The wetting phenomenon is an important issue in various technological processes. In some fields, liquids are desired to spread over solid surfaces, e.g., lubrication oils on metallic surfaces or paint on paper. On the other hand, it is necessary for hydrophobic coatings to repel water such as Teflon film on frying pans. The behavior of bubbles on solid surfaces immersed in liquid often has important effects on the performance of industrial apparatus dealing boiling or condensation. In these problems regarding wetting, it is known that the behavior of a drop or bubble on a solid surface is dependent on the three interfacial tensions between solid, gas, and liquid phases, as shown in Fig. 1. The tangential force balance between these interfacial tensions on the three-phase contact line leads to the following well-known Young’s equation [1]: σSV−σSL=σLV cos αY. (1) σSV, σSL, and σLV indicate solid-vapor, solid-liquid, and liquid-vapor interfacial tensions, respectively. The environmental atmosphere is assumed to be filled with saturated vapor of liquid. When a drop is exposed to air, however, σLV usually does not change because a thin layer of saturated vapor may be formed around the drop [2]. In the right-hand side of Eq. (1), αY is the angle between the solid surface and the liquid-vapor interface measured from the inside of the liquid phase and is called the contact angle. When the difference between the two interfacial tensions on the left-hand side of Eq. (1) is large enough to make αY on the right-hand side small, the solid is favorably wetted by the liquid. As the drop size becomes sufficiently small and the curvature of the solid-gas-liquid contact line becomes quite large, we should add a term representing the effect of line tension to the above equation [3].

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

No single step in the coating process has more impact on film adhesion than surface preparation. Film adhesion to a plastic is primarily a surface phenomenon and requires intimate contact between the substrate surface and the coating. However, intimate contact of that plastic surface is not possible without appropriate conditioning and cleansing. Plastic surfaces present a number of unique problems for the coater. Many plastics, such as polyethylene or the fluorinated polymers, have a low surface energy. Low surface energy often means that few materials will readily adhere to the surface. Plastic materials often are blends of one or more polymer types or have various quantities of inorganic fillers added to achieve specific properties. The coefficient of thermal expansion is usually quite high for plastic compounds, but it can vary widely depending on polymer blend, filler content, and filler type. Finally, the flexibility of plastic materials puts more stress on the coating, and significant problems can develop if film adhesion is low due to poor surface preparation.

Of the several techniques available for the surface modification, plasma processing has proved to be very appropriate. The low temperature plasma is a soft radiation source and it affects the material only over a few hundred Å deep, the bulk properties remaining unaffected. Plasma surface treatment also offers the advantage of greater chemical flexibility. PET films are widely used for packaging and electrical insulation. The studies of adhesion and printability properties are important. In the present study PET films are treated in air plasma for different time of treatment. The improvement in adhesion is studied by measuring T-peel and Lap shear strength. In addition, printability of plasma treated PET films is studied by cross test method. It has been found that printability increases considerably for plasma treatment of short duration. Therefore it is interesting to study the surface composition and morphology by contact angle measurement, ESCA and AFM. Surface energy and surface roughness can be directly correlated to the improvement in above-mentioned surface related properties. It has been found that the surface oxidation occurs containing polar functional groups such as CO, COO. A correlation of all such observations from different techniques gives a comprehensive picture of the structure and surface composition of plasma treated PET films.

The effect of oxygen plasma treatment on adhesion and surface properties of polypropylene (PP) was assessed. An oxygen rich modified PP layer, immiscible with bulk PP, was formed by the treatment. Contact angle measurements showed that macromolecular motions led with time to rearrangements of the surface layer drastically decreasing its wettability, while its composition, measured by XPS, remained unaffected.The shear strength of PP-epoxy joints increased after plasma treatment. The locus of failure was found to occur at the PP/epoxy interface for untreated PP, within PP in the case of oxygen-plasma-treated samples, close to the modified PP/bulk PP interface. This result suggests that the plasma treament improves the interaction at the PP/epoxy interface, but weakens the mechanical strength of the surface layer thereby creating a weak point at the modified PP/bulk PP interface.

Protective properties of zinc coating increase with an additional coating such as: chromate, phosphate, paint and polymer coating. Besides, additional treatment of zinc coating serves decorative purposes as well. The paper presents the influence of additional coating of zinc coating on their adhesive properties which are especially helpful in processes where adhesion plays an essential role. These processes include among others: gluing, painting or varnishing. Adhesive properties are characterized by the value of surface free energy.

The surface of the bio-material polymethyl methacrylate (PMMA) was treated with CO₂, Nd:YAG, excimer and high power diode laser (HPDL) radiation. The laser radiation was found to effect varying degrees of change to the wettability characteristics of the material depending upon the laser used. It was observed that interaction with CO₂, Nd:YAG and HPDL effected very little change to wettability characteristics of the PMMA. In contrast, interaction of the PMMA with excimer laser radiation resulted an increase in a marked improvement in the wettability characteristics. After excimer laser treatment the surface O₂ content was found to have increased and the material was seen to be more polar in nature. The work has shown that the wettability characteristics of the PMMA could be controlled and/or modified with laser surface treatment. However, a wavelength dependence of the change of the wetting properties could not be deduced from the findings of this work.

The paper describes an experimental investigation of the possibility of industrial modification of surface wettability and adhesion of polymers by the action of a plasma of a gliding discharge generated in air at atmospheric pressure in a simulated production process. The test material was polypropylene plates (PP). The modification was performed by a device with a multi-electrode (four pairs) system, which is not common. The quality of pre-processing and usability was evaluated primarily in terms of the industrial requirements, which means a change in wettability and adhesion expressed by the contact angle/surface free energy value in dependence to sample exposure time expressed by the conveyor belt speed. The surface free energy assessment of a treated polymeric surface by contact angle measurement was carried out by analyzing static sessile drops and evaluated by Owens–Wendt–Rabel–Kaelble (OWRK) model. The results determined a set of operating parameters at which the modification process meets the industrial requirements. By evaluating the change in surface free energy in relation to the storage time, the degree of hydrophobic recovery of the treated samples, i.e. the time stability of the plasma-treated surface, was also determined. It has been found that plasma-treated PP surface fully meets industrial demands and can be stored for at least 50 days.

The wetting (or dewetting) of a solid by a liquid is an integral part of many important processes such as coating, petroleum recovery, distillation and the handling of liquid fuels in low gravity conditions. Several experiments^1–4 have shown that wetting lines (where liquid, air and solid phases meet) which are straight at slow rates of movement over the solid have a sawtooth shape at sufficiently high speeds. We now offer a quantitative explanation for this phenomenon based on the postulate that, for a given system, there is a maximum rate at which wetting can proceed. The consequences of this interpretation are likely to be important, since, in many practical situations, the aim is to maximise the speed of wetting without entraining the displaced phase.