A Practical Procedure for Measuring Contact Angles in Wettability Studies by the Sessile Drop Method

This work sought to undertake a comprehensive investigation of the measurement errors associated with contact angle assessment of curved hydrogel contact lens surfaces. The contact angle coefficient of repeatability (COR) associated with three measurement conditions (image analysis COR, intralens COR, and interlens COR) was determined by measuring the contact angles (using both sessile drop and captive bubble methods) for three silicone hydrogel lenses (senofilcon A, balafilcon A, lotrafilcon A) and one conventional hydrogel lens (etafilcon A). Image analysis COR values were about 2 degrees , whereas intralens COR values (95% confidence intervals) ranged from 4.0 degrees (3.3 degrees , 4.7 degrees ) (lotrafilcon A, captive bubble) to 10.2 degrees (8.4 degrees , 12.1 degrees ) (senofilcon A, sessile drop). Interlens COR values ranged from 4.5 degrees (3.7 degrees , 5.2 degrees ) (lotrafilcon A, captive bubble) to 16.5 degrees (13.6 degrees , 19.4 degrees ) (senofilcon A, sessile drop). Measurement error associated with image analysis was shown to be small as an absolute measure, although proportionally more significant for lenses with low contact angle. Sessile drop contact angles were typically less repeatable than captive bubble contact angles. For sessile drop measures, repeatability was poorer with the silicone hydrogel lenses when compared with the conventional hydrogel lens; this phenomenon was not observed for the captive bubble method, suggesting that methodological factors related to the sessile drop technique (such as surface dehydration and blotting) may play a role in the increased variability of contact angle measurements observed with silicone hydrogel contact lenses.

The liquid drop captured at the capillary end, which is observed in capillary valve and pendant drop technique, is investigated theoretically and experimentally. Because of contact line pinning of the lower meniscus, the lower contact angle is able to rise from the intrinsic contact angle (θ*) so that the external force acting on the drop can be balanced by the capillary force. In the absence of contact angle hysteresis (CAH), the upper contact angle remains at θ*. However, in the presence of CAH, the upper contact angle can descend to provide more capillary force. The coupling between the lower and upper contact angles determines the equilibrium shape of the captured drop. In a capillary valve, the pinned contact line can move across the edge as the pressure difference exceeds the valving pressure, which depends on the geometrical characteristic and wetting property of the valve opening. When CAH is considered, the valving pressure is elevated because the capillary force is enhanced by the receding contact angle. For a pendant drop under gravity, the maximal capillary force is achieved as the lower contact angle reaches 180° in the absence of CAH. However, in the presence of CAH, four regimes can be identified by three critical drop volumes. The lower contact angle can exceed 180°, and therefore the drop takes on the shape of a light bulb, which does not exist in the absence of CAH. The comparisons between Surface Evolver simulations and experiments are quite well.

Effect of Ti content on the wetting behavior of Sn0.3Ag0.7Cu/AlN system

The dynamic behavior of impinging water droplets is studied in the context of varying surface wettability and wickability on smooth and nanostructured superhydrophilic surfaces. This study distinguishes the separate effects of wetting (contact angle), wickability, and inertia on the spreading and vaporization of water droplets deposited on nanoporous surfaces by considering experimental results in tandem with axisymmetric, volume of fluid (VOF) simulations of droplet spreading. High speed videos were obtained for water droplets spreading on nanoporous surfaces which exhibit very low (< 15°) contact angle and high wickability. In this study, the effect of wickability was assessed by comparing the experimental results, which include the low contact angle and high wickability effects, to predictions of the VOF model, which include only the ultralow contact angle. While a droplet touched to the nanostructured surface demonstrates spreading driven by wicking, droplets which hit the surface with a non-zero impact velocity demonstrate spreading characteristics similar to the smooth surface, which are driven by inertia and ultra-low contact angle. The presence of the nanoporous layer impacts the equilibrium position of the contact line and the final spread radius changes with impact velocity on the nanostructured surface. These results provide fundamental input for modeling of spray cooling systems with nanostructured surfaces.

