Contact angle hysteresis was measured for a variety of liquids on condensed monolayers of 17-(perfluoroheptyl)heptadecanoic acid adsorbed on polished chromium. The hysteresis was shown to be simply related to the molecular volume of the liquid and to result from the penetration of liquid molecules into the porous monolayer. However, contact angle hysteresis was negligible when the average diameter of the liquid molecules was larger than the average cross-sectional diameter of the intermolecular pores. It is shown that it is possible to estimate intermolecular pore dimensions of such adsorbed monolayers by contact angle hysteresis measurements on a series of liquids having gradations in molecular volume. The results of this investigation reveal that liquid penetration, even into pores of molecular dimensions, is a cause of significant contact angle hysteresis, and it is also shown how liquids can be selected for contact angle investigations on organic solid surfaces so that there will be freedom from this source of hysteresis. The results also suggest that under these experimental conditions, liquid water, on the average, behaves as if it were associated in clusters of about six water molecules. Similarly, both ethylene glycol and glycerol behave as associated clusters of about two molecules.

The contact angle of water on several polymer films was determined by three different methods; telescopic sessile drop, laser beam goniometry, and the Wilhelmy plate technique. The telescopic sessile drop method is the simplest, but the least accurate; whereas the laser beam goniometry compares favorably with the Wilhelmy plate in terms of accuracy, but cannot easily provide information on contact angle hysteresis.

The thermodynamic nature of interfaces and of adhesion is reexamined in the light of the Lifshitz theory of the forces acting across condensed phases. A new term is proposed, γ^LW, which consists of the sum of the terms heretofore ascribed to London, Debye, and Keesom forces, LW referring to Lifshitz-van der Waals. This term and a second term γ^SR account for the entirety of two-phase interactions in nonionic systems; SR refers to short range forces. This new analysis of forces is of value in explaining some important biological and other phenomena. The rather strong attachment of hydrophilic proteins, e.g., human serum albumin (HSA) and human immunoglobulin G (IgG), to low energy surfaces, e.g., polytetrafluoroethylene (PTFE) and polystyrene (PST), while immersed in H₂O, cannot be ascribed solely to Lifshitz-van der Waals forces (LW). For instance, it can be shown that the LW interaction would give rise to a repulsion between HSA and PTFE. The short range (SR) interactions, e.g., between H₂O and HSA, are due to H-bonds, which cannot directly account for interactions with PTFE. However, the combined SR interfacial tensions between the H-bonding liquid, the biopolymer, and the low energy surface still result in a strong attraction between PTFE and HSA, immersed in H₂O. This is analogous to the behavior of a liquid-air interface (where the fact that the direct interaction between a given solute and air is zero does not preclude the solute from being preferentially attracted to the interface). This SR attraction (minus the LW repulsion) between HSA and PTFE, in H₂O, is of the same order of magnitude as the adsorption energy derived from the Langmuir isotherm obtained for this system. Analogous results are found with IgG and PTFE, and also with HSA and IgG, with PST. Desorption patterns (obtained by changing the γ^LW and γ^SR of the liquid medium) allow an insight into the degree of local dehydration (or “denaturation”) of adsorbed proteins under various conditions. It is suggested that the term interfacial forces more aptly describes the underlying mechanism than “hydrophobic interactions.”

Low-temperature radiofrequency excited gas plasma was applied to the surfaces of a number of polymers. Polymers that are known to crosslink as well as those that only degrade under irradiation were included in the investigation. Surface changes were studied by viscosity, gel content, and contact-angle measurements. Changes in adhesive bond strength were used as a measure of overall practical effects of plasma treatment. In each case the response of the polymer surface to an oxidizing (oxygen) and a nonoxidizing (helium) plasma environment is discussed. Further indications of the nature of the surface changes were suggested by statistical treatment of the bond-strength data.

The paper presents a theory on determining the dynamic surface tension using two methods: the drop volume method and the maximum bubble pressure method.

