Sessile Drop Contact Angle Measurements Research Articles

Surface degradation of silicone rubber nanocomposites due to DC corona discharge

Corona discharge is recognized as one of the mechanisms that can influence the surface hydrophobicity of Silicone Rubber (SR) because of the chemical changes that occur on its surface. In this study SR samples were exposed to positive and negative DC corona for 25 and 50 hours using a needle-plane electrode system. Hydrophobicity changes were monitored using a sessile drop contact angle measurement facility. The physical changes on the surface were studied using Scanning Electron Microscopy (SEM) and surface roughness measurements. The effect of positive dc corona was found to be different from that of negative dc corona. Significant surface degradation and loss of hydrophobicity was found in the case of negative dc corona exposed samples. Significant improvement in the above mentioned properties were obtained by adding small quantities of nSIL into the SR matrix.

Flow of nanofluids through porous media: Preserving timber with colloid science

Understanding the flow of particles through accessible paths in timber is important to optimising the timber preservation process. In this paper, we identify inconsistencies between the previously established flow paths in timber for simple liquids, and those for particulate systems. We show that the flow paths of nanofluids are through the rays of Pinus sylvestris (Scots pine) sapwood, then into adjoining tracheids through ruptured cross-field pits with effective pore size of 1.75–3.0μm. We then present data from a custom-designed apparatus, with which we have studied the effect of size, charge and concentration of silica nanoparticles on their flow through pine sapwood. Our results show that particles smaller than 60nm passed well through timber irrespective of their zeta potential. The flow of positively charged particles was significantly reduced when particle diameter exceeded 100nm; whereas negatively charged particles with diameter of 250nm still passed through timber reasonably well, provided the concentration of particles was below 0.5% (w/w). Furthermore, we rationalise such flow data with AFM and sessile drop contact angle measurements, which gauge the interactions between the nanofluids and a functionalised silica surface as a model timber surface. Whilst negatively charged nanofluids showed better wettability on the model surface that the positive nanofluids, the wettability did not show any particle-size dependence. We suggest that such contact angle measurements, performed under quiescent conditions, could not fully predict the flow and deposition of nanofluids through timber, which would be more complex due to the presence of an applied external pressure that could affect inter-particle and particle-surface interactions.

ZnO deposition on metal substrates: Relating fabrication, morphology, and wettability

It is not common practice to deposit thin films on metal substrates, especially copper, which is a common heat exchanger metal and practical engineering material known for its heat transfer properties. While single crystal substrates offer ideal surfaces with uniform structure for compatibility with oxide deposition, metallic surfaces needed for industrial applications exhibit non-idealities that complicate the fabrication of oxide nanostructure arrays. The following study explored different ZnO fabrication techniques to deposit a (super)hydrophobic thin film of ZnO on a metal substrate, specifically copper, in order to explore its feasibility as an enhanced condensing surface. ZnO was selected for its non-toxicity, ability to be made (super)hydrophobic with hierarchical roughness, and its photoinduced hydrophilicity characteristic, which could be utilized to pattern it to have both hydrophobic-hydrophilic regions. We investigated the variation of ZnO's morphology and wetting state, using SEMs and sessile drop contact angle measurements, as a function of different fabrication techniques: sputtering, pulsed laser deposition (PLD), electrodeposition and annealing Zn. We successfully fabricated (super)hydrophobic ZnO on a mirror finish, commercially available copper substrate using the scalable electrodeposition technique. PLD for ZnO deposition did not prove viable, as the ZnO samples on metal substrates were hydrophilic and the process does not lend itself to scalability. The annealed Zn sheets did not exhibit consistent wetting state results.

Surface characteristics of selected carbon materials exposed to supercritical water

Interactions of supercritical water (SCW) with several carbon (including polycrystalline graphite, highly ordered pyrolytic graphite (HOPG), pyrolytic carbon, and diamond) and selected ceramic materials under oxic or anoxic conditions are investigated. Wettability and surface/material properties of samples, before and after the SCW exposure, are characterized with sessile drop contact angle measurement, profilometry, XPS, XRD, and ToF-SIMS. All tested ceramic materials became more hydrophilic during the SCW exposure, mainly due to hydrolysis reactions. Carbon samples exposed to oxic SCW became more hydrophilic as a result of surface oxidation. Carbon materials, except HOPG and diamond, became more hydrophobic when exposed to anoxic SCW because of degradation of hydrophilic oxygen functionalities. HOPG and diamond became more hydrophilic after anoxic SCW exposure mainly due to the removal of hydrophobic hydrocarbon contaminants. Hydrophobicity of different carbon samples exposed to SCW are explained based on the abundance of surface hydrophilic sites (i.e., oxygen functionalities, reactive carbon dangling bonds or defects), hydrocarbon impurities, and surface roughness.