Smart surfaces with externally controlled wettability patterns are ubiquitous building blocks for micro-/nanofluidic and lab-on-chip devices, among others. We develop hierarchical surfaces of ZnO nanorods grown on laser-microstructured silicon with reversible photo-induced and heat-induced wettability. The as-prepared surfaces are superhydrophilic, with very low water contact angles (~ 10°), and transition to a wetting state with high water contact angles (~ 150°) when annealed in vacuum. As the annealing temperature increases to 400 °C, the surfaces become completely water-repellent. Even though the annealed surfaces present high water contact angles, at the same time, they are very adhesive for water droplets, which do not roll off even when tilted at 90° or 180o (rose-petal effect), unlike standard hydrophobic surfaces which typically combine high water contact angles with low roll-off angles. The surfaces return to the superhydrophilic state when irradiated with UV light, which indicates a reversible wettability with external stimuli. Based on this transition, we demonstrate local modification of the wetting state of the surfaces by UV irradiation through a mask, which results in directed liquid motion, useful for microfluidic applications. The high contact angles obtained in this work are usually obtained only after chemical modification of the ZnO surface with organic coatings, which was not necessary for the hierarchical surfaces developed here, reducing the cost and processing steps of the fabrication route. These rose-petal surfaces can be used as “mechanical hands” in several applications, such as no-loss transport of small liquid volumes, precision coatings, spectroscopy, and others. Furthermore, the completely water-repellent surfaces, rarely reported elsewhere, may find important applications in frictionless liquid transport for microfluidic and other devices.

In this work, we have studied the wettability of zinc oxide (ZnO) nanorods grown on fluorine-doped tin oxide (FTO) by highlighting the effect of polar and non-polar ZnO facets on contact angle (CA) results. The variation in the wettability behaviors of the synthesized surfaces is mainly related to physical and chemical surface texturing which influenced the liquid drop penetration. Indeed, three main penetration states can be deduced: total, partial, and null-penetration. Where, low CA (100.9°) with high contact angle hysteresis (CAH) (13°) is observed for total penetration of the liquid drop. While, high CA (139.6°) with low CAH (7°) is observed for null-penetration. Moreover, we have found that the chemical texturing of ZnO, especially the hydrophilicity of ZnO tips, responsible for liquid drop sticking, prevents the liquid slipping over the surface. In order to promote the liquid rolling on the ZnO surface, we reported the physical modification of the ZnO structures. Therefore, the rolling of the liquid drop on the inclined surface of ZnO is achieved by using a new structure based on double scale roughness. This surface exhibits superhydrophobic behavior with a CA of 153° and CAH of 3°.

Template-assisted nanofabrication is a simple and effective method to produce various nanostructure morphologies by controlling the polymer, solvent, and template characteristics. In this study, the importance of the surface interactions between the solution and the template in controlling the morphology of the nanostructures has been highlighted. Contact angles between various polymer solutions and anodized aluminum oxide (AAO) templates have been determined. The morphology of the resultant nanostructures has been correlated with the measured contact angles between solution and template. It is generally observed that nanorods (diameter of 100–350 nm) are produced at low contact angles, whereas nanotubes (diameter of 200–400 nm) tend to form at high contact angles. Therefore, desired nanostructure morphology for a given application can be obtained by controlling the initial wetting interaction between solution and template.

