Changes In Water Contact Angle Research Articles

Wetting characteristics of vertically aligned graphene nanosheets

Vertically aligned graphene nanosheets (VAGNs) are a class of graphitic carbon in which few layers of graphene nanosheets are aligned perpendicular to the plane of the substrate. The change in water contact angle (from 103° to 135°) with VAGNs, as a function of change in the surface geometry, is analysed. Theoretical calculations and comparison with the experimental data shows that the apparent contact angle values of VAGNs are closer to that of the fully non-wetting mode or ideal Cassie mode of wetting. The ideal Cassie mode of wetting also explains the variation of the water contact angle of VAGNs with the surface morphology of the material and predicts how surface parameters can be modified to get the required wettability for a certain application of this material.

Macroscopic visual detection of phoxim by calix[4]arene-based host-guest chemistry

A new coumarin-appended calix[4]arene (C4C2) was synthesized and a self-assembled monolayers modified silicon surface (C4C2-SAMs) was constructed via click chemistry. The water contact angle measurements show that these C4C2-SAMs exhibited highly selective macroscopic response towards phoxim with a detection limit of 1 × 10−6 M. UV–vis and fluorescence titration further confirmed the high recognition selectivity of the C4C2-SAMs towards phoxim. The C4C2-SAMs are capable of detecting phoxim in real river water samples through visual water contact angle change, suggesting that the C4C2-SAMs can be used as a convenient and effective sensor for phoxim.

Investigations of Wettability of Wear Resistant Coatings Produced by Atmospheric Plasma Spraying

Changes in fluids contact angle in the interaction with materials surface can play a critical role in enhancement of hydro-machine components and pipelines efficiency and/or service lifetime. However most nowadays used materials and/or coatings are made from polymers or ceramic polymer composites produced by highly sophisticated and/or very expensive techniques. Unfortunately there are a lack of mechanical properties. With the aim to study the role of the surface topography on the water contact angle changes, the representatives of wear resistant coatings (WC10Co4Cr, Cr2O3+5SiO2+3TiO2and Al2O3) were produced by means of atmospheric plasma spraying. Wettability of the coatings surface was studied by adding the liquid droplet on as sprayed, grinded and polished coating's surface by measuring the changes of its contact angle. To estimate the coatings phase composition and topography XRD technique and optical profilometer were used. The contact angle of water was measured by sessile droplet method. To obtain the complex information of the cross-sectional coatings microstructure the conventional metallographic analysis approaches and optical microscopy were also used.

Tunable interlayer spacing of composite graphene oxide-framework membrane for acetic acid dehydration

Minute amount of poly(vinyl alcohol) (PVA) was used to cross-link graphene oxide (GO) nanosheets to prepare composite graphene oxide-framework (GOF) membranes with a nacre-like structure. The PVA provided additional increase in viscosity of GO suspension which made the solution casting of a large-area composite GOF membrane possible. The membranes were applied to separate an acetic acid-water mixture through pervaporation. The ATR-FTIR and XPS analyses confirmed the covalent bonding between GO and PVA as ether bonds were formed. This influenced the changes in water contact angle where it increased from 40.8° (pristine GO) to 62.7° (GO-PVA8wt%). X-ray diffraction showed that d-spacing of GOF was higher than GO due to the intercalation of PVA. The d-spacing of GO layer from dry to wet state has a difference of 2.59 Å but addition of PVA lowered the difference to 0.38 Å. The newly formed covalent bonds between GO and PVA might suppressed the stretching of d-spacing. It shows that the swelling was prevented and the interlayer spacing can be tuned. The composite GO-PVA8wt% membrane has a short d-spacing and provided an outstanding pervaporation performance at 80 °C with a permeation flux of 463.9 g m−2 h−1 and water concentration in permeate of 97.7%.

Fast Transport of Water Droplets over a Thermo-Switchable Surface Using Rewritable Wettability Gradient.

In spite of the reported temperature dependent tunability in wettability of poly(N-isopropylacrylamide) (PNIPAAm) surfaces for below and above lower critical solution temperature (32 °C), the transport of water droplets is inhibited by the large contact angle hysteresis. Herein, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a PNIPAAm grafted structured polymer surface using temperature-induced wettability gradient. Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this "transport zone" wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s. The theoretical calculations on the force measurements concur with dominating behavior of driving forces across the transport zone. The tunability in transport velocity by varying the temperature gradient along the surface or the inclination angle of the surface (maximum angle of 15° with a reduced velocity 0.4 mm/s) is also elucidated. In addition, as a practical application, coalescence of water droplets is demonstrated by using the temperature controlled wettability gradient. The presented results are expected to provide new insights on the design and fabrication of smart multifunctional surfaces for applications such as biochemical analysis, self-cleaning, and microfluidics.

