On the interpretation of contact angle hysteresis

The determination of solid surface free energy is still an open problem. The method proposed by van Oss and coworkers gives scattered values for apolar Lifshitz-van der Waals and polar (Lewis acid-base) electron-donor and electron-acceptor components for the investigated solid. The values of the components depend on the kind of three probe liquids used for their determination. In this paper a new alternative approach employing contact angle hysteresis is offered. It is based on three measurable parameters: advancing and receding contact angles (hysteresis of the contact angle) and the liquid surface tension. The equation obtained allows calculation of total surface free energy for the investigated solid. The equation is tested using some literature values, as well as advancing and receding contact angles measured for six probe liquids on microscope glass slides and poly(methyl methacrylate) PMMA, plates. It was found that for the tested solids thus calculated total surface free energy depended, to some extent, on the liquid used. Also, the surface free energy components of these solids determined by van Oss and coworkers' method and then the total surface free energy calculated from them varied depending on for which liquid-set the advancing contact angles were used for the calculations. However, the average values of the surface free energy, both for glass and PMMA, determined from these two approaches were in an excellent agreement. Therefore, it was concluded that using other condensed phase (liquid), thus determined value of solid surface free energy is an apparent one, because it seemingly depends not only on the kind but also on the strength of interactions operating across the solid/liquid interface, which are different for different liquids.     

A new technique for evaluating ink–cellulose interactions: initial studies of the influence of surface energy and surface roughness

Ink–cellulose interactions were evaluated using a new technique in which the adhesion properties between ink and cellulose were directly measured using a Micro-Adhesion Measurement Apparatus (MAMA). The adhesion properties determined with MAMA were used to estimate the total energy release upon separating ink from cellulose in water. The total energy release was calculated from interfacial energies determined via contact angle measurements and the Lifshitz–van der Waals/acid–base approach. Both methods indicated spontaneous ink release from model cellulose surfaces, although the absolute values differed because of differences in measuring techniques and different ways of evaluation. MAMA measured the dry adhesion between ink and cellulose, whereas the interfacial energies were determined for wet surfaces. The total energy release was linked to ink detachment from model cellulose surfaces, determined using the impinging jet cell. The influences of surface energy and surface roughness were also investigated. Increasing the surface roughness or decreasing the surface energy decreased the ink detachment due to differences in the molecular contact area and differences in the adhesiom properties.     

Contact angle measurements and interpretation: wetting behavior and solid surface tensions for poly(alkyl methacrylate) polymers

Low-rate dynamic contact angles of a large number of liquids were measured on a poly(ethyl methacrylate) (PEMA) polymer using an automated axisymmetric drop shape analysis profile (ADSA-P). The results suggested that not all experimental contact angles can be used for the interpretation in terms of solid surface tensions: eight liquids yielded non-constant contact angles and/or dissolved the polymer on contact. From the experimental contact angles of the remaining four liquids, we found that the liquid-vapor surface tension times the cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. γlv cos ζ depends only on γlv for a given solid surface (or solid surface tension). This contact angle pattern is again in harmony with those from other methacrylate polymer surfaces of different compositions and side-chains. The solid-vapor surface tension of PEMA calculated from the equation-of-state approach for solid-liquid interfacial tensions was found to be 33.6 ± 0.5 mJ/m2 from the experimental contact angles of the four liquids. The experimental results also suggested that surface tension component approaches do not reflect physical reality. In particular, experimental contact angles of polar and nonpolar liquids on polar methacrylate polymers were employed to determine solid surface tension and solid surface tension components. Contrary to the results obtained from the equation-of-state approach, we obtained inconsistent values from the Lifshitz-van der Waals/acid-base (van Oss and Good) approach using the same sets of experimental contact angles.     

Adhesion and Wettability Characteristics of Chemically Modified Banana Fibre for Composite Manufacturing

In this work banana fibre was chemically modified using various chemical agents. The surface energy of the fibre is an important parameter and one which governs the interaction of fibre with polymeric matrices. This paper describes the influence of various chemical treatments on the surface energy of the banana fibre investigated by contact angle measurements, spectroscopic analysis and surface morphology studies. The surface energy, work of adhesion, polarity, spreading coefficient, interfacial energy and interaction parameter were determined in the case of raw and chemically modified fibres. Chemical modification has been found to have a profound effect on the surface energy. The polar and dispersive components of the surface energy were also found to be dependent on the chemical treatment involved. The chemical modifications done in this work were: alkali treatment, silanation, benzoylation, formylation, potassium permanganate treatment and acetylation. Of all the modifications, the relative surface energy was found to be a maximum for alkali treated fibre and minimum for silanated fibre. Contact angle measurements were found to be an effective tool in predicting the possible interaction of the fibres with phenol formaldehyde matrix resin. Atomic force microscopy roughness analysis revealed a significant decrease in surface roughness for the chemically modified fibre. An increase both in fibre/matrix adhesion and interfacial shear strength has been observed for all surface modified fibres except for those modified by benzoylation and acetylation.     