Gas turbines are of great importance in industry. In the turbine section within a jet engine, thermal barrier coatings (TBCs) are utilized to protect the metal turbine blades, thus improve the efficiency of engine. However, this coating is extremely vulnerable to attack by injected particulates. This ingested particulate is often referred to as "CMAS" (Calcia-Magnesia-Alumina-Silica). Among all the CMAS materials, Volcanic Ash (VA) is the most common type which aeroengines may encounter during the flight. This type of CMAS material would melt in the combustion section by ultra high temperature and then impact the turbine blades with relatively high speed. Some of the particles would then stick on and bond with the TBC, thus cause degradation of the protecting coating. In this way, the jet engine would be permanently damaged. In the recent years, experiments have been done by different researchers to elaborate the effect of CMAS materials on TBCs. However, there is still a lack of knowledge in the bonding mechanism and physical adhesion between the CMAS particles (especially VA particles) and the substrate. A study of VA particle impingement is required in order to understand particle impingement, phase transition and heat transfer, bonding mechanism and splat morphology in detail. In this research, experiments were carried at Cambridge University, by Prof. Trevor William Clyne, Dr. James Dean and Dr. Catalina Taltavull to reproduce the VA-substrate impingement in jet engine. A Vacuum Plasma Spray (VPS) system was utilized to create a high-temperature, high-velocity flow field. Different types of Volcanic Ashes (VAs) were introduced into the experimental set-up. Sticking rate, Scanning Electron microscope (SEM) micrographs of deposition morphology were examined and collected. Chapter 3 elaborates the details of this experiment set-up and data collected from the experiment. This experiment set-up is utilized by the author for building numerical models and the result of this experiment is used to validate the numerical models. Three numerical models were built to perform a systematic study. Firstly, in Chapter 4, a Computational Fluid Dynamic (CFD) model was created to simulate the steady-state of the VPS flow field. The Discrete Particle Method (DPM) model was then utilized to simulate the injection of volcanic ash particles. After calculating the BI number, non-isothermal effects within the ceramic particles were simulated by introducing the heat transfer function by a user-defined function (UDF). This model gives the temperature gradient within and velocity of the in-flight VA particles at any time during the spray. It is shown that small particles (diameter < $10____ ____mu m$) would easily be melted and reach the iso-thermal state. However, these particles would be largely influenced by the flow field thus bypass the turbine blades. The large particles (diameter > $50____ ____mu m$) would easily impact the turbine blades, but would remain unmelted due to the large grain size. It is concluded that VA particles with diameter of $15____ ____mu m$ to $40____ ____mu m$ are the most "dangerous" particles, because these particles have both relatively high possibility to be melted, and high possibility to impact thus adhere on the substrate. Second of all in Chapter 5, systematic study of Yttria-stabilized Zirconia (YSZ) particle impingement and deposition on stainless steel in thermal spray process has been performed. A Coupled Eulerian and Lagrangian model was developed. This model contributes to simulate the process of semi-molten particle impact. By utilizing this model, both the large deformation of liquid part and the plastic deformation of the solid part could be extracted. One fully molten and two semi-molten(solid core with liquid shell and solid shell with liquid core) cases were studied. The results of the numerical model matches well with the experiment and analytical data. Interest parameters such as velocity, temperature, fraction of liquid part were varied. The contact area, splat morphology and local contact temperature were collected and studied. It is shown that, the larger the liquid fraction is, the larger the contact area would be. Moreover, effect of roughness of substrate is also studied. It is suggested that substrate roughness whose average asperity size is higher than the 1/10 of particle size is beneficial for adhesion. Third of all in Chapter 6, in order to simulate the impact for high-viscosity glass-state ceramic particles, Smoothed Particle Hydrodynamic (SPH) model was built. For high/ultra viscosity cases, traditional CFD method and Finite Element Method (FEM) would be extremely slow. SPH model transfers the Eulerian equations into Lagrangian equations. By utilizing this method, computational resources could be saved, and high viscosity impact could be simulated. The SPH algorithm was coded and equations for heat transfer was introduced to simulate the solidification of liquid. Systematic study were performed by utilizing this model. Viscosity, contact angle, velocity of particles were varied. Contact area, splat morphology and solidification at the contact area were examined. It is shown that, large contact angle would result in large contact area. However, particles impingement with low viscosity and high contact angle could result in the break up of the particles. The similar phenomena could be seen in experiment - small particles have lower viscosity and are approaching the substrate with a large contact angle. Therefore, the deposition of these types of particles show an obvious evidence of break up and oblique impact.

Polyvinylidene fluoride (PVDF) is a popular polymer material for making membranes for several applications, including membrane distillation (MD), via the phase inversion process. Non-solvent-induced phase separation (NIPS) and vapor-induced phase separation (VIPS) are applied to achieve a porous PVDF membrane with low mass-transfer resistance and high contact angle (hydrophobicity). In this work, firstly, the impacts of several preparation parameters on membrane properties using VIPS and NIPS were studied. Then, the performance of the selected membrane was assessed in a lab-scale direct-contact MD (DCMD) unit. The parametric study shows that decreasing PVDF concentration while increasing both relative humidity (RH) and exposure time increased the contact angle and bubble-point pore size (BP). Those trends were investigated further by varying the casting thickness. At higher casting thicknesses and longer exposure time (up to 7.5 min), contact angle (CA) increased but BP significantly decreased. The latter showed a dominant trend leading to liquid entry pressure (LEP) increase with thickness.

The wet spherical agglomeration technique with a small amount of bridging liquid which preferentially wets solid particles, is one of the accepted method to agglomerate the particles dispersed in liquid. In the present study, the effects of addition of ethanol and surface active agent to the suspension on the agglomeration behavior of sulfisomidine dispersed in chloroform were investigated. Addition of a suitable amount of ethanol, say 100ml, slightly increased the agglomerate size compared with the case without addition. When the ethanol was added more than this amount, the agglomerate size decreased significantly. While, the agglomeration did not occur absolutely when ionic surfactant was added.The contact angles of bridging liquid on solid particles in the same system as the agglomeration process were measured as a function of surfactant concentration and the methodology, i. e. sessile drop, hanging drop and Wilhelmy method. The contact angles obtained by the sessile drop method were reasonably correlated with the agglomerate size, which decreased with increasing the contact angle. The correlation of agglomerate size with both parameters of the interfacial tension between the bridging liquid and the dispersing medium and the contact angle were described linearly by equation (11) as seen in Fig. 6.

