Changes In Water Contact Angle Research Articles

Functional nano-textured titania-coatings with self-cleaning and antireflective properties for photovoltaic surfaces

Photoactive TiO2-only transparent coatings having self-cleaning and antireflection (AR) properties were prepared by forming first a nanosol through controlled hydrolysis of tetraisopropyl orthotitanate (TIPT), followed by deposition of this nanosol on glass substrates by dip-coating with a final calcination step to form the surface nano-textured thin film. The samples were characterized in terms of nano-structure and -texture by X-ray diffraction, UV–vis spectroscopy, scanning electron microscopy and atomic force microscopy, while AR properties were investigated by transmittance measurements. Self-cleaning properties were analyzed by measuring the changes of water contact angle, and by photocatalytic degradation of a dye. The aim was to analyze how to prepare these materials and the relation of the properties of titania with the surface nano-texture, particularly in relation to obtain the properties required to their use as functional coatings for PV cells. Films with good optical characteristics and high transmittance (<1% loss in transmittance) can be obtained at low speed of dip-coating (6mm/s) and high nitric acid concentration (0.5M). Under optimized conditions, calcination at low temperature (400°C) may already be sufficient to produce coatings with good functional properties, making the procedure compatible with the use of some flexible substrates. A preliminary mechanism of formation of the surface nano-texturing is also proposed.

Combined Situ Polymerization and Thermal Cross-Linking Technique for the Preparation of Ammonium Polyphosphate Microcapsules with Composite Shell

Ammonium polyphosphate (APP) was encapsulated with boron containing phenolic resin (BPF) by in situ polymerization and then modified with nanosize silica gel through thermal cross-linking with the goal of further improving its hydrophobicity, thermal stability, and shell mechanical property. The chemical and physical features of APP microcapsules were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetery (DSC), and laser particle sizing. The hydrophobicity was assessed by the water contact angle (WCA). The shell mechanical property was characterized by the change of water contact angle before and after the mechanical property test. The residues from thermogravimetric analyzer and muffle burner were investigated. The results showed that the APP microcapsules with a Si–B containing phenolic resin shell had been produced successfully (Si-BPFAPP). Compared with BPFAPP, the hydrophobicity, shell mechanical property, and therm...

Reversible manipulation of the adhesive forces of TiO2/polybenzoxazine nanoassembled coatings through UV irradiation and thermal treatment

In this study we mixed TiO2 nanoparticles (NPs) with 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (BA), as a precursor to a polybenzoxazine (PBA), to generate nanocomposite surfaces possessing low surface free energies. Because of extreme phase separation between the TiO2 NPs and BA, their mixtures featured nanoassembled structures on their surfaces. After thermal curing, we obtained PBA/TiO2 nanoassembled (PTN) surfaces possessing various TiO2 contents. The mixing of 30wt% TiO2 NPs into the PBA matrix generated a superhydrophobic surface (static water contact angle: >150° with a sliding angle of approximately 1°). We could convert this superhydrophobic TiO2/PBA nanoassembled surface, through photocatalytic oxidation, into a highly hydrophilic surface (static water contact angle: ca. 0°). Interestingly, we could convert the hydrophilic surface back into a superhydrophobic surface through heat treatment. Thus, both UV irradiation and oven treatment induced changes in the surface chemistry of these materials. Furthermore, we could tune the adhesive force of the PTN surfaces by varying the UV irradiation time, without significant changes in the static water contact angle. As a result, we could transport a water droplet among PTN surfaces that had been subjected to UV irradiation for various lengths of time.

Preparation and Characterization of Polyhedral Oligomeric Silsesquioxane-Containing, Titania-Thiol-Ene Composite Photocatalytic Coatings, Emphasizing the Hydrophobic-Hydrophilic Transition.

