Contact angle measurements and interpretation: wetting behavior and solid surface tensions for poly(alkyl methacrylate) polymers

Low-rate dynamic contact angles of a large number of liquids were measured on a poly(ethyl methacrylate) (PEMA) polymer using an automated axisymmetric drop shape analysis profile (ADSA-P). The results suggested that not all experimental contact angles can be used for the interpretation in terms of solid surface tensions: eight liquids yielded non-constant contact angles and/or dissolved the polymer on contact. From the experimental contact angles of the remaining four liquids, we found that the liquid-vapor surface tension times the cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. γlv cos ζ depends only on γlv for a given solid surface (or solid surface tension). This contact angle pattern is again in harmony with those from other methacrylate polymer surfaces of different compositions and side-chains. The solid-vapor surface tension of PEMA calculated from the equation-of-state approach for solid-liquid interfacial tensions was found to be 33.6 ± 0.5 mJ/m2 from the experimental contact angles of the four liquids. The experimental results also suggested that surface tension component approaches do not reflect physical reality. In particular, experimental contact angles of polar and nonpolar liquids on polar methacrylate polymers were employed to determine solid surface tension and solid surface tension components. Contrary to the results obtained from the equation-of-state approach, we obtained inconsistent values from the Lifshitz-van der Waals/acid-base (van Oss and Good) approach using the same sets of experimental contact angles.     

On the determination of the surface energetics of porous polymer materials

The solid surface tension γsv of hydrophobic polymer powders has been determined using the capillary penetration technique. By plotting Kγlv cos ζ, where K is a geometric factor, versus the liquid surface tension γlv, the following values of γsv were directly derived from the curves: poly(tetrafluoroethylene) γsv = 20.4 mJ/m², polypropylene γsv = 30.2 mJ/m², polyethylene γsv = 34.4 mJ/m², and polystyrene γsv = 27.5 mJ/m². These values are in good agreement with the γsv values obtained from contact angle measurements on flat and smooth solid surfaces of the same materials. If the contact angles were first calculated from the capillary penetration experiments, which is the usual procedure applied in the literature, distinctly higher contact angles were obtained. Obviously these angles are affected by the powder morphology and are therefore meaningless contact angles in terms of a surface energetic interpretation.     

DETERMINATION OF SOLID SURFACE TENSION FROM CONTACT ANGLES: THE ROLE OF SHAPE AND SIZE OF LIQUID MOLECULES

Accurate surface tension of Teflon® AF 1600 was determined using contact angles of liquids with bulky molecules. For one group of liquids, the contact angle data fall quite perfectly on a smooth curve corresponding to γ_sv = 13.61 mJ/m², with a mean deviation of only ±0.24 degrees from this curve. Results suggest that these liquids do not interact with the solid in a specific fashion. However, contact angles of a second group of liquids with fairly bulky molecules containing oxygen atoms, nitrogen atoms, or both deviate somewhat from this curve, up to approximately 3 degrees. Specific interactions between solid and liquid molecules and reorientation of liquid molecules in the close vicinity of the solid surface are the most likely causes of the deviations. It is speculated that such processes induce a change in the solid–liquid interfacial tension, causing the contact angle deviations mentioned above. Criteria are established for determination of accurate solid surface tensions.     

Low-rate Dynamic Contact Angles on Poly(methyl methacrylate/n-butyl methacrylate) and the Determination of Solid Surface Tensions

Low-rate dynamic contact angles of 12 liquids on a poly(methyl methacrylate/n-butyl methacrylate) P(MMA/nBMA) copolymer are measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It is found that 6 liquids yield non-constant contact angles, and/or dissolve the polymer on contact. From the experimental contact angles of the remaining 6 liquids, it is found that the liquid- vapour surface tension times the cosine of the contact angle changes smoothly with the liquid-vapour surface tension, i.e., γ_iv cos θ depends only on γ_iv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and noninert (polar and non-polar) surfaces [34-42, 51 -53]. The solid-vapour surface tension calculated from the equation-of-state approach for solid -liquid interfacial tensions [14] is found to be 34.4 mJ/m², with a 95% confidence limit of \pm 0.8mJ/m², from the experimental contact angles of the 6 liquids.     

