opposite effect on the droplet adhesion compared to the larger submicrometer and micrometer scale structures. Variation in the hierarchical structure of the nanoparticle surfaces contributed to varying droplet adhesion between the high- and low-adhesive superhydrophobic states. Nanoscale structures did not contribute to superhydrophobicity, and there was no evidence of the formation of the liquid-solid-air composite interface around the nanostructures. Therefore, larger submicrometer and micrometer scale structures were needed to decrease the liquid-solid contact area and to cause the superhydrophobicity. Our study suggests that a drastic wetting transition occurs on superhydrophobic surfaces at the nanometre scale; i.e., the transition between the Cassie-Baxter and Wenzel wetting states will occur as the liquid-solid-air composite interface collapses around nanoscale structures. Consequently, water adheres tightly to the surface by penetrating into the nanostructure. The droplet adhesion mechanism presented in this paper gives valuable insight into a phenomenon of simultaneous superhydrophobicity and high water droplet adhesion and contributes to a more detailed comprehension of superhydrophobicity overall.

Rapid fabrication of high aspect ratio silicon nanopillars for chemical analysis

In this study, a method for fabrication of high aspect ratio silicon nanopillars is presented. The method combines liquid flame spray production of silica nanoparticle agglomerates with cryogenic deep reactive ion etching. First, the nanoparticle agglomerates, having a diameter of about 100 nm, are deposited on a silicon wafer. Then, during the subsequent cryogenic deep reactive ion etching process, the particle agglomerates act as etch masks and silicon nanopillars are formed. Aspect ratios of up to 20:1 are demonstrated. The masking process is rapid, cheap and has the potential to be scaled up for large areas. Three other structured silicon surfaces were fabricated for comparison. All four surfaces were utilized as desorption/ionization on silicon (DIOS) sample plates. The mass spectrometry results indicate that nanopillar surfaces masked with the liquid flame spray technique are well suited as DIOS sample plates.

Nanoparticle Deposition from Liquid Flame Spray onto Moving Roll-to-Roll Paperboard Material

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 particles. Total mass yields of 5%–20% of the injected precursor material into the titanium dioxide nanocoating on the paperboard were achieved. With these yields, a highly hydrophobic surface was obtained by a mass loading of 10–50 mg/m2 of titanium dioxide on the paperboard.

ToF-SIMS analysis of UV-switchable TiO2-nanoparticle-coated paper surface.

The chemical composition of a TiO2 nanoparticle coated paper surface was analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) to study the interconnection between wettability and surface chemistry on the nanoscale. In this work, a superhydrophobic TiO2 surface rich in carboxyl-terminated molecules was created by a liquid flame spray process. The TiO2 nanoparticle coated paper surface can be converted by photocatalytic oxidation into a highly hydrophilic one. Interestingly, the hydrophilic surface can be converted back into a superhydrophobic surface by heat treatment. The results showed that both ultraviolet A (UVA) and oven treatment induce changes in the surface chemistry within a few nanometers of the paper surface. These findings are consistent with those from our previously reported X-ray photoelectron spectroscopy (XPS) analysis, but the ToF-SIMS analysis yields more accurate insight into the surface chemistry.

Creation of superhydrophilic surfaces of paper and board

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) <10°, were created mainly by modifying the surface chemistry of the paper and board by argon plasma or SiO2 coating. The nano- and microscale roughness existing on paper and board surfaces enabled the creation of the superhydrophilic surfaces. Furthermore, the benefits and limitations of the surface modification techniques are discussed and compared. For example, the SiO2 coating maintained its extreme hydrophilicity for at least six months, whereas the CAW of argon plasma-treated surface increased to about 20° already in one day.

Iron Oxide Doped Alumina-Zirconia Nanoparticle Synthesis by Liquid Flame Spray from Metal Organic Precursors

The liquid flame spray (LFS) method was used to make iron oxide doped alumina-zirconia nanoparticles. Nanoparticles were generated using a turbulent, high-temperature (𝑇max∼3000 K) H2-O2 flame. The precursors were aluminium-isopropoxide, zirconium-𝑛-propoxide, and ferrocene in xylene solution. The solution was atomized into micron-sized droplets by high velocity H2 flow and introduced into the flame where nanoparticles were formed. The particle morphology, size, phase, and chemical composition were determined by TEM, XRD, XPS, and N2-adsorption measurements. The collected particulate material consists of micron-sized aggregates with nanosized primary particles. In both doped and undoped samples, tetragonal phase of zirconia was detected in room temperature while alumina was found to be noncrystalline. In the doped powder, Fe was oxidized to Fe2O3. The primary particle size of collected sample was approximately from 6 nm to 40 nm. Doping was observed to increase the specific surface area of the powder from 39 m2/g to 47 m2/g.

Switchable water absorption of paper via liquid flame spray nanoparticle coating

Surface wetting/anti-wetting and liquid absorption are relevant properties of many porous solids including paper and other cellulose-based materials. Here we demonstrate how surface wetting by water and water absorption of commercially available kraft paper can be altered by thin nanoparticle coatings fabricated by liquid flame spray in facile and continuous one-step process. Surface wettability and absorption properties of paper increased with silica and decreased with titania (TiO2) nanoparticle coatings. Moreover, the water-repellent (superhydrophobic) TiO2 nanoparticle coated paper could be switched to superhydrophilic and water absorbing by ultraviolet illumination. The experiments revealed that although surface wetting and liquid absorption of nanoparticle coated paper are strongly related to each other, they are two distinct phenomena which do not necessarily correlate. We propose wetting regimes on the nanoparticle coated paper samples on the basis of the experimental observations.