“Etch and coat” methods, potentially useful for large scale production of superhydrophobic surfaces on metals, have been extended by introduction of a thin 20 nm Au interlayer which allowed thiol surface modifiers to be investigated. Fe, Ti and Zn surfaces modified in this way with a polyfluorothiol generally had lower contact angles, θ, than their silane modified equivalents but were more uniform and reproducible. The higher θ values for silanes appear to be associated with an increased effective roughness that is not seen in the thiol coatings, which follow the underlying surface. For Fe, Ti and Zn, θ increased to a maximum with etching time but then decreased significantly. Electron microscopy of samples etched for long times showed fewer grain boundaries, suggesting that the loss of hydrophobicity was related to reduced roughness. This was confirmed through quantitative measurements of roughness parameters which were directly correlated with the observed contact angles through the Wenzel equation. The observation of this behaviour for 3 different metals suggests it is a general effect and the assumption that increased etch times will give increased roughness is incorrect. Although etching , Au coating and thiol modification did not provide sufficient roughness for superhydrophobicity, electrodeposition of a Cu layer onto the metal substrates followed by electroless deposition of silver gave uniform surfaces with very high contact angles (up to θ = 168.9°) and low roll-off angles (α < 1°). This method is sufficiently simple and inexpensive to be suitable for application on industrial scales, for example in marine or aeronautical engineering.

Purpose: It is of major interest to understand how intraocular lens (IOL) surfaces interact with aqueous humor or tissues after implantation. Important parameters to describe the physicochemical surface properties are the contact angle and critical surface tension. Therefore, we performed measurements of the contact angle and critical surface tension of several commercially available IOLs. Setting: Department of Ophthalmology, Johannes Gutenberg University, Mainz, University Eye Hospital, Tübingen, and Max Planck Institute for Polymer Research, Mainz, Germany. Methods: A total of 17 commercially available standard quality IOL types (6 of each) of different materials from several manufacturers were studied. Scanning electron microscopy and contact angle measurements using the sessile drop method were made. Results: The sessile drop method differentiated all materials based on their surface forces. The contact angle (mean ± SD) ranged from 56.5 ± 3.8b 3Conclusion: Contact angles differed between the IOLs depending on the IOL material with the lowest contact angle in the heparin-surface-modified IOL. The contact angle measurement method generates useful information about the material surface and its potential biomaterial interactions.

Physicochemical, surface, and mechanical properties of three batches of T800 grade carbon fibers (CFs) treated with three kinds of sizing agents and Toray T800H CFs were characterized to study the effect of sizing agents on surface properties. Scanning electron microscopy for morphology, atomic force microscopy calculations, and results for the content of sizing agents showed that sizing agent B improved the surface roughness and CFs with high content of sizing agent always presented small surface roughness in a certain content range 1.2–1.6%. Surface energy of CFs was calculated by Young’s contact angle using the test results with water and glycol, and contact angles with LY-1 and modified-AC531 were also acquired. The results proved that CFs of sizing agent group B had the highest average surface energy and the lowest average contact angles with both LY-1 and modified-AC531. From both single-filament and tensile strength test results, the average strength of CFs of sizing agent group B was found to be the lowest, which indicated that sizing agent B had an influence on tensile strength decrease of T800 grade CFs. Comparing the results of interfacial shear strength both in a natural dry state and after hygrothermal treatment, high surface energy was found to be the key element to obtain high interfacial adhesion between T800 grade CFs and bismaleimide, and high surface roughness and low contact angle also played important roles. Among sizing agents A, B, and C, A had an effect on the interfacial shear strength decrease of CFs in the natural dry state, while C had that after hygrothermal treatment.

Surface energy components of a dye-ligand immobilized pHEMA membranes: Effects of their molecular attracting forces for non-covalent interactions with IgG and HSA in aqueous media

In this study, the contact angles of four lead-free solders, namely, Sn-3.5Ag, Sn-3.5Ag-4.8Bi, Sn-3.8Ag-0.7Cu, and Sn-0.7Cu (wt.%), were measured on copper substrates at different temperatures. Measurements were performed using the sessile-drop method. Contact angles ranging from 30° to 40° after wetting under vacuum with no fluxes and between 10° and 30° with rosin mildly activated (RMA) and rosin activated (RA) fluxes were obtained. The Sn-3.5Ag-4.8Bi exhibited the lowest contact angles, indicating improved wettability with the addition of bismuth. For all soldering alloys, lower contact angles were observed using RMA flux. Intermetallics formed at the solder/Cu interface were identified as Cu6Sn5 adjacent to the solder and Cu3Sn adjacent to the copper substrate. The Cu3Sn intermetallic phase was generally not observed when RMA flux was used. The effect of temperature on contact angle was dependent on the type of flux used.