A. De Virgiliis

Colloid-polymer mixtures between asymmetric walls: Evidence for an interface localization transition

We demonstrate via computer simulation that mixtures of colloids and polymers confined to thin films have the ability to undergo an interface localization transition. While one wall of the film is assumed to be hard for both particles, at the other wall, an additional repulsive potential acts, but on the colloids only. By varying the strength of this repulsion, a crossover from capillary condensation to interface localization is found. The latter occurs under conditions where in the bulk almost complete phase separation has occurred.

Corner wetting in the two-dimensional Ising model: Monte Carlo results

Square L ? L (L = 24?128) Ising lattices with nearest neighbour ferromagnetic exchange are considered using free boundary conditions at which boundary magnetic fields ? h are applied, i.e., at the two boundary rows ending at the lower left corner a field +h acts, while at the two boundary rows ending at the upper right corner a field ?h acts. For temperatures T less than the critical temperature Tc of the bulk, this boundary condition leads to the formation of two domains with opposite orientations of the magnetization direction, separated by an interface which for T larger than the filling transition temperature Tf (h) runs from the upper left corner to the lower right corner, while for T T > Tf (h) it scales as w ? L. The distribution P (?) of the interface position ? (measured along the z-direction from the corners) decays exponentially for T T > Tf (h). Furthermore, the Monte Carlo data are compatible with ? (Tf (h) ? T)?1 and a finite size scaling of the total magnetization according to M(L, T) = {(1 ? T/Tf (h))?? L} with ?? = 1. Unlike the findings for critical wetting in the thin film geometry of the Ising model, the Monte Carlo results for corner wetting are in very good agreement with the theoretical predictions.

Ising systems with pairwise competing surface fields

The magnetization distribution and phase behaviour of large but finite Ising simple cubic L × L × L lattices in d = 3 dimensions and square L × L lattices in d = 2 dimensions are studied for the case where four free boundaries are present, at which surface fields +Hs act on one pair of opposite boundaries while surface fields −Hs act on the other pair (in d = 3, periodic boundary conditions are used for the remaining pair). Both the distribution PL(m) of the global magnetization and also the distribution of the local magnetization m(x,z) are obtained by Monte Carlo simulations, where x and z denote the coordinates when the boundaries are oriented along the x-axis and z-axis (in d = 2); or along the xy-plane and zy-plane (in d = 3, where the periodic boundary condition applies in the y-direction). Varying the temperature T and linear dimension L it is found that a single bulk rounded phase transition occurs, which converges to the bulk transition temperature Tcb as , unlike other geometric arrangements of competing boundary fields, where a second transition occurs in the bulk due to interface formation or delocalization, related to wedge or corner filling or wetting transitions, respectively. In the present geometry, only precursors of wetting layers form on those boundaries where the field is oppositely oriented to the magnetization in the bulk and the thickness of these layers is found to scale like L1/2 (in d = 2) or lnL (in d = 3), respectively. These findings are explained in terms of a phenomenological theory based on the effective interface Hamiltonian and scaling considerations.

Substrate roughness-enhanced wetting in a two-dimensional Ising model with boundary fields

The wetting transition in a two-dimensional Ising model with a free rough surface at which a boundary magnetic field acts is studied using Monte Carlo simulations. In order to generate such rough surfaces we have used a simple one-dimensional solid-on-solid model. Apart from the basic thermodynamic quantities, such as susceptibilities, total average magnetizations, heat capacities, we have also recorded two magnetization distribution functions. The final results have been obtained by averaging simulation data over several replicas of the system of the same surface roughness. It has been shown that the wetting transition is located at a lower surface field than in the model with flat surfaces and that the critical value of the surface field decreases when the surface roughness becomes higher.