Critical wetting of surfaces in systems with long-range forces

Grain Boundary Complexions and Phase Transformations in Al- and Cu-Based Alloys

High-pressure torsion has been used to obtain the ultra-fine grained (UFG) state with a high specific area of grain boundaries (GBs) in Al-Zn, Al-Mg, Cu-Ag, Cu-Co, and Cu-Ni solid solutions with face-centered cubic (fcc) lattices. The UFG samples were heated in a differential scanning calorimeter (DSC). Small endothermic peaks in the DSC curves were observed in the one-phase solid-solution area of the respective phase diagrams, i.e., far away from the bulk solidus and solvus lines. A possible explanation of these endothermic peaks is based on the hypothesis of phase transformations between GB complexions. This hypothesis has been supported by observations with transmission electron microscopy and electron backscattering diffraction. The new lines of GB phase transformations have been constructed in the Al-Zn, Al-Mg, Cu-Ag, Cu-Co, and Cu-Ni bulk phase diagrams.

Theory of Liquid Film Growth and Wetting Instabilities on Graphene

We investigate wetting phenomena near graphene within the Dzyaloshinskii-Lifshitz-Pitaevskii theory for light gases of hydrogen, helium, and nitrogen in three different geometries where graphene is either affixed to an insulating substrate, submerged or suspended. We find that the presence of graphene has a significant effect in all configurations. When placed on a substrate, the polarizability of graphene can increase the strength of the total van der Waals force by a factor of 2 near the surface, enhancing the propensity towards wetting. In a suspended geometry unique to two-dimensional materials, where graphene is able to wet on only one side, liquid film growth becomes arrested at a critical thickness, which may trigger surface instabilities and pattern formation analogous to spinodal dewetting. The existence of a mesoscopic critical film with a tunable thickness provides a platform for the study of a continuous wetting transition, as well as the engineering of custom liquid coatings. These phenomena are robust to some mechanical deformations and are also universally present in doped graphene and other two-dimensional materials, such as monolayer dichalcogenides.

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.

Wetting in electrolyte solutions

Wetting of a charged substrate by an electrolyte solution is investigated by means of classical density functional theory applied to a lattice model. Within the present model the pure, i.e., salt-free solvent, for which all interactions are of the nearest-neighbor type only, exhibits a second-order wetting transition for all strengths of the substrate-particle and the particle-particle interactions for which the wetting transition temperature is nonzero. The influences of the substrate charge density and of the ionic strength on the wetting transition temperature and on the order of the wetting transition are studied. If the substrate is neutral, the addition of salt to the solvent changes neither the order nor the transition temperature of the wetting transition of the system. If the surface charge is nonzero, upon adding salt this continuous wetting transition changes to first-order within the wide range of substrate surface charge densities and ionic strengths studied here. As the substrate surface charge density is increased, at fixed ionic strength, the wetting transition temperature decreases and the prewetting line associated with the first-order wetting transition becomes longer. This decrease of the wetting transition temperature upon increasing the surface charge density becomes more pronounced by decreasing the ionic strength.

Effective exponents in the long-range critical wetting of alkanes on aqueous substrates

Alkanes on water show a two-stage wetting transition. Upon raising the temperature, a first-order transition from a molecularly thin to a mesoscopically thick liquid film is followed by a continuous divergence of the film thickness. This second transition is brought about by long-range interactions between adsorbate and substrate and is, therefore, referred to as long-range critical wetting. The divergence of the film thickness is theoretically expected to occur according to the asymptotic power law l approximately (Tw,c-T)betas, with betas=-1. This value has indeed been found for pentane on pure water; however, for hexane on salt solutions of different concentrations, betas=-0.73 was found for a 1.5M solution of NaCl and betas=-0.57 for a 2.5M salt solution. In addition, for hexane on a 2.5M solution of NaCl, an exponent of alphas=0.1 was found from contact-angle measurements, differing greatly from the theoretically expected value of alphas=-1. Using Dzyaloshinskii-Lifshitz-Pitaevskii theory, we calculate effective local exponents in order to explain the experimental findings. Taking into account the uncertainty of the exponents derived from experiments as well as the temperature range in which the measurements were carried out, a reasonable agreement between theory and experiment is found, thereby providing a consistent approach to resolving the apparently anomalous behavior of hexane on brine. The experimentally observed exponents betas=-0.57 and alphas=0.1 are also compatible with a long-range tricritical wetting transition, which is characterized by betas=-1/2 and alphas=0; this alternative explanation of the experimental findings is neither supported nor completely ruled out by our calculations.

Effect of a fluid-wall interaction on a drying layer

Density profiles of Lennard-Jones liquid confined in slit pore with weakly attractive or hard walls are studied along the pore coexistence curve. In the large pores, the thickness L0 of a drying layer increases approximately as the bulk correlation length xi- when approaching the bulk critical temperature Tc. Strengthening of the fluid-wall interaction suppresses the thickness of a drying layer and yields a logarithmic growth of L0 with tau=1-T/Tc. Existence of the two distinct regimes in the temperature behavior of L0 may reflect a partial drying transition expected for the weak long-range fluid-wall interactions. Confinement in small pores suppresses strongly the drying layer and its thickness grows as approximately ln(tau) upon heating with subsequent saturation. The sharpness of the interface between a liquid and a drying layer is studied in dependence on the fluid-wall interaction and pore width.