Investigation of long-term chemical stability of structured ZnO films in aqueous solutions of varying conditions

Nanostructured zinc oxide, ZnO, films feature attractive functional properties, but their long-term stability needs further investigation. Here, ZnO thin films with well-aligned rod-like structure were grown on stainless steel substrate. The long-term chemical stability of the ZnO films was investigated in solutions with varying pH values (3−11) to enhance knowledge about the durability of films in acidic and basic environments. The solubility and stability of the films in the solutions were investigated using atomic absorption spectrophotometry, scanning electron microscopy imaging and energy-dispersive X-ray spectroscopy analyses, as well as by monitoring changes in water contact angle of the films and in the pH values of the solutions. The ZnO film was found to be most stable at highest pH value, with the amount of dissolved zinc being lowest among the studied pH values and the changes observed with other characterization methods being minor compared to the samples immersed to other solutions. At the lowest pH, the film was removed rapidly from the substrate by dissolution. In solutions featuring pH values 5 and 9, the measured pH was unstable and changed constantly until it reached the value 7.2–7.6, i.e., until the equilibrium of different chemical species in the solution was achieved. These results are presented and discussed in this paper from the viewpoint of applicability of the ZnO films.

A Novel Way to Modify PTFE Membrane into Hydrophilicity

Polytetrafluoroethylene (PTFE) membrane, with high resistances to acid, alkali and other chemical corrosion, is the ideal filter material in water treatment, which can be used in a variety of harsh environment. However, the hydrophobictity of PTFE membrane limits its wide application in water treatment. Thus, the hydrophilic modification of PTFE membrane attracts much attention. In this work, the microporous PTFE membrane was treated by a novel strategy for enhancing its hydrophilicity. The pristine microporous PTFE membrane was modified by assembly of fluorocarbon surfactant. The structure and morphology of the membranes were characterized by attenuated Fourier Transform Infrared spectroscopy (ATR-FTIR) and scanning electron microscope (SEM). The variation of wettability was evaluated by the change of water contact angle (WCA). And the influences of the concentration of fluorocarbon surfactant and modification time on PTFE WCA were also investigated as well. The WCA of the pristine PTFE membrane is 156°. After modified by 5.0 g/L of fluorocarbon surfactants, the WCA reduced to 4.3°, indicating that the hydrophobic PTFE membrane was transformed into a hydrophilic one. FTIR and SEM results indicated that there were hydrophilic groups, such as carbonyl (-C=O) and hydroxyl (–OH) existing on the surface of PTFE membrane after modification. And WCA of the hydrophilic membrane showed a slight increase from 4.3°to 8° after a week, indicating that the hydrophilic PTFE membrane exhibiting a persistent durability.

Change of surface morphology, permeate flux, surface roughness and water contact angle for membranes with similar physicochemical characteristics (except surface roughness) during microfiltration

Abstract The aim of the present study is to confirm the extent to which smooth membrane surface (A2) improves antifouling characteristics compared with rough membrane surface (A1) during microfiltration. In the study, two membranes have similar physicochemical characteristics, except surface roughness (difference of 107.206 nm). The membranes with a rough surface (A1) and with a smooth surface (A2), respectively, were prepared by phase inversion under similar preparation conditions in order to obtain similar physicochemical properties such as material, average pore size, pore size distribution (PSD) and water contact angle values, for accurate and reliable comparisons of antifouling property. Based on the results, it was evident that final fluxes of A2 membrane with smooth surface were approximately 5% superior to those of A1 membrane with rough surface, during microfiltration of humic acid solution alone and together with calcium at 0.2 bar and 1 bar due to the difference of surface roughness. In addition to this, the extent of various antifouling characteristics on both the rough and smooth surfaces of membranes were also compared using field emission scanning electron microscope (FE-SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), and water contact angle measurement.

Erbium, Chromium:Yttrium-Scandium-Gallium-Garnet Laser Effectively Ablates Single-Species Biofilms on Titanium Disks Without Detectable Surface Damage.

