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1162. Ekevall, E., J.I.B. Wilson, and R.R. Mather, “The effect of ammonia and sulphur dioxide gas plasma treatments on polymer surfaces,” in Medical Textiles and Biomaterials for Healthcare, S.C. Anand, J.F. Kennedy, M. Miraftab, and S. Rajendran, eds., 491-498, Woodhead Publishing, Dec 2005.

Gas discharge plasma treatment can be used to modify the surface properties of biomaterials for a variety of biomedical applications. An established application is the use of oxygen and nitrogen plasmas to improve the hydrophilicity of surfaces, encouraging cell attachment and subsequent growth. The physical properties and surface chemistry of the biomaterial influences cell attachment and subsequent culture. In-situ cells are surrounded by a complex extracellular matrix (ECM) containing fibronectin, laminin, collagen types I-V, and proteoglycans. In this study, ammonia and sulphur dioxide gases have been chosen with the objective of incorporating carboxylic acid, sulphur and nitrogen containing groups on the surface.

1370. El-Bahy, M.M., and M.A.A. El-Ata, “Onset voltage of negative corona on dielectric-coated electrodes in air,” J. Physics D: Applied Physics, 38, 3403-3411, (Sep 2005).

This paper describes theoretical and experimental investigations of the effect of an electrode coating on the onset voltage of a corona on negatively stressed electrodes. Dielectric-coated hemispherically-capped rod-to-plane gaps positioned in air are investigated. The onset voltage is calculated based on the self-recurring single electron avalanche developed in the investigated gap. Accurate calculation of the electric field in the vicinity of a coated rod and its correlation to the field values near a bare rod of the same radius are obtained using the charge simulation method. The calculated field values are utilized in evaluating the onset voltage of the corona. Also, laboratory measurements of the onset voltage on bare and coated electrodes are carried out. The effects of varying the field nonuniformity, the coating thickness and its permittivity on the onset voltage values are investigated. The results show that coating the electrodes with a dielectric material is effective in increasing the onset voltage of the corona on its surface. The calculated onset voltage values for coated and bare electrodes agree satisfactorily with those measured experimentally.

1791. El-shimi, A., and E.D. Goddard, “Wettability of some low energy surfaces I: Air/liquid/solid interface,” J. Colloid and Interface Science, 48, 242-248, (Aug 1974).

The wettability of a number of low energy solid surfaces, including hoof keratin and human skin, has been examined using two liquids, water and methylene iodide, and employing Wu's empirical approach to obtain γsd and γsP, the dispersion and polar components of the solid “surface tension.” The sum of these parameters, (γsd + γsp) was found to be in good agreement with reported values of γc, the critical surface tension, based on Zisman plots. Using the latter method, γc values of solids selected from the above group were determined using aqueous ethanol solutions. The values were lower than those obtained using nonpolar liquids, thus confirming earlier findings. A compilation of our own data and data from the literature reveals that the derived values of γc show little or no dependence on the type of solid surface, the type of alcohol or its chain length. The results can be explained in terms of adsorption of alcohol at the surface of the solid.

2332. Elliott, G.E.P., T.A. Elliott, S.M. Rowan, and I.D. Severn, “The influence of the surface coating on the wettability of nylon 6 fibres,” in Wetting, Spreading and Adhesion, J.F. Padday, ed., 391-402, Academic Press, 1978.

1917. Ellison, A.H., H.W. Fox, and W.A. Zisman, “Wetting of fluorinated solids by hydrogen-bonding liquids,” J. Physical Chemistry, 57, 622-627, (1953).

1788. Ellison, A.H., and W.A. Zisman, “Wettability studies of nylon, polyethylene terephthalate and polystyrene,” J. Physical Chemistry, 58, 503-506, (1954).

1790. Ellison, A.H., and W.A. Zisman, “Wettability of halogenated organic solid surfaces,” J. Physical Chemistry, 58, 260-265, (1954).

633. Ellul, M.D., and D.R. Hazleton, “Chemical surface treatments of natural rubber and EDPM thermoplastic elastomers: effects on friction and adhesion,” Rubber and Chemical Technology, 67, 582-601, (Sep 1994).

