Highly hydrophobic surfaces have numerous useful properties; for example, they can shed water, be self-cleaning, and prevent fogging (1, 2). Surface hydrophobicity is generally characterized with contact angle (CA) goniometry. With a history of more than 200 years (3), the measurement of CAs was and still is considered the gold standard in wettability characterization, serving to benchmark surfaces across the entire wettability spectrum from superhydrophilic (CA of 0°) to superhydrophobic (CA of 150° to 180°). However, apart from a few reports [e.g., (4–8)], the inherent measurement inaccuracy of the CA goniometer has been largely overlooked by its users. The development of next-generation liquid-repellent coatings depends on raising awareness of the limitations of CA measurements and adopting more sensitive methods that measure forces.

Corona and chemical treatment worked cooperatively for increasing and stabilizing the polyethylene film surface energy. Gentle and varied corona discharge treatment conditions were applied for each polyethylene film to reach 40 dynes/cm. A rather low blending amount of additive could stabilize the film surface energy obviously. Compared with neat PE film, of which the surface energy decreased to 36 dynes/cm at the 12th day, films blended with 1000 ppm A7-OH or PE-PEG 4k -PE showed stable surface energy (36–38 dynes/cm) over 150 days. The influence of industrial applied slipping agent was investigated as well. Morphological and chemical changes were studied by X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscope (AFM). The surface energy was determined by the dyne pens. Mechanism investigation of hydrophilization and hydrophobic recovery processes showed that proper crystallization behavior and enough C[dbnd]O groups on the film surface guarantee satisfactory stability of the surface energy.

The carbon fibers (CFs) reinforced polytetrafluoroethylene (PTFE) composites have been modified using proton irradiation, and the surface energy, hardness and tribological properties have been investigated before and after irradiation. The CFs increased the hardness and the wear resistance. Proton irradiation led to defluorination and carbonization of the CF/PTFE composite surface, and decreased the surface wettability and the surface energy. The irradiation depth was 820 nm from the material surface calculated with SRIM software package. In addition, the wear resistance was improved after proton irradiation. Proton irradiation improved the wear resistance of the composite and induced the material transfer from Cu alloy surface to CF/PTFE. These significant improvements could enable potential applications in aeronautics and smart medical materials.

Hypothesis: Both the surface free energy (γ) and solubility (δ) parameters of substances are related to their cohesive energies which are decided by intermolecular interactions, and there should be some intrinsic relationships between the two parameters. Understanding of the γ-δ correlations is of great fundamental and practical importance. Several empirical γ-δ equations have been proposed so far, but their application to solids is limited. This is because the molar volume (V~) as a parameter exists in these equations while the V~ of solids is commonly hard to be obtained. Hence, the development of γ-δ equations without the parameter V~ is essential for solids.

Method: The γ and δ data of 21 solids including polymers and layered solid materials were chosen, and possible γ-δ relationships were systematically explored using the parameter data of solids by a trial and error fitting method.

Finding: Six γ-δ equations without the parameter V~ are proposed. The γ parameters include total (γ_t), dispersive (γ_d), and polar (γ_p) ones, and the δ parameters include the Hildebrand parameter (δ_t) and the Hansen dispersive (δ_d), polar (δ_p), and hydrogen-bonding (δ_h) ones. Interestingly, the so-obtained V~-free γ-δ equations are also valid for most liquids including nonpolar and polar ones. These γ-δ equations can provide a way to estimate non-measurable parameters from measurable parameters for solid materials, which is beneficial to the application of the characteristic parameters (γ and δ) for solid material engineering.

In this study, polyimide was treated by atmospheric pressure dielectric barrier discharge plasma using a helium and/or helium-oxygen mixture gasses. The polyimide was placed between copper electrodes with dielectric material installed on the cathode electrode. To investigate the surface treatment, the plasmas as a function of power, treatment time, and plasma gasses were introduced on the polyimide substrate. The experimental results show that the polyimide treated by dielectric barrier discharge plasma increases the wetting property. This property can be attributed to the surface roughness and the water compatible functional groups. The roughness increases by helium plasma treatment and can be further improved by increasing plasma power or the presence of oxygen in the helium-oxygen mixture plasma. On the other hand, the plasma surface treatment led to formation of oxygen related functional groups of -C=O and -OH.

