Sessile Contact Angle Research Articles

Formation and physical characterization of soy protein-isoflavone dispersions and emulsions

Proteins stabilize emulsions by adsorbing at the interface to prevent the natural tendency of droplet coalescence. Polyphenols (plant-derived secondary metabolites) are capable of complexing with proteins through structural interactions. Certain polyphenols have shown influence on interfacial properties of proteins, yet isoflavones specifically have not been widely investigated. Thus, the objective of this research was to study the effect of isoflavones on protein-stabilized oil-in-water emulsions. Soy protein fractions, assessed via SDS-PAGE and bicinchoninic acid assay, contained primarily β-conglycinin (7S) and glycinin (11S). Notable isoflavones identified from HPLC analysis were daidzin, genistin, and glycitin. Physical characteristics (size, zeta potential, polydispersity index, and wettability) of soy protein isolate-isoflavone (SPI–I) dispersions and emulsion droplets were measured with a Zetasizer Nano ZS and goniometer. Isoflavone addition significantly increased the contact angle of sessile drops towards neutral wettability. Heating appeared to significantly improve colloidal stability and emulsion droplets formed with heated SPI-I dispersions sustained size (∼200–245 nm) and zeta potential (∼-30-35 mV) over 7 days, with higher concentrations of isoflavones being generally correlated with smaller emulsion droplets. Collectively, these findings indicate that isoflavone addition improved emulsion stability compared to the control without isoflavones and that they can contribute to colloidal stability of food products.

Experimental Study of the Influence of the Adsorbate Layer Composition on the Wetting of Different Substrates with Water

Wetting is strongly influenced by adsorbate layers, which are omnipresent on surfaces. The influence of the composition and thickness of adsorbate layers on the water contact angle of sessile drops on different substrates was systematically investigated in the present work. Measurements were carried out for gold-sputtered substrates. These new results are compared to results from a previous study, in which corresponding measurements were carried out for technical steel and titanium substrates. In all experiments, different pretreatments of the samples were used to obtain variations of the adsorbate layer. The samples were either exposed to an oil bath or not, and different cleaning agents were used. The analysis of the adsorbate layer was carried out with X-ray photoelectron spectroscopy (XPS). The results for the different substrates reveal that the water contact angle depends mainly on the composition of the adsorbate layer. The substrate has only an indirect influence, as it influences the composition of the adsorbate layer. The thickness of the adsorbate layers was between 1.4 and 14 nm and was large enough to prevent a direct influence of the substrate on the water contact angle. It is shown that using the information on the adsorbate layer composition from XPS and the results for the water contact angle obtained for the gold samples alone, the water contact angles on the steel and titanium samples can be predicted.

Evolution of nanoliter size fluid droplet on micropatterned surface

Static shape of liquid droplets on textured surface draws significant attention from the standpoint of several engineering applications, ranging from heat transfer to bio-printing. This paper discusses the equilibrium behavior of a nanoliter size droplet dispensed on a micropatterned surface. For a given combination of intrinsic wettability of the surface, liquid surface tension and the geometric morphology of the textured surface, the liquid droplet prefers to be on Cassie-Baxter, Wenzel or an intermediate, hybrid state. Here we have carried out an energy-based simulation of nanoliter size droplets on micropatterned surface, using an open-source surface evolver fluid interface tool SE-FIT. A simplified periodic geometry of rectangular (straight or tapered) micro-pillars of specified dimensions is chosen for the micropatterned surface. For a given solid surface texture, we found that droplet prefers to transit from Wenzel to Cassie state beyond a threshold intrinsic sessile contact angle (which the liquid would have subtended on a microscopically smooth surface of the same solid material). This critical transition contact angle is plotted against the roughness parameter. Present study helps in designing the wettability-engineered surfaces for specific engineering applications.