Natural rubber thermoplastic elastomers (NRTPEs) made by dynamic vulcanization of natural rubber during its mixing with polypropylene were subjected to various halogenation surface treatments. Marked reduction in the coefficient of friction is possible depending on the chemical treatment employed, TPE composition and the presence of a lubricant. As a result of halogenation there is an increase in the microroughness and hardness of the NRTPE surface. These effects in part explain the large decrease in the friction coefficients since the contact area is decreased. Thus NRTPE can be employed in applications requiring low friction, such as certain types of seals. Another consequence of halogenation of NRTPE is the increase in its surface energy which in turn promotes adhesion to various polar substrates. Indeed it was determined that halogenation of NETPE is an effective way of priming the surface of these materials for adhesion to acrylic and other substrates. Ethylene Propylene Diene Monomer rubber-Polypropylene thermoplastic elastomers (EPTPEs) were used as a control in this study to assess how a low unsaturation EPDM-based TPE compares with the high unsaturation NRTPEs in different halogenation surface treatments.

1907. Elsner, C., M. Lenk, L. Prager, and R. Mehnert, “Windowless argon excimer source for surface modification,” Applied Surface Science, 252, 3616-3624, (Mar 2006).

The article describes applications of a novel windowless argon excimer source for surface modification. Experimental results on etching of polymeric surfaces, degradation of organic surface residues, surface activation and modification of gas permeability and selectivity of polymeric membranes are presented. Moreover, radical formation from the excimer source and surface curing of liquid acrylates are examined. Typical treatment times are in the range of minutes for photolytic decomposition effects and seconds for UV curing effects. The surface modification effects induced by the argon excimer source were analysed by XPS, ESR, IR-spectroscopy, white light reflection spectroscopy, scanning electron microscopy, micro-hardness and permeation measurements.

456. Elwes, E.H., and C. Delahaye, “Adhesion problems associated with coating polypropylene,” Polymer Paint Colour Journal, 181, 151-152, (Mar 1991).

2538. Encinas, N., B. Diaz-Benito, J. Abenojar, and M.A. Martinez, “Extreme durability of wettability changes on polyolefin surface by atmospheric pressure plasma torch,” Surface and Coatings Technology, 205, 396-402, (Oct 2010).

In the present work three common polyolefins: high density polyethylene (HDPE), low density polyethylene (LDPE) and polypropylene (PP) have been treated with an atmospheric pressure air plasma torch (APPT) in order to improve their wettability properties. The variations in surface energy (γs), as well as the durability of the treatment are determined by means of contact angle measurements for different aging times after plasma exposure (up to 270 days) using five test liquids which cover a wide range of polarities. The introduction of new polar moieties (carbonyl, amine or hydroxyl) is confirmed by Fourier transform infrared spectroscopy in attenuated total multiple reflection mode (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). Furthermore, scanning electron microscopy (SEM) provides information on the morphological changes and variation on surface roughness, revealing that smoother, lamellar and semispheric micrometric structures are created on the LDPE, HDPE and PP surfaces, respectively. Results show that APPT treatment enhances both the total and polar components of the γs under study, with an unprecedent stability (> 8 months) in time.

2253. Encinas, N., M. Pantoja, J. Abenojar, and M.A. Martinez, “Control of wettability of polymers by surface roughness modification,” J. Adhesion Science and Technology, 24, 1869-1883, (2010).

Most polymeric materials, particularly polyolefins and their derivatives, present a low surface energy which is the cause of their poor wettability and limits processes such as adhesive bonding, painting, or metalizing. Many methods have been developed and used to modify polymer surfaces for improved wetting, including mechanical treatments, wet-chemical treatments with strong acids or bases, and exposure to flames or corona discharge. In this paper the improvement of wetting properties of several polymeric materials widely used in the automotive industry, such as high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP) and silicone, is studied by means of surface mechanical abrasion using sandpapers of different grain sizes (1000, 180 and 80). Measurements of the surface roughness are performed using a Hommel Tester T8000 device equipped with a diamond stylus, which provides data on the arithmetic average roughness Ra parameter and Abbott–Firestone curve. Variations in the polymers surface energy (SE) are estimated through contact angle measurements using five test liquids of different polarities. Both components of the SE, dispersion (σD) and polar (σP), as well as total (σT) at different conditions of treatment are analyzed using the Owens–Wendt–Rabel–Kaelble (OWRK) method. Morphological changes induced in the surface are analyzed by Scanning Electron Microscopy (SEM). Additionally, measurements of the static friction coefficient (μs) are carried out by the standard method ASTM D 1894-08. A slight enhancement in surface wettability is found with the mechanical abrasion pre-treatment from the SE increase. Finally, a higher value of μs is achieved for the abraded specimens as the normal force acting onto the system is increased.

