Critical Wetting Research Articles

Wetting and emergence of long-range couplings in arrays of fluid cells.

Critical wetting is of crucial importance for the phase behavior of a simple fluid or Ising magnet confined between walls that exert opposing surface fields so that one wall favors liquid (spin up), while the other favors gas (spin down). We show that arrays of boxes filled with fluid and linked by channels with appropriately chosen opposing walls can exhibit long-range cooperative effects on a length scale far exceeding the bulk correlation length. We give the theoretical foundations of these long-range couplings by using a lattice gas (Ising model) description of a system.

Molecular Insights into the Wettability and Adsorption of Acid Gas-Water Mixture.

Sequestration of acid gas in geological formations is a disposal method with potential economic and environmental benefits. The process is governed by variables such as gas-water interfacial tension, wetting transition, and gas adsorption into water, among other things. However, the influence of the pressure and temperature on these parameters is poorly understood. This study investigates these parameters using coarse-grained molecular dynamics (CG-MD) simulations and density gradient theory (DGT). Simulations were carried out at 313.15 K and a pressure range of 0-15 MPa. A comparison was made against H2S-water systems to clarify the effects of adsorption on interfacial tension due to vapor-liquid-liquid equilibrium. The predicted H2S-water interfacial tension and phase densities by CG-MD and DGT matched the experimental values well. The adsorption can be quantified via the Gibbs Adsorption function Γ12, which correlated well with the three-phase transition. On the one hand, pressure increments below the three-phase transition revealed a significant adsorption of H2S. On the other hand, above the three-phase transition, the Gibbs Adsorption capacity remained constant, which indicated a saturation of H2S at the water surface due to liquid-liquid equilibrium. Finally, H2S behaves markedly differently in wetting transition, rather than the involved for CO2 to different molecular layers beneath the surface of aqueous solutions. In this respect, H2S is represented by a first-order wetting transition while CO2 presents a critical wetting. Finally, it has also been found that the preferential adsorption of H2S over the H2O interface is greater if compared to that of CO2, due to its strong interaction with water. In fact, we have also demonstrated that CO2 under triphasic conditions strongly influences the wetting of the ternary system.

Critical wetting in the (2+1)D solid-on-solid model

Critical wetting in the (2+1)D solid-on-solid model

Mechanical criteria and analytical modeling of mixed-state wetting on multiscale surfaces: Of the importance of nanoscale topography for water-repellency

Multiple natural surfaces exhibit excellent water-repellent properties. Such surfaces systematically demonstrate multiscale surface topographies. However, current mechanical criteria for composite-wetting equilibrium can only be applied on model micrometric pillars. This paper suggests a theoretical basis on the foundation of which we introduce multiscale mechanical criteria for mixed-state equilibrium on real surfaces. A method for the analytical modeling of mixed-state wetting on real textured surfaces and the retrieval of critical wetting parameters such as the total solid-liquid contact area and the total triple line perimeter at the effective anchorage depth is introduced. This method is applied to the case of the sacred Lotus leaf on which we predict the effective anchorage depth of the triple lines. Results show that the nanoscale topography of these leaves leads to the minimization of the effective anchorage depth of the droplet on the sides of the micrometric asperities, a determining parameter with regards to its effect on the total solid-liquid contact area and the total triple line perimeter, unveiling a little more the role of nanoscale wax crystals in nature’s strategies for water-repellency.

Liquid Repellence of Phobic Fiber Networks.

The wetting behavior of fiber networks, which are central to many research and industrial applications, can be difficult to predict accurately owing to their complex, heterogeneous structure. The cylindrical pore model, widely used to interpret and predict the forced wetting of hydrophobic porous materials, often does not yield correct results when working with fibrous networks like paper substrates and non-woven fabrics. This is because these materials exhibit variation in pore size, fiber length, and fiber diameter, as well as a reentrant pore geometry. Quantitative prediction of the critical wetting resistance of hydrophobized papers to arbitrary entrant liquids requires a more sophisticated analytical approach that considers this unique fibrous structure and the effect of stochastic variations within the pore matrix. In this work, we directly measure the critical breakthrough pressure for different porous substrates across various wetting entrant liquids. To isolate the effects of the structure and stochastics on critical wetting behavior of fibrous networks, we analyze additional materials strategically chosen for their subsets of structural features. Ultimately, we formulate a method that demonstrates physical reasonableness, numerical accuracy, and the ability to elucidate the effects of pore size, pore size distribution, fiber diameter, fiber diameter distribution, surface wettability, and liquid surface tension on critical breakthrough pressure of liquids through hydrophobic fibrous networks.

