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.     

Wettability and surface free energy of corona-treated biaxially-oriented polypropylene film

Several methods for the determination of both the surface free energy of polymer materials and the conditions necessary to perform contact angle measurements are discussed. The effects of the corona-treatment energy on the surface free energy and on the adhesion of acrylic adhesive were studied using a biaxially-oriented polypropylene film. The surface free energy was determined by the Owens-Wendt, and van Oss-Chaudhury-Good approaches, as well as with the wettability method, using different liquids. The presented results confirm that the surface free energy value depended on both the method used and the nature of probe liquids. Thus, it cannot be considered as a parameter characterizing unambiguously the surface layer of a corona-treated film. The values of the surface free energy for different film samples can be compared with one another only if determined using the same method and the same liquids. The variations of particular components of the surface free energy with the corona-treatment energy depend on e.g. the nature of probe liquids, which makes interpretation of the observed effects difficult.     

Molecular mechanisms of hydrophobic transitions

This review article provides new insights into the molecular mechanisms of hydrophobicity that emerge within the phenomenological framework of classical capillarity. The thermodynamic analysis of surface phenomena via the Gibbs adsorption equation that forms the basis of the experimental physical chemistry of liquid-fluid interfaces extends with its rigorous implications towards the wetting of solid surfaces. Observations on equilibrium contact angles, contact angle hysteresis, and the dynamics of wetting hitherto based on qualitative results that received fragmentary and eclectic interpretations are accounted for in terms of the values of adsorption pressures that act at the three-phase line. Hydrophobic surfaces and hydrophobic transitions induced by reversible solute adsorption qualify as two distinct limiting cases in this thermodynamic context. The equilibrium and kinetic aspects of the wetting of uniform and chemically heterogeneous surfaces by pure liquids and surfactant solutions is discussed with reference to reversible liquid-fluid interfaces. The importance of the solvent and solute adsorption at the solid-vapor interface for proper assessment of static and dynamic dewetting transitions is particularly addressed.     

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.     

Contact Angle Hysteresis on Polymer Surfaces: An Experimental Study

In order to characterize a solid surface, the commonly used approach is to measure the advancing and receding contact angles, i.e., the contact angle hysteresis. However, often an estimate of the average wettability of the solid–liquid system is required, which involves both the dry and wetted states of the surface. In this work, we measured advancing and receding contact angles on six polymer surfaces (polystyrene, poly(ethylene terephthalate), poly(methyl methacrylate), polycarbonate, unplasticized poly(vinyl chloride), and poly(tetrafluoroethylene)) with water, ethylene glycol and formamide using the sessile drop and captive bubble methods. We observed a general disagreement between these two methods in the advancing and receding contact angles values and the average contact angle determined separately by each method, although the contact angle hysteresis range mostly agreed. Surface mobility, swelling or liquid penetration might explain this behaviour. However, we found that the 'cross' averages of the advancing and receding angles coincided. This finding suggests that the cross-averaged angle might be a meaningful contact angle for polymer–liquid systems. Hence, we recommend using both the sessile drop and captive bubble methods.     

The combined effect of roughness and heterogeneity on contact angles: the case of polymer coating for stone protection

The individual effects of heterogeneity and roughness on contact angles have been repeatedly analysed in the literature, but the application of the accepted models to practical situations is often not correctly performed. In the present paper the combined effects of roughness and heterogeneity on the contact angles of water on stone surfaces protected by a hydrophobic polymer coating are considered. Two different kinds of calcareous stone with different surface roughnesses and porosities were protected against the effect of water absorption by two different polymer coatings. The contact angles of water on the protected stone surfaces were measured by the Wilhelmy and the sessile drop techniques. A comparison of the results obtained shows not only the limits of the static sessile drop technique, but also the combined effect of roughness and heterogeneity. Some considerations are developed on the application of commonly accepted models to surfaces with a combination of roughness and heterogeneity. Some other results obtained with techniques such as roughness measurements, mercury porosimetry, energy dispersive X-ray spectroscopy (EDXS), thermogravimetric analysis (TGA), water absorption by capillarity experiments (WAC), all able to show the structure and properties of the obtained films, are also compared with those obtained from contact angle measurements. It is concluded that the static contact angle is not well correlated with the degree of protection; on the contrary, the receding contact angles are well correlated with the degree of protection actually obtained. An ideal protecting agent should have a receding contact angle greater than 90°.     