634. Engel, J.H. Jr., and R.N. Fitzwater, “Adhesion of surface coatings as determined by the peel method,” in Adhesion and Cohesion, Weiss, P., ed., 89+, Elsevier, 1962.

1611. Erbil, H.Y., and R.A. Meric, “Determination of surface free energy components of polymers from contact angle data using nonlinear programming methods,” Colloids and Surfaces, 33, 85-97, (1988) (also in Interfaces in Polymer, Ceramic, and Metal Matrix Composites, H. Ishida, ed., Elsevier, 1988, p. 765-772).

1064. Eriksson, J.C., and S. Ljunggren, “Thermodynamics of curved interfaces in relation to the Helfrich curvature free energy approach,” in Surface and Interfacial Tension: Measurement, Theory, and Applications, Hartland, S., ed., 547-614, Marcel Dekker, 2004.

In 1878, Gibbs [1] published his celebrated “Theory of Capillarity,” the standard reference of surface thermodynamics ever since. In a rather compact-yet exhaustive and profound-manner, Gibbs treated fluid-fluid, as well as solid-fluid, interfaces and their equilibrium properties while representing the interfacial region in an Euclidean manner by a single dividing interface, preferably the so-called surface of tension. For this particular dividing surface, the standard Laplace (or Young-Laplace) equation [2]: ∆P=2Hγ (1) holds exactly for a majority of cases. Here H=(c1+c2)/2 denotes the mean curvature, γ is the interfacial tension, and ∆P is the pressure jump at the interface, and c1 and c2 are the principal curvatures of the surface of tension. Moreover, for any given interface, the interfacial tension γ attains a minimum value when the surface of tension is chosen to be the dividing surface, as may readily be verified.

699. Espana, J.M., D. Garcia, L. Sanchez, J. Lopez, and R. Balart, “Modification of surface wettability of sodium ionomer sheets via atmospheric plasma treatment,” Polymer Engineering and Science, 52, 2573-2580, (2012).

In this study, atmospheric plasma treatment has been used to modify the wetting properties of ethylene-methacrylic acid sodium ionomer. The effects of the plasma treatment on surface properties of this ionomer have been followed by contact angle measurements, Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). With the use of these techniques, the overall effects on activation–functionalization and surface topography changes have been determined in terms of the processing parameters of the atmospheric plasma treatment (rate and distance). The obtained results show a remarkable increase of the wetting properties and optimum balanced behavior is obtained for atmospheric plasma treatment with a rate of 100 mm/s and a distance of 6 mm; in this case, surface free energy is increased from 33 mJ/m2 (untreated ionomer) up to 62 mJ/m2, maintaining good transparency. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

1371. Esrom, H., R. Seebock, M. Charbonnier, and M. Romand, “Surface activation of polyimide with dielectric barrier discharge for electroless metal deposition,” Surface and Coatings Technology, 125, 19-24, (Mar 2000).

Electroless plating of non-conducting materials needs, prior to the metal deposition itself, to make the sample surface catalytically active. The route involving the chemical reduction of a thin solid metal–organic coating has, for this purpose, a significant potential in reducing the number of steps which are required today in conventional wet chemical metallization processes. In this work, a novel activation process using a dielectric barrier discharge (DBD) is described for the first time. This process is based on the plasma-induced chemical reduction at atmospheric pressure in air of palladium acetate (PdAc) layers resulting in the formation of palladium (Pd) on non-active surfaces. Fast surface activation of polymers like polyimide (PI) was found to occur in only a few seconds using a simple DBD device instead of expensive excimer UV lamps or complicated laser systems. The DBD-induced Pd layers on PI exhibit high activity with regard to initiation of the electroless copper plating. Indeed, copper deposition starts immediately after dipping the activated PI samples in the electroless solution without any inhibition time. Homogeneous copper coatings on PI were achieved under optimal plasma treatment conditions. The results are compared to those achieved with excimer UV lamps and excimer UV lasers.

837. Etzler, F.M., “Determination of the surface free energy of solids,” Reviews of Adhesion and Adhesives, 43, 3-45, (Feb 2013).