Casimir Contribution to the Interfacial Hamiltonian for 3D Wetting.

Previous treatments of three-dimensional (3D) short-ranged wetting transitions have missed an entropic or low-temperature Casimir contribution to the binding potential describing the interaction between the unbinding interface and wall. This we determine by exactly deriving the interfacial model for 3D wetting from a more microscopic Landau-Ginzburg-Wilson Hamiltonian. The Casimir term changes the interpretation of fluctuation effects occurring at wetting transitions so that, for example, mean-field predictions are no longer obtained when interfacial fluctuations are ignored. While the Casimir contribution does not alter the surface phase diagram, it significantly increases the adsorption near a first-order wetting transition and changes completely the predicted critical singularities of tricritical wetting, including the nonuniversality occurring in 3D arising from interfacial fluctuations. Using the numerical renormalization group, we show that, for critical wetting, the asymptotic regime is extremely narrow with the growth of the parallel correlation length characterized by an effective exponent in quantitative agreement with Ising model simulations, resolving a long-standing controversy.

Anticorrosion Efficiency of Inhibitor Coatings Based on Ammonium Cation with Different Substituents: The Influence of Wettability and Molecular Structure

In this work, different cationic surfactants with various aliphatic and aromatic ammonium cations were used to prepare inhibitor coatings and were characterized by different techniques such as IR spectroscopy and NMR. The inhibitor coatings were prepared by electrografting on the steel surface and their anticorrosion properties were evaluated in different media (HCl, H2SO4 and NaCl solutions). The electrochemical potentiodynamic polarization technique was used to study the inhibition efficiency of the prepared coatings. The dependence of the wetting properties of the electrografted layer and its homogeneity on the molecular structure of the prepared surfactants was studied. Particular attention was paid to the relationship between the properties of these surfactants in terms of critical micellar concentration, packing and wetting, and the anti-corrosion efficiency of their coatings. In this paper, we discuss the synergistic inhibition effect and the anticorrosion efficiency.

Preparation and evaluation of novel water soluble nonionic monoesters with expected surface activity (Dept.P)

A novel five series of monoesters were prepared two series by reaction of oxypropylated 1, 6 hexane diol with two types of fatty acid chloride (lauroyl and palmi and three series by the reaction of polyethylene glycol (MWs = 400, 1500, 2000) with three types of fatty acid chloride (decanoyl, lauroyl and palmitoyl chloride)]. The unique structural features of these surfactants were confirmed by spectroscopic tools (IR- 'H NMR). These nonionic monoesters have been found to exhibit excellent surface active properties including surface tension, interfacial tension, critical micelle concentration (CMC), low foaming, emulsion stability, and wetting. Also good biodegradability in river water, stability to hydrolysis in acidic and alkaline media, solubilization and dispersant properties in disperse dye systems were determined and evaluated. The antimicrobial and antifungal properties of these prepared monoesters were measured and evaluated .A comparison studies were done between the chemical structures and surface properties of such compounds.

Measuring failure pressure of porous superhydrophobic coatings via microfluidic method

The transition of critical wetting conditions on superhydrophobic surfaces, that is, from Cassie state to Wenzel state, often indicates the failure of super-hydrophobicity on the surface. Recognizing the critical hydrostatic pressure at which a superhydrophobic surface starts departing from the Cassie state is important in the application of surfaces used under water. However, the measurement of the critical pressure for a superhydrophobic surface often involves complicated methods and systems such as optical spectroscopy and pressure vessels. This paper reports a microfluidic method for the measurement of critical failure pressure of superhydrophobic porous coatings by allowing water to penetrate the coatings from cross-sectional direction in microchannels. Two types of superhydrophobic porous coatings with different hydrophobicities, namely, titanium oxide (TiO2) coatings and TiO2/single-walled carbon nanotube (SWNT) coatings, were fabricated inside microchannels by liquid-phase deposition and lift-off process. Experimental results show that the critical failure pressures of TiO2 coatings decrease from 36.6 to 28.0 mBar as the porosity increases. TiO2/SWNT coatings with porosity of 19% show a higher failure pressure of 46.1 mBar due to its higher contact angle and lower hysteresis in comparison with TiO2 coatings. The method presented here provides a simple way to measure the critical failure pressure of superhydrophobic coatings. Although only TiO2 and TiO2/SWNT coatings were tested, any porous coating material which is compatible with lift-off process can be measured by this microfluidic method.

