Contact angle interpretation: re-evaluation of existing contact angle data

Green Protective Geopolymer Coatings: Interface Characterization, Modification and Life-Cycle Analysis

In the interest of solving the resource and environmental problems of the construction industry, low-carbon geopolymer coating ensures great durability and extends the service life of existing infrastructure. This paper presents a multidisciplinary assessment of the protective performance and environmental impacts of geopolymer coating. Various parameters, such as main substance, water-solid (W/S) ratio, activator type and curing time, were investigated for their effects on interface characterization in terms of contact angle, surface energy, mechanical properties and microstructure. These parameters had negligible effects on the amounts and types of hydrophilic functional groups of geopolymer surfaces. A combination of organic surface modifiers and geopolymer coatings was shown to ensure hydrophobic surface conditions and great durability. Silicon-based modifiers exhibited better wetting performance than capillary crystalline surfactants by eliminating hydroxyl groups and maintaining structural backbone Si-O-T (Si, Al) on geopolymers’ surfaces. Finally, life-cycle analysis was conducted to investigate the environmental performance. Geopolymer coating yielded substantially lower environmental impacts (50–80% lower in most impact categories) than ordinary Portland cement (OPC) coating. Silicon-based modifiers had negligible influence due to their minimal usage. Increasing the W/S ratio diluted the geopolymer coating and decreased the environmental impacts, and slag-based geopolymer coating achieved lower environmental impacts than FA-based and MK-based varietie.

Experimental and molecular dynamics study into the surfactant effect upon coal wettability

In order to improve the wettability and permeability of coal seams, the water injection efficiency of coal seams has to be boosted, the amount of dust generation has to be reduced, and coal and gas outburst must be prevented, and a surfactant is used to modulate the coal surface wettability. In this work, taking coal samples from Pingdingshan mine in Henan as the research object, their surface chemistry was initially scrutinized and then coal surface engineering via surfactants was inspected by a contact angle test. The coal wettability was ameliorated with surfactants, particularly using the 1 wt% non-ionic surfactant Triton X-100, which elicited a 47% lower contact angle than the raw coal. The surface free energy of the coal sample modified by 1.0 wt% Triton X-100 was increased from 44.51 mN m^-1 to 49.52 mN m^-1. The microstructural characteristics of coal samples allowed leveraging the Wiser model to construct three kinds of surfactant-coal adsorption models to dissect the adsorption configuration of the system. The results indicate that the addition of surfactants increases both the interaction of water with the coal and the diffusion coefficient of water molecules, resulting in the coal surface transformation from hydrophobicity to hydrophilicity. Our current work can provide salutary guidance and reference for coal water injection and dust suppression.

Atmospheric Corrosion Protection Performance and Mechanism of Superhydrophobic Surface Based on Coalescence-Induced Droplet Self-Jumping Behavior

The coalescence-induced droplet self-jumping behavior on the superhydrophobic surface (SHS) provides a new way to achieve atmospheric corrosion protection. This work controls the droplet self-jumping behavior by regulating the SHS's surface energy and analyzes the relevant mechanism from the energy perspective, revealing the key pathway by which the surface energy impacts the droplet self-jumping behavior. On this basis, the electrochemical impedance spectroscopy testing technique is used to evaluate the effect of the droplet self-jumping behavior on the SHS corrosion protection performance, and the SHS atmospheric corrosion protection mechanism based on the coalescence-induced droplet self-jumping behavior is revealed. This study provides theoretical guidance for the development of SHS-based anticorrosion protection.

Axisymmetric Drop Shape Analysis (ADSA): An Outline

Drop shape techniques for the measurement of interfacial tension are powerful, versatile and flexible. The shape of the drop/bubble depends on the balance between surface tension and external forces, e.g. gravity. This balance is reflected mathematically in the Laplace equation of capillarity. Axisymmetric Drop Shape Analysis (ADSA) is a commonly used drop shape technique. A streamlined version of the development of ADSA over the past several decades is presented to illustrate its validity and range of utility. Several configurations of interest will be considered and presented systematically. Shape and surface tension will be linked to a shape parameter based on proper concepts of differential geometry. The resulting shape parameter will be shown to allow determination of the range of applicability of such a drop shape method.