The problem of the deformation of a quiescent air/liquid interface by a plunging solid surface is considered in the context of a differential force balance of the type used in withdrawal theory. Interfacial deformation and air entrainment which eventually arises at high speeds are discussed in terms of three separate regions: where the dynamic contact angle, θ, is >90°, 90° > θ > 180°, and θ → 180°. This latter condition leads to the development of a dimensionless correlation between Weber and Bond numbers correlating air entrainment data which were found to be in substantial agreement with the experimental results. The theoretical and experimentally measured profiles also showed good agreement, particularly for viscosities up to 6.77 P and dynamic contact angles less than 180°, for surface tensions in the range 34 < π < 65 dyn·cm⁻¹.

In the conventional methods of estimating the wetting characteristics of solids from contact angle experiments, the surface energetic properties of the solid are assumed to be identical in the environments of both the surrounding medium and probe fluids. While this assumption is suitable for solids which possess rigid surface structures (such as glasses, ceramics, and metals for example), it is generally inapplicable to polymeric solids, since the surfaces of the latter are relatively mobile so as to be able to adopt considerably different configurations in different environments. Based on a recognition of this feature of polymeric surfaces, a sequence of contact angle experiments is suggested to estimate: (a) the instantaneous as well as equilibrium surface energetic properties of a polymeric solid in an aqueous environment and (b) the time required for the polymeric surface to attain its equilibrium wetting characteristics in the aqueous environment. In order to illustrate the applicability of the suggested contact angle procedure, it is necessary to prepare model polymeric surfaces, which are smooth in surface texture, nonporous, and also chemically homogeneous. Such model surfaces were prepared in this study, by radio frequency sputter deposition of thin solid films of oxidized fluorocarbon compounds (from a Teflon FEP target) onto the smooth surfaces of highly polished, single crystal silicon substrates. The estimation of the wetting characteristics of the sputtered polymer films in an aqueous environment was then carried out by the suggested contact angle procedure. The results of the contact angle experiments indicate that the solid-water interfacial free energy of the sputtered polymer film which was initially equilibrated in an octane environment, decreases from an instantaneous value of 50.88 dyn/cm to an equilibrium value of 26.59 dyn/cm, over a duration of about 24 h. Such a change in the solid-water interfacial free energy of these model polymeric surfaces can arise due to a time-dependent reorientation of the buried polar groups of the solid from its bulk to its surface, when it is placed in contact with a strongly polar liquid like water. This interpretation was found to be consistent with the results of ESCA characterization, which indicated that the outer surface layers of the sputtered polymeric specimen contained a fair amount of the polar oxygen atoms that are capable of reorienting themselves from either the interior of the solid to its surface or vice versa, depending on their surrounding environment.

A method for measuring the dispersive part of the surface free energy γs^D of a high-energy solid, and its interaction energy with water and n-alkanes, W_SL, has been developed. It is based on the measurement of the contact angle of water on the solid under n-alkanes. Muscovite mica was chosen as a model high surface energy solid. The results obtained for γs^D and W_SL of mica are in good agreement with the results obtained by other techniques. The present method can be considered to be applicable for other solids.

A method of determining the polar term of the adhesion energy of several liquids to a high-energy solid, I_SL^P, has been developed, based on the measurement of the contact angle of water on a solid in a liquid medium. The I_SL^P values for mica are found to be a linear function of the square root of the polar term of the surface free energy of liquids. This finding agrees with the suggestion that the polar term of the energy of adhesion may be represented by the geometric mean of the polar term of the surface free energy of a solid and a liquid. The slope of the straight line provides the value of γ_S^P = 90 ergs/cm² for the polar term of the surface free energy of mica. The results were compared with those obtained by a cleavage method and also discussed in terms of each component of the surface free energy of mica. The present method is useful for the determination of the polar part of the energy of adhesion of a high-energy solid to liquids, and its surface free energy.