Control over wettability of textured surfaces by electrospray deposition

Microtextured surfaces were prepared by electrospray deposition (ESD) from hydrophilic and hydrophobic acrylic resin solutions. The surface morphologies and topologies were characterized using scanning electron microscopy and laser profile microscopy, respectively. Wetting behaviors on the surfaces were characterized by contact angle and sliding angle measurements. The contact angle of the water droplet on the hydrophilic resin‐coated surfaces decreased with an increase in the surface roughness. On the other hand, the contact angle on the hydrophobic resin‐coated surfaces increased with an increase in the surface roughness. In addition, a patterned surface composed of aligned fibers by ESD showed anisotropy of both wetting and sliding behaviors. These results indicate that ESD is a useful method for designing a textured surfaces and controlling the surface wettability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3811–3817, 2007     

Mussel-inspired catechol-based chemistry for direct construction of super-hydrophilic and waterproof coatings on intrinsic hydrophobic surfaces

Super-hydrophilic coatings are useful in many applications such as agricultural greenhouses. However, the direct modification of super-hydrophilic coatings on intrinsic hydrophobic surfaces is still a challenge, particularly without any pretreatment. Here, highly transparent super-hydrophilic and waterproof coatings on intrinsic hydrophobic surfaces were prepared via layer-by-layer (LbL) assembly catechol-grafted polymer-branched poly(ethylenimine) (bPEI)-3-(3,4-dihydroxyphenyl)propionic acid (denoted as C) and poly(acrylic acid) (PAA)-dopamine (denoted as D) inspired by the mussel. The catechol-functionalized polymer facilitates a mechanically robust coating that is tightly attached to the surface of the intrinsic hydrophobic polymers. This gives the coating excellent antifogging ability with a lowest contact angle of 0°. These coatings also demonstrated excellent stability after cross-linking with Fe³⁺ or alkali species. The super-hydrophilic and waterproof coatings on intrinsic hydrophobic surfaces can also be prepared by the spray method. The super hydrophilic coating exhibits favorable antifogging ability, making it potentially useful in numerous applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48013.     

A new model to determine contact angles on swelling polymer particles by the column wicking method

The contact angle determination on swelling polymer particles by the Washburn equation using column wicking measurements may be problematic because swelling occurs during the wicking process. The objective of this research was to develop a new model to more accurately determine contact angles for polymer particles that undergo solvent swelling during the column wicking process. Two phenomena were observed related to the swelling effect during the wicking process: (1) a temperature rise was detected during the wicking process when the swelling polymer particles interacted with polar liquids, and (2) a smaller average capillary radius (r) was obtained when using methanol (polar liquid) compared to using hexane (non-polar liquid). The particle swelling will induce both particle geometry changes and energy loss which will influence the capillary rise rate. The model developed in this study considered the average pore radius change and the energy loss due to the polymer swelling effect. Contact angle comparisons were conducted on wood with formamide, ethylene glycol, and water as test liquids, determined by both the new model and the Washburn equation. It was shown that the contact angles determined by the new model were about 4-37° lower than those determined by the Washburn equation for water, formamide, and ethylene glycol. Todetermine whether the polymer particles are swelling, two low surface tension liquids, one polar (methanol) and the other non-polar (hexane), can be used to determine the average pore radius (r values) using the Washburn equation. If the same r values are obtained for the two liquids, no swelling occurs, and the Washburn equation can be used for the contact angle calculation. Otherwise, the model established in this study should be used for contact angle determination.     

The effects of nanostructure and composition on the hydrophobic properties of solid surfaces

The effects of nanoroughness and chemical composition on the contact and sliding angles on hydrophobic surfaces were studied theoretically and experimentally. A theoretical model based on forces developed at the contact area between a liquid drop and hydrophobic smooth or nanoroughened surface was developed and compared with the existing models, which are based on forces developed at the periphery between the drop and the solid surface. The contact area based model gives rise to an interfacial adhesion strength parameter that better describes the drop-sliding phenomenon. Consequently, relationships were derived describing the dependence between the interfacial adhesion strength of the liquid drop to the surface of a given composition, the mass of the drop, the measured contact angles and the sliding angle. For a given surface chemistry, the sliding angle on a nanometric roughened surface can be predicted based on measurements of contact angles and the sliding angle on the respective smooth surface. Various hydrophobic coatings having different surface nanoroughnesses were prepared and, subsequently, contact angles and sliding angles on them as a function of drop volume were measured. The validity of the proposed model was investigated and compared with the existing models and the proposed model demonstrated good agreement with experimental results.     

On the surface properties of waterborne fluorinated coating polymers

In the present work the results of a study concerning the surface properties of coating materials obtained from fluorinated latices are reported. Better understanding is required regarding the influence of polymer structure, latex composition, and morphology on the surface properties of the resulting films. For this purpose, two commercial fluorinated waterborne polymer dispersions were compared with a model fluorinated acrylic latex purposely synthesized. The surface properties of mold samples, prepared from either the whole solid matter contained in the latices (dried latex) or the purified polymer, were determined by simple contact angle measurements. Two series of homologous liquids, namely H‐bonding alcohols and apolar hydrocarbons, were employed for determining the critical surface tension γ_c according to the method of Zisman. The results of the surface characterization indicate that the degree of fluorination plays a minor role in these materials, suggesting that the threshold above which the polymer surface is virtually saturated by CF₂ or CF₃ groups might have been largely exceeded. On the other hand, the effectiveness of the hydrophobic fluorinated coating resulted substantially unaffected by the presence of amphiphilic low molecular weight additives, such as surfactants and wetting agents, which can actually contribute to the reduction of the surface energy of these materials upon suitable thermal treatment.     