Surface characterization and adhesion of black-oxide-coated copper substrate: effect of surface hardening processes

The effects of surface-hardening processes on the changes in surface characteristics and adhesion of black copper oxide substrate with epoxy resins are studied. Various techniques, namely SEM, XPS, AFM, XRD, Auger electron spectroscopy, contact angle goniometry, D-SIMS and RBS, were used to identify the changes in surface characteristics. Dense, fibrillar cupric oxide crystals characterized the as-deposited oxide coating with high surface roughness. The surface-hardening process flattened and consolidated the fibrils without changing the compositional and thermodynamic characteristics of the coated surface. The surface-hardening process reduced the total thickness of copper oxide by approximately 50–150 nm. The reduction in oxide thickness was not a predominant factor for the reduced bond strength of the surface-hardened coating. The bond strengths of both the as-deposited and surface-hardened black oxide coatings increased with oxidation time, until saturation at about 120–150 s. For the as-deposited oxide coating, mechanical interlocking, high wettability and resistance to surface contamination are the three major sources for improved adhesion, amongst which the enhanced mechanical interlocking provided by the fibrillar cupric oxide is the most important. Surface hardening reduced the efficiency of mechanical interlocking mechanism. There was close functional dependence between the button-shear strength and surface characteristics, such as surface roughness, coating thickness and surface free energy.     

On the determination of the surface energetics of porous polymer materials

The solid surface tension γsv of hydrophobic polymer powders has been determined using the capillary penetration technique. By plotting Kγlv cos ζ, where K is a geometric factor, versus the liquid surface tension γlv, the following values of γsv were directly derived from the curves: poly(tetrafluoroethylene) γsv = 20.4 mJ/m², polypropylene γsv = 30.2 mJ/m², polyethylene γsv = 34.4 mJ/m², and polystyrene γsv = 27.5 mJ/m². These values are in good agreement with the γsv values obtained from contact angle measurements on flat and smooth solid surfaces of the same materials. If the contact angles were first calculated from the capillary penetration experiments, which is the usual procedure applied in the literature, distinctly higher contact angles were obtained. Obviously these angles are affected by the powder morphology and are therefore meaningless contact angles in terms of a surface energetic interpretation.     

MG63 osteoblast cell response on Zn doped hydroxyapatite (HAp) with various surface features

The clinical success of implant is governed by implant-surface and bone cell interaction that promote osseointegration and long term stability. Calcium hydroxyapatite (HAp) is a widely used bioceramic material for orthopedic and dental applications, which promotes bone tissue generation. Doped hydroxyapatite using various metallic ions is often reported to enhance this osteoconductive property. The objective of this study was to synthesize zinc doped HAp, to investigate the osteoblast cell response on this doped HAp and find out separately the effect of doping and different surface parameters on cell response. Slip casting technique was used to prepare pure and doped HAp specimens which were sintered at 1100 °C and 1250 °C. Different porosities, pore sizes were generated along with different surface roughness so as to understand the effect of these extrinsic parameters on cell culture. MG63 osteoblast cells were used for a maximum period of two weeks. Metabolic activity, adhesion and proliferation rate study of osteoblast cells on doped HAp showed significantly better response than pure HAp. Effect of doping was found to be more prominent than the effect of surface roughness.     

Determination of the acid-base characteristics of clay mineral surfaces by contact angle measurements-implications for the adsorption of organic solutes from aqueous media

The apolar and the polar (electron-acceptor and electron-donor, or Lewis acid-base) surface tension components and parameters of solid surfaces can be determined by contact angle measurements using at least three different liquids, of which two must be polar. With swelling clay minerals (e.g. smectite clay minerals), smooth contiguous membranes can be fabricated, upon which contact angles can be measured directly. With non-swelling clay minerals (e.g. talc), contact angles can be determined by wicking, i.e. by the measurement of the rate of capillary rise of the liquids in question through thin layers of clay powder adhering to glass plates. The apolar and polar (acid-base) surface tension components and parameters thus found for various untreated and quaternary ammonium base-treated clays allowed the determination of the net interfacial free energy of adhesion of human serum albumin onto the various clay particle surfaces immersed in water. The free energies of adhesion, thus found, correlate well with the experimentally observed degree of adsorption of human serum albumin.     