The surface free energy of polymeric films of polyvinylchloride (PVC) + poly(ethylene-co-vinylacetate) (EVA) blends was calculated using the van Oss treatment (Lifshitz and electron donor–electron acceptor components of surface free energy) and the Owens–Wendt treatment (dispersive and nondispersive components of surface free energy). Surface free energy results were found to be greatly dependent on the calculation method and on the number of standard liquids used for contact angle measurements. The nondispersive/donor–acceptor surface free energy component and the total surface free energy of polymeric films were always higher when the van Oss treatment was used compared to the Owens–Wendt treatment. Conversely, both methods led to similar apolar/Lifshitz components. All the calculation methods were in good agreement for the surface free energy of PVC; however, a discrepancy between the methods arose as EVA content in the blends increased. It seems that there is not yet a definite solution for the calculation of solid surface free energy. Further developments of existing models are needed in order to gain consistency when calculating this important physicochemical quantity.

The different methods available in the literature to calculate the surface tension of a solid from contact angle measurements are discussed and compared. The discussion is based on the contact angles of water, glycerol, and diiodomethane measured at 20°C on the surface of a side-chain liquid crystalline polymer. Some discrepancies exist among the results obtained with the different methods, mainly between the values yielded by Neumann's equation and those obtained with approaches that postulate the decomposition of the surface tension into several terms associated with different types of molecular interactions (methods of Owens and Wendt and of Good and van Oss). The physicochemical basis of these various treatments is discussed.

Several thermoplastic (technical, engineering, and high-performance) polymers were characterized using contact angle and electrokinetic measurements. From the measured contact angles of various test liquids on polymers, we calculated the solid surface tensions using the different approaches to determine them and compared the results. Zeta (ζ)-potential measurements gave information about the swelling behavior of the polymers in water, the surface chemistry, and the interactions with dissolved potassium and chloride ions. All investigated polymers displayed an acidic surface character. Comparing the results obtained from the ζ-potential measurements with the acid-parameter of the surface tension γ⁺ calculated from the measured “static” contact angles using the van Oss, Good, and Chaudhury approach revealed the same tendency. The correctness of the acid–base approach regarding the “overall” chemical surface character could be shown. However, it seems that the basic parameter γ⁻ obtained from the acid–base is greatly overestimated.

By means of an equation containing two adjustable coefficients it is possible to relate the surface free energy to the energy of vaporization, using the Hansen parameters from London force energy, polar energy, and hydrogen-bonding energy. The technique is applicable to simple organic liquids, mixtures of simple liquids, and most liquid metals. Hydroxy compounds, acidic and basic organic liquids, certain hexagonal and irregular metals, and most fused halides require special versions of the basic equation.

The effect of corona discharge on low-density polyethylene (LDPE) film was studied in terms of surface functionality and surface energetics of the film surfaces, improving the dyeability. The introduction of a polar group (OCO, CO, and CO) to a corona-treated LDPE film with acrylic acid could be confirmed by ESCA. The Owens–Wendt and Wu models using geometric means were studied to analyze the surface free energy of corona-treated film. It was found that the corona-treated LDPE film did lead to an increase in surface free energy, mainly due to the increase of its specific (or polar) component as the corona discharge power increased. Also, the K/S values were increased as the concentrations of dye increased. From the acid–base interaction point of view, it was found that the graft polymerization of acrylic acid onto the corona-treated LDPE film plays an important role in growing the acidic character which is one of the specific components of surface free energy, resulting in improving the dyeability with basic dyeing agent. A direct linear relationship is shown between the O_1s/C_1s ratio and the resulting K/S value or the specific component for this work.

Contact angles of 17 liquids on 3 hydrophobic solid surfaces, FC721, fluorinated ethylene propylene, and polyethylene terephthalate, were measured by using the Axisymmetric Drop Shape Analysis-Profile (ADSA-P) technique. Details of the surface preparation and the experiments are presented. The accuracy of these contact angle data is better than 0.2° in most cases. These data were used to calibrate an equation of state for interfacial tensions of solid—liquid systems. The end results of the analysis is an equation of state for interfacial tensions with a single parameter β = 0.0001247 (m²/mJ)², cf., Eqs. [22]–[24]. Within the experimental limitations, there is no evidence for the notion that β might change from system to system.