Coatings prepared from titania-thiol-ene compositions were found to be both self-cleaning, as measured by changes in water contact angle, and photocatalytic toward the degradation of an organic dye. Stable titania-thiol-ene dispersions at approximately 2 wt % solids were prepared using a combination of high-shear mixing and sonication in acetone solvent from photocatalytic titania, trisilanol isobutyl polyhedral oligomeric silsesquioxane (POSS) dispersant, and select thiol-ene monomers, i.e., trimethylolpropane tris(3-mercaptopropionate) (TMPMP), pentaerythritol allyl ether (APE), and 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TTT). The dispersed particle compositions were characterized by DLS and TEM. The synthetic methods employed yield a strongly bound particle/POSS complex, supported by IR, 29Si NMR, and TGA. The factors of spray techniques, carrier solvent volatility, and particle size and size distributions, in combination, likely all contribute to the highly textured but uniform surfaces observed via SEM and AFM. Polymer composites possessed thermal transitions (e.g., Tg) consistent with composition. In general, the presence of polymer matrix provided mechanical integrity, without significantly compromising or prohibiting other critical performance characteristics, such as film processing, photocatalytic degradation of adsorbed contaminants, and the hydrophobic-hydrophilic transition. In all cases, coatings containing photocatalytic titania were converted from superhydrophobic to superhydrophilic, as defined by changes in the water contact angle. The superhydrophilic state of samples was considered persistent, since long time durations in complete darkness were required to observe any significant hydrophobic return. In a preliminary demonstration, the photocatalytic activity of prepared coatings was confirmed through the degradation of crystal violet dye. This work demonstrates that a scalable process can be found to prepare titania-thiol-ene coatings having improved coating properties which also exhibit photocatalytic and self-cleaning attributes.

Biocatalyzed approach for the surface functionalization of poly(L-lactic acid) films using hydrolytic enzymes.

Poly(lactic acid) as a biodegradable thermoplastic polyester has received increasing attention. This renewable polyester has found applications in a wide range of products such as food packaging, textiles and biomedical devices. Its major drawbacks are poor toughness, slow degradation rate and lack of reactive side-chain groups. An enzymatic process for the grafting of carboxylic acids onto the surface of poly(L-lactic acid) (PLLA) films was developed using Candida antarctica lipase B as a catalyst. Enzymatic hydrolysis of the PLLA film using Humicola insolens cutinase in order to increase the number of hydroxyl and carboxylic groups on the outer polymer chains for grafting was also assessed and showed a change of water contact angle from 74.6 to 33.1° while the roughness and waviness were an order of magnitude higher in comparison to the blank. Surface functionalization was demonstrated using two different techniques, (14) C-radiochemical analysis and X-ray photoelectron spectroscopy (XPS) using (14) C-butyric acid sodium salt and 4,4,4-trifluorobutyric acid as model molecules, respectively. XPS analysis showed that 4,4,4-trifluorobutyric acid was enzymatically coupled based on an increase of the fluor content from 0.19 to 0.40%. The presented (14) C-radiochemical analyses are consistent with the XPS data indicating the potential of enzymatic functionalization in different reaction conditions.

Wettability properties of PTFE/ZnO nanorods thin film exhibiting UV-resilient superhydrophobicity

In this research, initially anodization process was used to fabricate ZnO nanorods on Zn substrate and then RF sputtering technique was applied to grow a thin layer of polytetrafluoroethylene (PTFE, Teflon) on the coated ZnO nanorods for producing a superhydrophobic surface. According to scanning electron microscopy (SEM) observations, ZnO nanorods were formed with average diameter and length of about ∼180nm and 14μm, respectively. Superhydrophilic property of ZnO nanorods and superhydrophobic property of PTFE/ZnO nanorods was investigated by water contact angle (WCA) measurements. It was found that the contact angle varied with the PTFE deposition time. The highest contact angle measurement was obtained at 160° for the PTFE (60min coating)/ZnO as optimum sample which indicates its superhydrophobic property. X-ray photoelectron spectroscopy (XPS) determined surface chemical composition and F/C ratio of about 1.27 for this sample. A change of water contact angle from 3° to 160° indicates transition from superhydrophilic to superhydrophobic state. Very low contact angle hysteresis (CAH) of ∼2° and sliding angle (SA) of ∼1° as well as unchanged contact angle under UV illumination was observed for the synthesized optimum PTFE/ZnO sample exhibits an excellent superhydrophobic property. Based on our data analysis, the ZnO nanorods and the PTFE/ZnO nanorods obey Wenzel and Cassie–Baxter model, respectively.

