A knowledge of the structural behavior of molecules confined in thin liquid films and the way in which they differ from bulk behavior is of great importance to a variety of technological applications. We discuss X-ray scattering studies of the conformation of liquid polymer wetting layers on laterally structured substrates in order to test theories of the conformality of the wetting layer to the substrate. Recent work on guided X-ray beams confined to the thin film opens up the possibility of detailed studies of ordering phenomena in molecular layers adjacent to solid surfaces. Recent experiments are discussed.

Normal alkanes of carbon number n>14 exhibit surface crystallization at their liquid-vapor interface. This has been investigated with x-ray reflectivity, grazing incidence scattering and surface tension measurements. The structure and thermodynamics of the surface layer is consistent with a monolayer of the bulk rotator phase occurring at the surface above the bulk melting temperature. On the other hand, thin films of alkanes on SiO2, exhibit a reduction of the melting temperature. The surface crystalline phase is observed for carbon number n>14. The vanishing of surface phase for small n may be due to a transition from surface freezing to surface melting behavior. These measurements can yield the relative surface energies of the various phases.

Using video-optical microscopy and image analysis software, the morphological development of phase separation of polystyrene (PS) and poly(vinyl methyl ether) (PVME) blends is monitored for film thickness ranging from 200 to 700nm's. In the current studies, films are confined between glass slides. For blends having a PS volume fraction of 0.30 (the critical composition), the area fraction of the PS-rich minority phase, A, decreases more rapidly as film thickness decreases. At long times, the final A achieves a constant value which is less than the bulk value. The correlation length of concentration fluctuations increases more rapidly than the bulk scaling prediction, which suggests that wetting plays a significant role in the phase separation kinetics of thin films. Because of confinement, the shape of the PS-rich phase is anisotropic, flattened along the film direction. The confinement restricts the phase growth perpendicular to the film plane, and thus hinders the development of phase separation at very late stages.

We report experimental measurements of the adhesion force between metallic substrates in undersaturated heptane vapor atmosphere, with a surface force apparatus. The attractive force between the substrates is strongly dependant of the condensation of a liquid bridge connecting the surfaces. The results show the importance of wetting phenomena for the maximum attractive force: we find that this maximum attraction varies as the power two-third of the curvature of the meniscus connecting the surfaces, in good agreement with the theory of Van der Waals wetting.

Surfactant self-assembly at the liquid-vapor, solid-liquid, and solid-vapor interfaces controls the wetting behavior of advancing surfactant solutions. While different surfactants exhibit different static and dynamic wetting properties, we show that these behaviors can be understood through an examination of microscopic structures driven by surfactant-surface interactions. We examine surfactant solutions exhibiting complete and partial static wetting as well as spreading by dendritic pattern formation and unsteady, stick-jump behavior. In each case, the observed behavior is related to the structure of the surfactant assemblies in the vicinity of the contact line.

The depinning of a contact line is studied as a dynamical critical phenomenon by a functional renormalization group technique. In D = 2 - ∈ “line” dimensions, the roughness exponent is ζ = ∈/3 to all orders in perturbation theory. Thus, ζ = 1/3 for the contact line, equal to the Imry-Ma estimate for equilibrium roughness. The dynamical exponent is z = 1 – 2∈/9 + O(∈2) < 1, resulting in unusual dynamical properties. In particular, a characteristic distortion length of the contact line depinning from a strong defect is predicted to initially increase faster than linearly in time. Some experiments are suggested to probe such dynamics.

The dynamics of phase separation of binary fluid mixtures in the presence of surfactants is studied via molecular dynamics simulations. In contrast to phase separation in pure binary fluid mixtures, which is characterized by an algebraic time dependence of the average domain size, mixtures containing surfactants exhibit nonalgebraic slow dynamics. At very late times, the average domain size saturates at a value inversely proportional to the surfactant concentration. Despite the fact that these systems only microphase separate, a dynamical scaling was observed at late times.

We have used infrared-visible sum-frequency generation (SFG) to spectroscopically probe the interfacial structure of a self assembled monolayer (SAM) of alkyl thiol on a Au(111) surface. The SFG spectra of the CH3 group measured as a function of azimuthal angle indicates that sulfur atoms cannot be situated at sites of a single type, e.g., hollow or bridge, but must be in a mixed arrangement. SFG is also used to study the reconstruction of the gold substrate by alkyl thiol.

The kinetics of de-wetting a polycarbonate (PC) film from a poly(styrene-coacrylonitrile) (SAN) copolymer film was monitored using optical microscopy. Whereas the SAN layer was stable upon annealing at 190°C, the PC layer dewetted the SAN and formed holes whose diameter increased linearly with time. Auger electron spectroscopy measurements confirmed that PC was fully removed from the interior of the hole. Upon varying the AN content, the dewetting velocity was found to be a minimum near 0.27 weight percent AN. This result is consistent with the interfacial thermodynamics between PC and SAN. Atomic force microscopy was used to provide a unique image of the hole profile.

The interaction of light with a system of molecules depends upon the polarisation induced by an external electric field, which depends not only upon the external field but also upon the local fields due to neighboring polarised molecules. These local fields result in the traditional Clausius-Mossotti (CM) dielectric constant for a molecule deeply imbedded in a medium. Near the surface the local fields are altered, and the dielectric constant becomes anisotropic and dependent upon depth into the medium. The local fields are shape dependent in small systems and differ substantially from the CM value.

A self-consistent computer calculation of the local fields has been implemented, and these effects will be shown using molecule positions and polarisabilities typical of liquids and crystals. The shape dependence of small systems, the reflection of light from liquids with fluctuating surfaces, and the effect of supporting substrates will be described.

