In this commentary we discuss the assay by M. Strobel and C. S. Lyons on contact angle measurements, critical and popular topic in surface/plasma science community. We agree with stressing the importance of dynamic contact angle measurements (i.e. the evaluation of both advancing and receding). However, we make some remarks about the meaning of angle hysteresis with particular regard to the concepts of roughness and chemical heterogeneity, on the basis of our experience in hydrophobic and super-hydrophobic surfaces. Further, we describe our different point of view in the dispute between Wilhelmy balance and sessile drop methods.

A theory, based on the presence of an adsorbed film in the vicinity of the triple contact line, provides a molecular interpretation of intrinsic hysteresis during the measurement of static contact angles. Static contact angles are measured by placing a sessile drop on top of a flat solid surface. If the solid surface has not been previously in contact with a vapor phase saturated with the molecules of the liquid phase, the solid surface is free of adsorbed liquid molecules. In the absence of an adsorbed film, molecular forces configure an advancing contact angle larger than the static contact angle. After some time, due to an evaporation/adsorption process, the interface of the drop coexists with an adsorbed film of liquid molecules as part of the equilibrium configuration, denoted as the static contact angle. This equilibrium configuration is metastable because the droplet has a larger vapor pressure than the surrounding flat film. As the drop evaporates, the vapor/liquid interface contracts and the apparent contact line moves towards the center of the drop. During this process, the film left behind is thicker than the adsorbed film and molecular attraction results in a receding contact angle, smaller than the equilibrium contact angle.

The surface chemistry of sheet molded composite (SMC) following interaction with a natural gas/air flame operated under reducing, stoichiometric, and oxidizing conditions has been investigated. The SMC surface chemistry is altered to contain in addition to hydrocarbon, ether, and ester functional groups, carbonyl and a greater carboxyl concentration. The extent of surface oxidation varies with the flame condition in the manner oxidizing ∼ stoichiometric > reducing. Lap shear tests carried out at 82°C (180°F) for coupons bonded with a urethane adhesive did not fail by fiber tear. Surface analysis results indicate failure at an oxidized SMC-adhesive/non-oxidized SMC interface and within the non-oxidized SMC surface.

The relationship between adhesion and surface energy is well established for systems where specific chemical interactions are unlikely, such as pressure sensitive adhesives. However, the relationship of wetting to adhesion in chemically reactive systems is not well understood. This work used atmospheric pressure plasma treatment in air of high density polyethylene to obtain surfaces with a range of electron donor and acceptor character prior to bonding with an amine cured epoxy. Adhesion correlated strongly with the electron donating character of surface energy, and the likely functional groups responsible for this adhesion were amines created by the plasma treatment process. These results indicate that wetting measurements may be useful in detecting the specific chemical interactions important to adhesion in reactive systems.

In the absence of weak boundary layers, surface energy can be an excellent indicator of the suitability of a fiber-reinforced composite surface for adhesive bonding. Mechanical surface treatments such as grit blasting are effective and commonly used to prepare composite surfaces, but the roughness introduced by these treatments makes quantification of the surface energy by contact angle methods difficult. This paper shows that the diameter of a small drop of a low-viscosity fluid chosen to have surface tension characteristics very similar to the adhesive can be used as an effective predictor of adhesive bond fracture energy. This technique could form the basis of a sensitive quality assurance tool for manufacturing.

The properties of the fiber/matrix interface in carbon fiber-reinforced composites play a dominant role in governing the overall performance of the composite materials. Understanding the surface characteristics of carbon fibers is a requirement for optimizing the fiber-matrix interfacial bond and for modifying fiber surfaces properly. Therefore, a variety of techniques for the surface treatment of carbon fibers have been developed to improve fiber-matrix adhesion as well as to enhance the processability and handling of these fibers. Many research groups have studied the effects of plasma treatments, correlating changes in surface chemistry with the interfacial shear strength. This article reviews the recent developments relative to the plasma surface modification of carbon fibers.

Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle analyses were performed on unsized and sized carbon fibers to better understand the mechanism of adhesion in carbon fiber/polymer matrix composites. AFM images and surface roughness analyses showed that the sizing changes the surface topography on a microscopic scale. The total surface energy decreased from 70 mJ/m² for unsized fiber to 54 mJ/m² for Ultem® sized and to 36 mJ/m² for PTPO sized fibers. The percentage of functional groups on the sized fibers decreased slightly compared to the unsized fibers. The surface functional groups and surface energies of fibers are critical properties in predicting fiber/matrix adhesion. Angle dependent XPS, voltage contrast XPS, and perimeter measurements revealed that the thickness of the poly(thioarylene phosphine oxide) (PTPO) sizing on the carbon fiber surface was greater than for the poly(etherimide) (Ultem®) sizing.

In moist environments biaxially-oriented corona-treated polyethylene terephthalate (BOPET) film undergoes a decay in surface energy with time. This decay is a significant and well-known problem and it considerably restricts the industrial application of BOPET film. In the present study the decay effect and the dynamics of corona-treated BOPET film in an aqueous environment have been studied using water contact angle and variable angle X-ray photoelectron spectroscopy (XPS) measurements. In addition the surface decay mechanism of the corona-treated BOPET film in aqueous environments was analyzed and a molecular moving model for the decay mechanism is proposed.

A critical review of published studies investigating the dielectric barrier discharge (DBD) treatment of four polymers widely employed in the packaging sector, namely: polyethylene (PE), polypropylene (PP), poly(ethylene terephthalate) (PET) and polystyrene (PS) is presented. The DBD treatment process operates at atmospheric pressure in air, and thereby offers a low cost method of enhancing the surface properties of polymers. The method is suitable for high volume in-line applications such as packaging. It has been reported that treatment doses as low as 0.01 J/cm² result in significant increases in surface energy and wettability, leading to enhanced adhesive bonding and printing performance. Two critical issues limit the improvements obtained via the DBD processing of polymers. Firstly, DBD processing can produce a poorly adhered surface layer of low molecular weight material, which can then interfere with bonding and printing processes. Secondly, the properties of DBD treated polymers tend to revert towards that of the untreated state during storage.

Dynamic Contact Angle Technique offers a unique, non-optical alternative to solid surface energy analysis. The technique provides advancing and receding hysteresis profile scans of a surface recorded in real time as the liquid meniscus traverses the solid surface. Changes in the wetting hysteresis scan can be used to characterize the qualitative effects of surface roughness, surface homogeneity, and surface polarity, as well as measure the quantitative surface energy of the solid. Applications in flexography in which wettability plays a critical role are numerous, and the switch from solvent-based inks to water-based inks gives impetus for future study.

Atmospheric pressure plasmas are commonly used to improve the wetting and adhesion properties of polymers. In spite of their use, the mechanisms for achieving these properties are unclear. In this regard, we report on a computational investigation of the gas phase and surface kinetics during humid-air corona treatment of polypropylene (PP) and the resulting modification of its surface properties while varying energy deposition, relative humidity (RH), web speed, and gas temperature. Using results from a global plasma chemistry model validated against experiments, we found that increasing energy deposition increased the densities of alcohol, carbonyl, acid, and peroxy radicals on the PP surface. In doing so, significant amounts of gas phase O₃ and N_xO_y are produced. Increasing the RH increased the production of peroxy and acid groups, while decreasing those of alcohol and carbonyl groups. Production of O₃ decreased while that of HNO₃ increased. Increasing the temperature decreased the concentrations of alcohol, carbonyl, and acid groups on PP while those of the peroxy radicals increased. For a given energy deposition, higher web speeds resulted in decreased concentrations of alcohols, peroxy radicals, carbonyl, and acid groups on PP.