Electrospun carboxylic-functionalized poly(arylene ether ketone) ultrafine fibers

As a high-performance polymer, carboxylic-functionalized poly(arylene ether ketone)s (PCA-PAEK) was electrospun into ultrafine fibers and characterized. To optimize the process, a set of experiments were designed. As a result, the optimum condition was obtained and the polymer concentration was determined as the most important factor. Incidentally, the PCA-PAEK fibers were found to exhibit superabsorbent behavior. In order to investigate processing characteristics, PCA-PAEK fibers fabricated from different solution concentrations were employed and characterized. The results showed that porosity and changes in water contact angle and water absorbency decreased with an increase in concentration. In addition to a high water absorption capacity, the fibers exhibited good water retention and repeated water absorbency as well. The study on kinetics of water absorption and swelling behavior showed that the mechanism was dependent on the polymer concentration from which the fibers were electrospun. Moreover, both dry and wet PCA-PAEK fibers showed high mechanical properties. Due to these properties, the PCA-PAEK ultrafine fibers are potentially useful as a water-absorbent material.

Effect of ethanol post-treatment on the bubble-electrospun poly(vinyl alcohol) nanofiber

Poly(vinyl alcohol) nanofibers were prepared by bubble electrospinning. After the ethanol post-treatment, poly(vinyl alcohol) nanofibers showed enhanced hydrophobicity with water contact angle change from 0 to 78.9?, and the break strength of poly(vinyl alcohol) nanofibers was dramatically improved from 8.23 MPa to 17.36 MPa. The facile strategy with improved hydrophobicity and mechanical properties of poly(vinyl alcohol) nanofibers will provide potential benefits for applications of this material, especially in filtration field.

Tetracycline tethered to titanium inhibits colonization by Gram-negative bacteria.

As peri-prosthetic infection is one of the most devastating complications associated with implant placement, we have reasoned that such infection can be largely subverted by development of antibacterial implants. Our previous work demonstrated that covalent coupling of vancomycin to titanium alloy prevented colonization by the Gram-positive pathogens, Staphylococcus aureus and Staphylococcus epidermidis. Some orthopedic devices, including permanent prosthesis anchors, and most dental implants are transcutaneous or transmucosal and can be prone to colonization by Gram-negative pathogens. We report here the successful covalent coupling of the broad-spectrum antibiotic, tetracycline (TET), to titanium surfaces (Ti-TET) to retard Gram-negative colonization. Synthetic progress was followed by changes in water contact angle, while the presence of TET was confirmed by immunofluorescence. Ti-TET actively prevented colonization in the presence of bathing Escherichia coli, both by fluorescence microscopy and direct counting. Finally, the Ti-TET surface supported osteoblastic cell adhesion and proliferation over a 72-h period. Thus, this new surface offers a powerful means to protect transcutaneous implants from adhesion of Gram-negative pathogens, decreasing the need for replacement of this hardware.

Change in Water Contact Angle on Electrospray-Synthesized SiO2 Coated Layers by Plasma Exposure

Hydrophilic SiO2 layers were obtained by the atmospheric-pressure plasma treatment. Superhydrophobic SiO2 layers were first deposited by the electrospray deposition method. The electrospunable solution that was prepared based on the solgel method was sprayed on Si (100) substrates. The surface of the electrosprayed SiO2 layers consisted of the agglomeration of nano-sized grains, which led to a very high roughness and revealed a very high contact angle to water droplets over 162˚. After having been exposed to the atmospheric Ar/O2 plasma, the observed superhydrophobicity of the SiO2 layers were greatly changed: a dramatic variation of the water contact angle from 162˚ to 3˚, namely realization of superhydrophillicity. Interestingly, the surface microstructure was almost preserved. According to the XPS analysis, it is more likely that thanks to the plasma exposure, the surface of SiO2 layers will be cleaned in terms of organic species that are hydrophobic-inducing, consequently leading to the hydrophilic nature observed for the plasma-exposed SiO2 layers.

