The effect of polymer surface on the wetting and adhesion of liquid systems

Young's equation describes the wetting phenomenon in terms of the contact angle between a liquid and a solid surface. However, the contact angle is not the only parameter that defines liquid–solid interactions, an additional parameter related to the adhesion between the liquid drop and the solid surface is also of importance in cases where liquid sliding is involved. It is postulated that wetting which is related to the contact angle, and interfacial adhesion, which is related to the sliding angle, are interdependent phenomena and have to be considered simultaneously. A variety of models that relate the sliding angle to the forces developed along the contact periphery between a liquid drop and a solid surface have been proposed in the literature. Here, a modified model is proposed that quantifies the drop-sliding phenomenon, based also on the interfacial adhesion that develops across the contact area of the liquid/solid interface. Consequently, an interfacial adhesion strength parameter can be defined depending on the mass of the drop, the contact angle and the sliding angle. To verify the proposed approach the adhesion strength parameter has been calculated, based on experimental results, for a number of polymer surfaces and has been correlated with their composition and structure. The interaction strength parameter can be calculated for any smooth surface from measurements of the contact and the sliding 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.     

Ice adhesion to rubber materials

The aim of this study was to investigate the interfacial shear strength between ice and rubbers. Different rubber materials containing only a polymer and curing agent (peroxide) were tested with regard to surface wettability and interfacial shear strength. The effect of different grades and amounts of carbon black filler was also studied. The wettability was determined from contact angles, using water and diiodomethane as test liquids, measured on carefully cleaned and mirror smooth rubber sheets. The test showed that there is a correlation between ice adhesion and rubber substrate wettability. Below a water contact angle of 90°, the interfacial shear strength of ice decreases linearly with increasing contact angle. For contact angles above 90°, the interfacial shear strength of ice stays practically the same. The presence of high surface energy additives such as reinforcing carbon black (e.g. N220 ISAF) significantly increases the interfacial shear strength. The highly hydrophobic behaviour of different plant surface textures was also investigated regarding ice adhesion strength. The combination of a submicrometer textured surface and a hydrophobic surface characteristic showed an abrupt decrease in the adhesion force of a water droplet at measured macroscopic contact angles above approximately 150°. Despite this water repellency, the ice adhesion strength is not nil. However, it was among the lowest values experienced in the test.     

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°.     

Development of a model for thin-film stability and spreading in solid–liquid–liquid systems

The presence of thin aqueous films and their stability has a profound effect on reservoir rock–fluids interactions involved in spreading and adhesion. The stability of thin wetting aqueous films on rock surfaces is governed by several variables including pH, brine and crude oil compositions, and capillary pressure. These variables govern the wetting states in the solid–liquid–liquid systems. The wetting states influence the residual oil saturation and the oil-water relative permeabilities and, consequently, the oil recovery. The objective of this study was to deduce a functional dependence of thin-film stability on the above parameters by considering intermolecular and surface interactions in rock–crude oil–brine systems. The surface forces are manifested as disjoining pressure in thin films. The disjoining pressure isotherms for the selected solid–liquid–liquid systems have been computed in terms of the bulk properties of the media. The equilibrium contact angles have also been computed from the integration of the Young–Laplace equation, which relates contact angle to the capillary pressure and disjoining pressure isotherm of the system. The contact-angle data obtained from sessile-drop experiments have been compared with the calculated results, as well as with other published results. Adhesion maps, which relate the film stability to brine pH and molarity, have been developed. The rock–fluids systems considered for this study consisted of smooth glass, quartz and Yates reservoir fluids. The DLVO theory has been used to model the intermolecular forces. The structural forces are incorporated to overcome the limitations of the DLVO theory. A charge regulation model has been used to analyze the crude oil–brine and glass–brine interfaces. The effects of multivalent ions have been incorporated using an equivalent molarity concept. The overall computational model developed in this study is aimed at providing a priori prediction capability of rock-fluids interactions in petroleum reservoirs for inclusion in reservoir simulators.     

Capillary and van der Waals force between microparticles with different sizes in humid air

It is well known that surface effect forces, such as van der Waals force and capillary force, are the major contributions to adhesion when microsized particles are in contact in humid environment. But it is very complex to calculate the adhesion force between two smooth unequal particles. In conventional approaches, the effective particle radius approximation and the constant half-filling angle assumptions are often used for computing the van der Waals forces between two microparticles. However, the approximation and the assumption are actually difficult to accurately model the forces between unequal particle sizes when the surfaces are with different properties. In this paper, we present a theoretical study of the van der Waals force and capillary force between two microparticles with different radii and the surface properties linked by a liquid bridge. The proposed model provides the adhesion force predictions in good agreement with the previous formula and existing experiment data. Considering the solid particles are partially wetted by the liquid bridge, the van der Waals force is calculated by divided the particle surface into a wetted part and a dry portion in our stimulation. Since the wetted surface portion of the particle is determined by the half-filling angle, the relationship between two half-filling angles of the unequal size particles is developed from the geometrical consideration, which is relate to the size ratio of the particles, the contact angle, and the separation distance. Then, the van der Waals force is determined using the surface element integration. Moreover, the influences of humidity, particles size, contact angle, and separation distance toward the adhesion forces are discussed using the proposed method. Simulations indicate that a higher relative humidity leads to bigger liquid bridges, suggesting a higher capillary force, but at the same time, the van der Waals force decreases due to the decrease in surfaces energy. As for the influence of contact angle, results show that a higher contact angle, that is, a more hydrophobic surface, reduces the capillary force but increases the van der Waals force (absolute value). The simulations also show that the both the capillary force and the van der Waals force (absolute value) increase as the particle size increases. When the particles are separated from each other, the capillary force and van der Waals force decreases gradually. These results are helpful to understand and utilize the adhesion interaction between particles with unequal sizes at the ambient condition.     