Characterization of epoxy resin surface energetics

This study has characterized the energetics of both the liquid state and the solid state of two commercially available epoxy resins: a DGEBA- and a TGMDA-based epoxy system. The surface properties of the liquid epoxies were evaluated by wetting measurements using a dynamic contact angle analysis (DCA). The Lifshitz-van der Waals components of the surface tension were found to be similar for both epoxy systems, while the acid-base components were found to be slightly different. Two different techniques were used to characterize the cured epoxy surface properties: wetting measurements and vapor adsorption measurements by means of inverse gas chromatography (IGC). The Lifshitz-van der Waals components of the surface energy were observed to be nearly the same for both epoxies, confirming that both resins have the same potential for non-specific interactions, in both liquid and solid states. Evaluations of the acid-base components of the work of adhesion by DCA and the Gibbs free energy change by IGC suggest that both cured epoxies show non-negligible specific interactions with both acidic and basic probes. However, computations of the accepticity and donicity parameters showed that both cured epoxies are predominantly basic, but also possess non-negligible acidity. It is likely that the presence of water on the solid surface contributes to the acidic character of the cured epoxies. The temperature dependence of the liquid surface tension for both epoxy systems was investigated. The same temperature dependence was observed: the surface tension decreased with temperature, following a linear regression. Corrections for viscous-drag effects on the liquid surface tension measurements were also made.     

粗糙表面上的移动接触线和动态接触角

提出一个粗糙表面上移动接触线和动态接触角的数理模型：毛细数较低时表观接触线前缘存在极薄的前驱膜,表观接触线在“湿”固体表面上移动,不同于传统模型中认为表观接触线在“干”固体表面上移动.在Moffatt角区内部流动解的基础上,通过引入接触线特征参数表征表观接触线在前驱膜上的滑移程度,导出动态接触角的速度关系.与不同研究者实验数据对比发现量纲1特征参数反映固体材料特性和表面特性对动态湿润过程的影响,与液相的性质无关.结合前期提出的滞后张力模型,对动态法和静态法测量静接触角产生的差异给出合理解释.     

Effects in Surface Free Energy of Sputter-Deposited VNx Films

Vanadium nitride (VN _x ) thin films have attracted much attention for semiconductor integrated circuit (IC) packaging molding dies, and forming tools due to their excellent hardness and, thermal stability. VN _x thin films with VN_0.45, VN_0.83, VN_1.22, VN_1.73, VN_2.06 were prepared using a radio frequency (RF) sputter technique. The experimental results showed that the contact angle at 20°C increases with increasing nitrogen content of the VN _x films, to 101.4° corresponding to VN_1.73 and then decreased. In addition, the contact angles decreased with increasing surface temperature, because an increase of the surface temperature disrupts the hydrogen bonds between water and the films and the water gradually vaporizes. The total surface fee energy (SFE) at 20°C decreased with nitrogen content of the VN _x films to 29.8 mN/m (VN_1.73) and then increased. This is because a larger contact angle means weaker hydrogen bonding which results in a lower SFE. The polar SFE component had the same trend as the total SFE, but the dispersive SFE component had the opposite trend. The polar SFE component is also lower than the dispersive SFE component. This is because hydrogen bonds are polar. The total SFE, dispersive SFE and polar SFE of the VN _x films all decrease with increasing surface temperature. This is because with increasing temperature, water evaporates from the surface, disrupting hydrogen bonds and hence increasing surface entropy. The film roughness has an obvious effect on the SFE and there is tendency for the SFE to increase with increasing film surface roughness. As a result the SFE and surface roughness can be expressed in terms of a simple ratio function.     