Measurement of contact angle under different gravity generated by a long-arm centrifuge

The contact angle, as a vital measured parameter of wettability of a material surface, is pertinent to adhesion, nucleation, capillary phenomena, and liquid management in space. With the development of space exploration, the relationship between the contact angle and gravity has attracted increasing attention. In this study, we designed and manufactured a long-arm centrifuge (d =6 m) and a matched low-rate dynamic contact angle measurement system, which has the capacity to measure the advancing and receding contact angles of sessile drops under 1G-8G. The advancing and receding contact angles of water on smooth and rough surfaces were measured, and the results showed that 1) this apparatus is reliable for measuring the advancing and receding contact angles of sessile drops under 1G-8G, 2) the contact angle is not changed by gravity, while the measured apparent contact angles are changed by gravity owing to they are slope angles of liquid-vapor interface away from the solid surface due to limitation of the resolution of measurement system, and 3) wetting state of the rough surface can be changed by gravity.

Assessment and Interpretation of Surface Wettability Based on Sessile Droplet Contact Angle Measurement: Challenges and Opportunities

AbstractSurface wettability, a property that governs the interaction between the solid and liquid phases, is central in various biological systems and technological applications. A typical approach to assess the wetting interaction is based on the sessile droplet contact angle. Despite its experimental simplicity, caution is required in the measurement and interpretation of the sessile contact angle as a wettability metric. In this work, the major challenges related to the use of the sessile droplet contact angle for characterizing surface wetting behavior are reviewed. The complexity of interpreting surface wettability using the contact angle value arising from the following factors is discussed: the role of solid–liquid interfacial area and three‐phase contact line, the existence of multiple metastable states with a range of contact angle values, and the droplet size dependence of contact angles. An overview of the emerging alternative wetting evaluation techniques and parameters that can complement the sessile contact angle measurement is outlined.

Deposition of TiO2 Thin Films on Wood Substrate by an Air Atmospheric Pressure Plasma Jet

In the present work, titanium dioxide (TiO2) coatings were deposited on wood surfaces by an atmospheric pressure plasma jet using titanium tetraisopropoxide (TTIP) as a precursor to improve the wood’s stability against ultraviolet (UV) light and its moisture resistance capability. The surface topology and morphology of the wood specimens were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Surface chemical compositions of the specimens were characterized by X-ray photoelectron spectroscopy (XPS) and by Fourier transform infrared (FTIR) spectroscopy. The wettability of the coated wood was investigated by measuring the sessile contact angle. SEM and AFM showed the presence of small globules of TiO2 with some areas agglomerated on the coated wood surface. The coated surface roughness increased with increasing deposition time. FTIR analysis showed the existence of a Ti–O–Ti band at 800–400 cm−1 on the coated wood surfaces. The results obtained from FTIR were confirmed by XPS measurements. The hydrophilic wood surfaces were transformed to become hydrophobic or superhydrophobic after coating with TiO2, depending on the deposition parameters. The changes of colour during UV-exposure for both uncoated and coated wood specimens were measured using the CIELab colour system. The TiO2 coated wood became more resistant to colour change after UV radiation exposure than did untreated wood.

The Influence of Gravity on Contact Angle and Circumference of Sessile and Pendant Drops has a Crucial Historic Aspect.

Normally, pendant drops adapt contact angles that are closer to 90° than their sessile analogues. This is due to the drop's weight that pulls the pendant drop and straightens its contact angles. In this paper, we show a case in which the opposite happens: sessile drops that adapt contact angles that are closer to 90° than their pendant analogues. To achieve these peculiar states, one needs to increase the effective gravity on the drops and then relax it again to 1 g. Apparently, this and other phenomena depend not only on the direction of the gravitational force but also on the drop's history. We show that the drop's contact angle (and resultant area) is affected by two types of histories: short-term history and long-term history. For example, if we gradually increase the effective gravity on the drop, decrease it back to 1 g, and then repeat this cycle again and again, we see that the first cycle is drastically different, whereas other cycles approach a plateau in their behavior. In addition to drop's history, we explain these observations in terms of volume conservation, drop contact area, and pinning effect. This study may be generalized for other body forces such as electrical and magnetic or accelerating systems.