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.     

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.     

Determination of dynamic contact angles in solid–liquid–liquid systems using the Wilhelmy plate apparatus

The purpose of this work was to carry out a systematic study of the effects of brine composition and rock mineralogy on rock-oil-brine interactions taking place in petroleum reservoirs. These terms are generally lumped into a single term called wettability in petroleum engineering. The extent of wetting of the rock surface by water or oil depends on the dynamic contact angles measured in such a mode as to enable movements of the three-phase contact line. The Wilhelmy plate technique has been used in this study to measure adhesion tension (which is the product of interfacial tension and cosine of the contact angle) at the solid-liquid interface. The water-advancing and water-receding contact angles have been calculated from the adhesion tensions by making independent measurements of the liquid-liquid interfacial tensions using a du Noüy ring tensiometer. The water-advancing and receding angles have been measured in this study for pure hydrocarbons against synthetic brines of different concentrations. Polished surfaces of glass slides and dolomite have been used to simulate the reservoir rock surfaces. A nonionic surfactant (ethoxy alcohol), which is being used in Yates reservoir in West Texas for enhancing oil recovery, was used to quantify its wettability effects. The results of the systematic experimental investigation of the effects of practical variables on wettability are presented. It is found that interactions between surface-active agents at the interface of two liquids have an effect on wettability alteration. The composition and concentrations of different organic and inorganic chemical species have a major effect in making a reservoir oil-wet or water-wet.     

Water Contact Angles on Poly(ethylene terephthalate) Film Exposed to Atmospheric Pressure Plasma

The poly(ethylene terephthalate), PET, film was exposed to atmospheric pressure plasma under various plasma processing parameters. The wettability of the PET film immediately after the exposure and after storage in air, which was determined by the sessile drop method, was strongly dependent on the plasma processing parameters. The contact angle hysteresis on the plasma-exposed PET film was examined by the Wilhelmy method. It was found that the hydrophobic recovery of the PET surface on storage after the plasma exposure was observed only for the advancing contact angle and that the receding angle remained almost the same. These experimental findings were explained on the basis of the calculation by Johnson and Dettre for the advancing and receding contact angles on model heterogeneous surfaces.     

Transparent ultra-hydrophobic surfaces

Self-cleaning surfaces have received a great deal of attention, both in research studies and commercial applications. Both transparent and non-transparent self-cleaning surfaces are highly desired, as they offer many advantages and their potential applications are endless. As in many other cases, also in the case of self-cleaning surfaces, nature found a solution before man. The Lotus flower is a symbol of purity in Asian cultures, even when rising from muddy waters it stays clean and untouched by dirt, organisms and pollutants. The Lotus leaf "self-cleaning" surface is hydrophobic and rough, showing a two-layer morphology. While hydrophobicity produces a high contact angle, the two-layer morphology reduces the adhesion of dirt and water drops to the surface. Because of this low adhesion, water drops easily slide across the leaf surface carrying the dirt particles with them. In the present work the Lotus leaf morphology was mimicked using hydrophobic chemistry and a two-layer topography, with a base layer of silica and a top layer of intrinsically nanostructured polyhedral oligomeric silsesquioxanes (POSS) particles. Results have indicated that, thus, a transparent ultra-hydrophobic coating can be obtained. When these materials were mixed and used as a single layer the hydrophobicity deceased significantly. The contact angle and sliding angle measurements were supported by AFM micrographs.     

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.     

Anisotropic Wetting of Liquids on Finely Grooved Surfaces

A photolithographically-prepared, parallel-grooved surface on silica has been employed as a model to study the influence of roughness on the spreading equilibrium of liquid drops. The equations generated by Oliver, Huh and Mason for cylindrically shaped drops were extended to account for wetting by liquid crystals. The observed drop shapes were dependent upon surface roughness. The equilibrium contact angles on a smooth surface can be calculated from the roughness, contact angles both parallel and perpendicular to the grooves, and the drop shape. Reasonably good agreement with experimental contact angles was obtained.     

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.     

