Superhydrophobic polymer films via aerosol assisted deposition — Taking a leaf out of nature's book

Aerosol assisted deposition of three sets of polymer films based on commercially available resins was achieved on various substrates. The films were characterised using a range of methods, including water contact and slip angle to determine water repellent properties. The aerosol assisted deposition inside the chemical vapour deposition reactor was unique in generating a highly rough superhydrophobic surface with water contact angles up to 170°. During the deposition process, two of the polymers were cured resulting in the development of high surface morphology. It was observed that the polymer that did not cure did not develop such a rough surface resulting in a lower water contact angle (∼ 99°). The superhydrophobic films had a Cassie-Baxter type wetting with water failing to penetrate the surface porosity, water spraying on the surface would bounce off. These films had exceptionally low slide angles of ca 1-2° from the horizontal.     

Initiated chemical vapor deposition of pH responsive poly(2-diisopropylamino)ethyl methacrylate thin films

Poly(2-(diisopropylamino)ethyl methacrylate) (PDPAEMA) thin films were deposited on low temperature substrates by initiated chemical vapor deposition (iCVD) method using tertbutyl peroxide as an initiator. Very high deposition rates up to 38 nm/min were observed at low filament temperatures due to the use of the initiator. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy show the formation of PDPAEMA films with high retention of tertiary amine functionality which is responsible for pH induced changes in the wetting behavior of the surfaces. As-deposited PDPAEMA thin films on flat Si surface showed a reversible switching of water contact angle values between 87° and 28°; after successive treatments of high and low pH water solutions, respectively. Conformal and non-damaging nature of iCVD allowed to functionalize fragile and rough electrospun poly(methyl methacrylate) fiber mat surfaces by PDPAEMA, which creates a surface with a switching behavior between superhydrophobic and approaching superhydrophilic with contact angle values of 155 ± 3°and 22 ± 5°, respectively.     

Superhydrophilic textured-surfaces on stainless steel substrates

Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) is used to produce micro/nano-textured surfaces on stainless steel substrates at low temperatures for altering the wetting property of the substrates. The micro/nano-textured surfaces were characterized using scanning electron microscopy, X-ray spectroscopy, and X-ray diffraction. The wetting properties of the textured surfaces were characterized by water contact angle measurements. It was found that AIC of a-Si changes the apparent contact angles of stainless steel substrates from 90° to about 0°, measured 0.5 s after a water droplet drops on the surfaces. The study also shows that a superhydrophilic textured surface can be converted to a highly hydrophobic surface with an apparent contact angle of 145° by coating the surface with a layer of octadecyltrichlorosilane.     

An investigation into bacterial attachment to an elastomeric superhydrophobic surface prepared via aerosol assisted deposition

Bacterial attachment is highly dependent on a surfaces microstructure. For example, some rough surfaces provide grooves suitable for bacterial adhesion. Superhydrophobic surfaces with a Cassie-Baxter wetting mechanism are shown to prevent contact between a bacterium and surface attachment points. The surface used in this study is a highly rough thin film made from a silicone elastomer via an aerosol assisted chemical vapour deposition (AACVD) process. The films had water contact angles averaging 165°, a very low slip angle, and were capable of duplicating the Lotus effect. The ability of bacteria (Escherichia coli and Methicillin-resistant Staphylococcus aureus) to adhere to this surface was tested by submersion in a bacterial suspension. The superhydrophobic elastomer surfaces reduced the attachment of the bacteria tested, relative to the control surfaces of plain glass, and flat elastomeric films. The reduction in bacterial adhesion, without the external action of chemicals, gives the elastomer surface deposited with AACVD possible applications in biomedical and catering industries. This progressive study of bacterial adhesion is carried out on an AACVD prepared surface and presents adhesion results from both smooth and highly roughened elastomeric surfaces.     

Effect of film thickness and agglomerate size on the superwetting and fog-free characteristics of TiO2 films

The effect of TiO₂ film thickness and agglomerate size on the non-UV activated superhydrophilic wetting and antifogging characteristics of TiO₂ films was investigated. Evidence from Atomic Force Microscopy analysis showed that surface roughness is the key parameter requiring control so as to retain the superhydrophilic wetting and antifogging behaviour of the synthesised films. Surface roughness can be tuned by simple manipulation of the multilayer assembly of TiO₂ nanoparticles through varying the film thickness and agglomerate size. A film thickness of ~ 140 nm yielded the optimum roughness (root mean square = 23 nm) to give the best superhydrophilic wetting behaviour. Thicker films reduced the film roughness and were detrimental to their superhydrophilic wetting properties. Smaller agglomerate size was also found to be important in retaining film roughness.     