The effects of geometric non-additivity on the wetting of symmetric mixtures at a wall

The paper discusses the behavior of symmetric binary mixtures of Lennard-Jones particles in contact with non-selective smooth walls. Using the Monte Carlo simulation method in the grand canonical ensemble, we have elucidated the effects of negative and positive geometric non-additivity on the wetting behavior in a series of systems characterized by different strength of the fluid-wall interaction.It has been shown that even in the case of mixtures which do not exhibit demixing transition the geometric non-additivity considerably changes the wetting behavior of symmetric mixtures. In particular, it has been demonstrated that the structure and stability of the bulk solid and liquid phases play an important role, and lead to non-monotonous changes of the wetting temperature when the geometric non-additivity changes. It has been also shown that the systems with negative geometric non-additivity exhibit a triple-point wetting over a certain range of the surface potential strength. In such cases, the first-order wetting transition occurs when the wetting temperature is higher than the bulk triple point temperature and becomes a continuous transition (critical wetting) when the triple wetting occurs. On the other hand, the systems with positive geometric non-additivity have not been observed to show the triple point wetting, and the wetting temperature decreases smoothly when the strength of the surface field increases. For sufficiently strong surface fields, the adsorbed film develops in a layer-by-layer mode, independently of the geometric non-additivity.

Drying and wetting transitions of a Lennard-Jones fluid: Simulations and density functional theory.

We report a theoretical and simulation study of the drying and wetting phase transitions of a truncated Lennard-Jones fluid at a flat structureless wall. Binding potential calculations predict that the nature of these transitions depends on whether the wall-fluid attraction has a long ranged (LR) power law decay or is instead truncated, rendering it short ranged (SR). Using grand canonical Monte Carlo simulation and classical density functional theory, we examine both cases in detail. We find that for the LR case wetting is first order, while drying is continuous (critical) and occurs exactly at zero attractive wall strength, i.e., in the limit of a hard wall. In the SR case, drying is also critical but the order of the wetting transition depends on the truncation range of the wall-fluid potential. We characterize the approach to critical drying and wetting in terms of the density and local compressibility profiles and via the finite-size scaling properties of the probability distribution of the overall density. For the LR case, where the drying point is known exactly, this analysis allows us to estimate the exponent ν∥, which controls the parallel correlation length, i.e., the extent of vapor bubbles at the wall. Surprisingly, the value we obtain is over twice that predicted by mean field and renormalization group calculations, despite the fact that our three dimensional system is at the upper critical dimension where mean field theory for critical exponents is expected to hold. Possible reasons for this discrepancy are discussed in the light of fresh insights into the nature of near critical finite-size effects.

Soil aggregate breakdown in response to wetting rate during the inter-rill and rill stages of erosion in a contour ridge system

Wetting rate is one of the major factors determining the magnitude of slaking force in soils. Differences in wetting rate result in varying degrees of soil aggregate breakdown that alters the distribution of particle sizes in sediment. The objective of the present study was to determine how various wetting rates affected soil aggregate breakdown and soil fraction distribution in a contour ridge system. In the simulated rainfall experiment, the breakdown of 16 sizes soil aggregate was evaluated under five wetting rates (10, 20, 30, 60, and 90mmh−1) at different stages of erosion. The greatest fraction of sediment was microaggregate (accounting for 63.66%–69.19% of the total), and 20–50μm and 50–100μm classes were dominant within microaggregate fraction. The obviously increased loss of sediment from these two sizes occurred mainly as a result of the breakdown of 2–5mm and 1–2mm fractions during the inter-rill stage of erosion, and it was induced by the collapse of 0.25–0.5mm, 0.5–1mm, and 100–200μm aggregates during rill erosion. Critical wetting rate thresholds of 20mmh−1 controlled the breakdown of 2–5mm and 1–2mm fractions during the inter-rill stage. During the rill stage, when the wetting rate exceeded 10mmh−1, the breakdown of 0.25–0.5mm and 0.5–1mm sizes intensified and the critical threshold for the collapse of 100–200μm aggregate was 20mmh−1. These findings enhance our understanding of the breakdown mechanism of soil aggregates and supply guidance for implementing contour ridge systems.