Progress in understanding wetting transitions on rough surfaces

The abrupt change in the apparent contact angle occurring on a rough surface is called wetting transition. This change may be spontaneous or promoted by external stimuli such as pressure or vibration. Understanding the physical mechanism of wetting transitions is crucial for the design of highly stable superhydrophobic and omniphobic materials. Wetting regimes occurring on rough surfaces are introduced. Experimental methods of study of wetting transitions are reviewed. Physical mechanisms of wetting transitions on rough surfaces are discussed. Time and energy scaling of wetting transitions are addressed. The problem of the stability of Cassie wetting on inherently hydrophobic and hydrophilic surfaces is discussed. The origin and value of a barrier separating the Cassie and Wenzel wetting states are treated in detail. Hierarchical roughness increases the value of the energy barrier. The stability of Cassie wetting observed on re-entrant topographies is explained. The irreversibility of wetting transitions is explained, based on the asymmetry of the energy barrier, which is low from the side of the metastable (higher-energy) state and high from the side of the stable state. The critical pressure necessary for a wetting transition is introduced. The problem of "dimension" of wetting transition is discussed. Reducing the micro-structural scales enlarges the threshold pressure of a wetting transition. The roles of gravity and air compressibility in wetting transitions are treated. The dynamics of wetting transitions is reviewed. The results of molecular simulations of wetting transitions are presented. The trends of future investigations are envisaged.

Theoretical models for surface forces and adhesion and their measurement using atomic force microscopy

The increasing importance of studies on soft matter and their impact on new technologies, including those associated with nanotechnology, has brought intermolecular and surface forces to the forefront of physics and materials science, for these are the prevailing forces in micro and nanosystems. With experimental methods such as the atomic force spectroscopy (AFS), it is now possible to measure these forces accurately, in addition to providing information on local material properties such as elasticity, hardness and adhesion. This review provides the theoretical and experimental background of afs, adhesion forces, intermolecular interactions and surface forces in air, vacuum and in solution.

Wetting transitions on biomimetic surfaces

Biomimetic hierarchical surfaces demonstrate a potential for a variety of green technologies, including energy conversion and conservation, owing to their remarkable water repellence. The design of such surfaces allowing emerging green applications remains a challenging scientific and technological task. Understanding the physical mechanism of wetting transitions (WTs) is crucial for the design of highly stable superhydrophobic materials. The main experimental and theoretical approaches to WTs are reviewed. Reducing the micro-structural scales is the most efficient measure needed to enlarge the threshold pressure of WTs. The trends of future investigations are envisaged.

Poly(trifluoroethylene) adsorption and heterogeneous photochlorination reactions

Heterogeneous (gas-solid) photochlorination reactions of poly(trifluoroethylene) (PF 3E) films were studied as a function of reaction time and light intensity. The rate of chlorination was found to be faster in high-intensity light when compared to the reaction in ambient light. PF 3E irreversibly adsorbed to oxidized silicon and covalently attached amine monolayers supported on silicon, producing hydrophobic thin films in the thickness range of 8-40 A. Adsorption conditions such as polymer concentration and solvent composition were investigated. Radical grafting of maleic anhydride to the polymer backbone resulted in increased adsorption on oxidized silicon.

Drop size effect on contact angle explained by nonextensive thermodynamics. Young's equation revisited

We applied the concepts of nonextensive thermodynamics [Turmine et al., J. Phys. Chem. B 108 (2004) 18980], to describe the equilibrium of a liquid drop placed on a solid substrate. This approach provides a consistent formal framework for analyzing the contact angle according to the nature and structure of the substrate, and also the size of the drops. The introduction, for the solid/liquid interface, of the concept of "fuzzy interface" characterized by a thermodynamic dimension, different from 2/3 (surface) allowed description of the case of substrates with ill-defined geometry (such as porous, structured systems, fractal structure systems, etc.) and straightforward explanation of the phenomenon of super hydrophobicity without using a layer of trapped air in possible anfractuosities. The application of the nonextensive thermodynamics relationships, allowed explanation of all the usual behaviors described in the literature (Young, modified Young, Wenzel, Cassie-Baxter) including the case of the composite interfaces made of materials with different natures and spatial structures (smooth, rough, homogeneous and heterogeneous surfaces). We show that the contact angle can vary with the drop volume according to a power law, and this was validated against values published in the literature. This study also has consequences for the relations between "thermodynamic dimension" and "fractal dimension."

Prediction of solid–water–hydrocarbon contact angle

The reliability of a recently developed solid-vapour and solid-liquid interfacial tension models has been investigated by applying them to predict liquid-vapour and liquid-liquid interfacial tension values. The impact of the geometrical molecular packing and the molecular orientations near the surface on the predicted values are discussed. The mutual solubility data are shown to be adequate for calculation of the interaction parameters in the solid-liquid model and a new equation, using this information, is developed for prediction of water-hydrocarbon interfacial tension. The model has been applied to recent data on water-methane-n-decane and water-methane-cyclohexane-n-decane interfacial tensions at elevated temperature and pressure and its reliability demonstrated. It is shown that the solid-liquid interfacial tension model is solely adequate for predicting the contact angle by applying it to mercury-water-benzene and stearic acid-water-n-decane systems.