Knowledge of the surface free energy of solids is important to understanding a number of processes involving wetting and adhesion to solid surfaces. The measurement of surface free energy has been a subject of active interest for at least 50 years. Despite the importance of the problem to a variety of industries a universally accepted method or set of methods for determination of solid surface free energies has not been agreed upon. In this review article various methods that have been used for the calculation of surface free energies are discussed. The limitations and concerns for employment of each of these methods are furthermore highlighted. Of principal concern is the use of contact angles that meet the requirements to be Young’s contact angles and the mixing of quantities obtained by contact angle measurements with those obtained by IGC, as surface free energies obtained by IGC tend to be larger than those obtained from contact angle measurements. Calculated values from IGC data are presumably larger than those from contact angle data as IGC data are often collected at very low surface coverages.

1097. Etzler, F.M., “Characterization of surface free energies and surface chemistry of solids,” in Contact Angle, Wettability and Adhesion, Vol. 3, K.L. Mittal, ed., 219-266, VSP, Nov 2003.

The surface chemistry and surface energetics of materials are important to the performance of many products and processes—sometimes in as yet unrecognized ways. This review is written for the researcher interested in exploring the nature of surfaces and their relation to processes involving spreading, wetting, liquid penetration and adhesion. Researchers concerned with many types of products including pharmaceuticals, printing and the making of composite materials should have interest in this topic. More specifically, this work is a review of the literature concerning the surface free energy of solids. Both theoretical approaches for understanding the surface free energy of solids are explored and contrasted, as are experimental methods for measuring surface free energy of solids. Experimental methods that offer insight into the chemical nature of surfaces but do not measure surface free energy are also discussed as these two subjects are intertwined.

1424. Etzler, F.M., “Surface free energy of solids: A comparison of models,” in Contact Angle, Wettability and Adhesion, Vol. 4, K.L. Mittal, ed., 215-236, VSP, Jul 2006.

An understanding of the surface free energy and surface chemistry of solids is needed for investigation into the nature of processes involving adhesion, wetting and liquid penetration. Frequently the contact angles of several probe liquids on a given solid are used for calculation of solid surface free energy. Models by Fowkes, Kwok and Neumann, van Oss, Chaudhury and Good, as well as by Chang and Chen have been used for such calculations. Each of the above models has been championed in the literature. It has been noted by the present author and others that the use of different models may lead to different qualitative interpretations of the nature of a solid surface. A disinterested comparison of the various available models has not been made. In the present paper, a comparison of the calculations is undertaken in order to better understand the limitations of each model. Particular attention to the assumptions required for contact-angle data to be used for surface free energy calculations is given. The effect of the degree to which the experimental contact-angle data meet the required assumptions have on the calculated surface free energy is addressed in this work. When data meeting the theoretical assumptions common to the various published models are used, all of the published models fit the data, to a good approximation, equally well. A poor fit of the experimental data is an indicator that at least one liquid does not fully meet the assumptions re-quired by the chosen model. Differences in the acid—base character of the solid surface appear to re-sult from the acid—base scale used by the model. The paper is intended to raise the awareness of the difficulties in assigning surface free energy and predicting wetting behavior.

957. Etzler, F.M., “Determination of the surface free energy of solid surfaces: Statistical considerations,” in Advances in Contact Angle, Wettability and Adhesion, Vol. 3, K.L. Mittal, ed., 299-329, Scrivener, Feb 2018.

2019. Etzler, F.M., “Determination of the surface free energy of solid surfaces:Can the best model be found,” in Advances in Contact Angle, Wettability and Adhesion (Vol. 4), K.L. Mittal, ed., 73-98, Scrivener, Oct 2019.

In order to determine the surface free energy of a solid, it is necessary to measure contact angles of a variety of liquids on a given solid. The models investigated, here, include those proposed by Zisman, Kwok and Neumann; Owens and Wendt; van Oss, Chaudhury and Good, as well as Chen and Chang. In this chapter, the relative merits of these models are explored. The use of an overdetermined data set allows one to assess the statistical quality of the model and the estimated parameters. Liquids that show unusual behaviors (eg stick-slip) are unsuitable for determination of surface free energy. In this work, it will not be possible to examine the quality of each contact angle measurement. Rather, a relative assessment of various models is made. The results reported here indicate that no more than two adjustable parameters can be statistically justified. The Zisman, Kwok-Neumann models and a version of the van Oss, Chaudhury and Good model where the value of γ+ for the solid surface equals zero appear to be statistically viable. γ+ is the parameter that assesses the acidic character of the surface. These models yield similar values for the total surface free energy of the polymer surfaces.