Comparison of drag characteristics of self-polishing co-polymers and silicone foul release coatings: A study of wettability and surface roughness

An experimental study has been carried out to evaluate the drag characteristics of different self-polishing co-polymers (SPC) (tin based and tin-free) and a silicone foul release (FR) coating. Drag measurements have been performed on a smooth aluminum cylinder connected to a rotor device. Various coatings on cylinders were examined and differential length technique was also used to avoid the end effects during rotation. Surface energy of the coated samples was determined using static contact angle measurement. Characteristic roughness measurements of the coated surfaces were evaluated with atomic force microscopy (AFM) technique.Drag measurements showed that the frictional resistance of the FR coated cylinders was lower than that of SPC coated samples.Contact angle results showed that the critical surface tension and its polar component for silicone FR coating are less than SPC coatings. This prevents firm adhesion of fouling organisms on underwater hulls.AFM studies revealed a lower surface roughness for silicone FR coating as compared to SPC coatings. Also, its surface texture is considerably different from SPC coatings.It can be concluded that the drag characteristics of a surface are affected by its free energy and roughness parameters.     

Adhesion Improvement of ABS Resin to Electroless Copper by H2SO4–MnO2 Colloid with Ultrasound-Assisted Treatment

The adhesion strength between electroless copper and acrylonitrile-butadiene-styrene (ABS) resin can be improved significantly by an environmentally friendly etching system containing H₂SO₄–MnO₂ colloid as a replacement for conventional chromic acid etching solutions. In this paper, the effects of the H₂SO₄ concentration and ultrasound-assisted treatment (UAT) on the surface roughness and adhesion strength were investigated. When the H₂SO₄ concentration was 11.8～12.7 M, good etching was obtained. With UAT, many uniform cavities formed on the ABS surface with the average surface roughness (R _a) and maximum roughness (R _max) of ABS substrates decreasing from 386 and 397 nm to 278 and 285 nm, respectively, which were much lower than that etched by CrO₃–H₂SO₄ colloid (420 and 510 nm, respectively). The average adhesion strength increased from 1.29 to 1.39 kN/m, which was close to that obtained with chromic acid etching treatment (1.42 kN/m). The surface contact angle measurement indicated that the density of the polar groups on the ABS surface increased with increasing time of UAT. The results indicated that surface etching with UAT not only improved the uniformity of cavities, but also enhanced the oxidation rate of ABS resin, which in turn resulted in greater adhesion strength and a lower surface roughness.     

Spreading of liquid drops over solid substrates: 'like wets like'

The classic hydrodynamic wetting theory leads to a linear relationship between spreading speed and the capillary force, being determined only by the surface tension of the liquid and its viscosity. Both equilibrium and dynamic processes of wetting are important in adhesion phenomena. The theory appears to be in good agreement with the results generated from experiments conducted on the spreading of poly(dimethylsiloxane) (PDMS) on soda-lime glass substrate and fails to account for the behavior of other liquids. In this study, the spreading kinetics of four different liquids (hexadecane, undecane, glycerol and water) was determined on three different solids, namely, soda-lime glass, poly(methyl methacrylate) (PMMA) and polystyrene (PS). Droplets from the same liquid allowed to spread under identical conditions on three different substrates produce distinctly different behaviors. The results show that the equilibrium contact angles are qualitatively ranked in accordance with the critical surface tension of wetting (γ _c) of the respective solid, i.e., high-γ _c solids caused the low surface tension liquids to assume the least equilibrium spreading (largest contact angle). On the other end, low-γ _c solids with the lowest surface tension liquid produce the most wetting (smallest contact angle). The results suggest that equilibrium spreading could be explained on the basis of the axiom 'like wets like'; in other words, polar surfaces tend to be wetted by polar liquids and vice versa.     

Numerical Simulation of the Adhesive Contact between a Slightly Wavy Surface and a Half-Space

In this work, the adhesive contact between a slightly wavy surface and a half-space has been investigated numerically. The surface traction was described by the Lennard–Jones potential with the Derjaguin's approximation. The deformation was first obtained by using the formula for the line contact and then, using the arc-length continuation algorithm, the relation between the contact half-width and the total force per asperity was obtained. The pull-off forces were then determined. The numerical simulation presented in this paper can be used to simulate all adhesive contacts ranging from the JKR contact to a rigid contact.     