An interpretation of solid surfaces is generated based on physical considerations and the laws of thermodynamics. Like the widely used Owens–Wendt (OW) method, the proposed method uses liquids for characterization. Each liquid provides an absolute lower bound on the surface energy with some uncertainty from measurement variations. If multiple liquids are employed, the largest lower bound is taken as the most accurate, with uncertainty due to measurement errors. The more liquids used, the more accurate is the greatest lower bound. This method links generalizations of the Good–Girifalco equation with a general thermodynamic inequality relating the three-interfacial tensions in a three-phase equilibrium system. The method always satisfies this inequality with better than a 65% certainty. However, the OW seldom, if ever, conforms to this inequality and even then, the degree of satisfaction is insignificant. A reconciliation of the two methods is proposed based on rescaling the OW surface energies to conform to the inequality. This enables interpretations of dispersion and polar components of the surface energy, which are thermodynamically self-consistent. The proposed method is also capable of dealing with material exchange between liquid and solid phases, when the surface tension and contact angle of the saturated liquids can be measured.

Critical surface tension (CST) is a measure of solid surface tension and is mainly determined by measuring the contact angle of a droplet on a target solid surface. The concept of CST makes it possible to determine solid surface tension without any unprovable assumptions such as the Fowkes hypothesis. However, it requires somewhat special devices and skills for measuring the contact angle. In this work, we propose a simple method to determine the CST of a solid by measuring the droplet spreading area. This method is developed by combining the conventional CST with a simple analytical droplet model. The difference in estimated CSTs between our method and the conventional one is within 3.0%. Our method enables a quick and simple evaluation of the solid surface tension without special devices for measuring the contact angle.

Improvement of paint adhesion to aluminium surfaces is one of the main challenges in many industrial applications. In this paper, we introduce the atmospheric pressure air plasma jet as an appropriate candidate for preparation of 5052 aluminium surface alloy to improve paint adhesion in the industrial level. The employed plasma jet can promote paint adhesion to aluminium surface at the treatment velocity of 2 m/min and plasma size of 10 mm. Based on the cross-cut test, adhesion of polyurethane paint to the surface greatly increases from 1B to 5B level due to the plasma treatment. According to the results, the surface wettability increases under the influence of the plasma treatment so that water droplet contact angle reduces from 79.0°±2.0°–27.5°±2.0° after the treatment. Dyne test ink also denotes the increment of surface energy to the greater than 72 mN/m. Besides, we employ various analytical methods to investigate the physical and chemical changes arise from the plasma processing to the surface. Atomic force microscopy (AFM) results show a twofold increase in the roughness parameters of plasma treated surface which can result in a stronger paint and surface interlocking. Chemical analysis of the surface reveals that plasma treatment of the aluminium surface leads to the surface cleaning and formation of hydrophilic functional groups that attract much more water towards the surface and improves the paint adhesion.

The atmospheric pressure He- H 2 O plasma jet has been analyzed and its effects on the Kapton polyimide surface have been investigated in terms of discharge power effect. The polyimide surfaces before and after plasma treatment were characterized using atomic force microscopy (AFM), X-ray photoelectrons spectroscopy (XPS) and contact angle. The results showed that, increasing the discharge power induces remarkable changes on the emission intensity, rotational and vibrational temperatures of He- H 2 O plasma jet. At the low discharge power ≤5.2 W, the contact angle analysis of the polyimide surface remarkably decrease owing to the abundant hydrophilic polar C=O and N–C=O groups as well as increase of surface roughness. Yet, plasma treatment at high discharge power ≥5.2 W results in a slight decrease of the surface wettability together with a reduction in the surface roughness and polar groups concentrations.