An atmospheric pressure plasma system has been used to treat amorphous polyethylene terephthalate (APET) to enhance its healseal properties to a polyethylene terephthalate (PET) film. The plasma treated APET sheet material was thermoformed into trays for use in the food packaging industry and heatsealed to a PET film. The heatsealing properties of the resulting package were assessed using the burst test technique. It was found that the plasma treatment significantly enhanced the adhesive properties and an increase in burst pressure from 18 to 35 kPa was observed for plasma treated food trays. The APET surface chemistry was assessed after plasma treatment where it was found that the plasma treatment had affected an increase in oxygen and an addition of nitrogen species to the polymer surface. The surface roughness (R_a) of the plasma treated samples was also observed to increase from 0.4 to 0.9 nm after plasma treatment.

The interfacial forces between a polyethylene particle and a toner substrate in alkaline aqueous solutions were studied using an atomic force microscope colloidal probe technique. Measurements were taken at pH 9 in water and solutions of 5 × 10^-4 M CaCl₂, 1 × 10^-4 M Na oleate, and 1 × 10^-4 M Na oleate plus 5 × 10^-4 M CaCl₂ in order to mimic the conditions present during de-inking flotation. A polyethylene particle was used to represent the air bubble. The observed interaction forces were described by the extended DLVO theory. An energetic barrier caused by electrical double-layer repulsion was observed in water and Na oleate solutions but was greatly diminished in CaCl₂ solution. A long-range attractive force was found to be present in these systems and was described using a simple exponential function. The long-range attractive force was virtually the same in water and CaCl₂ solution but decreased significantly in Na oleate solution because of the reduced hydrophobicity of the interacting surfaces caused by the adsorbed carboxylate layer. However, in the presence of oleate and calcium ions the observed attraction was even stronger and of longer range than in water and CaCl₂ solutions. Moreover, no energetic barrier was observed. These results can be attributed to the presence of precipitated calcium oleates on the interacting surfaces.

Sessile-drop and captive-bubble techniques were used for contact angle measurements. The advancing and receding contact angles were measured for water and ethylene glycol at self-assembled monolayer surfaces of dodecanethiol, for water at methylated quartz surfaces, and for water at roughened polyethylene and polytetrafluoroethylene surfaces. It was found that for each technique used, sessile-drop and captive-bubble, different advancing contact angles and different receding contact angles were frequently obtained for nonideal systems with rough and heterogeneous solid surfaces. The disagreement between contact angles, as measured with the two different techniques, increased with increasing imperfection of the solid surface. Also, it was observed that solid surface roughness and heterogeneity affected a variation of the advancing and receding contact angles with drop (bubble) size. No contact angle change with respect to drop (bubble) size (in the range 1–7 mm base diameter) was observed when smooth and homogeneous solid surfaces were well prepared. It is possible that metastable states, which are responsible for the contact angle hysteresis, also affect the contact angle/drop (bubble) size relationship. These three-phase systems with sessile drop and captive bubble at heterogeneous and/or rough solid surfaces are complex because solid surface heterogeneity and roughness cause contortions in the shape of the three-phase contact line and the drop (bubble) surface in the vicinity of the three-phase contact line. These contortions may affect a variation of the internal free energy of the liquid drop (gas bubble). It is shown that a slight variation in the advancing contact angle value over a few millimeters change in drop (bubble) diameter does not guarantee a high-quality surface state. Measurements of the receding contact angles provide more information on the quality of the solid surface and they should always be included with the measurements of advancing contact angles.

The current broad interest in wetting characterization of solid surfaces is driven by recent advances in the formulation of surfaces and coatings that are superhydrophobic, superhydrophilic, oleophobic, oleophilic and so on. Unfortunately, the contact angle data presented in many publications raise some concerns among the surface chemists and physicists who work with contact angle measurement techniques on a regular basis. In those articles, best practices are often ignored, and the data presented are limited to the static contact angles measured for small droplets, a few times smaller than typically recommended. The reported contact angles are neither advancing nor receding, and their reproducibility in different laboratories is therefore questionable. In this note, guidelines to measurements of reproducible and reliable advancing and receding contact angles are summarized.

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.