Wetting and layering in critical binary fluid mixtures

Critical energy-density profile near walls

We examine critical adsorption for semi-infinite thermodynamic systems of the Ising universality class when they are in contact with a wall of the so-called normal surface universality class in spatial dimension d=3 and in the mean-field limit. We apply local-functional theory and Monte Carlo simulations in order to quantitatively determine the properties of the energy density as the primary scaling density characterizing the critical behaviors of Ising systems besides the order parameter. Our results apply to the critical isochore, near two-phase coexistence, and along the critical isotherm if the surface and the weak bulk magnetic fields are either collinear or anticollinear. In the latter case, we also consider the order parameter, which so far has yet to be examined along these lines. We find the interface between the surface and the bulk phases at macroscopic distances from the surface, i.e., the surface is "wet." It turns out that in this case the usual property of monotonicity of primary scaling densities with respect to the temperature or magnetic field scaling variable does not hold for the energy density due to the presence of this interface.

Layering and prewetting in experimental systems: an assessment cross-referred to current theories

The first aim of this review is to assess the experimental data available about surface phase transitions at the contact between fluid bulk phases and either their vapour in the case of liquids (free interfaces) or solid substrates in general and to test their "universal" behaviour out of any consideration about critical laws, which constitute a topic by themselves. As, here, the level under consideration is only the microscopic one, wetting transitions are excluded. Our second goal is to compare these experimental data to their description by models, which are far more numerous. This imposed us to privilege, here, Cahn's and DFT theories because their way of reasoning is very close to classical thermodynamics and so, the discussion of the experimental data under examination here is facilitated with no exclusion of decisive contribution by other kinds of theories. A thorough analysis of available experiments and theoretic works makes layering appear as a general and non-wetting-dependent phenomenon exhibited by simple gases or gas mixtures on solids and also by liquid mixtures. On the other hand, though prewetting is not deeply different in essence, it is fully wetting-dependent. Both phenomena can be found in systems where interactions are very different by nature and, according to very recent models and experiments, the interplay between these interactions may lead to intertwining between both surface phenomena.

Local-functional theory of critical adsorption

Local-functional methods are applied to critical adsorption in three-dimensional Ising-like systems. The universal order-parameter-profile scaling functions, P(+/-)(x), along the critical isochore (+) and phase boundary (-), are calculated along with their associated universal amplitudes. Good agreement is found with the results of Monte Carlo simulations. General properties of P(+/-)(x) for small and large scaled distance, x, are elucidated. An order-parameter scaling function, P(c)(x), for critical adsorption along the critical isotherm is introduced and calculated.

Dynamics of wetting layer formation

We study the formation and growth of wetting layers in the binary liquid mixture cyclohexane-methanol. By progressively deuterating the methanol we can tune the equilibrium wetting layer thickness. Hysteresis of the transition is observed for large thicknesses and is absent for thinner ones. This can be understood by calculating the activation energy for wetting layer nucleation as a function of the film thickness. We also show that the late-stage growth of the wetting layer after the nucleation process follows a power law in time, in agreement with a diffusion-limited growth mechanism proposed theoretically.