783. Etzler, F.M., J. Simmons, N. Ladyzhynsky, V. Thomas, and S. Maru, “Assesment of acid-base character of polymer surfaces from contact angle and other surface chemical data,” in Acid-Base Interactions: Relevance to Adhesion Science and Technology, Vol. 2, K.L. Mittal, ed., 385-394, VSP, Dec 2000.

1693. Etzler, F.M., J.F. Bobalek, and M.A. Weiss, “Surface free energy of paper and inks: Printability issues,” in Proceedings from the TAGA International Conference, 225-237, TAGA, 1993.

2782. Etzler, F.M., M. Buche, J.F. Bobalek, and M.A. Weiss, “Surface free energy of paper and inks: Printability issues,” in 1995 Papermakers Conference Proceedings, 383-394, TAPPI Press, 1995.

1978. Evans, J.M., “The influence of oxygen on the nitrogen corona treatment of polyolefins,” J. Adhesion, 5, 9-16, (Jan 1973).

The resultant surface activation of polymers by corona discharges has been found to be markedly influenced by the type and purity of gases used in the corona. In this work it is shown that for the nitrogen gas corona treatment (15 KV, 15 mins) of polyethylene and polypropylene, traces of oxygen, >0.5% and <0.15% respectively, are sufficient to produce chemical changes in the polymer surface.

1979. Evans, J.M., “Nitrogen corona activation of polyethylene,” J. Adhesion, 5, 1-7, (Jan 1973).

Experiment has shown that the nitrogen corona-induced autohesion of polyethylene and the nitrogen-corona induced sorption of iodine by polyethylene both follow similar mechanisms. The controlling factor is postulated to be the formation of short-lived electrets within the polymer surface.

1972. Evans, J.R.G., and D.E. Packham, “Adhesion of polyethylene to metals: The role of surface topography,” J. Adhesion, 10, 177-191, (1979).

Previous work established the importance of the fibrous substrate topography in obtaining good adhesion of polyethylene to matt black oxide films formed on copper in alkaline solution. In this paper the effect of the very rough surface topography is shown to be general. Anodising treatments for copper and zinc and a high temperature oxidation for steel are described which give a very rough surface consisting (respectively) of fibrous, dendritic and blade-like growths. The peel strength of polyethylene to these substrates is high even under circumstances, for example when the polymer is stabilised with anti-oxidant, where adhesion to a chemically similar smooth surface is low. The high peel strength is associated with large amounts of energy being dissipated during peeling in plastic deformation of the polymer near the interface. It is suggested that this is caused by the development of high shear stress concentration at the fibre ends causing yielding in a large volume of polymer.

749. Everaert, E.P., H.C. van der mei, and H.J. Busscher, “XPS analyses of plasma-treated silicone rubber,” in Surface Modification of Polymeric Biomaterials, B.D. Ratner and D.G. Castner, eds., 89-96, Plenum Press, Mar 1997.

Silicone polymers exhibit good mechanical properties for a variety of biomedical and industrial applications. For instance, silicone rubber has been used for voice prostheses, urinary catheters, contact lens material, and icing coating materials. However, their inherently high hydrophobicity limits certain applications of this material despite its favorable mechanical properties6. Plasma treatment of silicone polymers may affect their hydrophobicity and therewith their boundability to other materials without affecting the bulk properties. Plasma treatment often involves progressive oxidation of the surface and cross-linking of surface molecular groups which inhibits migration of low molecular weight oligomers to the surface. Various gases have been used to modify silicone polymers by plasma treatment, such as oxygen, helium, ammonia, carbon dioxide, nitrogen and argon. Frequently a thin cross-linked, sometimes water washable, silica-like surface layer was produced by plasma treatment, but there is no consensus about the nature of the chemical groups produced at the outermost surface. The surface hydrophilicity created by plasma treatment is often lost over time. This so-called hydrophobic recovery can be influenced by the storage conditions, whether in air or in liquid, temperature or subsequent adsorption of a surfactant.