Surface modification of Ti-6Al-4V alloy for osseointegration by alkaline treatment and chitosan-matrix glass-reinforced nanocomposite coating

Individual and combined treatments of alkaline (NaOH) and chitosan/bioactive glass (SiO2-CaO-MgO) nanocomposite coating were applied on Ti-6Al-4V alloy surfaces with the objective of investigating and improving i) wettability, ii) apatite-formation ability, iii) corrosion resistance, and iv) biocompatibility. Individual applications of each surface treatment were found to enhance hydrophilicity, apatite-forming ability, corrosion resistance, and cytocompatibility (MG-63 cells). The most improved properties, except apatite-formation ability, were obtained using the combined treatment yielding a reduction of 64 and 93% in sessile contact angle and corrosion rate, respectively, than the untreated substrate. These improvements originate from the desired roughness and apatite-formation ability of the alkaline treatment and the appropriate biocompatibility and corrosion protection of the nanocomposite coating. That is, the combined application of alkaline treatment and chitosan nanocomposite coating is demonstrated as a promising surface-engineering strategy for hard-tissue replacement applications.

On the equilibrium contact angle of sessile liquid drops from molecular dynamics simulations.

We present a new methodology to estimate the contact angles of sessile drops from molecular simulations by using the Gaussian convolution method of Willard and Chandler [J. Phys. Chem. B 114, 1954-1958 (2010)] to calculate the coarse-grained density from atomic coordinates. The iso-density contour with average coarse-grained density value equal to half of the bulk liquid density is identified as the average liquid-vapor (LV) interface. Angles between the unit normal vectors to the average LV interface and unit normal vector to the solid surface, as a function of the distance normal to the solid surface, are calculated. The cosines of these angles are extrapolated to the three-phase contact line to estimate the sessile drop contact angle. The proposed methodology, which is relatively easy to implement, is systematically applied to three systems: (i) a Lennard-Jones (LJ) drop on a featureless LJ 9-3 surface; (ii) an SPC/E water drop on a featureless LJ 9-3 surface; and (iii) an SPC/E water drop on a graphite surface. The sessile drop contact angles estimated with our methodology for the first two systems are shown to be in good agreement with the angles predicted from Young's equation. The interfacial tensions required for this equation are computed by employing the test-area perturbation method for the corresponding planar interfaces. Our findings suggest that the widely adopted spherical-cap approximation should be used with caution, as it could take a long time for a sessile drop to relax to a spherical shape, of the order of 100 ns, especially for water molecules initiated in a lattice configuration on a solid surface. But even though a water drop can take a long time to reach the spherical shape, we find that the contact angle is well established much faster and the drop evolves toward the spherical shape following a constant-contact-angle relaxation dynamics. Making use of this observation, our methodology allows a good estimation of the sessile drop contact angle values even for moderate system sizes (with, e.g., 4000 molecules), without the need for long simulation times to reach the spherical shape.

Controlled Growth of MoS2 Flakes from in-Plane to Edge-Enriched 3D Network and Their Surface-Energy Studies

Controlled and tunable growth of chemically active edge sites over inert in-plane MoS2 flakes is the key requirement to realize their vast number of applications in catalytic activities. Thermodynamically, growth of inert in-plane MoS2 is preferred due to fewer active sites on its surface over the edge sites. Here, we demonstrate controlled and tunable growth from in-plane MoS2 flakes to dense and electrically connected edge-enriched three-dimensional (3D) network of MoS2 flakes by varying the gas flow rate using tube-in-tube chemical vapor deposition technique. Field emission scanning electron microscope results demonstrated that the density of edge-enriched MoS2 flakes increase with increase in the gas flow rate. Raman and transmission electron microscopy analyses clearly revealed that the as-synthesized in-plane and edge-enriched MoS2 flakes are few layers in nature. Atomic force microscopy measurement revealed that the growth of the edge-enriched MoS2 takes place from the in-plane MoS2 flakes. On the basis of the structural, morphological, and spectroscopic analysis, a detailed growth mechanism is proposed, where in-plane MoS2 was found to work as a seed layer for the initial growth of edge-enriched vertically aligned MoS2 flakes that finally leads to the growth of interconnected 3D network of edge-enriched MoS2 flakes. The surface energy of MoS2 flakes with different densities was evaluated by sessile contact angle measurement with deionized water (polar liquid) and diiodomethane (dispersive liquid). Both liquids show different nature with the increment in the density of the edge-enriched MoS2 flakes. The total surface free energy was observed to increase with increase in the density of edge-enriched MoS2 flakes. This work demonstrates the controlled growth of edge-enriched vertically aligned MoS2 flakes and their surface-energy studies, which may be used to enhance their catalytic activities for next-generation green fuel production.