Increasing evidence implicates biofilms, consisting of species such as Porphyromonas gingivalis (Pg), in the etiology of peri-implantitis. Multiple approaches to ablate biofilms on failing implants have been proposed and include use of lasers, most recently the erbium, chromium:yttrium-scandium-gallium-garnet (Er,Cr:YSGG) laser. The purpose of this study is to establish an in vitro single-species biofilm model on implant surfaces and determine power settings of the Er,Cr:YSGG laser that remove biofilm without causing physical damage to disks. Single-species biofilms consisting of Pg strain 381 were grown on titanium disks, including: 1) sandblasted, large-grit, acid-etched (SLA); 2) calcium phosphate nano-coated (CaP); 3) anodized; or 4) machined surfaces. Power settings from 0 to 1.5 W using an Er,Cr:YSGG laser equipped with radial firing tip were used. Biofilm formation/removal was quantitated using confocal and scanning electron microscopy. Surface changes in temperature, microroughness, and water contact angle were analyzed. Results show confluent Pg biofilm coating all disk surfaces. The laser removed biofilms from all surfaces, with CaP and SLA surfaces requiring power setting of 1.0 to 1.5 W for ablation of bacteria coating the disks. Within this power range, and with water spray, there were no changes in surface temperature, surface roughness, or contact angle on any surfaces tested. The Er,Cr:YSGG laser with radial firing tip and water spray was able to effectively ablate ≥95% of biofilm on all types of tested titanium surfaces, using clinically relevant power settings, without causing measurable physical changes to surfaces.

Enhanced Photoelectrocatalytic Activity in Ni-TiO2 Nanocomposites Fabricated By Pulse Potential Deposition

The Ni-TiO2 single layer nanocomposites were fabricated on copper substrates by the pulse plating (PP) and pulse reverse plating (PRP) techniques. Morphological studies conducted by field emission scanning electron microscopy (FESEM) revealed that pulse potentials and durations affected the microstructure of coatings. The chemical composition of coatings was studied by energy dispersive X-ray system (EDX) and it was found that by the prolongation of current-off durations in PP and anodic cycles in PRP, deposits containing 11.58 wt.% and 13.16 wt.% TiO2 were produced, respectively. The influence of heat treatment temperature on the phase structure of Ni-TiO2 nanocomposites was investigated using X-ray diffraction and it was demonstrated that the anatase to rutile transformation occurred at temperatures higher than 450°C. The hydrophilicity of Ni-TiO2 coatings was evaluated by monitoring the change of water contact angle under the UV illumination. It was observed that the nanocomposite prepared by PRP exhibited the maximum hydrophilicity after 3 hours of heat treatment at 450°C, for which the contact angel prior to UV irradiation was 78.96° and after 1h of UV irradiation it reduced to 13.65°. Surface hydroxyl groups density which account for the photo-induced hydrophilicity of the Ni-TiO2 coating was defined by X-ray photoelectron spectroscopy (XPS), before and after 1h of UV illumination. The results indicated that under the optimized condition, for the coatings prepared by PP and PRP, 12.6% and 15.3% increases were detected in the density of hydroxyl groups. To evaluate the effect of pulse plating parameters on the photocatalytic activity of the Ni-TiO2 coatings prepared by PP and PRP, photo-degradation experiments of methyl orange aqueous solution were carried out under the UV light. The results indicated that TiO2 content, irradiation duration and sintering temperature affected the photocatalytic efficiency of the coatings, and the optimized PP and PRP deposited coatings were capable of disintegrating the methyl orange up to 76.21% and 51.89% after 1 hours of UV illumination. The photoelectrocatalytic degradation of methyl orange by Ni-TiO2 coatings was studied and the effect of different applied potentials was determined. The amounts of methyl orange degraded by the PP and PRP deposited composites under the anodic potential of 600 mV along with 1 hours of simultaneous UV illumination were 75.30% and 89.61%, respectively.

Impact of atmospheric pressure nonequilibrium plasma discharge on polymer surface metrology

Due to the attraction of plasma technologies as a clean and efficient means of surface modification, significant research has gone into the physical and chemical aspects of polymer functionalization. In this study, it was shown that the use of an atmospheric plasma jet can efficiently modify the surface of polyethylene terephthalate samples and change their hydrophobic properties to more hydrophilic characteristics. The dependence on the changes with respect to time, distance, and atomic oxygen (O I) intensity were considered as factors. It was found that with closer proximity to the plasma source (without causing thermal degradation) and with increasing levels of O I, that the changes of water contact angle and surface free energy can be maximized. It was also observed that the electron energy distribution function, for a given chemistry, significantly differed with changes in distance from the jet nozzle. This shows that for this type of plasma jet system, the bulk of the chemical reactions occur in the nozzle of the jet and not in the surrounding atmosphere. Therefore, this leads to more efficient energy transfer, higher gas temperatures, and better surface activation of samples when compared to systems that produce external chemical reactions due to more diffusion in the surrounding atmosphere and loss of reactive species to other atoms and molecules that are present.

