Surface film, mechanical tension

Based on this low surface tension feature and the commonly observed insolubiUty of defoamers, two related antifoam mechanisms have been introduced (29) (/) The agent dispersed in the form of fine drops enters the Hquid film between bubbles and spreads as a duplex film. The tensions created by this Spreading lead to the mpture of the original Hquid film. (2) A droplet of the agent enters the Hquid film between bubbles, but rather than spreading produces a mixed monolayer on the surface. This monolayer, if of less coherence than the original film-stabilizing monolayer, causes destabilization of the film. 

Although silicone oils by themselves or hydrophobic particles (e.g., specially treated silica) are effective antifoams, combinations of silicone oils with hydrophobic silica particles are most effective and commonly used. The mechanism of film destruction has been studied with the use of surface and interfacial tensions, measurements, contact angles, oil-spreading rates, and globule-entering characteristics for PDMS-based antifoams in a variety of surfactant solutions.490 A very recent study of the effect of surfactant composition and structure on foam-control performance has been reported.380 The science and technology of silicone antifoams have recently been reviewed.491... 

Mechanism of Action A bulk-producing laxative that decreases surface film tension by mixing liquid and bowel contents. Therapeutic Effect Increases infiltration of liquid to form a softer stool. 

As was stressed by Professor Ubbelohde, in the process of cell recognition not only the lateral diffusion of the binding sites has to be considered, but also the mechanical effects resulting from the local change of surface tension, inducing convection at the cell surface. It is well known, in the cell-to-cell contact inhibition of motion, in tissue culture, that a cell approaches another cell by touching it by means of microvilli and that this process can be affected when adding surfactants to the culture. Now the point is, What is the relative importance of both diffusion and convection Well, in binary surface films, it was observed that the transport process induced by two-dimensional convection is much more rapid than the two-dimensional diffusion. 

From this description it becomes obvious that a mechanical tension must develop in the surface film, because the atoms will tend to assume a closer packing. Hence any adsorbed molecule or atom which can improve the screening of the solid will decrease their state of tension and cause the surface film to expand and release some of the pressure which it exerted upon the subjacent layers. Adsorption of screeners, even of inert gas atoms such as argon, causes many porous solids to expand. 

From a chemical viewpoint, the interior of a crystal and its surface can be looked upon as if they were different individuals, comparable to two modifications of a substance or to a metal which is under mechanical stresses. The phenomenon of stress corrosion reveals that the part under tension differs in its chemical reactivity from the part under compression. Different modifications of silica can have very different reactivities. Therefore, it must be expected that the surface film can affect the apparent stability of a solid. Indeed, the surface structures of some hydrated salts are in equilibrium with the ambient atmosphere and prevent these crystals from losing water (efflorescing), even if the bulk of the crystal has a higher water vapor pressure than the atmosphere. The formation of a fresh highly asymmetrical surface may also cause substances to detonate. 

Our objective in this study is to elucidate the complex phenomena occurring during the process of three phase foam thinning, to identify the interaction mechanisms between the oil droplets, the thinning foam film and the Plateau-Gibbs borders and the role of surface and interfacial tension gradients in foam stability, and to examine the implications upon crude oil displacement by foam in pourous media. 

Surface Films. Insoluble polar molecules (e.g., long-chain fatty acids) exhibit an extreme kind of adsorption at liquid surfaces. That is, they can be made to concentrate in one molecular layer at the surface. These interfacial films often provide the stabilizing infiuence in emulsions because they can both lower interfacial tension and increase the interfacial viscosity. Increasing interfacial viscosity provides a mechanical resistance to coalescence. Such systems also lend themselves to the study of size, shape, and... 

The rupture mechanisms of thin liquid films were considered by de Vries [15] and by Vrij and Overbeek [16]. It was assumed that thermal and mechanical disturbances (having a wavelike nature) cause film thickness fluctuations (in thin films), leading to the rupture or coalescence of bubbles at a critical thickness. Vrij and Overbeek [16] carried out a theoretical analysis of the hydrodynamic interfacial force balance, and expressed the critical thickness of rupture in terms of the attractive van der Waals interaction (characterised by the Hamaker constant A), the surface or interfacial tension y, and the disjoining pressure. The critical wavelength, for the perturbation to grow (assuming that the disjoining pressure just exceeds the... 

Several authors have in the past stretched the term surface tension to imply that liquids have a saturated film in their surfaces, some mechanism such as a stretched membrane or contractile skin. However we should be careful since this view can lead to great problems when the structure of the so-called skin is considered in terms of molecules. Some writers propose that the surface molecules have their force-fields so deflected that they form a kind of linked skin in the surface, and the attractions between the surface molecules are directed along the surface instead of equally in all directions. This is wrong on two points ... 

The transport of inclusion cations in electrochemical cells is affected by various influences, such as mechanical stress [1, 2], phase transformation of host materials [26], and the electric field generated during the cation intercalation/deintercalation. Other mechanical stresses may be generated on the electrode films, such as the electrostrictive force [27], formation of surface film, and surface tension change due to the adsorption of species from the electrolyte [28]. However, the contribution of other stresses to the total mechanical stress on the electrode film is small for example, the electrostrictive stress is about 1.29 x 10 % of the total on a film of LiMn204 [29]. Hence, the major contribution of mechanical stress on the electrode is due to the intercalation stress induced by volume changes of the hosting material. 

Hardy W B 1912 The tension of composite fluid surfaces and the mechanical stability of films of fluids Proc. R. See. A 86 610-35... 

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