Continuous wetting transitions in Xe adsorbed on NaF and on plated Cs and Rb substrates

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.

Adsorption of Ne on alkali surfaces studied with a density functional theory

A density functional formalism is applied to investigate the wetting behavior of Ne adsorbed on planar substrates. The study is performed over the complete range of temperatures spanned from the triple point T_{t} up to the critical one T_{c} . For this purpose, an effective attractive pair potential was built on the basis of a separation procedure. This approach yields a good description of properties of the liquid-vapor interface at coexistence in the whole range of temperatures T_{t} < or = T < or = T_{c} . The adsorption of Ne on alkali metals and the alkaline-earth metal Mg is analyzed. This sequence of substrates exhibit increasing attractive strength leading to a variety of wetting situations throughout the interval T_{t}<-->T_{c} . A comparison with experimental data and other microscopic calculations is done. The predictions of a simple model are discussed. For NeRb we were able to resolve prewetting lines. Results obtained from a density functional are reported for NeK and NeMg . In the case of the latter system the interesting behavior occurs close to T_{t} . According to our results, Ne wets surfaces of Na and Li, and this statement is in agreement with the whole picture of the analyzed substrates.

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.

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.

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.