Controlling the contact angle of biological sessile drops for study of their desiccated cracking patterns.

Current exploration of cracking patterns of desiccated biological sessile drops as a new approach of scientific research is progressing rapidly. It has been proposed that biological fluids are naturally capable of storing information. Cracking patterns of desiccated biological sessile drops have the potential to provide a facile means to study the links between compositions of biofluids, their structures and their functions. This potential is, however, limited by our current inability to control the influences of non-pathological factors on cracking patterns. Among the non-pathological factors, the initial sessile drop contact angle has a strong influence on cracking patterns through affecting the material transport and stress distributions within the drop. In this work, we developed a method to control the initial drop contact angle on a glass surface to enable the investigation of the contact angle-induced pattern changes in a biological sessile drop. Human blood was selected as the biofluid in this study, because of its richness in cracking patterns. It has been found that the increase in the initial contact angle enlarges the orthoradial cracks close to the drop edge and compresses the width of the peripheral region. We have also concluded that the number of cracks in the central region of the desiccated pattern can be correlated with the drop contact angle. This work also provides a novel protocol for fabricating standardized substrates for studies of desiccation patterns of biological and other complex colloidal fluids.

Oxygen plasma treatments of polydimethylsiloxane surfaces: effect of the atomic oxygen on capillary flow in the microchannels

Modification of polydimethylsoloxane/water interaction, to promote a spontaneous water flux through the microchannels, is a crucial task in microfluidic applications. For that reason, in this research, the authors study the hydrophilicity improvement induced by low-power oxygen plasma treatments (15 W) on the polydimethylsiloxane (PDMS) microchannel. The effects of the oxygen plasma treatments on wettability and water-work of adhesion on PDMS surfaces have been studied by sessile contact angle. The chemical composition of the plasma has been investigated by means of optical emission spectroscopy. The results indicate that the improvement of wettability on treated PDMS is led by the percentage of atomic oxygen in the plasma discharge. Super-hydrophilic surfaces (contact angle < 5°) have been obtained optimising the atomic oxygen percentage in the plasma discharge varying only the plasma working pressure. Super-hydrophilic PDMS microchannels show the highest spontaneous capillary flow in the channels while the hydrophilic microchannel shows only a small capillary flow.

A Disposable Amperometric Sensor Based on High-Performance PEDOT:PSS/Ionic Liquid Nanocomposite Thin Film-Modified Screen-Printed Electrode for the Analysis of Catechol in Natural Water Samples.

A conducting polymer-based composite material of poly(3,4-ethylenedioxythiophene) (PEDOT): poly(4-styrenesulfonate) (PSS) doped with different percentages of a room temperature ionic liquid (IL), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), was prepared and a very small amount of the composite (2.0 µL) was drop-coated on the working area of a screen-printed carbon electrode (SPCE). The SPCE, modified with PEDOT:PSS/IL composite thin-film, was characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), profilometry and sessile contact angle measurements. The prepared PEDOT:PSS/IL composite thin-film exhibited a nano-porous microstructure and was found to be highly stable and conductive with enhanced electrocatalytic properties towards catechol, a priority pollutant. The linear working range for catechol was found to be 0.1 µM–330.0 µM with a sensitivity of 18.2 mA·mM·cm−2 and a calculated limit of detection (based on 3× the baseline noise) of 23.7 µM. When the PEDOT:PSS/IL/SPCE sensor was used in conjunction with amperometry in stirred solution for the analysis of natural water samples, the precision values obtained on spiked samples (20.0 µM catechol added) (n = 3) were 0.18% and 0.32%, respectively, with recovery values that were well over 99.0%.