1927. Evieux, J., P. Montois, V. Nassiet, Y. Baziard, J.A. Petit, and R. Dedryv, “Study of bonded plasma-treated polyetherimide components for power integration: Durability in a hot/wet environment,” J. Adhesion, 80, 263-290, (Apr 2004).

This work deals with the study of the durability, in a hot/wet environment, of structural adhesively bonded polyetherimide (PEI) assemblies used in power electronics packaging technology. An overall approach is proposed, for which the epoxy joint-PEI substrates assembly on the one hand, and the adhesive system components (substrate surface and bulk adhesive) on the other hand, are studied separately with different analytical techniques. The first part of this work was devoted to the substrate surface state and to its modification using a cold plasma treatment of the PEI surface. Then for chosen parameters (power, duration) contact angle measurements indicated an increased surface tension resulting from surface decontamination (removal of release agent and carbon contaminants) and from the creation of polar species, such as esters or carboxylic acid groups, on the PEI surface (XPS analyses). The second part of this study concerned the bulk adhesive ageing in an ethylene glycol-water solution at 70°C. Mass uptake measurements versus time showed the liquid diffusion in the bulk adhesive associated with a microscopic damage of the epoxy system. An overall plasticizing of the adhesive with a considerable decay of the α-transition temperature of one of the two adhesive epoxy-amine networks (TGDDM-BAPP) was also highlighted using rheometry. However, in these ageing conditions, the adhesive glassy modulus decreases slighty because of the thermomechanical stability of the other epoxy network. In the third part, the asymmetric wedge test showed the beneficial effect of the cold plasma treatment on the epoxy/PEI interface durability in the aggressive medium.

2027. Ewane-Ebele, F., and H.P. Schreiber, “Measurement and use of surface tension data in film-forming polymers,” J. Oil and Colour Chemists Association, 60, 249-255, (Jul 1977).

Describes a method of measuring the critical surface tension of film forming polymers and the effect of temperature on the surface tension. The method gave reliable results for polyethylene, polystyrene, and polymethyl methacrylate. Changes in polymer properties due to aging can be monitored by the method, and the effect of glass transition temperatures and the effect of plasticizers in a styrene/acrylic copolymer were also studied.

676. Extrand, C.W., “Water contact angles and hysteresis on polyamide surfaces,” J. Colloid and Interface Science, 248, 136-142, (Apr 2002) (also in Contact Angle, Wettability and Adhesion, Vol. 2, K.L. Mittal, ed., p. 289-297, VSP, Sep 2002).

1096. Extrand, C.W., “A thermodynamic model for wetting free energies from contact angles,” Langmuir, 19, 646-649, (Jan 2003) (also in Contact Angle, Wettability and Adhesion, Vol. 3, K.L. Mittal, ed., p. 211-218, VSP, Nov 2003).

2715. Extrand, C.W., “Uncertainty in contact angle measurements from the tangent method,” J. Adhesion Science and Technology, 30, 1597-161, (2016).

The uncertainty in contact angles from sessile drops measured by the tangent method was estimated using a standard error propagation technique involving partial derivatives. If contact angles are <60°, then uncertainty of the tangent method appears to be quite small,≤ ± 2°. However, as θ values approach 90°, uncertainty increases asymptotically and can exceed  ±5°.

2717. Extrand, C.W., “Uncertainty in contact angle estimates from a Wilhelmy tensiometer,” J. Adhesion Science and Technology, 29, 2515-2520, (2015).

The uncertainty in contact angles from the Wilhelmy tensiometer was analyzed using standard error propagation techniques involving partial derivatives across the full range of wettability, from completely wetting to non-wetting surfaces. Uncertainties in force, sample perimeter, and liquid surface tension of 1% were shown to yield uncertainty in contact angles of a few degrees over the middle range of wettability, but exceeded 10° at the extremes.

3031. Extrand, C.W., “Work of wetting associated with spreading of sessile drops,” in Contact Angle, Wettability and Adhesion, Vol. 6, K.L. Mittal, ed., 81-94, VSP, 2009.