Absence of drop size dependence of contact angles of sessile drop systems is sometimes observed in experiments. The contact angle data sometimes fluctuate periodically about a horizontal line. Moreover, in cases where a drop size dependence of contact angles exists, the contact angle data often scatter significantly. These fluctuations may be caused by surface roughness. In this paper, two idealized rough surface models are developed. The mean contact angle of a sessile drop in each rough solid surface model is calculated. The fluctuations of the drop size dependence of contact angles produced by these models resemble those obtained experimentally and the fluctuations may therefore be a consequence of the roughness on solid surfaces. It is also concluded that the apparent absence of drop size dependence of contact angles does not necessarily imply zero or extremely low line tension.

The kinetics of modification of a fluoropolymer coating (FC 722, 3M Company) during its contact with octane, dodecane, and hexadecane is studied via measurement of quasi-static (velocity independent) advancing and receding dynamic contact angles. A decrease in both angles with the time of contact between solid and liquid is observed and it is interpreted as the result of swelling of the polymer. By means of a theoretical extrapolation of the θ_R(t) data tot= 0, based on an equation relating θ_R(t) to swelling kinetics, the experimentally inaccessible receding contact angle on dry coating, θ⁰_R, is determined. The contact angle hysteresis on such a surface, θ⁰_A− θ⁰_R, is found to be less than the hysteresis, θ^∞_A− θ^∞_R, obtained on samples that were soaked in the alkanes long enough to reach saturation. This increase is thought to be due to loosening of the polymer chains during the swelling, leading to an exposure of higher-energy segments to the nonpolar liquid and to an enlargement of the solid surface pores filled with liquid. The contact angle data are also interpreted in terms of interfacial free energies.

Low-rate dynamic contact angles of nine liquids on a poly(methyl methacrylate) (PMMA) polymer are measured by an automated axisymmetric drop shape analysis—profile (ADSA-P). It is found that two liquids dissolved the polymer on contact. From the experimental contact angles of the other seven polar and nonpolar liquids, it is found that the liquid–vapor surface tension times cosine of the contact angle changes smoothly with the liquid–vapor surface tension (i.e., γ_lνcos θ depends only on γ_lνfor a given solid surface). The dependence of γ_lνcos θ on γ_sνis explicitly illustrated by replacing the solid surface from the PMMA to other methacrylate polymers: such a procedure shifts the curves in a very regular manner. Thus, because of Young's equation, γ_sldepends only on γ_lνand γ_sν. This contact angle pattern is in harmony with those from other inert and noninert (polar and nonpolar) surfaces. The solid–vapor surface tension of PMMA calculated from the equation of state approach for solid–liquid interfacial tensions is found to be 38.5 mJ/m², with a 95% confidence limit of ±0.5 mJ/m²from the experimental contact angles of the seven liquids.

By employing a new strategy, we show that axisymmetric drop shape analysis (ADSA) can be used to determine simultaneously the surface tension and the density of polymer melts from sessile drops at elevated temperatures. To achieve this, two developments were necessary. First, the ADSA algorithm had to be modified to replace the density by the mass of the drop as an input parameter. Since ADSA also yields the volume, the density became output rather than input. Second, a closed high-temperature chamber whose temperature could be precisely controlled and a sample holder that allowed the formation of highly axisymmetric sessile drops at elevated temperatures had to be developed. For a typical polymeric material (polystyrene), it is demonstrated that measurements with sessile drops yield essentially the same surface tension values and temperature coefficients as measurements with pendant drops. The densities determined with ADSA are comparable to independent PVT results.

A theoretical treatment of the effect of surface heterogeneity on contact angles is given, by means of a model employing the capillary rise of a liquid in contact with a stripwise heterogeneous surface. Local contortions of the liquid-vapor surface are postulated to conform to an assumed periodic shape of the three-phase line. A minimum of free energy is found in a configuration in which Young's equation is obeyed locally. When the width of the strips is below some value of the order of 0.1 μ, the amplitude of the periodic contortion of the three-phase line is less than about 10 A, which is operationally indistinguishable from a straight line. Extension of this model is made to a patchwise heterogeneous surface, and a mechanism for hysteresis is developed. For patches smaller than about 0.1 μ, it is shown that heterogeneity should make a negligible contribution to hysteresis.