Live Imaging of Micro-Wettability Experiments Performed for Low-Permeability Oil Reservoirs

Low-permeability (unconventional) hydrocarbon reservoirs exhibit a complex nanopore structure and micro (µm) -scale variability in composition which control fluid distribution, displacement and transport processes. Conventional methods for characterizing fluid-rock interaction are however typically performed at a macro (mm) -scale on rock sample surfaces. In this work, innovative methods for the quantification of micro-scale variations in wettability and fluid distribution in a low-permeability oil reservoir was enabled by using an environmental scanning electron microscope. Live imaging of controlled water condensation/evaporation experiments allowed micro-droplet contact angles to be evaluated, while imaging combined with x-ray mapping of cryogenically frozen samples facilitated the evaluation of oil and water micro-droplet contact angles after successive fluid injection. For the first time, live imaging of fluids injected through a micro-injection system has enabled quantification of sessile and dynamic micro-droplet contact angles. Application of these combined methods has revealed dramatic spatial changes in fluid contact angles at the micro-scale, calling into question the applicability of macro-scale observations of fluid-rock interaction.

In Situ Determination of the Water Condensation Mechanisms on Superhydrophobic and Superhydrophilic Titanium Dioxide Nanotubes.

One-dimensional (1D) nanostructured surfaces based on high-density arrays of nanowires and nanotubes of photoactive titanium dioxide (TiO2) present a tunable wetting behavior from superhydrophobic to superhydrophilic states. These situations are depicted in a reversible way by simply irradiating with ultraviolet light (superhydrophobic to superhydrophilic) and storage in dark. In this article, we combine in situ environmental scanning electron microscopy (ESEM) and near ambient pressure photoemission analysis (NAPP) to understand this transition. These experiments reveal complementary information at microscopic and atomic level reflecting the surface wettability and chemical state modifications experienced by these 1D surfaces upon irradiation. We pay special attention to the role of the water condensation mechanisms and try to elucidate the relationship between apparent water contact angles of sessile drops under ambient conditions at the macroscale with the formation of droplets by water condensation at low temperature and increasing humidity on the nanotubes' surfaces. Thus, for the as-grown nanotubes, we reveal a metastable and superhydrophobic Cassie state for sessile drops that tunes toward water dropwise condensation at the microscale compatible with a partial hydrophobic Wenzel state. For the UV-irradiated surfaces, a filmwise wetting behavior is observed for both condensed water and sessile droplets. NAPP analyses show a hydroxyl accumulation on the as-grown nanotubes surfaces during the exposure to water condensation conditions, whereas the water filmwise condensation on a previously hydroxyl enriched surface is proved for the superhydrophilic counterpart.

Surface modification of birch veneer by peroxide bleaching

This study examined effects of surface modification with hydrogen peroxide (H2O2) on adhesive bond performance in birch veneer bonded with phenol–formaldehyde resin. The veneer was treated with 5% of H2O2 at 80 °C in the presence of alkali with the objective of improving adhesive bond performance and reducing the resin demand. The effects of the surface modification were determined by surface color measured with a spectrophotometer, bond performance tests with ABES (automated bonding evaluation system) and surface hydrophobicity with sessile contact angle measurements. Results demonstrated that veneer surface became significantly whiter, which also increased in lightness and decreased in redness and yellowness. ABES test revealed that a remarkable increase in bond performance in the treated veneer and the maximum bond strength with the treatment at 60 min, which was nearly twice as high as reference sample (5.42 → 9.94 N/mm2), was obtained. A notable decrease in contact angle measurements was also observed in the treated veneers (0° in 0.5 s). The surface modification of birch veneer with H2O2 + alkali demonstrated greater potential for the improvement of physical characteristics in veneer surface. Other aspects of H2O2 consumption during the treatment are also discussed.

Contact Angle of Sessile Drops in Lennard-Jones Systems

Molecular dynamics simulations are used for studying the contact angle of nanoscale sessile drops on a planar solid wall in a system interacting via the truncated and shifted Lennard-Jones potential. The entire range between total wetting and dewetting is investigated by varying the solid-fluid dispersive interaction energy. The temperature is varied between the triple point and the critical temperature. A correlation is obtained for the contact angle in dependence of the temperature and the dispersive interaction energy. Size effects are studied by varying the number of fluid particles at otherwise constant conditions, using up to 150,000 particles. For particle numbers below 10,000, a decrease of the contact angle is found. This is attributed to a dependence of the solid-liquid surface tension on the droplet size. A convergence to a constant contact angle is observed for larger system sizes. The influence of the wall model is studied by varying the density of the wall. The effective solid-fluid dispersive interaction energy at a contact angle of θ = 90° is found to be independent of temperature and to decrease linearly with the solid density. A correlation is developed that describes the contact angle as a function of the dispersive interaction, the temperature, and the solid density. The density profile of the sessile drop and the surrounding vapor phase is described by a correlation combining a sigmoidal function and an oscillation term.