Surface tension parameters of ice obtained from contact angle data and from positive and negative particle adhesion to advancing freezing fronts

From contact angle data obtained on flat ice surfaces with a number of liquids, combined with data on particle and macromolecule adhesion or non-adhesion to advancing freezing fronts, the apolar (Lifshitz-van der Waals or LW) and polar (Lewis acid-base or AB) surface tension (γ) components and parameters have been determined. At 0°C these are γ^LW _iee = 26.9 and γ^AB _ice = 39.6 mJ/m². The latter consists of an electron-acceptor (γ^⊕) and an electron-donor (γ^?) parameter: γ^⊕ = 14 and γ^? = 28 mJ/m².     

Measurement of the Surface Free Energy of Amorphous Cellulose by Alkane Adsorption: A Critical Evaluation of Inverse Gas Chromatography (IGC)

Surface energies of amorphous cellulose “beads” were measured by IGC at different temperatures (50 to 100°C) using n-alkane probes (pentane to undecane). The equation of Schultz and Lavielle was applied which relates the specific retention volume of the gas probe to the dispersive component of the surface energy of the solid and liquid, γ^d _s and γ^d _l, respectively, and a parameter (“a”) which represents the surface area of the gas probe in contact with the solids. At 50°C, γ^d _s was determined to be 71.5 mJ/m², and its temperature dependence was 0.36 mJ m^?2 K^?1. Compared with measurements obtained by contact angle, IGC results were found to yield higher values, and especially a higher temperature dependence, d(γ^d _s)/dT. Various potential explanations for these elevated values were examined. The surface energy, as determined by the Schultz and Lavielle equation, was found to depend mostly on the parameter “a”. Two experimental conditions are known to affect the values of “a”: the solid surface and the temperature. While the surface effect of the parameter “a” was ignored in this study, the dependence of the surface energy upon temperature and probe phase was demonstrated to be significant. Several optional treatments of the parameter “a” were modeled. It was observed that both experimental imprecision, but mostly the fundamental difference between the liquid-solid vs the gas-solid system (and the associated theoretical weakness of the model used), could explain the differences between γ^d _s and d(γ^d _s)/dT measured by contact angle and IGC. It was concluded that the exaggerated temperature dependence of the IGC results is a consequence of limitations inherent in the definition of parameter “a”.     

Comparison of the Lifshitz–van der Waals/acid–base and contact angle hysteresis approaches for determination of solid surface free energy

Total surface free energy, γ_S ^TOT, for several solids (glass, PMMA, duralumin, steel and cadmium) was calculated from the surface free energy components: apolar Lifshitz–van der Waals, γ_S ^LW, and acid–base electron–donor, γ_S ^-, and electron–acceptor, γ_S ⁺. Using van Oss and coworkers' approach (Lifshitz–van der Waals/acid–base (LWAB) approach), the components were determined from advancing contact angles of the following probe liquids: water, glycerol, formamide, diiodomethane, ethylene glycol, 1-bromonaphthalene and dimethyl sulfoxide. Moreover, receding contact angles were also measured for the probe liquids, and then applying the contact angle hysteresis (CAH) approach very recently proposed by Chibowski, the total surface free energy for these solids was calculated. Although the thus determined total surface free energy for a particular solid was expected to depend on the combination of three probe liquids used (LWAB approach), as well as on the kind of the liquid used (CAH approach), surprisingly the average values of the surface free energy from the two approaches agreed very well. The results obtained indicate that both approaches can deliver some useful information about the surface free energy of a solid.     

Influence of different alkyl side chains on solid surface tension of polymethacrylates

Low‐rate dynamic contact angles on poly(t‐butyl methacrylate) (PtBMA) were measured by an automated axisymmetric drop shape analysis profile (ADSA‐P). The solid surface tension of PtBMA is calculated to be 18.1 mJ/m², with a 95% confidence limit of ±0.6 mJ/m². This value was compared to previous results with different homopolymeric polymethacrylates [poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(n‐butyl methacrylate) (PnBMA)] and with copolymeric polymethacrylates {poly(methyl methacrylate/ethyl methacrylate, 30/70) [P(MMA/EMA, 30/70)] and poly(methyl methacrylate/n‐butyl methacrylate) [P(MMA/nBMA)]}. It was found that increasing length and size of the alkyl side chain decrease the solid surface tension, as expected. Comparison with pure alkyl surfaces suggests that the surface tension of PtBMA is dominated by the very hydrophobic t‐butyl group. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2493–2504, 2000     