The stress tensor for polar solutions is constructed starting from a nonlocal and nonlinear Landau free energy functional. Explanations of some puzzling properties of liquids near interfaces are proposed.

Monte Carlo simulations are used to study various properties of a new lattice model of microemulsions. In particular, we calculate the critical exponent β of the order parameter (water concentration) and the correlation length exponent ν, and find them to be in excellent agreement with those of the 3D Ising model, and also in agreement with the experimental measurements. However, when the same exponents are calculated for the microemulsions in a porous medium, they do not agree with those of either the dilute or the random-field Ising model.

We report here on the application of the new technique of the X-Ray Surface Forces Apparatus (XSFA) to the study of the smectic liquid crystal 8CB (4-cyano-4′-octylbiphenyl) and a zwitterionic polyisoprene melt. The XSFA allows one to study the structure of fluid films under confinement and flow using intense synchrotron x-ray radiation. The above systems were investigated with the distances between the confining surfaces ranging from 0.4μm to a few tens of microns. Two different kinds of confining surfaces were used leading to different structural behavior of the samples as a function of the confining gap.

The shear rheology of molecularly-thin films of fluids has been studied experimentally as it depends on sinusoidal frequency (linear response) or on sliding velocity (nonlinear response). Building upon previous identification of a solidlike state that is induced by confinement, we find the shearinduced transition to a sliding state in which the viscous dissipation is essentially velocity-independent. The mechanism appears to involve wall slip but Fourier transforms of the response reveal fluctuations, intrinsic to the sliding state, over all accessible frequencies. Other ongoing studies involve shear-induced changes in the fluorescence of confined fluorescent probes, shear dilatancy, and the contrast between the shear of simple nonpolar fluids, and block copolymers.

In this study we used molecular dynamics simulations to investigate the structural changes in nanoscopically thin n-octane films confined between smooth solid surfaces as a result of a) increasing solid-methylene unit affinity and b) increasing pressure. Increasing solid-methylene unit energetic affinity resulted in the solidification of the film at a critical value. A similar transition is observed at a critical value of pressure. Bulk n-octane was a liquid in both cases. The transition was signaled by an abrupt increase in the intermolecular order and was facilitated by a precipitous extension of the octane molecules, which adopted almost fully extended configurations. A discontinuous jump in the film density at the critical values of solid affinity and pressure was evident. The characteristics of the transitions showed that it was a mild first order transition from a highly ordered liquid to a poorly organized solid.

Shear thinning of confined liquids is studied in the framework of the time dependent Ginzburg-Landau equation coupled to a shear-induced velocity field. Scaling relationship between the effective viscosity and the shear rate is analytically derived with an exponent which depends on the velocity profile within the liquid and on the boundary conditions. The velocity profile is derived in the limit of low shear rate. Thinning is observed for shear rates faster than typical liquid relaxation rates. Relevance to existing systems and predictions amenable to new experiments are discussed.

The grand canonical Monte Carlo method is used to study a binary mixture of Lennard-Jones atoms confined to various corrugated slit-micropores which are in thermodynamic equilibrium with their bulk phase counterpart. The micropore walls have the structure of the (100) face of an fcc lattice. In addition to this atomic scale structure, one wall possesses nanoscale structure in the form of rectilinear grooves (corrugation). The grooved surface divides the confined fluid film into two strip shaped regions. The confined film is studied in each region as a function of groove width, bulk phase composition, and the size of the wall atoms.

Drainage of circular foam films is much more rapid when the drainage is asymmetric. The same basic mechanism is responsible for asymmetric drainage of thin circular films and marginal regeneration. A linear stability analysis showed that these phenomena are caused by a hydrodynamic instability that is produced by a surface-tension-driven flow and stabilized by surface viscosity, surface diffusivity and system length scale. A criterion for the onset of this instability was derived. Experiments performed on small circular films of aqueous solutions of SDS and SDS:l-dodecanol demonstrated the strong stabilizing effect of surface viscosity. Experimental results were found to be in good agreement with the predictions of the linear stability analysis. Finite difference simulations demonstrate the validity of the linear stability analysis for when the radius of curvature of the “barrier ring” is large compared to the transverse wave length of the instability. These simulations also show the circulation cells that relax the surface tension gradient and thus accelerate the drainage of the film.

We have conducted a real time, two-dimensional light scattering study of the nonlinear dynamics of field-induced structures in an electrorheological fluid subjected to oscillatory shear. We have developed a theoretical description of the observed dynamics by considering the response of a fragmenting/aggregating particle chain to the prevailing hydrodynamic and electrostatic forces. This structural theory is then used to describe the nonlinear rheology of ER fluids.

When a rigid two-dimensional triangular crystalline layer of colloidal spheres confined between two smooth repulsive walls is gradually given freedom to move out of plane, it buckles dynamically undergoing several Peierls transitions involving different soft phonon modes before forming a two layer crystal with square in-plane symmetry. We have mapped out the complex phase diagram of the buckling transitions as a function of sphere density and wall separation. Digital imaging is used to study the instantaneous particle positions and trajectories of the uniform, highly charged 0.3 μm diameter polystyrene spheres that comprise the crystalline layer in water suspension. Brownian motion of the spheres creates a true thermodynamic system with a real temperature, which is studied using video microscopy. We follow the collective dynamics of the system as well as individual particle motions and the motions and rearrangements of topological defects and domains. At sufficiently low sphere densities the system melts into a fluid. As the wall separation increases to the point of two layer formation we observe square symmetry in the fluid correlation volumes.