Influence of heated surfaces and fluids on pool boiling heat transfer

The influence of heated surfaces and fluids on pool boiling heat transfer was investigated experimentally. As the test specimen, Plain Surface (PS) and Nano-Structured Surface (NSS) were used. As the test fluid, pure water, an aqueous Lithium Bromide (LiBr) solution, and an aqueous Sodium Dodecyl Sulfate (SDS) solution were tested. Under the saturated water and aqueous LiBr solution conditions, the NSS showed a lower Onset of Nucleate Boiling (ONB) and higher Nucleate Boiling Heat Transfer Coefficient (NBHTC) than the PS. On the other hand, under aqueous SDS boiling conditions, the NBHTC was increased on the PS, but decreased on the NSS, as compared with under pure water boiling conditions. It was found that the interaction and combination between the heated surface and fluid can be an important parameter affecting the boiling heat transfer performance, and both the surface tension and surface wettability (i.e., contact angle) should be taken into account. The change of water contact angle induced by the chemical reaction between the heated surface and fluid was examined and discussed. As the test surface, copper PS, titanium PS, and titanium dioxide NSS were prepared. Then, they were boiled under saturated water and butanol conditions for 8h, and their water contact angles were compared with the fresh (i.e., pre-boiled) surfaces. The water-boiling process remarkably decreased the water contact angles for all surfaces, as compared with the butanol-boiling process. This implies that the chemical reaction between the heated surfaces and boiling fluids affects the surface wettability, which can strongly influence the pool boiling heat transfer performance.

Effect of methanol ratio in mixed solvents on optical properties and wettability of ZnO films by cathodic electrodeposition

ZnO thin films were prepared in the electrolyte with different methanol ratio by cathodic electrodeposition method. Microstructure, surface morphology, optical properties and wettability of the thin films were investigated by X-ray diffractometer, field-emission scanning electron microscope, ultraviolet–visible spectroscope, fluorescence spectrometer and water contact angle apparatus. Increase of methanol ratio in the solvents may restrain the (002) plane preferential orientation in some extent. Change of current density curves with the ratio of methanol in the solution play a vital role on electrochemical reaction kinetics, microstructure and/or surface morphology of ZnO thin films. With the methanol ratio increase from 0 to 0.8, the surface morphology changes from nanorods to net-like nanostructure. The adsorbed NO3− ions on the polar planes hinder the crystal growth along the c-axis and redirect the growth direction along the nonpolar planes. The maximum and minimum band gaps have been obtained in the ZnO thin films with the methanol ratio of 0.4 and 0.6, respectively. Change of contact angle before UV irradiation may be related to surface morphology and oxygen vacancies. The maximum light-induced water contact angle change has been observed in the sample with the methanol ratio of 0.8. The results may be attributed to the higher surface roughness and net-like morphology.

Surface engineering of titanium alloy substrates with multilayered biomimetic hierarchical films to regulate the growth behaviors of osteoblasts

Osseointegration is essential for the long-term survival of orthopedic implants. Inspired by the hierarchical structure of natural bone, we fabricated a hierarchical structure with osteoinduction potential on titanium alloy (Ti6Al7Nb) substrates via a spin-assisted layer-by-layer assembly technique, with hydroxyapatite nanofibers as the intercalated materials and gelatin and chitosan as the polycation and polyanion, respectively. The as-synthesized hydroxyapatite nanofibers were characterized using scanning electron microscopy (SEM), transmission electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The change of water contact angle corresponding to different layers indicated the formation of a multilayered structure, since different components have their inherent wettability natures. The multilayered lamellar structure was revealed by the cross-sectional view of SEM, suggesting that the film was successfully deposited onto Ti6Al7Nb substrates. Osteoblasts cultured on the hierarchical structure deposited Ti alloy substrates displayed significantly higher cell viability (P<0.01) and better adhesion, a higher production level of alkaline phosphatase, mineralization, genes expressions of osteocalcin and osteopontin (P<0.01 or P<0.05) compared to those of native Ti6Al7Nb substrates after culture for 4, 7 or 14days. These results indicated that the lamellar structure was beneficial for the biological functions of osteoblasts, establishing the basis for osseointegration of a titanium alloy implant.

