PTFE was treated with oxygen plasma, and the effects of treatment time were evaluated by XPS, SEM, and the contact angles of water and CH₂l₂. Advancing and receding angles were interpreted in the light of current theories on contact angle hysteresis. It was found that at short treatment time wettability reflects chemical modification of the surface, while at longer treatment times surfaces are deeply etched and contact angles are controlled by roughness. With water as the wetting liquid, the typical behavior of composite surfaces was observed.

The dynamic wetting behavior of poly(tetrafluoroethylene) (PTFE), polyethylene (PE), polypropylene (PP), poly(ethylene terephthalate) (PET), nylon 6, poly(ether urethane) (PU), poly(vinyl alcohol) (PVA), and cellulose was studied by the Wilhelmy balance technique at speeds of immersion from 1 to 50 mm/min. The Wilhelmy method was modified so as to determine contact angles without extrapolation of the loop to the zero immersion depth, employing a rectangular flat sample having a rectangular hole. This modification of the method allowed us to determine the advancing and receding contact angles on the very narrow sample area close to the lower (first) and the upper (second) sample-hole boundaries, theta1 and theta2, respectively. The interaction time of the sample part located at the lower boundary with the wetting liquid (water) was twice as long as that of the upper boundary. No difference was observed between the advancing contact angles measured at the lower and the upper parts of the sample (theta(ADV,1) = theta(ADV,2)) for all the Polymers, displaying that the dried polymer surfaces had no difference in wettability along the sample length. However, the lower part of the sample became more hydrophilic than the upper part during the wetting measurement for PET, PU, nylon 6, PVA, and cellulose, resulting in the difference between the receding contact angles (theta(REC,1) < theta(REC,2)). The effect was attributed to the time-dependent surface reorientation of hydrophilic and hydrophobic groups, occurring upon immersion of the polymer sample in water. A close correlation was observed between the hysteresis of the contact angle and the underwater surface reconstruction of polymers: the strongest hysteresis corresponds to the greatest wettability gradient generated by the time-dependent reorientation process. However, even when the effect of reorientation was zero (PTFE, PE, and PP) or very low (cellulose), the observed hysteresis was still as high as 27-degrees. The contribution of the surface reorientation of polar groups to the observed hysteresis was estimated to amount to 0-25-degrees, depending on the chemical structure of the polymer investigated. The speed of the sample immersion had no detectable effect on the wettability of PTFE, PE, and PP. On the other hand, the advancing contact angle on PET, PU, and nylon 6 increased while the receding contact angle decreased, as the immersion speed became higher. This behavior may be accounted for by referring to a model of macromolecular dynamics at the three-phase boundary.

Properties of polymer surfaces are very sensitive to minute quantities of impurity and different preparation procedures. Baier and Zisman reported that wettabilities of polypeptides and polyamides such as Nylon 2,6, and 11 are different when the polymers are cast from different solvents. They attributed this difference to the existence or absence of the surface hydrogen bonding site. They also proposed a Zisman plot split criterion for the recognition of hydrogen-bonding functionality in a polymer specimen's surface. However, we found that this criterion does not always work. We then apply our new model for acid-base interactions to interpret their data. The model fits data well. Moreover, it is noticed that cases with exposed surfacehydrogen bonding sites are all bipolar and surfaces without hydrogen bonding sites are all monobasic.

In this paper, we report our unexpected finding about the correlation between Lewis acid−base surface interaction components and linear solvation energy relationship (LSER) solvatochromic parameters α and β. In 1987, van Oss, Chaudhury, and Good proposed to split the asymmetric acid−base parts of a bipolar system into separate surface tension components:  Lewis acid (electron acceptor) γ⁺ and Lewis base (electron donor) γ^-. It was assumed that the ratio of γ⁺ and γ^- for water at 20 °C was to be 1.0. With that ratio as a reference, the base components, γ^- for other liquids, biopolymers, polymers, and solids appeared to be overestimated. Recently, we unexpectedly found a correlation for liquids between γ⁺ and γ^-, and α (solvent hydrogen-bond-donating ability) and β (solvent hydrogen-bond-accepting ability) introduced since 1976 by Taft and Kamlet. From that correlation, we obtained a more realistic ratio for the normalized α and β values for water at ambient temperature to be 1.8 instead of 1.0. Based on this new ratio, we calculated total surface tensions for related materials at 20 °C. They are generally unchanged as expected, despite the considerable, favorable change in the γ⁺ and γ^- values in the direction of lowering the Lewis basicity. The predicability of solubility with interfacial tension is also unaffected. For example, the sign of those negative interfacial tensions that favor solubility remains the same. In addition, the implications of other LSER parameters, e.g. Π* and δ_H², on surface properties will be briefly mentioned.

