Microscopic thin liquid films

Correspondingly, emulsion, foam and wetting films have been studied [4—10]. The model of a microscopic thin liquid film (radius 100 pm) allows one to obtain films at very low concentrations of polymeric surfactant and to study their formation and stability as well as to establish and to distinguish the surface (interaction) forces in them. 

The measuring cell, in which the microscopic thin liquid films are formed and studied, is the basic part of micro interferometric apparatus. Figure 6.1 presents the main details of three measuring cells. In the Scheludko-Exerowa cell (Figure 6.1a) the film is formed in the middle of a biconcave drop at constant capillary pressure. This is a horizontal round film of radius r of about 50-100 pm. A small portion of the liquid is sucked out of the drop through the capillary using a micro-metrically driven... 

Figure 6.1 Main parts of three experimental cells used to Investigate microscopic thin liquid films.

Microscopic foam films are most successfully employed in the study of surface forces. Since such films are small it is possible to follow their formation at very low concentrations of the amphiphile molecules in the bulk solution. On the other hand, the small size permits studying the fluctuation phenomena in thin liquid films which play an important role in the binding energy of amphiphile molecules in the bilayer. In a bilayer film connected with the bulk phase, there appear fluctuation holes formed from vacancies (missing molecules) which depend on the difference in the chemical potential of the molecules in the film and the bulk phase. The bilayer black foam film subjected to different temperatures can be either in liquid-crystalline or gel state, each one being characterised by a respective binding energy. 

Two theories, macroscopic and microscopic, are involved in the calculation of the van der Waals component of disjoining pressure in thin liquid films. According to the microscopic theory, first treated by Kallman and Willstatter [145], de Boer [146] and Hamaker [147], the... 

Microscopic foam films from amphiphilic ABA triblock copolymers have been used to assess steric interactions. Most of the work on copolymers [128,129] has been carried out with the Thin Liquid Film-Pressure Balance Technique (see Chapter 2, Section 2.1.8). Nevertheless, some intriguing results have been obtained with the dynamic method for surface force measurement [127]. 

In order to investigate the influence of external pressure (disjoining pressure) experiments with single foam films have been carried out using the Thin Liquid Film -Pressure Balance Technique, described in Section 2.1.8 [e.g. 47,48], The radius of the microscopic foam films was close to the initial film radius in a real polyhedral foam (about 0.2 to 0.3 mm). Fig. 7.7 presents a histogram of the distribution by size (diameter) of films in the foam. The most probable film size (under these conditions) has permitted us to choose a suitable radius for the single foam films for these experiments. 

Foam (5) is a collection of gas bubbles with sizes ranging from microscopic to infinite for a continuous gas path. These bubbles are dispersed in a connected liquid phase and separated either by lamellae, thin liquid films, or by liquid slugs. The average bubble density, related to foam texture, most strongly influences gas mobility. Bubbles can be created or divided in pore necks by capillary snap-off, and they can also divide upon entering pore branchings (5). Moreover, the bubbles can coalesce due to instability of lamellae or change size because of diffusion, evaporation, or condensation (5,8). Often, only a fraction of foam flows as some gas flow is blocked by stationary lamellae (4). 

As already mentioned, if the van der Waals force (or other attractive force) is not predominant, first a dimple forms in the thinning liquid films. Usually the dimple exists for a short period of time initially it grows, but as a result of the swift outflow of liquid it decreases and eventually disappears. The resulting plane-parallel film thins at almost constant radius R. When the electrostatic repulsion is strong, a thicker primary film forms (see point 1 in Figure 5.13). From the viewpoint of conventional DLVO theory, this film must be metastable. Indeed, the experiments with microscopic foam films, stabilized with sodium octyl sulfate or sodium dodecyl sulfate in the presence of different amount of electrolyte, show that a black spot may suddenly form and a transition to... 

Spectroscopic techniques based on the optical microscope are being used with increasing success in photophysics. Microscopic fluorescence decay measurements have been made on both thin liquid films and droplets of concentrated dye solutions.Illustrative data are given for rhodamine B in 20 pm films. A luminescence lifetime microscope spectrometer based on time-correlated single photon counting with an avalanche diode detector has measured... 