Influence of annealing temperature on the structural, mechanical and wetting property of TiO2 films deposited by RF magnetron sputtering

TiO₂ films have been deposited on silicon substrates by radio frequency magnetron sputtering of a pure Ti target in Ar/O₂ plasma. The TiO₂ films deposited at room temperature were annealed for 1 h at different temperatures ranging from 400 °C to 800 °C. The structural, morphological, mechanical properties and the wetting behavior of the as deposited and annealed films were obtained using Raman spectroscopy, atomic force microscopy, transmission electron microscopy, nanoindentation and water contact angle (CA) measurements. The as deposited films were amorphous, and the Raman results showed that anatase phase crystallization was initiated at annealing temperature close to 400 °C. The film annealed at 400 °C showed higher hardness than the film annealed at 600 °C. In addition, the wettability of film surface was enhanced with an increase in annealing temperature from 400 °C to 800 °C, as revealed by a decrease in water CA from 87° to 50°. Moreover, the water CA of the films obtained before and after UV light irradiation revealed that the annealed films remained more hydrophilic than the as deposited film after irradiation.     

Preparation and characterization of increased-efficiency photocatalytic TiO2-2xNx thin films

We report here on the characteristics of RF-sputtered 300 nm thick films of TiO_2-2xN_x prepared on glass substrates at 350 °C, by adjusting the N₂:Ar partial pressure ratio in the deposition chamber between 0.00 and 0.33. XRD, XPS, AFM and contact angle data were used to derive film structure, elemental composition and oxidation state of Ti, surface morphology and hydrophilicity, respectively. The band gap was derived from spectral data in the 350-450 nm range. Film structure and composition were changed by adjusting the partial pressure of the reactive gases during sputtering and by post-deposition annealing at 400 °C in air, for 90 min. The values of the contact angle of films' surface with de-ionized water and of surface free energy per unit area show that films are super-hydrophilic for high-nitrogen content. Correlations are made between film structure, elemental composition, electronic and wettability properties.     

Superhydrophobic surface directly created by electrospinning based on hydrophilic material

We describe a method to form hydrophobic surfaces using PHBV (Poly (hydroxybutyrate-co-hydroxyvalerate))—a kind of intrinsically hydrophilic material. The concentration of polymer solutions was varied to control the surface morphology and resultant wetting property. The as-prepared films were characterized by micro-scale valley-and-hill structure, which was formed by aggregating of electrospun beads. The bead morphology changed from smooth to porous and popcorn-like with decreased concentrations. The shape of water droplet on these surfaces had contact angles ranging from 110.7 to 158.1°, with a maximum standard deviation of 2.5°. It was found that both the micro and nanostructure were important to create a superhydrophobic surface.     

Laser‐Induced Graphene in Controlled Atmospheres: From Superhydrophilic to Superhydrophobic Surfaces

The modification of graphene‐based materials is an important topic in the field of materials research. This study aims to expand the range of properties for laser‐induced graphene (LIG), specifically to tune the hydrophobicity and hydrophilicity of the LIG surfaces. While LIG is normally prepared in the air, here, using selected gas atmospheres, a large change in the water contact angle on the as‐prepared LIG surfaces has been observed, from 0° (superhydrophilic) when using O₂ or air, to >150° (superhydrophobic) when using Ar or H₂. Characterization of the newly derived surfaces shows that the different wetting properties are due to the surface morphology and chemical composition of the LIG. Applications of the superhydrophobic LIG are shown in oil/water separation as well as anti‐icing surfaces, while the versatility of the controlled atmosphere chamber fabrication method is demonstrated through the improved microsupercapacitor performance generated from LIG films prepared in an O₂ atmosphere.     

Superhydrophilic surface treatment for thin film NiTi vascular applications

A variety of surface treatment methods were evaluated to modify the hydrophilic nature of thin film nitinol (NiTi). It has been suggested that increasing hydrophilicity reduces the prevalence of platelet adhesion and thrombosis in the vascular system. In this study, thin film NiTi was treated with three pretreatments cleaning, buffered oxide etchant (BOE), and BOE/nitric acid (HNO₃), followed by one surface treatment. The three surface treatment studied were UV irradiation, thermal treatment, or hydrogen peroxide. Two surface treatments, i.e., thermal at 600 °C for 30 min and 30% hydrogen peroxide treatment for 15 h, produced superhydrophilic surfaces, i.e., wetting angle = 0°. However, the superhydrophilic surface produced by the thermal treatment also embrittled the thin film due to the relative thickness of the oxide grown. Long term studies in air showed that all surface treatments trend toward hydrophobic natures. However, storage of the surface treated thin film NiTi in Deionized (DI) water preserved even the superhydrophilic surfaces indefinitely.     