Using the molecular-kinetic theory of wetting, we analyze the dynamic contact angle of a sessile drop of squalane spreading spontaneously on Langmuir−Blodgett multilayer substrates (behenic acid on glass). This allows the effect of microscale roughness on the parameters appearing in the theory to be determined. In particular, it is shown that the jump frequency of liquid molecules at the wetting line decreases with microroughness, supporting the idea that surface defects induce additional pining potentials. The increase in pinning potential can be explained in terms of a linear increase in the activation free energy of wetting with increasing RMS microroughness.

Axisymmetric adhesion tests are used to probe the adhesion between a carboxylated, poly(n-butyl acrylate) (PNBA) elastomer and a variety of substrates. The elastomer is in the form of a hemispherical cap, which is pressed against a flat substrate to give a circular contact area. A standard fracture mechanics approach is used to relate the applied load and the radius of the contact area to the energy release rate, 𝒢, which can be viewed as an adhesion energy. For a given substrate, 𝒢 is a function of the crack velocity, v, defined as the time derivative of the contact radius. The contact pressure does not appear to play a role in the development of the adhesive forces, and the specific relationship between 𝒢 and v depends on the substrate. In all cases this relationship can be adequately described by the empirical expression, 𝒢(v) = 𝒢₀(1 + (v/v*)^0.6). Values of 𝒢₀ are within a factor of about 2 of the expected thermodynamic work of adhesion, but values of v* vary from 5.5 to 215 nm/s, depending on the detailed nature of the substrate. Decreases in the adhesion energy, as quantified by a decrease in 𝒢₀ and/or an increase in v*, are determined primarily by the segmental mobility of the substrate molecules. Comparison of results for free and grafted PNBA chains indicates that translational diffusion of molecules at the substrate is not required in order to substantially reduce the adhesion energy.

The surface of oxygen-plasma-treated polystyrene (PSox) was investigated using X-ray photoelectron spectroscopy (XPS), streaming potential measurements and a dynamic study of the wetting properties at different pH (Wilhelmy plate method). The PSox surface is functionalized with various oxygen-containing groups, including carboxyl functions, and must be viewed as covered by a polyelectrolyte which swells depending on pH. The wetting hysteresis, its evolution upon repeated cycles and the influence of pH are controlled by the dissolution of functionalized fragments and the retention of water upon emersion; the retained water may evaporate progressively and allow macromolecule compaction and/or reorientation. Modification of the PSox surface upon aging in dry atmosphere, humid atmosphere, and water was studied using XPS and dynamic wetting measurements. Aging in water provoked the dissolution of PSox macromolecular chains, as indicated by adsorption of released fragments on a check PS sample placed nearby. However, the concentration of functionalized molecules at the surface of water-aged PSox was still sufficient to allow swelling at pH 5.6 and 11.0. Hydrophobicity recovery was faster in humid air (R. H. 95%) compared to dry air (R. H. 5%), due to the plasticizing effect of water. Hydrophobicity recovery upon aging in air was reversed quickly by immersion at pH 5.6 or 11.0, due to deprotonation and swelling.

Films of poly(methyl methacrylate) (PMMA) of both medium and high molecular weight have been prepared by casting onto clean glass. The difference in water contact angle of the surface originally in contact with glass. and air and the variation over time of this parameter have been studied. By use of the surface analytical techniques X-ray photoelectron spectroscopy (XPS) and, particularly, static secondary ion mass. spectroscopy (SSIMS), it has been shown that migration of low molecular weight impurities from the bulk of the film to the film/air interface is responsible for the contact angle behavior.

Surface free energies have been calculated for solid fluorocarbon materials by employing a method that utilizes dielectric data and theoretical predictions of van der Waals (dispersion) interactions. Excellent agreement between the results of direct force measurements and those of the theory for retarded van der Waals interactions supports the methodology. Two relatively new fluorocarbon polymers have been identified as having the lowest known surface free energies of all bulk homogeneous polymeric solids. This study provides confirmation that estimates of solid surface free energies based on contact angle measurements with dispersive organic liquids depend on the dielectric properties of both the liquids and the solid.

A detailed X-ray photoelectron spectroscopy study of a plasma-modified polystyrene (PS) surface was carried out after N₂ plasma treatment. PS surfaces were found to be highly hydrophilic and reactive as it readily picks up oxygen giving rise to oxyfunctionalities on the surface. The plasma treatment also led to a slow chain scission with carboxyl, forming carbonate linkage.

Polystyrene, a polymer extensively used in the biomedical field, causes a problem for some applications because of its surface hydrophobicity. Nitrogen plasma could transform this shortage through polar group attachment. To understand the role of hydrogen during surface functionalization in the nitrogen cold plasma, the effects of the nitrogen and the mixture of N₂/H₂ plasma are investigated by both the examinations of the densities of attached amine groups and the in-situ diagnostic analyses such as optical emission spectroscopy and mass spectrometry. An increase of functionalization has been proved to be controlled by the gaseous NH radical formation when H₂ is added.