Coating of titanium plate by photocurable azidophenyl chitosan derivative for application to implants

Titanium (Ti) is widely used as an implant material. There are various kinds of Ti surface modification methods to facilitate effective osseointegration. In this study, UV‐curable azido‐LMC (azidophenyl low molecular weight chitosan) was used to modify the Ti surface. Here we suggest a novel Ti coating material that confers complex, diverse improvements to the Ti surface through a simple process. First, a cytotoxicity test of azido‐LMC against osteoblast MG‐63 was performed. The curing ratio dependent on UV irradiation time and concentration was determined by a comparison of weight. A 5% azido‐LMC solution, which showed a wide curing ratio range, was used to investigate the surface properties. The contact angle value was measured to compare hydrophilicity, and osteoblast MG‐63 cells were cultured on the coating surface. The Bradford assay was used to assess the protein immobilization capability. Hydroxyapatite, which has a beneficial influence on osseointegration, was included on the coating surface and observed by scanning electron microscopy. Surface roughness was measured by atomic force microscopy. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Comparison of drag characteristics of self-polishing co-polymers and silicone foul release coatings: A study of wettability and surface roughness

An experimental study has been carried out to evaluate the drag characteristics of different self-polishing co-polymers (SPC) (tin based and tin-free) and a silicone foul release (FR) coating. Drag measurements have been performed on a smooth aluminum cylinder connected to a rotor device. Various coatings on cylinders were examined and differential length technique was also used to avoid the end effects during rotation. Surface energy of the coated samples was determined using static contact angle measurement. Characteristic roughness measurements of the coated surfaces were evaluated with atomic force microscopy (AFM) technique.

Drag measurements showed that the frictional resistance of the FR coated cylinders was lower than that of SPC coated samples.

Contact angle results showed that the critical surface tension and its polar component for silicone FR coating are less than SPC coatings. This prevents firm adhesion of fouling organisms on underwater hulls.

AFM studies revealed a lower surface roughness for silicone FR coating as compared to SPC coatings. Also, its surface texture is considerably different from SPC coatings.

It can be concluded that the drag characteristics of a surface are affected by its free energy and roughness parameters.     

Analysis and surface energy estimation of various model polymeric surfaces using contact angle hysteresis

Wetting of hydrophobic polymer surfaces commonly employed in electronic coatings and their interaction with surfactant-laden liquids and aqueous polymer solutions are analyzed using a contact angle hysteresis (CAH) approach developed by Chibowski and co-workers. In addition, a number of low surface tension acrylic monomer liquids, as well as common probe liquids are used to estimate solid surface energy of the coatings in order to facilitate a thorough analysis of surfactant effects in adhesion. Extensive literature data on contact angle hysteresis of surfactant-laden liquids on polymeric surfaces are available and are used here to estimate solid surface energy for further understanding and comparisons with the present experimental data. In certain cases, adhesion tension plots are utilized to interpret wetting of surfaces by surfactant and polymer solutions. Wetting of an ultra-hydrophobic surface with surfactant-laden liquids is also analyzed using the contact angle hysteresis method. Finally, a detailed analysis of the effect of probe liquid molecular structure on contact angle hysteresis is given using the detailed experiments of Timmons and Zisman on a hydrophobic self-assembled monolayer (SAM) surface. Hydrophobic surfaces used in the present experiments include an acetal resin [poly(oxymethylene), POM] surface, and silane, siloxane and fluoro-acrylic coatings. Model surfaces relevant to the literature data include paraffin wax, poly(methyl methacrylate) and a nano-textured surface. Based on the results, it is suggested that for practical coating applications in which surfactant-laden and acrylic formulations are considered, a preliminary evaluation and analysis of solid surface energy can be made using surfactant-laden probe liquids to tailor and ascertain the quality of the final coating.     

铜表面物理化学有序度对蒸汽冷凝特性的影响

研究了经不同粒度砂纸研磨后不同粗糙度纯铜表面的润湿性和蒸汽冷凝性能,分析了表面物理粗糙度和氧化引起的表面化学成分异质性对润湿性和冷凝传热性能的影响规律。结果表明,当铜表面粗糙度小于0.15 μm,增大粗糙度有利于增大接触角,而当粗糙度增大到0.15 μm后,继续增大粗糙度反而会减小其表面接触角。铜表面氧化后造成的化学成分异质性有利于增大接触角。表面微观结构和化学成分异质性的共同作用决定了铜表面的润湿性与蒸汽在其表面的冷凝模式。当粗糙度为0.15 μm左右时,氧化后铜表面表现出最佳的滴状冷凝状态,其冷凝传热系数达到Nusselt理论模型的3.5倍左右。但是表面粗糙结构同时也会增大液滴在金属表面的运动阻力,对冷凝传热性能产生不利影响。