Deposition of Ni-CGO composite anodes by electrostatic assisted ultrasonic spray pyrolysis method

Deposition of composite films of Ni and Gd-doped ceria was carried out using the electrostatic assisted ultrasonic spray pyrolysis method for the first time. The composite films were highly homogeneous, as revealed by element mapping via energy-dispersive spectrometry. Scanning electron microscope examinations revealed that deposition temperature and electric field strength had profound influence on resultant microstructure, while composition of the precursor solution had little effect. A highly porous cauliflower structure ideal for solid oxide fuel cell anode performance was obtained with a deposition temperature of 450 °C under an electric field introduced by an applied voltage of 12 kV. Films obtained with a lower deposition temperature of 250 °C or a higher applied voltage of 15 kV resulted in denser films with low porosity, while lower applied voltages of 7 or 5 kV resulted in thinner or discontinuous films due to the insufficient electrostatic attraction on the aerosol droplets. As revealed by AC impedance measurement, the area specific resistances of the Ni-CGO anode with porous cauliflower structure were rather low and a value of 0.09 Ω cm² at 550 °C was obtained.     

Surface treatment of polycarbonate and polyethersulphone for SiNx thin film deposition

Silicon nitride thin films were deposited with good adhesion on plasma treated polyethersulphone (PES) and polycarbonate (PC) substrates by in-situ rf magnetron sputtering. The surfaces of the PES and PC substrates were performed by plasma treatment at various rf powers and processing time in Ar, O₂ atmosphere. From the X-ray Photoelectron Spectroscopy (XPS) examination of the surface of the treated substrates, it was found that the ratio of oxide containing bonds increased with increasing rf power. The surface roughness of the PES and PC substrates increased with increasing rf power. The plasma treated surface of the substrates became hydrophilic as measured by the water contact angle. The water contact angle for the PES and PC substrates decreased with increasing rf power and processing time, significantly. The lowest value of the contact angle of 14.09° was observed at rf power of 200 W. It was observed that the adhesion properties between the SiN_x films and substrates were enhanced by the plasma treatment.     

Low temperature atomic layer deposition of titania thin films

This paper presents a comprehensive study of atomic layer deposition of TiO₂ films on silicon and polycarbonate substrates using TiCl₄ and H₂O as precursors at temperatures in the range 80-120 °C. An in-situ quartz crystal microbalance was used to monitor different processing conditions and the resultant films were characterised ex-situ using a suite of surface analytical tools. In addition, the contact angle and wettability of as-deposited and UV irradiated films were assessed. The latter was found to reduce the contact angle from ≥ 80° to < 10°. Finally, the effect of surface pre-treatment on film toughness and adhesion was investigated and the results show a significant improvement for the pre-treated films.     

Low temperature annealing effects on the structure and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on glass substrates at room temperature (RT) ∼500 °C by pulsed laser deposition (PLD) technique and then were annealed at 150-450 °C in air. The effects of annealing temperature on the microstructure and optical properties of the thin films deposited at each substrate temperature were investigated by XRD, SEM, transmittance spectra, and photoluminescence (PL). The results showed that the c-axis orientation of ZnO thin films was not destroyed by annealing treatments; the grain size increased and stress relaxed for the films deposited at 200-500 °C, and thin films densified for the films deposited at RT with increasing annealing temperature. The transmittance spectra indicated that E_g of thin films showed a decreased trend with annealing temperature. From the PL measurements, there was a general trend, that is UV emission enhanced with lower annealing temperature and disappeared at higher annealing temperature for the films deposited at 200-500 °C; no UV emission was observed for the films deposited at RT regardless of annealing treatment. Improvement of grain size and stoichiometric ratio with annealing temperature can be attributed to the enhancement of UV emission, but the adsorbed oxygen species on the surface and grain boundary of films are thought to contribute the annihilation of UV emission. It seems that annealing at lower temperature in air is an effective method to improve the UV emission for thin films deposited on glass substrate at substrate temperature above RT.     