Wetting 12Cr18Ni10Ti and EK-173 reactor steels using a eutectic melt of PbBi and alloys of it with lithium

Results from systematic experimental studies of the temperature dependences of the surface wetting of a number of EK-173, 12Cr18Ni9Тi, and 12Cr18Ni10Тi structural steels using liquid lead, bismuth, and a eutectic melt of PbBi with lithium under vacuum and/or an argon protective atmosphere. Critical wetting temperatures are found on the polytherms of the wetting of EK-173, 12Cr18Ni9Тi, and 12Cr18Ni10Тi steels using lead, bismuth, and a eutectic melt of PbBi under vacuum. It is shown that steel of EK-173 grade is more stable than 12Cr18Ni9Тi structural steel when in contact with a eutectic melt of PbBi at higher temperatures. Steel wetting is considerably higher under vacuum than in a protective argon atmosphere. Study of the wetting of 12Cr18Ni10Тi steel using liquid alloys of (PbBi)eut with lithium revealed a tendency toward improved wetting of steel with higher lithium content in a eutectic melt of PbBi.

Phase transitions of fluids in heterogeneous pores

We study phase behaviour of a model fluid confined between two unlike parallel walls in the presence of long range (dispersion) forces. Predictions obtained from macroscopic (geometric) and mesoscopic arguments are compared with numerical solutions of a non-local density functional theory. Two capillary models are considered. For a capillary comprising of two (differently) adsorbing walls we show that simple geometric arguments lead to the generalized Kelvin equation locating capillary condensation very accurately, provided both walls are only partially wet. If at least one of the walls is in complete wetting regime, the Kelvin equation should be modified by capturing the effect of thick wetting films by including Derjaguin's correction. Within the second model, we consider a capillary formed of two competing walls, so that one tends to be wet and the other dry. In this case, an interface localized-delocalized transition occurs at bulk two-phase coexistence and a temperature $T^*(L)$ depending on the pore width $L$. A mean-field analysis shows that for walls exhibiting first-order wetting transition at a temperature $T_{w}$, $T_{s}>T^*(L)>T_{w}$, where the spinodal temperature $T_{s}$ can be associated with the prewetting critical point, which also determines a critical pore width below which the interface localized-delocalized transition does not occur. If the walls exhibit critical wetting, the transition is shifted below $T_{w}$ and for a model with the binding potential $W(\ell)=A(T)\ell^{-2}+B(T)\ell^{-3}+\cdots$, where $\ell$ is the location of the liquid-gas interface, the transition can be characterized by a dimensionless parameter $\kappa=B/(AL)$, so that the fluid configuration with delocalized interface is stable in the interval between $\kappa=-2/3$ and $\kappa\approx-0.23$.

Critical wetting, first-order wetting, and prewetting phase transitions in binary mixtures of Bose-Einstein condensates

An ultralow-temperature binary mixture of Bose-Einstein condensates adsorbed at an optical wall can undergo a wetting phase transition in which one of the species excludes the other from contact with the wall. Interestingly, while hard-wall boundary conditions entail the wetting transition to be of first order, using Gross-Pitaevskii theory we show that first-order wetting as well as critical wetting can occur when a realistic exponential optical wall potential (evanescent wave) with a finite turn-on length $\lambda$ is assumed. The relevant surface excess energies are computed in an expansion in $\lambda/\xi_i$, where $\xi_i$ is the healing length of condensate $i$. Experimentally, the wetting transition may best be approached by varying the interspecies scattering length $a_{12}$ using Feshbach resonances. In the hard-wall limit, $\lambda \rightarrow 0$, exact results are derived for the prewetting and first-order wetting phase boundaries.

Hydro-Oleophobic Property of Butterfly Wing Surface and Biomimetic Fabrication of Hydrophobic Silver Film

The hydrophobicity and oleophobicity(methanol repellency) of butterfly wing surfaces were measured by a video-basedcontact angle (CA) meter. The multi-dimensional microstructure of the wingsurfaces was characterized by a scanning electron microscope (SEM) and an atomic force microscope (AFM). The wingsurface exhibits superhydrophobicity (water CA 150.4~159.2°) and low adhesion (water sliding angle 1~3°). Meanwhile, the wingsurface displays high repellency against methanol. The critical concentrationsfor wetting and spreading-wetting of methanol solution on the wing surface are60% and 80%, respectively. The butterfly wing surface is ofhydro-oleophobicity. The wing surface possessescomplicated hierarchicalmicrostructures. Using the butterfly wing as a bio-template, the hydrophobicsilver films were prepared. Water CA increases from metal silver’s intrinsicCA 63.0° maximally to 139.2° (Speyeria aglaja, 5 nm silver film). The microstructures on thewing surface result in the transition of metal silver from hydrophilic tohydrophobic. The butterfly wing can be used as a template for design of smartinterface and functional surface.