Continuous manufacturing of reentrant structures via roll-to-roll process

Reentrant structures have been deemed necessary for repelling low-surface-tension liquids. In this article, a novel manufacturing methodology using the roll-to-roll process to enable continuous manufacturing of reentrant structures over large areas on a polymeric film was demonstrated. A two-stage process, composed of embossing followed by post-compression, allowed the fabrication of reentrant structures, which cannot be easily fabricated using other processes over large areas in a continuous manner. The treatment of the reentrant surface structures with an additional coating of 1H,1H,2H,2H-perfluorooctyltrichlorosilane layer provided liquid repellency with contact angles of 155 and 140°, respectively, for water (Y_lv = 72 mN m⁻¹) and diiodomethane (Y_lv = 50 mN m⁻¹). © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46980.     

A Study on the Impact of the Adhesion of Penicillium expansum on the Physicochemical Surface Properties of Cedar Wood

The sessile drop technique was used to investigate the evolution of the physicochemical properties of cedar wood as a function of contact time with the Penicillium expansum spores. The most important finding showed that the impact of different contact periods (2, 4, 6, 8, 10, and 24 hr) on the wood surface were very indicative. In fact, after 2 hr of contact, the results have shown a significant impact of the bioadhesion of spores to the substrate on both the hydrophobic character (θ_W = 108.5°; ΔGiwi = ?28.25 mJ/m²), the electron donor (γ^? = 13.63 mJ/m²), and the electron acceptor (γ⁺ = 4.35 mJ/m²) parameters that were significantly reduced compared to the initial wood (θ_W = 118.5°; ΔGiwi = ?6.29 mJ/m²; γ^? = 32.1 mJ/m²; and γ⁺ = 9.1 mJ/m²). In addition, this decrease of parameters continued over time to stabilize after 10 hr of contact. Indeed, after 24 hr, the acid/base properties were almost zero and the contact angle with water decreased to 30°. Moreover, it was found that the coefficient of correlation (r²) was strong between the contact angle with water, the surface energy, and the electron acceptor character with the contact time parameter with values (r² = 0.65), (r² = 0.79), and (r² = 0.68), respectively.     

Quantification of Bacillus subtilis and Bacillus sp. Adhesion on Fez Medina Cedar Wood

The adhesion of Bacillus subtilis and Bacillus sp. isolated from Fez cedar wood decay has been investigated. Furthermore, the physicochemical proprieties including hydrophobicity and electron donor/electron acceptor (Lewis acid–base) of both bacteria and substrata were evaluated using contact angle measurements. The results show that Bacillus subtilis has a hydrophobic character (ΔG _iwi = –20 mJ/m²). In contrast, Bacillus sp. exhibits a hydrophilic (ΔG _iwi = –20 mJ/m²), electron donating (γ^–) and weakly electron accepting (γ⁺) character. With respect to the substrata surface, we found that the cedar wood used in this work, was hydrophobic in character, having relatively more electron-donor that electron-acceptor properties (γ^– = 6 ± 4 mJ/m²; γ⁺ = 0 ± 3 mJ/m²). The phenomena of adhesion were observed by environmental scanning electron microscopy (ESEM) and cell adhesion was quantified using a Matlab program. The analysis of images obtained by ESEM show that the both cells was able to adhere to the wood substrata and the quantitative adhesion results showed that the surface coverage by Bacillus sp. (90%) was higher than that by the Bacillus subtilis strain (40%).     

Surface energy of liquid copper and single-crystal sapphire and the wetting behavior of copper on sapphire

The wetting behavior of liquid copper on sapphire is affected by the crystallographic orientation of the sapphire surface, the oxygen partial pressure, and the temperature. The influences of each of these conditions have been studied by the sessile drop technique over the oxygen partial pressure range 10^-2-10^-20 atm at temperatures of 1100 and 1250°C. The effect of oxygen partial pressure on the liquid copper surface energy follows the Gibbs-Langmuir law. The contact angle varies with the crystallographic orientation of the sapphire surface. This variation is more significant at higher oxygen partial pressures, but is eliminated at higher temperatures. The liquid copper surface energy was determined to be γ_lv = 1.757-3.3 x 10^-4T(°C) J/m². The solid surface energy of sapphire was estimated as γ_sv = 1.961-4.7x 10^-4T(°C) J/m², which applies only to the temperature range 927-2077°C.     