Controlling wettability of PECVD-deposited dual organosilicon/carboxylic acid films to influence DNA hybridisation assay efficiency.

Herein, plasma polymerisation of a dual-layer tetraethyl orthosilicate (TEOS) and acrylic acid (AA) film under a specific recipe is performed. Newly deposited films are found to retain a weakly-bound soft layer of partially polymerised acrylic acid (wbAA), which can be ultimately removed by washing. However, when on the surface, this soft layer is shown to be influential in manipulating the properties of a robust covalently-bound AA (cbAA) underlayer when treated appropriately. Specifically, treatment of the as-deposited dual-layer TEOS/AA films via timed incubation in a humidity-controlled environment results in changes in the water contact angle (WCA) of the cbAA, and ultimately the surface of the TEOS/AA, enabling tuning of the wettability of the acrylic acid layer. Through the use of a controlled incubation environment of the TEOS/AA, followed by washing, we have demonstrated that carboxylic-acid containing surfaces with a WCA between 85° and 10° can be routinely generated, using basic apparatus and simple methodology. Moreover, these surfaces not only retain their AA functionality, demonstrated by covalent-linking of amine-terminated single-stranded DNA, but also strongly inhibit non-specific binding of the DNA strands. The efficiency of these surfaces to be used in DNA direct-binding hybridisation assays has been demonstrated, with limits of detection of 1.11 and 1.66 nM being measured.

Fast and Reproducible Wettability Switching on Functionalized PVDF/PMMA Surface Controlled by External Electric Field

In this paper, the novel material with controllable wettability switching triggered by external electric field is developed. The material is based on the piezopolymer blend of poly(methyl methacrylate) (PMMA) and polyvinylidene difluoride (PVDF), grafted with various hydrophobic and hydrophilic molecules to alter the initial wettability of polymer films. Organic functional groups are grafted on the blend surface via formation of covalent bonds between PMMA and arenediazonium tosylates. The surface chemical structure and morphology of prepared samples are studied by X‐ray photoelectron spectroscopy, UV–vis, and attenuated total reflection infra red (ATR‐IR) spectroscopies, atomic force microscopy (AFM), and confocal microscopy. Attachment of gold nanoparticles to grafted thiol chemical groups is additionally used to confirm the modification procedure. Application of external stimuli in the form of the electric field leads to dramatic changes in water contact angle. Wettability switching is found to be extremely fast and completely repeatable without the destructions of the polymer film. Mechanism of switching is attributed to the morphological smoothening of piezoelectric responsible composite PVDF–PMMA under the application of external stimuli.

Aeronautical livery coating with icephobic property

The main problem for the aircrafts is the ice formation on critical components (leading edges, slats, vertical tails, etc … ) that decreases reliability and safety of the flights. The objectives of this work are mainly two. The former is to design and develop a new multifunctional coating with aesthetical (livery effect) and icephobic properties other than high adhesion and abrasion resistance. The latter is to design and realise a new tool to use with classical contact angle measurement techniques. This tool allows to replicate the same thermodynamic conditions (in terms of pressure and temperature) present at flight altitude. In fact it was demonstrated that the water contact angle changes, varying at the same time pressure and temperature. The experimental results corroborated the influence of temperature and pressure on the shape of the supercooled water droplet applied on the new icephobic coating. The high durability of the new anti-ice livery coating was determined thorough several mechanical tests such as cutting and tape test, pull-off tests and nanoindentation tests.

Doping-Induced Tunable Wettability and Adhesion of Graphene.

We report that substrate doping-induced charge carrier density modulation leads to the tunable wettability and adhesion of graphene. Graphene's water contact angle changes by as much as 13° as a result of a 300 meV change in doping level. Upon either n- or p-type doping with subsurface polyelectrolytes, graphene exhibits increased hydrophilicity. Adhesion force measurements using a hydrophobic self-assembled monolayer-coated atomic force microscopy probe reveal enhanced attraction toward undoped graphene, consistent with wettability modulation. This doping-induced wettability modulation is also achieved via a lateral metal-graphene heterojunction or subsurface metal doping. Combined first-principles and atomistic calculations show that doping modulates the binding energy between water and graphene and thus increases its hydrophilicity. Our study suggests that the doping-induced modulation of the charge carrier density in graphene influences its wettability and adhesion [corrected]. This opens up unique and new opportunities for the tunable wettability and adhesion of graphene for advanced coating materials and transducers.