Acid/base theory has, over the last decade or so, been developed to describe interfacial free energies, or tensions, in wetting theory. An approach put forward by van Oss and co-workers, involving van der Waals/Lifshitz and Lewis electron acceptor/donor contributions to surface/interfacial free energies, has often been employed. The present study considers use of this theory for evaluating surface data for various polymeric surfaces employing known, characterized liquid probes for obtaining contact angle data. Results are analyzed using extended matrix analyses, originally proposed for treating the dispersive/polar interpretation of wetting results, and good agreement with literature values is obtained. By “inverting” the system, i.e., by treating the known solids as probes and rederiving surface data for liquids, inconsistencies are found to arise. Results for wetting of the same polymers and mica, using a two-liquid system (n-octane/water), are exploited to attempt to rederive the surface characteristics of water. Again, serious incoherence is manifest. Despite the conceptual interest of acid/base theory, clearly the mathematical formulation is presently inadequate. Copyright 1999 Academic Press.

Photomicrographs were taken of the organic drops formed on surfaces of vertical cylindrical fibers in water. The organic liquids used were 96% paraffin oil + 4% tetrabromoethane, paraffin oil, and 80% paraffin oil + 20% heptane. The fibers studied consised of plyester, a fluoroethylene-propylene copolymer (FEP), and nylon. The upper and lower contact angles, θ_t and θ_b, formed by the drops on the fiber surface were measured as a function of the dimensionless maximum drop radius, N, and length, $\text{L}$, from the projected images. As N increased so did θ_t, whereas θ_b only initially increased and then became more or less constant. Furthermore, θ_b decreased slightly close to the highest investigated values of N for systems involving FEP fiber. For a given system, the difference between the values of θ_t and θ_b increased as N increased, confirming that gravity forces affect the drop shape and contact angles. Good agreement is found between the measured values of θ_t and θ_b, and those obtained by using the theory for the shape of a drop on a vertical fiber [A. Kumar and S. Hartland, J. Colloid Interface Sci. 124, 67 (1988)].

The current work incorporates a microscopic study of the effect of fiber orientation on the fiber wetting process and flow of liquid droplets along filter fibers when subjected to airflow and gravity forces. Glass filter fibers in various combinations were oriented at various angles within a plane defined by the airflow direction and were supplied with distilled water in aerosol form. The behavior and flow of the liquid collected by the fibers were observed and measured using a specially developed microscope cell, detailed in the paper. The experimental results were compared to a theoretical model developed to describe the behavior. The theory and experimental results showed good agreement. The developed theory allows an optimum angle to be determined for the internal filter fiber structure in the design of wet filters. A sensitivity analysis of the model was conducted to determine the most important parameters. This will aid design of wet filtration systems such that maximal self-cleaning can be accomplished with minimal water use.

A new method to measure the contact angle θ of sessile drops deposited onto solid substrates is presented. The crux of the method is to use the drop as a convex mirror for large, collimated, incident laser beams. The reflected light is intercepted on a screen in the far field. It appears as a bright circular spot, the diameter of which is simply related to the beam angular divergence 4θ. Overall measuring accuracies can be made better than 0.1° for θ angles less than 45θ. Reproducibility is also excellent because of the natural averaging over the entire three-phase line boundary of the drop. It is estimated to be ±0.25°. An extension of the method, valid for all angles less than 90°, is to use the drop as a plano-convex lens. This is of course applicable only if the solid substrate is optically clear. The refracted light is then intercepted on a screen and analyzed to yield the contact angle through simple geometrical optics considerations. These two methods are demonstrated on a series of short-chain alkanes (hexane to hexadecane) in contact with hydrophobic glass slides and/or polytetrafluoroethylene plates.

We report contact angle measurements of five n-alkanes, dodecane through hexadecane, on Teflon (FEP) as a function of drop size. In all cases the contact angles decreased by approximately 5° when the drop size was increased from approximately 1 to 4 mm contact radius. A complete solution to the problem of mechanical equilibrium of a sessile drop on a solid surface indicates that the dependence of the contact angle on drop size may be explained by including the effect of line tension in the Young equation. The observed drop size dependence of the contact angle yields a line tension of (2.5 ± 0.5) × 10⁻⁶ J/m. Over the range of n-alkanes studied it was not possible to discern any dependence of the line tension on liquid surface tension.