Critical wetting in the two-dimensional Ising ferromagnet confined between inhomogeneous walls

We present a numerical study of the critical wetting behavior of an Ising magnet confined between two walls, separated by a distance L, where short-range inhomogeneous surface magnetic fields act. So, samples are assumed to have a size L × M, L being the width and M the length, respectively. By considering surface fields varying spatially with a given wavelength or period (λ), H 1(x,λ) with 1 ≤ x ≤ M, we found that the wetting temperature is given by the exact result of Abraham [D.B. Abraham, Phys. Rev. Lett. 44, 1165 (1980)] provided that an effective field given by the spacial average value $$\left( {H_{eff} \equiv \tfrac{1} {\lambda }\int_0^\lambda {H_1 (x,\lambda )dx > 0} } \right)$$ is considered. The above results hold in the low wavelength regime, while for λ → ∞ and a bivaluated surface field (i.e., H max for x ≤ M/ 2, and δ H max for x>M/ 2, with 0 <δ< 1), one observes two almost independent wetting transitions, both being compatible with Abraham’s exact results corresponding to H max and δ H max, respectively. On the other hand, for H 1(x,λ) ≠ 0 but H eff = 0 bulk standard critical behavior results is observed.

Critical and Tricritical Wetting in the Two-Dimensional Blume–Capel model

The two-dimensional Blume–Capel model with free surfaces where a surface field $$H_1$$ acts and the “crystal field” (controlling the density of the vacancies) takes a value $$D _s$$ different from the value $$D$$ in the bulk, is studied by Monte Carlo methods. Using a recently developed finite size scaling method that studies thin films in a $$L \times M$$ geometry with antisymmetric surface fields $$(H_L=-H_1)$$ and keeps a generalized aspect ratio $$c = L^2/M$$ constant, surface phase diagrams are computed for several typical choices of the parameters. It is shown that both second order and first order wetting transitions occur, separated by tricritical wetting behavior. The special role of vacancies near the surface is investigated in detail.

Order of wetting transitions in electrolyte solutions.

For wetting films in dilute electrolyte solutions close to charged walls we present analytic expressions for their effective interface potentials. The analysis of these expressions renders the conditions under which corresponding wetting transitions can be first- or second-order. Within mean field theory we consider two models, one with short- and one with long-ranged solvent-solvent and solvent-wall interactions. The analytic results reveal in a transparent way that wetting transitions in electrolyte solutions, which occur far away from their critical point (i.e., the bulk correlation length is less than half of the Debye length) are always first-order if the solvent-solvent and solvent-wall interactions are short-ranged. In contrast, wetting transitions close to the bulk critical point of the solvent (i.e., the bulk correlation length is larger than the Debye length) exhibit the same wetting behavior as the pure, i.e., salt-free, solvent. If the salt-free solvent is governed by long-ranged solvent-solvent as well as long-ranged solvent-wall interactions and exhibits critical wetting, adding salt can cause the occurrence of an ion-induced first-order thin-thick transition which precedes the subsequent continuous wetting as for the salt-free solvent.

Renormalization group calculations for wetting transitions of infinite order and continuously varying order: Local interface Hamiltonian approach

We study the effect of thermal fluctuations on the wetting phase transitions of infinite order and of continuously varying order, recently discovered within a mean-field density-functional model for three-phase equilibria in systems with short-range forces and a two-component order parameter. Using linear functional renormalization group calculations within a local interface Hamiltonian approach, we show that the infinite-order transitions are robust. The exponential singularity (implying 2-α(s)=∞) of the surface free energy excess at infinite-order wetting as well as the precise algebraic divergence (with β(s)=-1) of the wetting layer thickness are not modified as long as ω<2, with ω the dimensionless wetting parameter that measures the strength of thermal fluctuations. The interface width diverges algebraically and universally (with ν([perpendicular])=1/2). In contrast, the nonuniversal critical wetting transitions of finite but continuously varying order are modified when thermal fluctuations are taken into account, in line with predictions from earlier calculations on similar models displaying weak, intermediate, and strong fluctuation regimes.