Low temperature preparation of TiO2 nanodot film on substrates

Herein, we report a photoinduced transition of hydrophobicity to high hydrophilicity of TiO₂ nanodot films in applications of cell sheet engineering. A phase-separation-induced self-assembly process was adopted to prepare a TiO₂ nanodot gel film on a substrate. Subsequently, a hydrothermal treatment (with ethanol/water at 140 °C for 2 h) was used to convert the nanodot gel film to TiO₂ nanodot solid film. The resulting TiO₂ dots were amorphous with adjustable size and density. The amorphous TiO₂ nanodot film showed a conversion from a good hydrophobic surface, with a water contact angle (WCA) of 67.6 ± 2.0°, to a highly hydrophilic one, with a WCA of 5.3 ± 2.0° (i.e. almost superhydrophilic) after UV irradiation. A good reversibility was also observed.     

A novel patterning technique using super-hydrophobic PTFE thin films by Cat-CVD method

Super-hydrophobic poly-tetrafluoroethylene (PTFE) films, with a water contact angle of over 160°, are formed by catalytic chemical vapor deposition (Cat-CVD) under high catalyzer temperature or pressure. Hydrophobicity of the PTFE films is maintained even after annealing up to 300 °C. We demonstrate a novel method for forming metal lines using super-hydrophobic PTFE films. Water-based functional liquid containing silver nanoparticles dropped on the patterned PTFE film localizes only on hydrophilic regions, resulting in formation of metal lines after annealing up to 150 °C.     

Reversible wettability of ZnO nanostructured thin films prepared by pulsed laser deposition

This work reports on the photoinduced wettability changes of high quality nanostructured ZnO films grown on Si by pulsed laser deposition (PLD) under different growth parameters. The wetting behavior of the resulting films can be reversibly switched from hydrophobic to hydrophilic, through alternation of UV illumination and dark storage. The kinetics of this wetting transition are studied by monitoring the time evolution of the corresponding contact angles. Finally, the influence of the film properties over the observed wetting response is discussed.     

Fabrication of ZnO submicrorod films with water repellency by surface etching and hydrophobic modification

Superhydrophobic ZnO submicrorod films have been fabricated on zinc sheets through an H₂O₂-assisted surface etching process and subsequent surface modification with a monolayer of 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FDS). The crystal structure, chemical compositions, morphologies, and wettability of the resultant ZnO films were analyzed by means of X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and water contact angle measurements. It is found that the surface of the as-prepared ZnO films on zinc substrate was hydrophobic with a water contact angle of 95 ± 2°, whereas after modification with FDS, the film exhibited superhydrophobicity and the water CA increased to 154 ± 2°. It is shown that both the higher surface roughness and the lower surface free energy play an important role in creating the superhydrophobic films.     

Vapor phase surface functionalization under ultra violet activation of parylene thin films grown by chemical vapor deposition

Various reactive gas phase treatments have been investigated as surface functionalization dry processes with the goal to improve the wettability of parylene C films, keeping good optical properties in the visible range. The films were grown on different substrates by chemical vapor deposition with thicknesses ranging from 300 to 1630 nm. The polymer surface was treated under ultra violet (UV) irradiation at 254 nm in reactive atmospheres including He, H₂O, H₂O₂, O₂ and ambient air. The UV/O₂ treatment is the most efficient since the water contact angle decreases from 100° to 6° while the transmittance is maintained at 90% in the visible wavelengths. Furthermore it exhibits long life stability. The functionalization mechanism is discussed in relation with previous reports.     

Hyperhydrophilic rough surfaces and imaginary contact angles

The complete wetting of rough surfaces is only poorly understood, since the underlying phenomena can neither be described by the Cassie‐Baxter nor the Wenzel equation. An experimental accessiblility by the sessile drop method is also very limited. The term “superhydrophilicity” was an attempt to understand the wetting of rough surfaces, but a clear definition is still forthcoming, mainly because non‐superhydrophilic surfaces can also display a contact angle of zero. Since the Wilhelmy balance is based on force measurements, it offers a technology for obtaining signals during the whole wetting process. We have obtained evidence that additional forces occur during the complete wetting of rough surfaces and that mathematically contact angles for a hydrophilicity beyond the contact angle of zero can be defined by imaginary numbers. A hydrophilized TPS‐surface obtained by chemical wettability switching from a superhydrophobic surface has been previously characterized by dynamic imaginary contact angles of 20i°–21i° and near‐zero hysteresis. Here an extremely high wetting rate is demonstrated reaching a virtual imaginary contact angle of Θ_V,Adv > 3.5i° in less than 210 ms. For a rough surface displaying imaginary contact angles and extremely high wetting rates we suggest the term hyperhydrophilicity. Although, as will be shown, the physical basis of imaginary contact angles is still unclear, they significantly expand our methodology, the range of wettability measurements and the tools for analyzing rough hydrophilic surfaces. They may also form the basis for a new generation of rationally constructed medicinal surfaces.