Surface Modification of Fluorinated Polymers by Microwave Plasmas

We developed a new plasma treating method, incorporating the use of microwaves generated by an electronic cooking range. Using this method, polytetrafluorethylene (PTFE) and a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP) were treated. Dialkylphthalates (DAP) were used as the standard liquids of contact angle measurements for evaluation of the wetting properties of plasma treated polymers. The components of surface tension (γ_L) due to the dispersion force (γ^d _L) and the polar force (γ^P _L) of DAP were calculated by Fowkes' equation from the contact angles (θ) on polypropylene. After plasma treatment cos θ of several standard liquids on PTFE and FEP increased. The linear relationship between γ_L(1 + cos θ)/(γ^d _L)^½ and (γ^P _L/γ^P _L)^½ was verified. γ_s and γ^d _s and γ^d _s of the plasma treated PTFE and FEP also increased. From the results of ESCA analysis, it was found that a significant amount of oxygen was introduced to the polymer surface by the plasma treatment. Peel strengths of a pressure sensitive adhesive bonded to PTFE and FEP increased approximately two-to threefold if the plasma treatment was used prior to bonding.     

On the Predominant Electron-Donicity of Polar Solid Surfaces

The reasons for the predominant electron-donicity of almost all solid polar surfaces and its implications are discussed in this paper. By contact angle or interfacial tension measurements, the electron-accepting as well as the electron-donating properties of polar liquids can be ascertained, through the interplay between their energies of adhesion and cohesion. For the solid-liquid interface, direct interfacial tension measurements are not possible, but indirectly, solid/liquid interfacial tensions of polar systems can be obtained by contact angle measurement. However, as the energy of cohesion of a solid does not influence the contact angle formed by a liquid drop placed upon its surface, one can only measure the solid surface'ks residual polar property, manifested by the energy of adhesion between solid and liquid. This residual polar property is of necessity the dominant component; in most cases this turns out to be its electron donicity. When, by means of contact angle measurements with polar liquids, both electron-accepting and electron-donating potentials are found on a polar solid, it is most likely still partly covered with a polar liquid: usually water. The amount of residual water of hydration of a polar solid follows from its polar (Lewis acid-base) surface tension component (γ^AB). The degree of orientation of the residual water of hydration on a polar solid can be expressed by the ratio of the electron-donating to electron-accepting potentials (γ^?/γ^⊕), measured on the hydrated surface.     

DETERMINATION OF SOLID SURFACE TENSION FROM CONTACT ANGLES: THE ROLE OF SHAPE AND SIZE OF LIQUID MOLECULES

Accurate surface tension of Teflon® AF 1600 was determined using contact angles of liquids with bulky molecules. For one group of liquids, the contact angle data fall quite perfectly on a smooth curve corresponding to γ_sv = 13.61 mJ/m², with a mean deviation of only ±0.24 degrees from this curve. Results suggest that these liquids do not interact with the solid in a specific fashion. However, contact angles of a second group of liquids with fairly bulky molecules containing oxygen atoms, nitrogen atoms, or both deviate somewhat from this curve, up to approximately 3 degrees. Specific interactions between solid and liquid molecules and reorientation of liquid molecules in the close vicinity of the solid surface are the most likely causes of the deviations. It is speculated that such processes induce a change in the solid-liquid interfacial tension, causing the contact angle deviations mentioned above. Criteria are established for determination of accurate solid surface tensions.     

Influence of the pH of Water on its Electron-Accepticity and Donicity

By means of contact angle measurements on dry layers of electrostatically neutral dextran with pure water (pH 6.1), water acidified with HCl (to pH 1.94) and water made alkaline with NaOH (to pH 12.8), it could be shown that there was essentially no change as a function of pH in the ratio of γ⁺/γ^? of water as compared with the aqueous acid and alkaline solutions. (Here γ⁺ is the Lewis acid parameter of the polar surface tension component of water and γ^? is its Lewis base parameter). In contrast, with contact angles measured with the same liquids on negatively charged clean glass, a significant decrease in contact angle was observed with water at pH 12.8, which was caused by the fact that at this alkaline pH an increase in surface hydrophilicity took place. This is because surfaces that have a given surface electrical potential at neutral pH generally acquire an even higher surface potential under more alkaline conditions which, concomitantly, also gives rise to an increase in surface hydrophilicity, and thus to lower contact angles with water. Finally, contact angles with acid water, pure water, and alkaline water, deposited on hydrophobic Parafilm surfaces, were exactly the same.     

