Roughness film-liquid interface

Roughness, generally speaking, is not easily incorporated into the multilayer formahsm because it violates the assumption of lateral heterogeneity. On the other hand, it is certainly essential. The chapter (M. Urbakh et al. 2006, in this volume) describes various ways to deal with roughness. Roughness may very well occur not only at a quartz-Hquid interface, but also at a film-liquid interface. There is a logical extension of the formahsm treated to the case of multilayers (M. Urbakh et al. 2006, in this volume). One uses an impedance of the hquid of the form ... 

For a closer comparison with experiments, thermal fluctuations have to be included in the model as an additional source of roughness of the liquid interface ). Even in the limit where we predict a smooth and flat liquid interface, e.g. when is big, thermal fluctuations will always cause a r.m.s. roughness of the order of a few Angstroms. They have been found to be about SA for thick water films ... 

By analogy with monomolecular films at liquid-air interfaces, surfactants at the liquid-liquid interface will normally form monolayers with various molecular packing densities ranging from relatively loosely packed arrangements normally associated with fluid phases (gases and liquids) to the close-packed solid phase. Classically, amphiphilic adsorption at interfaces has been roughly classified in terms of film types related to normal states of matter (Figure 9.13) ... 

The second system is the thin liquid films (45-90A) of a nearly spherical, nonpolar molecule, tetrakis(2-ethylhexoxy) silane. The XR analysis demonstrated direct monitoring of internal layering in the thin liquid films. Model-independent fitting to the XR data found that there are three electron density oscillations near the solid-liquid interface, with a period of 10A (consistent with the molecular dimensions). The oscillation amplimde has a strong inverse dependence on the substrate surface roughness. 

In the context of the structural perturbations at fluid-solid interfaces, it is interesting to investigate the viscosity of thin liquid films. Eaily work on thin-film viscosity by Deijaguin and co-workers used a blow off technique to cause a liquid film to thin. This work showed elevated viscosities for some materials [98] and thin film viscosities lower than the bulk for others [99, 100]. Some controversial issues were raised particularly regarding surface roughness and contact angles in the experiments [101-103]. Entirely different types of data on clays caused Low [104] to conclude that the viscosity of interlayer water in clays is greater than that of bulk water. 

The LOFO approach, based on capillary interactions induced by liquid-solid interfaces, is used for transferring prefabricated thin solid metal films onto molecu-larly modified solid substrates. In spite of the fact that the glass/metal pad during the lift-off process leaves a relatively rough (1 nm) surface, several types of device have been fabricated by LOFO [154-156]. 

In fact a liquid surface in the absence of external perturbations such as mechanical vibrations is perhaps the most smooth (disordered) surface that can be achieved by the action of gravity. Clearly, LB films could not be grown in the absence of gravity, e.g., in a Space Shuttle. In the case of water, the flat water/air interface has a roughness of 0.3 nm, a value determined by X-ray reflectivity measurements (Braslau et al, 1985). A further advantage of water is its relatively high surface tension yo. when compared to other liquids, which amounts to 72.8 mN m for pure water at RT. This value originates in the formation of a network of weak... 

The few investigators who have attempted to use the original Harkins-Junt method have encountered a number of inherent difficulties. A major problem is that it is virtually impossible to avoid some interparticle capillary condensation as p/p° —+ 1. This inevitably reduces the extent of the available liquid-vapour interface (Wade and Hackerman, 1960), Moreover, the thickness of a pre-adsorbed film as p/p° — 1 is highly dependent on the shape, size and roughness of the particles. 

The interface boundary inside the chaimel is shown on figures 4 and 5 as a function of the liquid Reynolds number. At large liquid flow rate the considerable part of the liquid flows in the corners and the film is thinned both on the long and short sides of the channel. This enhances the heat transfer in comparison with uniform film. At small liquid Reynolds numbers the minimum film thickness becomes the same as the wall roughness and film rupture occurs leading to stable rivulet flow. Dry areas exist on the wall, which are not wetted by liquid. This reduces the heat transfer. The... 

A model of deformed evaporating liquid film, which is moved hy co-current vapor flow and gravity, is developed. Shear stress from co-current vapor is included as a boundary condition on interface. Intermolecular forces are taking into account as disjoining pressure component and surface roughness is considered also. 

Fig. 7 Calculated peak to valley surface roughness due to thermal fluctuations at the liquid vapor interface of Zdol (Mw = 3100g/mol) as a function of the average film thickness.

Figure 13. Reflectivity functions for idealized interfaces (vacuum/solid or vacuum/liquid) for quartz (qtz), thin film kapton (plast) and water at X-ray energy of 7100 eV. All materials are infinitely thick and have rms roughness = 0.0.

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