Evidence for phase separation of ethanol-water mixtures at the hydrogen terminated nanocrystalline diamond surface

Interactions between ethanol-water mixtures and a hydrophobic hydrogen terminated nanocrystalline diamond surface, are investigated by sessile drop contact angle measurements. The surface free energy of the hydrophobic surface, obtained with pure liquids, differs strongly from values obtained by ethanol-water mixtures. Here, a model which explains this difference is presented. The model suggests that, due to a higher affinity of ethanol for the hydrophobic surface, when compared to water, a phase separation occurs when a mixture of both liquids is in contact with the H-terminated diamond surface. These results are supported by a computational study giving insight in the affinity and related interaction at the liquid-solid interface.

Preparation of polyvinylidene fluoride membrane via dual coagulation bath system and its wettability study

AbstractHydrophobic polyvinylidene fluoride (PVDF) membrane has been successfully developed using dual coagulation bath system in which first coagulation bath produced membrane with nodular structure whereas second water coagulation bath provided membrane with porous structure. The effects of coagulation time, polymer concentration, and isopropanol/water ratio in first coagulation bath on membrane wettability were studied. The membrane surface morphology such as pore size and porosity were studied using field emission scanning electron microscope and porosimeter. The wettability of the membrane was determined using sessile drop contact angle measurement. Using dual‐coagulation bath system, it was found that membrane with bicontinuous structure tends to have higher water contact angles. This membrane could be produced through short immersion time of 18 wt % PVDF solution in soft nonsolvent bath followed by coagulation in water nonsolvent bath. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Hydrophobic switching nature of methylcellulose ultra-thin films: thickness and annealing effects

We have studied the thermosensitive property of methylcellulose (MC) thin films supported on Si substrate by static sessile drop contact angle measurements, and their surface properties and thin film structure by x-ray reflectivity (XRR) and atomic force microscopy (AFM) techniques. From the static sessile drop contact angle measurements, the MC thin films showed the characteristic hydrophilic-to-hydrophobic transition at ∼70 °C, which is the lower critical solution temperature of the bulk solution volume phase separation transition. For films with thickness d ≤ Rg, the onset of such a transition is affected by the film thickness while very thick films, d ≫ Rg, yielded higher contact angles. Annealing the MC thin films with thicknesses ∼200 Å (near the radius of gyration, Rg, of the polymer) below the bulk glass transition temperature (Tg ∼ 195 ° C) would not change the hydrophobic switch nature of the film but annealing ‘at’ and above the bulk Tg would change its surface property. From surface topography images by AFM, there were no significant changes in either the roughness or the film texture before and after annealing. With XRR data, we were able to determine that such changes in the surface properties are highly correlated to the film thickness changes after the annealing process. This study, we believe, is the first to examine the thermal annealing affects on the thermal response function of a thermoresponsive polymer and is important for researching how to tailor the hydrophobic switching property of MC thin films for future sensing applications.

Effect of oxides on the reaction kinetics during hot-dip galvanizing of high strength steels

Selective oxidation of the minor alloying elements during annealing prior to hot-dip galvanizing cause poor wetting with liquid zinc, resulting in bare spot defects. The effect of surface coverage, morphology (i.e., size and distribution) of the oxides on the reaction kinetics was examined. The Zn wettability has been determined by sessile drop contact angle measurement method and the Fe–Al interfacial layer was investigated by the use of a novel hot temperature spinning process. It has been found that besides the overall surface oxide coverage, size of the oxide islands is a dominant factor in affecting the reaction kinetics.

Creating gradients of two proteins by differential passive adsorption onto a PEG-density gradient

Many fundamental biological processes, including early embryo development, immune responses and the progression of pathogens, are mediated by gradients of biological molecules. Understanding these vital physiological processes requires the development of biomaterial platforms that can mimic them in-vitro. Such platforms include laboratory generated surface gradients of biological molecules. In this work, we report a method for the generation of surface gradients of two proteins. We used a surface grafting density gradient of polyethylene glycol (PEG) to control protein adsorption. In addition, we used protein size as a tool to control the position and the adsorbed amount of both proteins. To demonstrate our concept, we used fibrinogen as an example of a large protein and lysozyme as an example of a small protein. However, we speculate that the same strategy could be extended to any other pair of large and small proteins. We used X-ray photoelectron spectroscopy and sessile drop contact angle measurements to determine the chemical composition and wettability of the gradients. Protein adsorption was studied by surface plasmon resonance imaging.