A method for estimating interfacial tension of liquid crystal embedded in polymer matrix forming PDLC

A simple and convenient method based on sessile drop technique for measuring surface tensions of polymer and nematic liquid crystal (LC) is described. Contact angles formed by drops of probe liquids and a nematic LC on a photocurable polymer were measured. The surface energies were evaluated using the Fowkes method, Neumann's equation, and new equations developed based on Neumann's approach. The values of surface tensions were used to evaluate the interfacial interaction in term of work of adhesion between the LC and polymer. Further, the effect of dichroic dye on the extent of interaction and work of adhesion was examined by measuring contact angle in consequence of dye addition. A difference in work of adhesion between the lower and higher dye‐doped LC droplets gave an indication of affinity relationship between polymer and LC molecules. A change in work of adhesion resulted in variability of nematic director configurations inside phase separated LC droplets embedded in polymer matrix; when viewed under polarizing optical microscope. Thus, our approach of estimating surface energy of polymer and LC has found to be useful in determining interaction at polymer–LC interface. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41137.     

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.     

Adhesion of solid particles to gas bubbles. Part 2: Experimental

In slurry bubble columns, the adhesion of solid catalyst particles to bubbles may significantly affect the G-L mass transfer and bubble size distribution. This feature may be exploited in design by modifying the hydrophilic or hydrophobic nature of the particles used. Previously we have proposed a generalised model, describing the adhesion of particles to G-L interface under stagnant conditions. In this work, we studied the adhesion of particles characterised by different degree of hydrophobicity and porosity: non-porous polystyrene and glass beads, unmodified and hydrophobised mesoporous silica, and activated carbon particles. Images recorded at high optical magnification show the particles adhering to gas bubbles individually or as aggregates. In aqueous media, higher liquid surface tension and particle surface hydrophobicity increase the adhesion strength and the tendency of particles to agglomerate, in agreement with the model. The adhesion of non-porous rough-surface particles to gas bubbles can be characterised by the receding contact angle. The advancing contact angle represents better the adhesion of the same particles to liquid droplets. We found that the “effective” contact angle of porous particles is much lower than an “intrinsic” contact angle calculated from the heat of immersion in water, or measured by sessile drop method. An equivalent contact angle derived from the Cassie rule explains the wetting behaviour of particles having the pores filled with liquid.     

Hydrodynamics and mechanism of hydrophobic foam column tray: Contact angle hysteresis effect

Interfacial wettability adjustment is a new method for intensifying vapor–liquid mass transfer process. Contact angle effect has been well investigated but not complete due to interfacial wettability consisting of both static behavior (contact angle) and dynamic behavior (contact angle hysteresis). Here, methods of adjusting contact angle hysteresis (CAH) were proposed, and then, the CAH effect on the hydrodynamics was investigated. A multiscale analysis of CAH effect, from interfacial force and wettability to single-bubble and gas–liquid two-phase flow inside the foam to bubble swarm hydrodynamics, was conducted, and thus, the hydrodynamic performance criteria were derived. The interfaces had similar contact angles, whereas a significant difference in the CAH was prepared by using the developed sol dip-coating and spray coating methods. Subsequent experiments revealed that lower CAH can decrease the pressure drop, homogenize the gas distribution, and increase the weeping rate, which are consistent with the derived criteria.     

Wetting behavior of thermally bonded polyester nonwoven fabrics: The importance of porosity

The wetting properties of thermally bonded polyester nonwoven fabrics with different basis weights were studied. These nonwovens had the same composition: 85% poly(ethylene terephthalate) and 15% poly(butylene terephthalate) fibers. Two techniques, the 3S wicking test and sessile drop method, yielded similar water contact angles for all the nonwovens, but these results differed from the values obtained with the single fibers. In the nonwoven fabrics, the pore structure played a dominant role in the wetting properties: the existence of large pores in the thinner nonwovens reduced the dimensions of the liquid–solid interfacial perimeter. Compared with the water contact angle of the constituent single fibers, the contact angle of the fabrics was increased. A crenellated surface model was created to quantify the influence of pores on the wettability of nonwovens. It was possible to deduce the surface porosity of the fabric with this model, but only in the case of contact with nonwetting liquids such as water: this surface porosity corresponded only to the outermost layers of the fabric structure. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 387–394, 2006     

Interfacial phenomena in Al2O3—liquid metal and Al2O3—liquid alloy systems

The sessile drop technique has been used to measure the contact angle of liquid metals and liquid alloys in contact with polycrystalline alumina. The experiments were carried out in argon atmosphere at various temperatures. The measured contact angles exhibit no wettability (θ > 90°). The linear temperature functions of the work of adhesion as well as of the interfacial energy were also determined in the investigated systems. The partial high values of the work of adhesion in several Al₂O₃-liquid metal systems can be attributed to a chemical bond establishment at the interface. The values of the interfacial energies at the melting point of the metals, for non-reactive Al₂O₃-liquid metal systems, vary in a restricted region (2·35–2·75) J m⁻²). An empirical relation is proposed for evaluation of the interfacial energy of the metals at their melting point. The agreement between experimental and calculated values is satisfactory.