Wetting of methanol on the n-alkanes: Observation of short-range critical wetting

Wetting and surface forces

In this review we discuss the fundamental role of surface forces, with a particular emphasis on the effect of the disjoining pressure, in establishing the wetting regime in the three phase systems with both plane and curved geometry. The special attention is given to the conditions of the formation of wetting/adsorption liquid films on the surface of poorly wetted substrate and the possibility of their thermodynamic equilibrium with bulk liquid. The calculations of contact angles on the basis of the isotherms of disjoining pressure and the difference in wettability of flat and highly curved surfaces are discussed. Mechanisms of wetting hysteresis, related to the action of surface forces, are considered.

Critical wetting of a class of nonequilibrium interfaces: a computer simulation study

Critical wetting transitions under nonequilibrium conditions are studied numerically and analytically by means of an interface-displacement model defined by a Kardar-Parisi-Zhang equation, plus some extra terms representing a limiting, short-ranged attractive wall. Its critical behavior is characterized in detail by providing a set of exponents for both the average height and the surface order-parameter in one dimension. The emerging picture is qualitatively and quantitatively different from recently reported mean-field predictions for the same problem. Evidence is shown that the presence of the attractive wall induces an anomalous scaling of the interface local slopes.

Formation of n-alkane layers at the vapor/water interface

We present a study on the initial wetting behaviors of two low molecular weight alkanes, heptane and octane, at the vapor/water interface using both neutron and X-ray reflectometry. Combined X-ray and neutron reflectivity studies data showed that a uniform film, which has never been reported, was formed continuously at 25 degrees C. As the adsorptive deposition continued, each adsorbed film was saturated at a specific equilibrium thickness: 48 and 36 A for deuterated heptane and octane, respectively, and 21 A for hydrogenated octane. The thickness of the adsorbed layer measured by neutron reflectivity is in agreement with that measured using X-ray reflectivity. Our observations of continuous and saturated adsorption behaviors are analyzed qualitatively using a kinetic adsorption model.

Order parameter modeling of fluid dynamics in narrow confinements subjected to hydrophobic interactions

A novel phase-field approach is developed for quantitative modeling of the complex thermophysics over reduced length scales in narrow fluidic confinements, as induced by the surface roughness-hydrophobicity coupling and the consequent hydrodynamic interactions. The method is tested for flows on micro- and nano- corrugated surfaces in narrow confinements, and the agreement with molecular dynamics and lattice Boltzmann simulations is found to be quantitative.

Optical reflectivity of thin rough films: application to ellipsometric measurements on liquid films

Equations to calculate the optical properties of a flat but rough interface are given. This interface is supposed to be much thinner than the wavelength of the light, and made of N-1 layers of refractive indices n{m} (for layer m ) between two media of refractive indices n{0} and n{N}. The interfaces separating two layers are rough. The roughness is supposed to have weak slopes (mid R:nablaZmid R:(z)1). These equations can be solved very simply for any N values. The explicit solution for N=2 (one rough layer) is given for the incidence angle equal to the Brewster angle in order to apply the result to ellipsometric measurements, specially on liquid interfaces whose roughness originates from thermal fluctuations and is depending on a small number of parameters.

Wedge filling and interface delocalization in finite Ising lattices with antisymmetric surface fields

Theoretical predictions by Parry et al. for wetting phenomena in a wedge geometry are tested by Monte Carlo simulations. Simple cubic LxLxL(y) Ising lattices with nearest neighbor ferromagnetic exchange and four free LxL(y) surfaces, at which antisymmetric surface fields +/-H(s) act, are studied for a wide range of linear dimensions (4</=L</=320, 30</=L(y)</=1000), in an attempt to clarify finite size effects on the wedge filling transition in this "double-wedge" geometry. Interpreting the Ising model as a lattice gas, the problem is equivalent to a liquid-gas transition in a pore with quadratic cross section, where two walls favor the liquid and the other two walls favor the gas. For temperatures T below the bulk critical temperature T(c) this boundary condition (where periodic boundary conditions are used in the y direction along the wedges) leads to the formation of two domains with oppositely oriented magnetization and separated by an interface. For L,L(y)--> infinity and T larger than the filling transition temperature T(f)(H(s)), this interface runs from the one wedge where the surface planes with a different sign of the surface field meet (on average) straight to the opposite wedge, so that the average magnetization of the system is zero. For T<T(f)(H(s)), however, this interface is bound either to the wedge where the two surfaces with field -H(s) meet (then the total magnetization m of the system is positive) or to the opposite wedge (then m<0). The distance l(0) of the interface midpoint from the wedges is studied as T-->T(f)(H(s)) from below, as is the corresponding behavior of the magnetization and its moments. We consider the variation of l(0) for T>T(f)(H(s)) as a function of a bulk field and find that the associated exponents agree with theoretical predictions. The correlation length xi(y) in the y direction along the wedges is also studied, and we find no transition for finite L and L(y)--> infinity. For L--> infinity the prediction l(0) proportional, variant (H(sc)-H(s))(-1/4) is verified, where H(sc)(T) is the inverse function of T(f)(H(s)) and xi(y) proportional, variant (H(sc)-H(s))(-3/4), respectively. We also find that m vanishes discontinuously at the filling transition. When the corresponding wetting transition is first order we also obtain a first-order filling transition.

Is a position-dependent stiffness relevant for the wetting phase diagram?

In this paper we determine the wetting phase diagram for three-dimensional systems with short-range forces assuming the presence of a position-dependent stiffness contribution as recently proposed [M.E. Fisher and A.J. Jin, Phys. Rev. Lett. 69, 792 (1992)]. We predict a discontinuous transformation of the phase diagram immediately upon moving beyond the mean-field approximation. However, in contrast to Fisher and Jin we find that a renormalization group calculation yields fluctuation-induced second-order transitions rather than fluctuation-induced first-order ones. As a consequence, in all fluctuation regimes we recover the same qualitative phase diagram as predicted in the absence of a position-dependent stiffness coefficient. Furthermore, recent predictions for tricritical wetting behavior remain unaffected by the stiffness contribution.

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.