Contact angle measurements to determine the rate of surface oxidation of artists’ alkyd paints during accelerated photo-ageing

The rate of surface photo-oxidation and changes in polarity of the surfaces of artists’ alkyd paint films were studied by a rapid and novel approach using sessile drop contact angle measurements. The contact angles of distilled water and diiodomethane drops on the film surfaces were measured over a period of 25–2000 h of artificial ageing at 40 °C under an indoor light filtered Xenon light source. Chemical changes on the surfaces of the films were also followed using Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR). Titanium white artists’ paint samples from three different manufacturers were investigated: Winsor & Newton, Ferrario and Da Vinci Paint Co. As the films aged, there was a decrease in the contact angle and an increase in the polar component of the free surface energy of the paint film surfaces indicating the formation of photo-oxidation products. Such changes correspond well to the chemistry of oil paints, and suggested chemistry of long-alkyd paints, because of their relatively high weight percent of fatty acids.

Controlling unconfined yield strength in a humid environment through surface modification of powders

Capillary forces between particles caused by the condensation of water from the atmosphere can lead to a significant increase in bulk powder strength and in turn affect the flowability and dispersability of the powder. The effect of relative humidity on powder strength was investigated for a spherical aluminum powder with a mean particle size of 4 µm. The surface hydrophobicity of the aluminum powder was modified with n-octadecyltrimethoxysilane (ODTMS) and tridecafluoro-1,1,2,2-tetrahydrooctyl-1-triethoxysilane (TDFS). Sessile drop contact angle measurements on ideal aluminum surfaces treated with ODTMS and TDFS respectively, were used to determine optimum conditioning time to produce adequately hydrophobic surfaces. It was found that one hour of treatment resulted in advancing contact angle increase from 67° for bare aluminum to 114° and 113° for ODTMS and TDFS treated surfaces respectively. The powder flow properties of the bulk powders were then subsequently measured at three humidity levels using and automated annular shear cell powder tester. An increase in the unconfined yield strength, f c, was seen at a relative humidity as low as 50 percent for the untreated aluminum powder. Further increasing the relative humidity to 80 percent resulted in a 100 percent increase in the unconfined yield strength of the untreated aluminum powder. The aluminum powders modified with TDFS and ODTMS demonstrated a smaller increase in the unconfined yield strength of 30% and 20% respectively.

PEGylation of Porous Silicon Using Click Chemistry

Porous silicon has received considerable interest in recent years in a range of biomedical applications, with its performance determined by surface chemistry. In this work, we investigate the PEGylation of porous silicon wafers using click chemistry. The porous silicon wafer surface chemistry was monitored at each stage of the reaction via photoacoustic Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, whereas sessile drop contact angle and model protein adsorption measurements were used to characterize the final PEGylated surface. This work highlights the simplicity of click-chemistry-based functionalization in tailoring the porous silicon surface chemistry and controlling protein-porous silicon interactions.

Thermosensitive Copolymer Coatings with Enhanced Wettability Switching

Expanded cross-linked copolymers of poly(N-isopropylacrylamide) (PNiPAAm) and poly(acrylic acid) (PAAc) of varying monomer ratios were grafted from a crystalline silicon surface. Surface-tethered polymerization was performed at a slightly basic pH, where electrostatic repulsion among acrylic acid monomer units forces the network into an expanded polymer conformation. The influence of this expanded conformation on switchability between a hydrophilic and a hydrophobic state was investigated. Characterization of the copolymer coating was carried out by means of X-ray photoelectron spectroscopy (XPS) ellipsometry, and diffuse reflectance IR. Lower critical solution temperatures (LCSTs) of the copolymer grafts on the silicon surfaces were determined by spectrophotometry. Temperature-induced wettability changes were studied using sessile drop contact angle measurements. The surface topography was investigated by atomic force microscopy (AFM) in Milli-Q water at 25 and 40 degrees C. The reversible attachment of a fluorescently labeled model protein was studied as a function of temperature using a fluorescence microscope and a fluorescence spectrometer. Maximum switching in terms of the contact angle change around the LCST was observed at a ratio of 36:1 PNiPAAm to PAAc. The enhanced control of biointerfaces achieved by these coatings may find applications in biomaterials, biochips, drug delivery, and microfluidics.