PTFE treatment by remote atmospheric Ar/O2 plasmas: a simple reaction scheme model proposal

Polytetrafluoroethylene (PTFE) samples were treated by a remote atmospheric pressure microwave plasma torch and analyzed by water contact angle (WCA) and X-ray photoelectron spectroscopy (XPS). In the case of pure argon plasma a decrease of WCA is observed meanwhile an increase of hydrophobicity was observed when some oxygen was added to the discharge. The WCA results are correlated to XPS of reference samples and the change of WCA are attributed to changes in roughness of the samples. A simple kinetics scheme for the chemistry on the PTFE surface is proposed to explain the results.

Direct laser writing of nanorough cell microbarriers on anatase/Si and graphite/Si

The formation of hierarchical structures consisting of microstripe barriers decorated with nanorough ablated materials prepared by direct laser writing is described. Linear features of circa 25μm width and 12μm height are achieved on amorphous and crystalline titania and graphitic carbon films deposited on silicon. Ablated protrusions build up barriers decorated by nanoscale Si-film reconstructions, as indicated by EDX maps and micro-Raman spectroscopy. Wettability tests show a dramatic change in water contact angle, which leads to almost full wetting after irradiation, irrespective of the original film composition. Fluorescence microscopy images of human mesenchymal stem cells cultured on 1D and 2D structures demonstrate the short term biocompatibility of the ablated surfaces. It is shown that cells adhere, extend and polarize on feature edges, independently of the type of surface, thus suggesting that the created nanoroughness is at the origin of the antifouling behavior. In particular, irradiated anatase and graphite surfaces demonstrate an increased performance of crystalline films for the creation of cell guiding and trapping devices. The results suggest that such laser processing of films may serve as a time-and-cost-efficient method for the design of few-cells analytical surfaces.

Spin-Casting Polymer Brush Films for Stimuli-Responsive and Anti-Fouling Surfaces.

Surfaces modified with amphiphilic polymers can dynamically alter their physicochemical properties in response to changes of their environmental conditions; meanwhile, amphiphilic polymer coatings with molecular hydrophilic and hydrophobic patches, which can mitigate biofouling effectively, are being actively explored as advanced coatings for antifouling materials. Herein, a series of well-defined amphiphilic asymmetric polymer brushes containing hetero side chains, hydrophobic polystyrene (PS) and hydrophilic poly(ethylene glycol) (PEG), was employed to prepare uniform thin films by spin-casting. The properties of these films were investigated by water contact angle, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and quartz crystal microbalance (QCM). AFM showed smooth surfaces for all films with the roughness less than 2 nm. The changes in water contact angle and C/O ratio (XPS) evidenced the enrichment of PEG or PS chains at film surface after exposed to selective solvents, indicative of stimuli- responsiveness. The adsorption of proteins on PEG functionalized surface was quantified by QCM and the results verified that amphiphilic polymer brush films bearing PEG chains could lower or eliminate protein-material interactions and resist to protein adsorption. Cell adhesion experiments were performed by using HaCaT cells and it was found that polymer brush films possess good antifouling ability.

Enhancing proliferation and osteogenic differentiation of HMSCs on casein/chitosan multilayer films

Creating a bioactive surface is important in tissue engineering. Inspired by the natural calcium binding property of casein (CA), multilayer films ((CA/CS)n) with chitosan (CS) as polycation were fabricated to enhance biomineralization, cell adhesion and differentiation. LBL self-assembly technique was used and the assembly process was intensively studied based on changes of UV absorbance, zeta potential and water contact angle. The increasing content of chitosan and casein with bilayers was further confirmed with XPS and TOF-SIMS analysis. To improve the biocompatibility, gelatin was surface grafted. In vitro mineralization test demonstrated that multilayer films had more hydroxyapatite crystal deposition. Human mesenchymal stem cells (HMSCs) were seeded onto these films. According to fluorescein diacetate (FDA) and cell cytoskeleton staining, MTT assay, expression of osteogenic marker genes, ALP activity, and calcium deposition quantification, it was found that these multilayer films significantly promoted HMSCs attachment, proliferation and osteogenic differentiation than TCPS control.

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.