Mobile Film Surfaces

Another unique phenomenon exhibited by Hquid helium II is the Rollin film (62). AH surfaces below the lambda point temperature that are coimected to a helium II bath are covered with a very thin (several hundredths llm) mobile film of helium II. For example, if a container is dipped into a helium II bath, fiUed, and then raised above the bath, a film of Hquid helium flows up the inner waH of the container, over the Hp, down the outer waH, and drips from the bottom of the suspended container back into the helium II bath. SinHlady, if the empty container is partiaHy submerged in the helium II bath with its Hp above the surface, the helium film flows up the outer waH of the container, over its Hp, and into the container. This process continues until the level of Hquid in the partiaHy submerged container reaches that of the helium II bath. 

A second way of classifying the material is on the basis of the experimental methods involved. For mobile interfaces, surface tension is easily measured. For these it is easiest to examine the surface tension-adsorption relationship starting with surface tension data. When insoluble surface films are involved, we shall see how the difference in y between a clean surface and one with an adsorbed film may be measured directly. For solid surfaces, surface tension is not readily available from experiments. In this case adsorption may be measurable directly, and the relationship between adsorption and surface tension may be examined from the reverse perspective. 

Both the adsorption of neon and its surface flow are small. The accuracy of determining the activation energy for surface diffusion is therefore poor. An average value of 1250 cal. is calculated from the data in Figure 8. This value is considerably greater than RT, indicating that we do have activated diffusion and not a mobile film. 

Radiochemical methods are applied for the study of a wide range of electrochemical surface processes. The most important areas are as follows - adsorption and -> electrosorption occurring on the surface of electrodes the role of electrosorption in -> electrocatalysis -> deposition and dissolution of metals - corrosion processes the formation of surface layers, films on electrodes (e.g., polymer films), and investigation of migration processes in these films study of the dynamics of - electrosorption and - electrode processes under steady-state and equilibrium conditions (exchange and mobility of surface species) electroanalytical methods (e.g., radiopolarog-raphy). 

A very suitable method for measurement of the lateral diffusion of molecules adsorbed at the foam film surfaces is Fluorescence Recovery after Photobleaching (FRAP) ([491-496], see also Chapter 2). Measurements of the lateral diffusion in phospholipid microscopic foam films, including black foam films, are of particular interest as they provide an alternative model system for the study of molecular mobility in biological membranes in addition to phospholipid monolayers at the air/water interface, BLMs, single unilamellar vesicles, and multilamellar vesicles. 

This model approach is most reliable for the description of high expansion ratio foams, the geometry of which is rather simple and have minimum tangential mobility of surfaces. Kruglyakov et al. [11-13] have derived analytical equations that do not contain empirical constants. They have also found an experimental relation between the tangential mobility of liquid/gas interface and the border radius and foam film type. 

The expression for foam viscosity Eq. (8.1 la) contains two terms x 0/y which is the elasticity component, related to the demolition of foam structure, and r, which is a dissipation term, related to the liquid flow through films and borders during the deformation process. The models of Khan and Armstrong [14] and Kraynik and Hansen [43] imply that the continuous phase is in the films, no liquid exchange occurs and the film surfaces are mobile, thus predicting a very small contribution of the viscous dissipation in the films, rj = 13[Pg.584]

J.A. Venables and G.D.T. Spiller, Nucleation and Growth of Thin Films, Surface Mobilities on Solid Materials Fundamental Concepts and Applications, NATO ASI series, 1983, pp.341-404. 

As discussed earlier, SAMs of alkanethiols on gold surfaces offer access to highly ordered, surface-confined molecular structures. The use of these assemblies as stable, well-defined spacers for studies of electron transfer between the gold electrode and redox eenters attached to the film surface [50, 73-83] or in solution [44, 46, 84] is depicted in Figure 5. The densely packed monolayer maintains a precise separation between a gold electrode surface and the pendant or dissolved redox center, and effectively eliminates conformational mobility that can complicate electron-transfer rate studies [73]. An advantage of using electrode-confined spacers is that... 

Originally. TOA was devised and used for analyses of chain mobility in polymers (7) and polymer blends (8.91 by monitoring birefringence disappearance during programmed heating in scratches scribed on film surfaces. Lenz (10.Ill has reported the use of TOA in the study of liquid crystal polymers, and recently we have used the technique in determining melt- and isotropization temperatures for thermotropic fully aromatic liquid crystal polymers (4.51. 

Analysis of Heterogeneity. The monolayer analysis consists of three elements an adsorption isotherm equation, a model for heterogeneous surfaces, and an algorithm such as CAEDMON, which uses the first two elements to extract the adsorptive energy distribution and the specific surface from isotherm data. Morrison and Ross developed a virial isotherm equation for a mobile film of adsorbed gas at submonolayer coverage (6) ... 

The expression for coalescence probability (sometimes named efficiency) of binary fluid particle collisions in dispersions is normally related to the physical phenomena that are considered to determine the overall coalescence process. The coalescence process in stagnant fluids is generally assumed to occur in three consecutive stages, as sketched in Fig 9.2. First, bubbles collide, trapping a small amount of liquid between them under the action of the continuous phase. Second, this liquid drains over a period of time from an initial thickness until the liquid film separating the bubbles reaches a critical thickness, under the action of the film hydrodynamics. The hydrodynamics of the film depends on whether the film surface is mobile or immobile, and the mobility, in turn, depends on whether the continuous phase is pure or a solution. Third, at this point, film rupture occurs due to film instability resulting in instantaneous coalescence. 

There are various cases of particle-interface interactions, which require separate theoretical treatment. The simpler case is the hydrodynamic interaction of a solid particle with a solid interface. Other cases are the interactions of fluid particles (of tangentially mobile or immobile interfaces) with a solid surface in these cases, the hydrodynamic interaction is accompanied by deformation of the particle. On the other hand, the colloidal particles (both solid and fluid) may hydrodynamically interact with a fluid interface, which thereby undergoes a deformation. In the case of fluid interfaces, the effects of surfactant adsorption, surface diffusivity, and viscosity affect the hydrodynamic interactions. A special class of problems concerns particles attached to an interface, which are moving throughout the interface. Another class of problems is related to the case when colloidal particles are confined in a restricted space within a narrow cylindrical channel or between two parallel interfaces (solid and/or fluid) in the latter case, the particles interact simultaneously with both film surfaces. 

Electrophoretic Mobility. To estimate the possible charge on the sulfoxide film surface, we investigated the effects of sodium chloride and potassium thiocyanate on the electrophoretic mobility of dodecane droplets stabilized with DMS (see Figure 4). In the sodium chloride system the droplets were essentially uncharged at low concentrations, but between 3-5 X 10"3 mole/dm3, the mobility increased to —1.2 mfx cm/V sec. In the potassium thiocyanate system, the droplets had a mobility of... 

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