Anisotropic surface chemistry of crystalline pharmaceutical solids

The purpose of this study was to establish the link between the wetting behavior of crystalline pharmaceutical solids and the localized surface chemistry. A range of conventional wetting techniques were evaluated and compared with a novel experimental approach: sessile drop contact angle measurements on the individual facets of macroscopic (>1 cm) single crystals. Conventional measurement techniques for determining surface energetics such as capillary rise and sessile drops on powder compacts were found not to provide reliable results. When the macroscopic crystal approach was used, major differences for advancing contact angles, theta(a), of water were observed-as low as 16 degrees on facet (001) and as high as 68 degrees on facet (010) of form I paracetamol. theta(a) trends were in excellent agreement with X-ray photoelectron spectroscopy surface composition and known crystallographic structures, suggesting a direct relationship to the local surface chemistry. Inverse gas chromatography (IGC) was further used to probe the surface properties of milled and unmilled samples, as a function of particle size. IGC experiments confirmed that milling exposes the weakest attachment energy facet, with increasing dominance as particle size is reduced. The weakest attachment energy facet was also found to exhibit the highest theta(a) for water and to be the most hydrophobic facet. This anisotropic wetting behavior was established for a range of crystalline systems: paracetamol polymorphs, aspirin, and ibuprofen racemates. theta(a) was found to be very sensitive to the local surface chemistry. It is proposed that the hydrophilicity/hydrophobicity of facets reflects the presence of functional groups at surfaces to form hydrogen bonds with external molecules.

The Effects of Milling on the Surface Properties of Form I Paracetamol Crystals

The effects of milling and particle size on surface energies of form I paracetamol crystals are reported. Paracetamol crystals (75-850 microm) were obtained by cooling methanol and acetone saturated solutions. Additionally, macroscopic (>2 cm) single crystals were grown by slow solvent evaporation from saturated solutions, ball milled and sieved into different particle size fractions. Surface properties were characterised using Inverse Gas Chromatography and compared with those calculated from sessile drop contact angle measurements on macroscopic single crystals. Dispersive surface energies, gamma(d)SV for milled samples increased by 20% with decreasing particle size. With decreasing particle size acceptor numbers, KA values were constant but donor numbers, KB decreased. For unmilled materials KB was comparable to KA but with a significantly lower gamma(d)SV of only 33 mJ/m(2). Milling resulted in fracture along the crystal's lowest attachment energy plane (010), exposing facets of different surface chemistry to that of the native external facets theta for the (010) fracture plane confirmed a higher gamma(d)SV compared to external facets such as (011) of single crystals. Milling exposes a hydrophobic surface for paracetamol form I crystals which becomes increasingly more dominant with decreasing particle size.

Cell mimetic monolayer supported chitosan-haemocompatibility studies

Chitosan is a natural polymer, widely explored for biomedical and tissue engineering applications. However the thrombogenic nature limits their application in blood contacting devices and implants. Here, we have attempted to understand the haemocompatibility of chitosan by immobilizing a monolayer of cell mimetic lipid compositions. The phosphatidylcholine/cholesterol/galactocerebroside lipid composition (PC/Chol/GalC, 1:0.35:0.125) was deposited onto the chitosan films. Characterization of the modified surface was done by sessile drop contact angle measurement. The contact angle of the chitosan film reduced from 80.65 +/- 1.4 to 23.5 +/- 1.9 after the surface modification. Swelling nature of chitosan seemed to influence the orientation and packing of the lipid monolayer. In vitro calcification studies with metastable salt solution indicated increased calcification on the modified surface. This may be due to formation of nuclei for calcification on the expanding monolayer. The preliminary haemocompatibility studies with washed platelets, leukocytes and erythrocytes showed overall reduction in blood cell adhesion to the modified surfaces. Scanning electron microscopy was used for morphological characterization of platelet adhesion and activation on the surfaces. On the bare chitosan surface, fully spread platelets with extending pseudopodia indicated platelet activation. The smooth surface of the modified film did not activate platelets. These studies showed that, though the lipid monolayer on chitosan film is able to reduce the over all blood cell adhesion and platelet activation it is prone to calcification.

Properties of compression-densified wood, Part II: surface energy

Surface energy of yellow-poplar wood from three wood treatment groups was studied using sessile-drop contact-angle measurements and the Chang acid–base model for surface energy calculation. Water, glycerol, formamide, ethylene glycol and α-bromonaphthalene were used as probe liquids. Wood treatments were control, hygro-thermal treatment and densification. Contact angle trends for densified and hygro-thermally treated wood were found to be the same. Total surface energy decreased with both hygro-thermal treatment and densification.