Influence of the coating method on the formation of superhydrophobic silicone–urea surfaces modified with fumed silica nanoparticles

Effect of the coating method on the formation of superhydrophobic polydimethylsiloxane–urea copolymer (TPSC) surfaces, modified by the incorporation of hydrophobic fumed silica nanoparticles was investigated. Four different coating methods employed were: (i) layer-by-layer spin-coating of hydrophobic fumed silica dispersed in an organic solvent onto TPSC films, (ii) spin-coating of silica–polymer mixture onto a glass substrate, (iii) spray coating of silica/polymer mixture by an air-brush onto a glass substrate, and (iv) direct coating of silica–polymer mixture by a doctor blade onto a glass substrate. Influence of the coating method, composition of the polymer/silica mixture and the number of silica layers applied on the topography and wetting behavior of the surfaces were determined. Surfaces obtained were characterized by scanning electron microscopy (SEM), white light interferometry (WLI) and advancing and receding water contact angle measurements. It was demonstrated that superhydrophobic surfaces could be obtained by all methods. Surfaces obtained displayed hierarchical micro-nano structures and superhydrophobic behavior with static and advancing water contact angles well above 150° and fairly low contact angle hysteresis values.     

蒸汽冷凝型态的表面自由能差判据

提出了蒸汽在固体表面上冷凝方式的表面自由能差判据 ,即冷凝温度下液体表面自由能与固体表面的表面自由能差大于 33.3mJ·m^-2 时蒸汽在该表面上呈现滴状冷凝的必要条件 .当表面自由能差在 0与 33.3mJ·m^-2范围内 ,表面强化冷凝传热的效果将取决于表面自由能差值的大小 ,差值越大 ,强化效果越明显 .这对深化表面涂层强化冷凝传热的机理以及选择传热表面涂层材料具有指导意义 .通过与文献报道实验结果的比较证实 ,该判据排除了静态接触角判据中测量温度的影响 .     

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.     

Relation between the cleanability of bare or polysiloxane-coated stainless steels and their water contact angle hysteresis

Cleaning of bare or coated stainless steel surfaces is investigated using some specific techniques for both particulate soil and oil removal. Particulate soil is removed from the surface by a water drop sliding, whereas oil is eliminated by shear flow of a commercial detergent. The cleanability performance is found to depend both on surface energy and topography. In general, the water contact angle hysteresis, which itself is related to the advancing contact angle and the surface roughness, is found to be an appropriate criterion for characterizing the cleaning performance. This finding is discussed in terms of retention and removal forces during the cleaning process and could provide in the future a criterion for material selection for industrial use of stainless steel surfaces.     

Time‐dependent changes of surface properties of polyether ether ketone caused by air plasma treatment

The effect of air plasma treatment on wetting and energy properties, surface composition and morphology of polyether ether ketone (PEEK) was investigated. The influence of the storage time on the surface properties of plasma‐treated polymer plate was also examined. The properties were determined by advancing and receding contact angle measurements, Fourier transform infrared spectroscopy supported by theoretical spectrum modelling, X‐ray photoelectron spectroscopy and optical profilometry. Three theoretical approaches were used in the determination of the apparent surface free energy of the untreated and plasma‐treated PEEK samples from the measured contact angles of probe liquids (water, formamide, diiodomethane): the contact angle hysteresis method, the Owens and Wendt approach and the Lifsthitz ? van der Waals acid–base approach. It was found that air plasma treatment of PEEK causes significant chemical and morphological changes of the polymer surface, which are reflected in the decrease of contact angles from 83.4° to 11.7° for water after 180 s plasma treatment. This is due to the formation of polar functional groups resulting in the increase of the surface hydrophilicity. After plasma treatment the apolar component of the surface free energy practically does not change, while the polar component increases significantly, especially for plates treated for 180 s, from 0 to 19.6 mJ m^?2. In addition, the modified PEEK surface is not stable during storage and it acquires more hydrophobic character. © 2016 Society of Chemical Industry     

Coatings to prevent frost

The ability of hydrophobic, organic–inorganic hybrid coatings to decelerate frost propagation was investigated. Compared to a bare aluminum surface, the coatings do not significantly reduce the freezing probability of supercooled water drops. On both surfaces, the probability for ice nucleation at temperatures just below 0°C, for example at ?4°C, is low. Freezing of a single drop on aluminum leads, however, to instant freezing of the complete surface. On hydrophobic coatings, such a freezing drop is isolated; the frozen area grows slowly. At ?4°C surface temperature in a +12°C/90% relative humidity environment, on surfaces providing a water contact angle hysteresis of about 10° and receding water contact angles higher than 90°, a rate for the growth of the average radius of the frozen area of about 2 µm/s was observed. Submitting the surface to an airflow of 1 m/s led to faster frost spreading in flow direction. Although the airflow compromised the anti-ice properties to some extent, the application of the hydrophobic coating in a heat recovery ventilation experiment extended the time interval between defrosting cycles by a factor of 2.3.