Importance of Surface Tension Characterization for Food, Pharmaceutical and Packaging Products: A Review

This article reviews the various theoretical approaches that have been developed for determination of the surface tension of solids, and the applications to food industrial products. The surface tension of a solid is a characteristic of surface properties and interfacial interactions such as adsorption, wetting or adhesion. The knowledge of surface tension is thus of great interest for every domain involved in understanding these mechanisms, which recover a lot of industrial investigations. Indeed, it is the case for the packaging industry, the food materials science, the biomedical applications and the pharmaceutical products, cleaning, adhesive technology, painting, coating and more generally all fields in relation with wettability of their systems. There is however no direct method for measurements of surface tension of solids, except the contact angle measurements combined with an appropriate theoretical approach are indirect methods for estimation of surface tension of solids. Moreover, since the publication by Young (1805) who developed the basis of the theory of contact angle some two hundred years ago, measurements and interpretations are still discussed in scientific literature, pointing out the need to better understand the fundamental mechanisms of solid-liquid interfacial interactions. Applications of surface tension characterization in the field of food materials science are detailed, especially for packaging and coating applications, which recover different actual orientations in order to improve process and quality.

Recent developments in superhydrophobic surfaces and their relevance to marine fouling: a review

In this review, a brief synopsis of superhydrophobicity (i.e. extreme non-wettability) and its implications on marine fouling are presented. A short overview of wettability and recent experimental developments aimed at fabricating superhydrophobic surfaces by tailoring their chemical nature and physical appearance (i.e. substratum texture) are reviewed. The formation of responsive/"smart" surfaces, which adjust their physico-chemical properties to variations in some outside physical stimulus, including light, temperature, electric field, or solvent, is also described. Finally, implications of tailoring the surface chemistry, texture, and responsiveness of surfaces on the design of effective marine fouling coatings are considered and discussed.

The influence of flotation agent concentration on the wettability and flotability of polystyrene

The fundamental flotation process is the formation of a flocculant by air bubbles and solid particles in an aqueous solution. The behavior of plastic particles is significantly influenced by the wettability of the plastics. In this article the reciprocal relationship between the flotability and wettability of polystyrene was studied at different concentrations of flotation agents, particularly terpineol, polyethylene glycol dodecyl ether, tannic acid, and calcium lignosulfonate. The conclusions obtained demonstrate the dissimilar action of flotation depressants, what means different adhesion mechanisms on a plastic surface.

Prediction of solid–fluid interfacial tension and contact angle

Two simple equations have been developed using the lattice theory and the regular solution assumption to predict the solid-vapor and solid-liquid interfacial tension. The required parameters are the liquid critical temperature and volume, the solid melting temperature and the molar volume of liquid and solid compounds. To confirm the models, the predicted solid-fluid interfacial tension values have been used to predict the contact angle of the liquid drop on the solid surface applying Young's equation. Agreement of the predicted contact angle with the experimental data reveals the reliability of the developed models.

The solid surface free energy calculation

This paper follows the "defense" of the Good-van Oss-Chaudhury (GvOC) acid-base approach made in Part I and carries out a detailed analysis of the Zisman critical surface energy and, mainly, of the Neumann equation-of-state (EQS) theory. The analysis is made on both a "practical" and a theoretical basis, trying to highlight the acceptable fitting results of axisymmetric drop shape analysis (ADSA) methods and their independence of the assumed thermodynamic foundations of EQS. Some new and original criticisms of the EQS approach are raised and it is shown that other purely semiempirical models, represented by different fitting equations with the same number of parameters, can represent the data measured by ADSA method with the same goodness as EQS. The equation of state appears as one of many semiempirical approaches for the evaluation of surface free energy of solids. Independent of the previous analysis, the criteria used in ADSA measurements are evaluated and some comments made on them.

Thermodynamic method for prediction of surfactant-modified oil droplet contact angle

A model applying surfactant self-assembly theory and classical thermodynamics has been developed to aid in the prediction of solid surface cleaning by aqueous surfactant solutions. Information gained from a combination of surfactant self-assembly behavior and cleaning system parameters, such as oil species, surfactant type, temperature, alkalinity, and solid surface type has been shown to provide insight into surface cleaning. The model combines minimization of free energy, pertinent component distribution mechanisms, and surfactant self-assembly processes to provide a methodology for the predicting of oil droplet contact angles. Such predictive capabilities will allow for the development of beneficial environmental and economic changes to industrial and commercial surface cleaning and degreasing processes. Results from the model will be compared to experimental data to verify the capability of the theory to account for the effect of solutions parameters on oil droplet behavior. The model, while approximate in nature, has shown a remarkable quantitative predictive ability.