IR−visible sum frequency generation (SFG) spectroscopy was used to study surface modification of polystyrene by its exposure to a UV light source or plasma. It was found that the polystyrene surface underwent dramatic changes after exposure to these treatments, as evidenced by marked changes in the surface SFG spectra. Before the treatments, the surface spectrum showed a pronounced peak at 3068 cm^-1 which is characteristic of the symmetric stretch of the aromatic C−H of polystyrene. This peak decreased markedly, and other vibrational bands associated with the CH₂ and CH₃ groups appeared after the treatments. The observed spectral changes provided direct evidence of surface reactions involving the aromatic ring. In addition, our data showed that the degrees of oxidation of the polystyrene surface were different with the two processes. The oxidation to a higher oxidation state, resulting in the formation of carbonyl/carboxyl species, was observed with plasma treatment but not with UV irradiation. This difference was also reflected in contact angle measurements. Before the treatments, the contact angle was 95 ± 4°. It decreased to 45 ± 4° and to less than 10° with UV irradiation and plasma treatment, respectively. The different pathways for the two treatments are discussed. In addition, the kinetics of photooxidation of the polystyrene surface was measured in situ, yielding a half-life of 15 min, which is much shorter than that of the bulk.

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.

Comments are made concerning the recent use of adjectives to describe solid surfaces that exhibit anomalously high water contact angle values. We suggest that the meaning of the word hydrophobic be resolved before it is modified, for example, to superhydrophobic and further modified, for example, to sticky superhydrophobic and before the definitions of these new words become issues of contention. The case is made that the first statement in the title is appropriate with experiments that demonstrate significant attractive interaction between liquid water and the surface of solid Teflon. Four types of experiments are described: the interaction of a silicon-supported covalently attached perfluoroalkyl monolayer (a model Teflon surface) with a sessile water drop (1) and with a thin film of water on a clean silicon wafer surface (2), the interaction of 1 and 12 microm diameter solid Teflon particles with a water droplet surface (3), and the interaction of a thin (<5 microm) Teflon film with a water droplet (4). The concepts of shear and tensile hydrophobicity are introduced, and the recommendation that two numbers, advancing and receding contact angle values, should be considered necessary data to characterize the wettability of a surface. That the words hydrophobic, hydrophilic, and their derivatives can and should only be considered qualitative or relative terms is emphasized.

We respond to a recent report in this journal that criticizes our experiments, which disproved the Wenzel and Cassie theories. The criticism is that we measured contact angles “with drops that were too small, ignoring the indications of existing theoretical understanding.” We take a step back to give an explanation of what we believe to be the reason that the “existing theoretical understanding” is wrong. We explain that the teaching of surface science has led generations of students and scientists to a misunderstanding of the wetting of solids by liquids. This continues as evidenced by this recent criticism and numerous recent papers. We describe several demonstrations that were designed to help teachers, students, and scientists overcome the widespread learning disability that is rooted in their faulty intuition and to help them regard wetting from the perspective of lines and not areas.

We review our 2006−2009 publications on wetting and superhydrophobicity in a manner designed to serve as a useful primer for those who would like to use the concepts of this field. We demonstrate that the 1D (three-phase, solid/liquid/vapor) contact line perspective is simpler, more intuitive, more useful, and more consistent with facts than the disproved but widely held-to-be-correct 2D view. We give an explanation of what we believe to be the reason that the existing theoretical understanding is wrong and argue that the teaching of surface science over the last century has led generations of students and scientists to a misunderstanding of the wetting of solids by liquids. We review our analyses of the phenomena of contact angle hysteresis, the lotus effect, and perfect hydrophobicity and suggest that needlessly complex theoretical understandings, incorrect models, and ill-defined terminology are not useful and can be destructive.

The line energy associated with the triple phase contact line is a function of local surface defects (chemical and topographical); however, it can still be calculated from the advancing and receding contact angles to which those defects give rise. In this study an expression for the line energy associated with the triple phase contact line is developed. The expression relates the line energy to the drop volume, the interfacial energies, and the actual contact angle (be it advancing, receding, or in between). From the expression we can back calculate the equilibrium Young contact angle, θ ₀, as a function of the maximal advancing, θ _A, and minimal receding, θ _R, contact angles. To keep a certain maximal hysteresis between advancing and receding angles, different line energies are required depending on the three interfacial energies and the drop's volume V. We learn from the obtained expressions that the hysteresis is determined by some dimensionless parameter, script K sign, which is some normalized line energy. The value of script K sign required to keep a constant hysteresis (θ _A - θ _R) rises to infinity as we get closer to θ ₀ = 90°.