We have established experimentally that, for certain liquid-solid systems, the contact angle formed by a drop of liquid varies with drop size, below a critical diameter which is probably a function of the nonuniformity of the solid surface. This effect has been observed with water on Teflon FEP and on polymethylmethacrylate (PMMA), in regard to both advancing and receding contact angles. The decrease was about 8° in θ_a and 16° in θ_r for water on Teflon FEP, between diameters of 4 mm and about 1 mm; and for other systems, a comparable or even greater effect was observed. For water on PMMA, θ_r decreased from 51° to 26°, between diameters of 8 and 3 mm. With ethylene glycol on Teflon FEP, the decrease in θ with drop size was observed in the retreating angle only. With n-decane on Teflon FEP, the contact angle was independent of drop size, between diameters of 1 and 12 mm. For all liquid-solid systems studied, the limiting contact angles for large drops were in good to excellent agreement with the values obtained by the vertical plate method. Qualitatively, this effect could be explained by hypothesizing the existence of a large, negative line tension. It was found that there was quantitative disagreement between observed results and predictions based on this hypothesis. A theory is proposed to explain the experimental results, based on the known heterogeneity of many polymer surfaces and a previously discussed theory of hysteresis. A negative pseudo-line tension can be used in a phenomenological description of the motion and contortion of the three-phase line.

We report on the surface modification of Sylgard-184 poly(dimethyl siloxane) (PDMS) networks by ultraviolet (UV) radiation and ultraviolet/ozone (UVO) treatment. The effects of the UV light wavelength and ambient conditions on the surface properties of Sylgard-184 are probed using a battery of experimental probes, including static contact angle measurements, Fourier transform infrared spectroscopy, near-edge X-ray absorption fine structure, and X-ray reflectivity. Our results reveal that when exposed to UV, the PDMS macromolecules in the surface region of Sylgard-184 undergo chain scission, involving both the main chain backbone and the side groups. The radicals formed during this process recombine and form a network whose wetting properties are similar to those of a UV-modified model PDMS. In contrast to the UV radiation, the UVO treatment causes very significant changes in the surface and near-surface structure of Sylgard-184. Specifically, the molecular oxygen and ozone created during the UVO process interact with the UV-modified specimen. As a result of these interactions, the surface of the sample contains a large number of hydrophilic (mainly –OH) groups. In addition, the material density within the first ≈5 nm reaches about 50% of that of pure silica. A major conclusion that can be drawn from the results and analysis described in this work is that the presence of the silica fillers in Sylgard-184 does not alter the surface properties of the UVO- and UV-modified Sylgard-184.

The characterization of polymer-water interfaces by contact angle measurement is performed using water-immiscible liquids. It gives the dispersive and the nondispersive components of surface free energy as a function of functional group of copolymer hydrogels. Using the method of Rusanov, the contactangle-induced deformation of the three-phase region in our systems was examined. The systems used were poly(2-hydroxyethyl methacrylate) (HEMA), poly(2-hydroxyethyl methacrylate-methyl methacrylate) (HEMA-MMA), poly(2-hydroxyethyl methacrylate-methoxyethoxyethyl methacrylate) (HEMA-MEEMA), poly(2-hydroxyethyl methacrylate-aminoethyl methacrylate) (HEMA-AEMA), and poly(2-hydroxyethyl methacrylate-diethylaminocthyl methacrylate) (HEMA-DEAMA). The deviation of contact angle due to the surface deformation was found to be appreciable in case of poly(HEMA-AEMA) and poly(HEMA-DEAMA).