Spreading of liquid drops over solid substrates: 'like wets like'

The classic hydrodynamic wetting theory leads to a linear relationship between spreading speed and the capillary force, being determined only by the surface tension of the liquid and its viscosity. Both equilibrium and dynamic processes of wetting are important in adhesion phenomena. The theory appears to be in good agreement with the results generated from experiments conducted on the spreading of poly(dimethylsiloxane) (PDMS) on soda-lime glass substrate and fails to account for the behavior of other liquids. In this study, the spreading kinetics of four different liquids (hexadecane, undecane, glycerol and water) was determined on three different solids, namely, soda-lime glass, poly(methyl methacrylate) (PMMA) and polystyrene (PS). Droplets from the same liquid allowed to spread under identical conditions on three different substrates produce distinctly different behaviors. The results show that the equilibrium contact angles are qualitatively ranked in accordance with the critical surface tension of wetting (γ _c) of the respective solid, i.e., high-γ _c solids caused the low surface tension liquids to assume the least equilibrium spreading (largest contact angle). On the other end, low-γ _c solids with the lowest surface tension liquid produce the most wetting (smallest contact angle). The results suggest that equilibrium spreading could be explained on the basis of the axiom 'like wets like'; in other words, polar surfaces tend to be wetted by polar liquids and vice versa.     

Interfacial tension from the height and diameter of sessile drops and captive bubbles with an arbitrary contact angle

A method has been developed to calculate the interfacial tension of sessile drops and captive bubbles of arbitrary contact angle by measuring the drop diameter and vertical distance to the apex at arbitrary horizontal planes within the drop. The procedure works in theory for any contact angle with an accuracy on the order of 0.1%. However, practical limitations reduce the range of angles to roughly 50°–180° but do not restrict the range of interfacial tensions (at least 0.01 mJ/m² to 72.0 mJ/m²). The optimal strategy is to use the method at several points on a single drop and to calculate the mean and standard deviation of the resulting interfacial tensions.     

Contact angle,wetting, and adhesion: a critical review

The theory of the contact angle of pure liquids on solids, and of the determination of the surface free energy of solids, γ_s, is reviewed. The basis for the three components γ^LW _s, γ⊕_s, and γ?_s is developed, and an algebraic expression for these properties in terms of measured contact angles is presented. The inadequacy of the 'two-liquid' methodology (which yields a parameter, 'γ^p') is demonstrated. Attention is given to contact angle hysteresis and to the film pressure, π_e. Some recommendations are made with regard to contact angle measurements. A new treatment of hydrophilicity, and of the scale of hydrophobic/hydrophilic behavior, is proposed. It is shown that there are two kinds of hydrophilic behavior, one due to Lewis basicity (electron-donating or proton-accepting structures) and the other due to Lewis acidity (electron-accepting or proton-donating structures). The properties γ^? and γ^⊕ are the quantitative measures of these types of behavior and they are structurally independent of each other. A triangular diagram, with γ^LW at the hydrophobic corner, and γ^⊕ and γ^? at the two hydrophillic corners, is suggested.     

Calendar of events

This review reports the successful synthesis of novel oligomeric silanes having end-capped fluoroalkyl groups. Glass surface was effectively modified by these oligomeric silanes. In particular, oligomeric silanes were more reactive and effective in the surface fluoroalkylation than monomeric silanes. From contact angle measurements, surface free energies were reduced to 15–20 and 1–3 mJ/m² for the dispersive and the polar components, respectively, and the surfaces were shown to be both highly water- and oil-repellent. Modified glass surface was analyzed using XPS. A linear correlation was observed between the dispersive component of surface free energy γ_S ^d and the area ratio of the F1s peak to the Si2p peak. The structure of the siloxane layer on the modified glass surface is discussed in terms of a network interphase model.