Influence of molecular weight of polydimethylsiloxane precursors and crosslinking content on degree of ethanol swelling of crosslinked networks

Using PDMS (polydimethylsiloxane) as a basic polymeric matrix to the preparation of ethanol-permselective pervaporation membranes is a vibrant field of research. In this paper, a detailed study of the effects of the molecular weight of PDMS precursors and the content of the TEOS (tetraethyl orthosilicate) crosslinker on the degree of swelling in ethanol and ethanol contact angle is reported. Five PDMS precursors with molecular weights of 26.6K, 35.5K, 50.2K, 71.7K, and 110.4K, and five crosslinking contents (1wt%, 2wt%, 5wt%, 10wt%, and 15wt%) were chosen to prepare twenty-five PDMS networks. Considering only the maximum tensile strength of the networks, the optimum molecular weight of the precursor was found to be 35.5K and the optimum crosslinker content was 5wt%. The average Young’s modulus of the PDMS network prepared under these conditions reached 0.63MPa after using toluene to extract the network. Some uncrosslinked precursors always occur in the networks, and have some influence on the molecular weight of the precursors and the crosslinker content that is used. It was found that the content of the uncrosslinked precursors has direct effect on the contact angle of ethanol sessile drops at the surface of the extracted PDMS networks, and higher extraction corresponded to a smaller ethanol contact angle. A combined parameter (S), defined as the quotient of the extraction amount (AE) and the tensile elastic modulus (EY), gives a good linear relationship with the increase in weight of networks swelled in ethanol. This means that the degree of equilibrium swelling of the networks is simultaneously strongly influenced by the tensile modulus and the content of the uncrosslinked precursors.

Experimental Investigation of Dynamic Contact Angles on Horizontal and Inclined Surfaces Part I: Flat Silicon Oxide Surfaces

Abstract In the last decades, many attempts have been made to investigate influence quantities on the formation of contact angles and their interpretation. The effect of gravitation on the measured contact angles is a controversially discussed mater. In 1876 J. W. Gibbs already formulated conditions for equilibrium and therefore even for the equilibrium contact angle θ e which describe the acting gravitational field on a fluid component. Nevertheless, these relations often are not considered. The presented study illustrates differences between the apparent contact angles of sessile drops which are obtained by dynamical measurements with the inclined-plate and the needle-in method. This fact demonstrates that the force distibution at the triple line additionally differs in dependence on the used measuring method. The force distribution at the triple line depends on the relative position in the gravitational field (e.g. inclined or horizontal sample position) and on the curvature of the liquid-vapour interface (interfacial tension). Hence, theses effects on the apparent contact angles have to be considered for a reasonable interpretation of the measurement data. For this purpose, an approach to describe the effect of forces related to gravity effects on contact angles will be presented.

Bubbles and Drops on Curved Surfaces

Surface curvature affects the shape, stability, and apparent contact angle of sessile and pendant drops. Here, we develop an approximate analytical solution for non-axisymmetric perturbations to small spherical drops on a flat substrate, assuming a fixed contact angle and fixed drop volume. The analytical model is validated using numerical solutions of the Laplace equation from the Surface Evolver software. We investigate the effects of surface curvature on drop shape, pressure, and surface energy, ascertaining the energy-gradient force that drives lateral drop migration. By balancing this force with the viscous resistance/drag force, in the lubrication approximation, we predict velocities of the order of 0.1 mm s(-1) for 1 mm diameter drops of water with a 30° contact angle on a substrate with a curvature gradient of 0.01 mm(-2), achieved, for example, on a harmonic surface with a wavelength of 4 cm and an amplitude of 4 mm.