Corrosion protection of mild steel by one-step electrodeposition of superhydrophobic silica film

Superhydrophobic silica films were deposited onto mild steel substrate for the purpose of corrosion protection. The superhydrophobicity was obtained by one-step electrodeposition of inorganic/organic hybrid sol–gel films from tetraethoxysilane and dodecyltriethoxysilane mixed sol–gel precursors, as a result of simultaneous generation of high roughness and low-energy surface. The corrosion performance was investigated by monitoring the change of water contact angle, by the electrochemical impedance spectroscopy measurements and by iron dissolution test over the whole immersion time. The results show that the superhydrophobic film provides an effective barrier coating for the mild steel interface.

Self-Cleaning Properties, Mechanical Stability, and Adhesion Strength of Transparent Photocatalytic TiO2–ZnO Coatings on Polycarbonate

Transparent layers containing TiO2 have been intensively studied because of their interesting application potential including photocatalytically active and self-cleaning surfaces. In the present work, transparent TiO2-ZnO thin films on a SiO2 interlayer were successfully deposited on the surface of polycarbonate to provide polymeric sheets with a self-cleaning, superhydrophilic, and photocatalytically active surface layer. To ensure a good adhesion of the SiO2 interlayer, the polycarbonate sheets were first modified by irradiation with UV(C) light. The prepared films were characterized by UV/vis spectrophotometry, SEM, XRD, Raman spectroscopy, ellipsometry, and water contact-angle measurements. All prepared films are transparent, have thicknesses in the range between 120 and 250 nm, and possess superhydrophilic properties. Moreover, they exhibit good adhesion qualities as defined quantitatively by cross-cut tests. However, their mechanical strengths, checked by felt-abrasion tests, differ by changing the molar TiO2-ZnO ratio. The photocatalytic activity, expressed as photonic efficiency, of the coated surfaces was estimated from the kinetics of the photocatalytic degradation of methylene blue and methyl stearate. The combination between superhydrophilic properties and photocatalytic activity was determined by studying the change of water contact angle during the storage of the prepared films in the dark under an ambient atmosphere and under an atmosphere containing either acetone or isopropanol followed by UV(A) irradiation. In addition, self-cleaning properties were examined by determining the changes in the contact angle during the irradiation time after applying oleic acid to the surface. The results show that increasing the molar ratio of ZnO in TiO2 coatings up to 5% yields maximum photonic efficiency values of 0.023%, as assessed by the photocatalytic degradation of methylene blue. Moreover, the superhydrophilic coating with a molar TiO2-ZnO ratio of 1:0.05 exhibits the best self-cleaning properties combined with a good mechanical stability and a very good stability against UV irradiation.

Activation of PET Using an RF Atmospheric Plasma System

The plasma treatment of polymer surfaces is routinely used to enhance surface properties prior to adhesive bonding or biomolecule interaction. This study investigates the influence of plasma treatment conditions on the surface activation of polyethylene terephthalate (PET) using the SurFx Atomflo™ 400L plasma source. In this study the effect of applied plasma power, processing speed, gas composition and plasma applicator nozzle to substrate distance were examined. The level of polymer surface activation was evaluated based on changes to the water contact angle (WCA) of PET samples after plasma treatment. PET surface properties were also monitored using surface energy and X-ray photoelectron spectroscopy (XPS) analysis. The heating effect of the plasma was monitored using thermal imaging and optical emission spectroscopy (OES) techniques. OES was also used as a diagnostic tool to monitor the change in atomic and molecular species intensity with changes in experimental conditions in both time and space. XPS analysis of the PET samples treated at different plasma powers indicated that increased oxygen content on samples surfaces accounted for the decreases observed in WCAs. For the first time a direct correlation was obtained between polymer WCA changes and the OES measurement of the atomic hydrogen Balmer Hα and molecular OH line emission intensities.

Enhanced Hydrophilic Property of TiO2Thin Film Deposited on Glass Etched with O2Plasma

TiO2 films were deposited on glass substrates with and without O2 plasma etching by using the RF-magnetron sputtering method. We focused on the effect of surface structure on the photoinduced hydrophilic properties of TiO2 films, fabricated on different surface conditions according to the presence or absence of the O2 plasma treatment on glass substrates. The wettability and photoinduced hydrophilic properties of the TiO2 films were investigated according to the changes in water contact angles under UV light irradiations with a very low intensity of 0.1 mW/cm 2 . The photoinduced hydrophilic properties on the TiO2 formed above the plasma treated glass were also superior to those on the TiO2 formed above the bare glass. This enhanced TiO2 film has been used practically for self cleaning and anti-fogging glasses.