Figure 3 Principle of a foam film microinterferometry cell. Single, thin liquid films are formed above a bubble attached to an air-water surface in a tightly closed cell placed in the field of a metallographic microscope (x 200). and illuminated by reflected heat-filtered monochromatic light. The microscope can be focused either on the film or on the bubble diameter. The interferometry patterns change with the film local thickness. In Figures 4 -6, the bubble diameter is 1.21 mm and the film diameter is 0.42 mm...

The apparatus used for studying thin liquid films is schematically depicted in Fig. 6. This device, commonly known as a thin-film balance, allows drainage patterns of single foam, emulsion, or wetting films to be recorded. The film is formed in a specially constructed cell that is placed on the state of an inverted microscope. The reflected light from the film is split into two parts, one directed to a CCD camera and another to a fiber-optic probe tip located in the microscope eyepiece. The radius of the tip is only about 20... 

In order to reduce the adhesion of dust particles and powders to painted surfaces, the surfaces may be insulated by the application of some sort of thin liquid or solid film or by the use of multilayer coatings. Then the adhesion of microscopic particles will be determined not by the properties of the paint material, but by the nature of the film. When a thin liquid film is applied, we are essentially replacing adhesion in gas by adhesion in a liquid, and this will reduce the adhesive forces (see Chapter VI). 

In summary, using cantilevers as sensors we have discovered an effect arising with microscopic, pinned drops which to the best of our knowledge was never observed using other methods. Our tentative explanation is that a thin liquid film wets the surface, reduces the surface tension on the top side, and causes the cantilever to bend towards the bottom side. 

Historically the measurement of forces between solid and fluid interfaces began at approximately the same time. However, with the advent of the Surface Force Apparatus (SFA) and Atomic Force Microscope (AFM) over the last few decades, studies concerning the direct measurement of interactions between solid interfaces have received more attention than those concentrated on fluid interfaces (i.e. thin-liquid films - films which have at least one fluid-fluid interface such as foam, emulsion... 

In addition to electrostatic double-layer forces, London-van der Waals dispersion forces have long been recognized as being important in thin-liquid films. The calculation of these forces has been approached in two different ways, namely microscopically and macroscop-ically. 

The thickness of the membrane phase can be either macroscopic ( thick )—membranes with a thickness greater than micrometres—or microscopic ( thin ), i.e. with thicknesses comparable to molecular dimensions (biological membranes and their models, bilayer lipid films). Thick membranes are crystalline, glassy or liquid, while thin membranes possess the properties of liquid crystals (fluid) or gels (crystalline). 

Cryoelectron microscopy makes it possible to have a direct view into the frozen sample without additional preparation [100]. With the aid of a cryogen (e.g., liquid nitrogen-cooled liquid ethane), the sample is plunge frozen as a very thin aqueous film prepared on a microscopic grid. Subsequently, the vitrified specimen is directly transferred into a precooled electron microscope. Because the specimens are usually ) 2005 by CRC Press LLC... 

Study of microscopic O/W films has been performed by Velev et. al. [514-516] and a new phenomenon spontaneous cyclic formation of a dimple (thicker lens-like formations) in O/W emulsion films stabilised by a non-ionic surfactant (Tween 20) was observed. This phenomenon was described as a diffusion dimple formation in contrast to the dimple created as a result of hydrodynamic resistance to thinning in liquid films [55,56,63,237,517], The dimple shifted from the centre to the periphery and periodically regenerated. Photos of the different periods of a dimple growth are shown in Fig. 3.115 and the process is schematically presented in Fig. 3.116. 

The principle of the method, shown in Figure 3, is the following the sample is placed at the bottom of the cell made of optical glass which is filled with a protein solution of a given concentration. An air bubble is formed in the solution by means of a capillary tube. The pressure of the air bubble is maintained constant using a mercury pump adjusted with a manometer. The distance, h, between the capillary tube and the sample may be adjusted so as to obtain a thin film of the solution between the air and the sample. The film thickness, h, varies between 20-150 nm, while its diameter is about 300 m. The cell is fixed on the table of a metallographic microscope and the kinetics of the failure or of the formation of the liquid film is observed directly or photographed by means of a movie camera. 

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