Formation and mobility of droplets on composite layered substrates

A mesoscale fluid film placed on a solid support may break up and form droplets. In addition, droplets may exhibit spontaneous translation by modifying the wetting properties of the substrate, resulting in asymmetry in the contact angles. We examine mechanisms for droplet formation and motion on uniform and terraced landscapes, i.e., composite substrates. The fluid film stability, droplet formation and velocity are studied theoretically in the isothermal case using a lubrication approach in one spatial dimension. The droplet properties are found to involve contributions from both the terraced layer thickness and molecular interactions via the disjoining potential.

Theoretical description of the adsorption and the wetting behavior of alkanes on water

The wetting behavior of alkanes of medium chain length (e.g., pentane, hexane, and heptane) on water is more complex than the usually observed first-order wetting transition from partial to complete wetting by showing a sequence of two transitions. In this sequential-wetting scenario, a first-order transition from a microscopically thin to a mesoscopically thick layer of liquid on the substrate surface is followed by a continuous divergence of the film thickness upon increase of the temperature. This critical transition to complete wetting at T(w,c) is solely determined by long-range interactions between substrate and adsorbate, which are well-described by Dzyaloshinskii-Lifshitz-Pitaevskii [Adv. Phys. 10, 165 (1961)] theory in terms of the static dielectric constants and the refractive indices of the media involved. The first-order thin-thick transition, however, which occurs at a lower temperature T(w,1), results from an interplay of short-range and long-range forces and is notoriously more difficult to describe because a satisfactory theory of the short-range interactions between substrate and adsorbate is still missing. The approach presented in this paper attempts to account for the short-range interactions in an effective way: Within a Cahn-type [J. Chem. Phys. 66, 3667 (1977)] theory that has been augmented for long-range interactions and modified to treat the first layer of adsorbed molecules in a lattice-gas approach, the contact energy is deduced from the surface pressure, which in turn is calculated using a two-dimensional van der Waals equation of state and an expression for the Henry's law constant that was derived by Hirasaki [J. Adhes. Sci. Technol. 7, 285 (1993)]. The method uses only the dielectric properties of the isolated bulk media and simple assumptions on the size and the shape of the adsorbed alkane molecules and leads to satisfactory results for the transition temperatures T(w,1) and T(w,c).

Long-range critical wetting: observation of a critical end point

Alkanes deposited on aqueous substrates exhibit two different types of wetting behavior: alternatively to the usual first-order wetting transition, a sequential-wetting scenario of a long-range critical wetting transition preceded by a first-order thin-thick transition may be observed. Here, we present the first successful experimental attempt to locate the transition point between the standard first-order wetting and the long-range critical wetting: a critical end point, observed in a mixture of pentane and hexane which is deposited on an aqueous solution of glucose. Furthermore, we present the first direct measurement of the contact angle in the intermediate wetting state (frustrated-complete wetting) in the sequential-wetting scenario of hexane on brine and compare to theoretical predictions.

Crossover from first-order to critical wetting: short-range tricritical wetting

We study wetting in liquid mixtures of methanol and the n-alkanes. Mixing alkanes of different chain lengths, we can examine the crossover between critical (continuous) and first-order (discontinuous) wetting transitions. Measurements of the film thickness and surface specific heat exponent indicate that for carbon number n between 11 (undecane) and 9 (nonane), there is a crossover from first-order to critical wetting with a tricritical wetting point between an effective alkane carbon number of 9.6 and 10. The observed variation of the specific heat exponent in the tricritical region agrees fairly well with the predictions of a simple mean-field model with only short-range interactions.

Density functional theory of long-range critical wetting

The wetting properties of a fluid adsorbed at a solid substrate are studied by means of density functional theory. Explicit calculations of the substrate-liquid and substrate-gas density profiles are carried out and used to evaluate the asymptotic expansion for the interface potential of a system with long-range interactions. The range of validity of the asymptotic expansion is checked by comparing it with the interface potential obtained numerically through the constrained minimization of the density functional free energy. Depending on the parameters of the fluid-fluid and substrate-fluid interactions we find first-order or critical wetting transitions. In a limited range of parameters, the critical wetting transition is preceded by a first-order transition between a microscopic and a mesoscopic film, thus corroborating previous calculations and experiments for alkanes on brine. We find that the behavior of the alkanes on brine is not universal, since it requires fine-tuning of the fluid-fluid and substrate-fluid interactions. Finally, we investigate the influence of the short- and long-range forces on the location of the first-order transition. We find that for the models studied, the long-range forces cannot be treated perturbatively. Thus for this type of model it is not possible to separate the effects of short- and long-range forces as done in Landau theories, where the long-range forces are treated perturbatively.

First-order and critical wetting of alkanes on water

Ellipsometry measurements of the wetting behavior of different alkanes on water show a sequence of two wetting transitions: a first-order (discontinuous) transition followed by a critical (continuous) one. We report temperature-induced wetting transitions for different alkanes and a novel pressure-induced wetting transition for an alkane mixture. The experiments enable us to determine the global wetting phase diagram as a function of chain length and temperature which we subsequently calculate theoretically. The two transition lines are found to be approximately parallel, in accordance with basic theoretical arguments.