Tunable pH and temperature response of weak polyelectrolyte brushes: role of hydrogen bonding and monomer hydrophobicity

We report on weak polyacid brushes with highly tunable pH and temperature response characteristics. This was achieved by synthesizing a series of homo- and copolymers which contain 2-alkylacrylic acids (aAAs) of increased hydrophobicity, i.e. acrylic acid (AA), methacrylic acid (MAA), or 2-ethylacrylic acid (EAA), using surface-initiated reversible addition–fragmentation chain transfer (SI-RAFT) polymerization. As revealed by contact angle measurements, in situ ellipsometry and AFM studies of brush swelling, the pH-response of PAA and PMAA brushes was similar, with brushes remaining highly swollen (swelling ratio 2.5–3.0) at low pH values. The PEAA brush, however, was unique as it showed low degrees of water uptake ( 70°) contact angles in the entire pH region from 2 to 8. Copolymer brushes of aAAs with N-isopropylacrylamide (NIPAM), denoted as P(AA-co-NIPAM), P(MAA-co-NIPAM) and P(EAA-co-NIPAM), demonstrated dual pH and temperature response, which was strongly dependent on the type of aAA co-monomer. P(AA-co-NIPAM) and P(MAA-co-NIPAM) brushes underwent large-amplitude pH-induced changes in brush swelling and water contact angle in the range of pH from 3 to 6, and were only weakly responsive to temperature in the transition region. In contrast, more hydrophobic P(EAA-co-NIPAM) brushes demonstrated both pH and temperature responses at physiologically relevant neutral/basic pH values even when the content of EAA units in the copolymer was as high as ∼50%. We discuss the role of inter- and intra-molecular hydrogen bonding and monomer hydrophobicity and ionization (quantified by FTIR) in determining pH ranges for brush response. These findings might enable control of molecular/cellular adhesion and flow at interfaces potentially useful in microfluidic and biomedical applications.

Significant change in water contact angle of electrospray‐synthesized SiO2 films depending on their surface morphology

Amorphous SiO2 films were deposited by means of an electrospray technique. The relation between the water contact angle (WCA) of the deposited SiO2 films and the surface morphology is investigated. The feeding rate of the electrospray process greatly affects the morphology of the synthesized SiO2 films. There is also a significant change in the WCA on the surface of the films: the rougher the surface, the greater the WCA. A model based on the Cassie–Baxter formulation is used to explain the change observed in the WCA on the SiO2 films. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Side-on Surface Modification of Polystyrene with an Atmospheric Pressure Microplasma Jet

An atmospheric-pressure microplasma jet operating in helium gas has been used in a side-on configuration to modify locally the surface energy of polystyrene (PS). The plasma plume and gas outflow emanating from the capillary-type discharge (8 kVp–p at 30 kHz excitation) were directed parallel to (side-on) and just above the sample surface, rather than head-on as in conventional treatment. This side-on treatment method has been shown to prevent surface damages, pitting or roughing caused by head-on treatment methods. Schlieren photography has been used to identify regions of laminar and turbulent flow in the exiting gas stream and the nature of their interaction with the substrate surface. Correlation of these observations with spatially resolved water contact angle (WCA) measurements of the treated PS, show clearly that substantial reductions in WCA (∼50–60°) occur in downstream regions where the turbulent gas mixed with air impinges the surface. In contrast, only small changes in WCA occur (∼10–20°) in regions closer to the exit orifice where the helium (He) flow is still laminar across the surface. Little or no treatment is observed for distances greater than 5 cm from the source. In addition, low levels of treatment were observed in the regions where the visible plasma glow of the jet is adjacent and parallel to the surface. The results indicate that excited air species (either mixed or entrained in the He gas flow) are the main agents causing surface covalent bond breaking leading to surface modification.