Biocompatible carbon-based screen-printed electrodes for the electrochemical detection of nitric oxide

We report in this study on the first use of carbon-based screen-printed electrodes (SPCEs) for the development of inexpensive, disposable and biocompatible electrochemical sensors for NO in solution. First, the screen printing electrode process was performed on two types of substrates and the chemical modification of the obtained electrodes was achieved in order to endow them with the desired sensitivity, selectivity and biocompatibility. The first important step in building the sensor system consisted into transposing existing chemical modifications to design selective and sensitive coatings towards NO. The second major and novel step of this work was devoted to confer to the electrode material the desired biocompatibility character by overcoating the whole chemically modified SPCEs with an additional layer of poly-oxyethyleneglycol (PEG) polymers. Since the biocompatible property of materials is generally linked to their hydrophilic character, the biocompatibility of the designed electrodes was assessed by sessile drop contact angle measurements. Finally, the performances of the electrodes towards NO were evaluated by chronoamperometric measurements.

Amphiphilic Gold Nanoparticles Grafted with Poly(N-isopropylacrylamide) and Polystyrene

Two types of amphiphilic gold nanoparticles (AuNP-1 and -2) grafted with a mixture of poly(N-isopropylacrylamide) (PNIPAM) and polystyrene (PS) chains in two different compositions have been successfully prepared with the “grafting-to” method in a homogeneous THF phase. These AuNPs were thoroughly characterized by FTIR, 1H NMR, UV−vis, high-resolution transmission electron microscopy, thermogravimetric analysis, and dynamic light scattering to determine the total number of polymer chains bound to the gold nanoparticles, the ratio between PNIPAM and PS chains, and the size of the gold core. Langmuir monolayer experiments at the air−water interface of the two types of AuNPs revealed different compression isotherms of the surface pressure vs particle area (π−A curve) conducted at 20 °C. These amphiphilic gold nanoparticles can be regarded as analogues of amphiphilic diblock copolymers at the air−water interface. The compression isotherm of AuNP-2 with a PNIPAM:PS ratio of 2:1 showed several characteristic regions that can be attributed to the polymer conformational transitions from the pancake, the pancake to brush transition, to the brush. However, the monolayer of AuNP-1, with a ratio of 5:1 of PNIPAM:PS, never reaches a brush stage but showed an extension of the pseudoplateau region upon compression. These differences may be due to the more hydrophilic nature and the more stretched PNIPAM chains. Furthermore, the sessile drop contact angle measurements, conducted at room temperature on both upper and lower surfaces of the AuNP-2 monolayer transferred at 35 mN/m onto either hydrophilic or hydrophobic substrates, are slightly different, 82 ± 2° and 77 ± 2°, respectively. After comparing with the literature data of the contact angles of water on either the pure PS film or the PNIPAM brush, we concluded that the chemically different PNIPAM and PS chains grafted on the surface of the gold core tend to be phase-separated.

Acrylonitrile-Based Copolymer Membranes Containing Reactive Groups: Surface Modification by the Immobilization of Poly(ethylene glycol) for Improving Antifouling Property and Biocompatibility

The antifouling property and biocompatibility of a polyacrylonitrile-based copolymer membrane were improved by the immobilization of poly(ethylene glycol) (PEG) on the membrane surface. The studied membranes were fabricated from poly(acrylonitrile-co-maleic acid), in which the carboxyl groups could be conveniently conversed into anhydride and then esterified with poly(ethylene glycol). Chemical and morphological changes on the membrane surface were characterized by Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA), scanning electron microscopy (SEM), and sessile drop contact angle measurements (CA). It was found that the water contact angle of the membrane was reduced and the biocompatibility corresponding to platelets adhesion and protein adsorption was improved significantly with the immobilization of PEG chains on the membrane surface. Furthermore, the permeation behaviors of the base and modified membranes were investigated by bovine serum albumin (BSA) filtration experiments. Membranes containing hydrophilic carboxyl groups or PEG chains showed higher solution flux, lower BSA adsorption, and better flux recovery after cleaning than those of polyacrylonitrile membranes. Particularly, compared with polyacrylonitrile membranes, the PEG-immobilized membrane showed a 6-fold increase in BSA solution flux, 63% reduction in total fouling, and 67% reduction in BSA adsorption.