Contact angle measurements on polymer hydrogels were performed at various temperatures, and we obtained the dispersive (γ_s^d) and nondispersive (γ_s^p) components of the surface tension of polymer hydrogel at each temperature. Utilizing the temperature dependence values of γ_s^d and γ_s^p, we obtained the surface entropies of polymer hydrogels. The polymer hydrogels used were isotactic and syndiotactic poly(2-hydroxyethyl methacrylate) (HEMA), poly(2-hydroxyethyl methacrylate + aminoethyl methacrylate) (HEMA + AEMA), poly(2-hydroxyethyl methacrylate + N-vinyl pyrrolidone) (HEMA + VP), poly(2-hydroxyethyl methacrylate + methyl methacrylate) (HEMA + MMA), poly(2-hydroxyethyl methacrylate + methoxyethyl methacrylate) (HEMA + MEMA), and poly(2-hydroxyethyl methacrylate + methoxyethoxyethyl methacrylate) (HEMA + MEEMA), respectively. The contact angles were also measured by using droplets of water-immiscible liquids under conditions in which the polymer hydrogel was fully hydrated.

The surface tensions of polyethylene, polyisobutylene, and polyvinyl acetate, and the interfacial tensions of polyethylene/polyvinyl acetate and polyisobutylene/polyvinyl acetate systems have been measured by the pendent drop method in the temperature range up to 200°C. The results are analyzed in terms of the equations of Fowkes and of Girifalco and Good, and suggest that the conformational restriction of polymer molecules imparts a limitation on the extent of interfacial contacts and sharp phase boundaries in these systems. Several quantities of interest in adhesion, such as contact angle, spreading coefficient, and work of adhesion are also discussed.

Contact angles of polar liquids on various nonpolar solids immersed in a nonpolar liquid have been measured. They agreed satisfactorily with values calculated by combining Young's equation either with measured contact angle data of the liquids on the substrates in air, or with the Fowkes relation valid at each of the interfaces. The latter method also applies when one of the liquids spreads on the substrate in air. Polyethene, polystyrene, polytetrafluorethene and glyceryltristearate were used as nonpolar solids; paraffin oil and heptane as nonpolar liquids; water and glycerol as polar liquids.

Digital image processing techniques are applied toward the determination of polymer surface tension by pendant drop measurements. Experimental values for poly(dimethylsiloxane) as a function of molecular weight and temperature correspond well with previous measurements of poly(dimethylsiloxane) surface tension, testifying to the applicability of the new technique. Current thermodynamic treatments are found to provide excellent predictions of poly(dimethylsiloxane) surface tension for molecular weights of 3900 and 75,000 at temperatures ranging from 20 to 120°C. Theories developed by K. M. Hong and J. Noolandi (Macromolecules, 14, 1223, 1981) and Y. Rabin (J. Polym. Sci. Polym. Lett. Ed. 22, 335, 1984) yield predictions within 5% of t he experimental results for the materials and conditions studied.

The purpose of this research was to classify fluoropolymer surfaces with regard to their wettability by various liquids. Thin solid blades, coated with various fluoropolymers, were suspended from a force transducer balance. The blades penetrated various hydrocarbon/air and hydrocarbon/water interfaces, and the curves of force acting on the blade versus displacement were recorded. The contact angles hydrocarbon/solid/air and hydrocarbon/solid/water were calculated using: $\text{COS}\text{θ =}\text{t}\text{γL}$ and $\text{COS}{}_{\mathrm{i}}\text{θ =}\text{T}{}_{\mathrm{i}}\text{γi}$ where θ, θ_i: contact and interfacial contact angle; γL, γi: surface and interfacial tension; and τ, τ_i: adhesion and interfacial adhesion tension. Cos θ versus γ_L for various γ_L were plotted and γ_c (for cos θ = 1) was determined. It was found that there are two critical surface (γ_c) and critical interfacial (γ_cⁱ) tensions due to contact angle hysteresis. Three different fluoropolymer surfaces were investigated. When the surface can be used in the form of a well-defined blade and when enough liquid to be tested is available, the method was found to be useful, rapid, and reproducible. Under these circumstances it can be used in place of Zisman's well-known sessile drop method.

A modified Wilhelmy plate technique has been developed for the measurement of surface tensions of viscous polymers. The method requires no knowledge of liquid density and provides a means of assuring a zero contact angle for the polymer on the plate. The surface tensions of several silicone polymers with viscosities as high 106 centipoises have been measured. The method has also been used to determine the surface tensions of several molten polyethylenes as a function of temperature over the range 115° to 215°C.