In this theoretical study, the work done on a sessile drop during spreading was estimated. It was found that the energy required to stretch the contact line is much greater than the energy needed to stretch the air–liquid interfacial area. The model shows that wetting energies are relatively small for large contact angles, but increase dramatically as contact angles tend towards zero. For a given drop volume, more work is needed to spread higher surface tension liquids than lower surface tension ones. Similarly, larger drops require more energy to spread than smaller ones. The work of wetting estimated here for sessile drops is comparable to energies from other wetting geometries, such as capillary bridge and sphere tensiometry. This work provides theoretical support for the experimental observation that interactions at the contact line dominate the wetting behavior of spreading sessile drops.

2895. Extrand, C.W., and S.I. Moon, “Contact angles of liquid drops on super hydrophobic surfaces: Understanding the role of flattening of drops by gravity,” Langmuir, 26, 17090-17099, (Oct 2010).

Measurement of contact angles on super hydrophobic surfaces by conventional methods can produce ambiguous results. Experimental difficulties in constructing tangent lines, gravitational distortion or erroneous assumptions regarding the extent of spreading can lead to underestimation of contact angles. Three models were used to estimate drop shape and perceived contact angles on completely nonwetting super hydrophobic surfaces. One of the models employed the classic numerical solutions from Bashforth and Adams. Additionally, two approximate models were derived as part of this work. All three showed significant distortion of microliter-sized drops and similar trends in perceived contact angles. Liquid drops of several microliters are traditionally used in sessile contact angle measurements. Drops of this size are expected to and indeed undergo significant flattening on super hydrophobic surfaces, even if the wetting interactions are minimal. The distortion is more pronounced if the liquid has a lesser surface tension or greater density. For surfaces that are completely nonwetting, underestimation of contact angles can be tens of degrees. Our modeling efforts suggest that accurate contact angle measurements on super hydrophobic surfaces would require very small sessile drops, on the order of hundreds of picoliters.

2041. Extrand, C.W., and Y. Kumagai, “An experimental study of contact angle hysteresis,” J. Colloid and Interface Science, 191, 378-383, (Jul 1999).

Advancing and receding contact angles of four organic liquids and water were measured on a variety of polymer surfaces and silicon wafers using an inclinable plane. Contact angles varied widely from liquid to liquid and from surface to surface. Surface roughness was relatively unimportant. Instead, the contact angles seemed to be more closely tied to the chemical nature of the surfaces. In general, contact angles increased with the liquid surface tension and decreased with the surface tension of the solid. Several definitions were used to calculate contact angle hysteresis from the experimental data. Although hysteresis is usually considered an extensive property, we found that on a given surface a wide range of liquids gave a unique value of reduced hysteresis. Apparently, reduced hysteresis represents an intrinsic parameter describing liquid–solid interactions.

766. Fainerman, V.B., and R. Miller, “The maximum bubble pressure tensiometry,” in Drops and Bubbles in Interfacial Research, Mobius, D., and R. Miller, eds., 279-326, Elsevier, Jun 1998.

This chapter focuses on the maximum bubble pressure method (MBPM) and deals with the physico-chemical and hydrodynamic processes taking place at various stages of the growth of a bubble and its separation from a capillary. Particular emphasis is made on theoretical problems like surface tension calculation from the measured excess pressure, surface tension calculation from the measured excess pressure, splitting of time interval between consecutive bubbles into lifetime and deadtime, and calculation of these characteristic times involving inertial and viscous properties of liquid and gas, non-stationarity of flows, etc. Emphasis is also made on experimental details like measurements of pressure and bubble formation frequency and optimisation of the geometry of capillary and measuring system related to the application of the MBPM. The results presented in this chapter contribute both to an improvement of the commercially available devices, and to a better understanding of the method by the users, helping them in the application of the MBPM and in a correct interpretation of the results.

2285. Fang, C., and J. Drelich, “Theoretical contact angles on a nano-heterogeneous surface composed of parallel apolar and polar strips,” Langmuir, 20, 6679-6684, (2004).

Neumann−Good's parallel strip model (J. Colloid Interface Sci. 1972, 38, 341) was used to analyze the contact angle hysteresis for a liquid on a heterogeneous surface composed of alternatively aligned horizontal apolar (θ = 70°) and polar (θ = 0°) strips. The critical size of the strip width, below which the contact angle hysteresis disappears, was determined on the basis of the analysis of the activation energy for wetting to be from 6 to 12 nm. This calculated value of the critical strip size is 1 order of magnitude smaller than that of 0.1 μm, which has been commonly considered as the limit of heterogeneity size causing the appearance of the contact angle hysteresis.

 

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