Polyamide 12 (PA12) films have been modified by CF₄ and CF₄+H₂ (50/50 v/v) microwave plasma with different treatment times. The surface modifications have been followed versus plasma exposure duration by water contact angle measurements and atomic force microscopy (AFM) Pervaporation measurements were carried out to characterize the effects of these plasma treatments on water transport through PA12 films. From these measurements, water permeability was determined for each duration time of treatment. The efficiency of these plasma treatments in reduced permeability is compared. For both plasma treatments (CF₄ and CF₄+H₂), the analysis of the experimental data shows an increase and then a decrease of the permeability coefficient P with treatment durations. These observations are related with the evolution of the surface versus treatment time. From all these experimental results, it is clearly shown that the barrier effect to water in plasma-treated layers of PA12 is improved significantly, especially with CF₄.

We examined the effect of hydrogen content in various polymers in a N₂/H₂ discharge for surface amine functionalization. Three polymers (polyethylene (PE), polyvinylidene fluoride (PVDF), and poly(tetrafluoroethylene) (PTFE)) containing various amounts of hydrogen and fluorine were treated with an atmospheric pressure dielectric barrier discharge (DBD). While surface modification was observed on the PE and the PVDF in a pure N₂ discharge, adding H₂ in a N₂ discharge was necessary to observe the fluorine etching on the surface of the PVDF and PTFE polymers. The presence of a slight amount of hydrogen in the gas mixture was also a prerequisite to the formation of amino groups on the surface of all three polymers (max NH₂/C ∼ 5%). Aging revealed that the modified polymer surfaces treated in a N₂−H₂ discharge were less prone to hydrophobic recovery than were surfaces treated in pure N₂, due primarily to the presence of a higher density of polar groups on the surfaces. We demonstrated that H atoms in the discharge are necessary for the surface amine functionalization of polymers in a N₂ atmospheric pressure DBD, regardless of polymer chemical composition. It is therefore possible to control the plasma functionalization process and to optimize the concentration and specificity of NH₂ grafted onto polymer surfaces by varying the H₂ concentration in a N₂ atmospheric pressure DBD.

The surfaces of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were treated with an atmospheric-pressure oxygen and helium plasma. Changes in the energy, adhesion, and chemical composition of the surfaces were determined by contact angle measurements, mechanical pull tests, and X-ray photoelectron spectroscopy (XPS). Surface-energy calculations revealed that after plasma treatment the polarity of PET and PEN increased 6 and 10 times, respectively. In addition, adhesive bond strengths were enhanced by up to 7 times. For PET and PEN, XPS revealed an 18-29% decrease in the area of the C 1s peak at 285 eV, which is attributable to the aromatic carbon atoms. The C 1s peak area due to ester carbon atoms increased by 11 and 24% for PET and PEN, respectively, while the C 1s peak area resulting from C-O species increased by about 5% for both polymers. These results indicate that oxygen atoms generated in the plasma rapidly oxidize the aromatic rings on the polymer chains. The Langmuir adsorption rate constants for oxidizing the polymer surfaces were 15.6 and 4.6 s(-1) for PET and PEN, respectively.

Static contact angle and dynamic (advancing and receding) contact angles of water on polymeric surfaces were investigated using microscope cover glasses coated with various plasma polymers of trimethylsilane and oxygen. By variation of the mole fraction of the TMS/oxygen mixture, glass surfaces having varying degrees of wettability were prepared. The advancing contact angle of a sessile droplet, which is independent of the droplet volume, is considered as the static contact angle of water on a polymeric surface, θ_S, which is a parameter characteristic to a polymeric surface. The dynamic contact angle of water refers to the contact angle of which three-phase contact line is in motion with respect to the surface. The dynamic advancing (immersing) contact angle, θ_D,a, and receding (emerging) contact angle, θ_D,r, were measured by the Wilhelmy balance. The difference between θ_D,a and θ_D,r is mainly due to the direction of dynamic force acting on the three-phase contact line. The discrepancy between the immersion and the emersion buoyancy lines and the corresponding values of contact angles can be used to indicate the hysteresis due to the dynamic factor (the dynamic hysteresis). The dynamic hysteresis is largely determined by the critical immersion depth in which the three-phase contact line remains at the same place on the surface while the shape of meniscus changes when the motion of the sample is reversed. The dynamic hysteresis may contain the contribution of the change of static contact angle due to the surface-configuration change caused by the wetting of the surface (the intrinsic hysteresis). The dynamic hysteresis varies according to the value of cos θ_S, with the maximum at the threshold value around 0.6 and linearly decreases above this value, as the emersion line approaches the limiting buoyancy line determined by the surface tension of the liquid. The intrinsic hysteresis follows the same trend with the maximum at around 0.8. The three contact angles are related by cos θ_S = (cos θ_D,a + cos θ_D,r)/2.