Thin liquid films hydrodynamic forces

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian liquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin liquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a laige number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the IUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

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... 

Saffman had interests in turbulence, viscous flows, vortex motion and water waves. He made valuable theoretical contributions to different areas of low-Reynolds-number hydrodynamics. These included the lifting force on a sphere in a shear flow at small but finite Reynolds numbers, the Brownian motion in thin liquid films, and particle motion in rapidly rotating flows. Saffinan s other contributions include dispersion in porous media, average velocity of sedimenting suspensions, and compressible low-Reynolds-number flows. 

It is now generally recognized that the presence of surfactants plays an important role for the drainage velocity of thin liquid films and the hydrodynamic forces in these films. The... 

In this section, we consider the properties of equilibrium thin liquid films. Section VI is devoted to the surface forces of intermolecular origin acting in the liquid films or between the particles in dispersions. Finally, in Secs. VII-IX we describe the role of surfactants in the hydrodynamic processes in liquid films and dispersions, which are coupled with convective and diffusion transfer of surfactant. 

For nondeformable particles, the theories describing the interaction forces are well advanced. So far, most of the surface force measnrements between planar liquid surfaces (TFB) have been conducted under conditions such that the film thickness is always at equilibrium. In the absence of hydrodynamics effects, the forces are correctly accounted considering classical theories valid for planar solid surfaces. When approached at high rate, droplets may deform, which considerably complicates the description it is well known that when the two droplets are sufficiently large, hydrodynamic forces result in the formation of a dimple that flattens prior to film thinning. Along with the hydrodynamic interactions, the direct... 

As is well known, a lot of effects of surfactants, like damping of surface waves, the rate of thinning of liquid films, foaming and stabilisation of foams and emulsions, cannot just be described by a decrease in interfacial tension or by van der Waals and electrostatic interaction forces between two interfaces. The hydrodynamic shear stress at an interface covered by a surfactant adsorption layer is a typical example for the stimulation of an important surface effect. This effect, shown schematically in Fig. 3.9., is called the Marangoni effect. 

When a liquid phase becomes very thin, both faces of this film interact. The nature of the interactions may be electrical or of shorter range of interaction [55l (attractive van der Waals forces) or even of very short range of interaction (steric repulsive forces between hydrocarbon chains in lipid bilayers or repulsive hydration forces between oriented water molecules around polar heads of molecules merging in aqueous films between two lipid drop or in soap films). When two faces of such films approach one another, repulsive and attractive forces are unbalanced, giving rise to a constraint, corresponding to a non-equilibrium value of the thickness of the liquid film h. Hydrodynamic instabilities of planar films (dielectric or aqueous) have been widely investigated in the last ten years [54] [55] r59l They are... 

Common conceptual models for liquid distribution and transport in variably saturated porous media often rely on oversimplified representation of media pore space geometry as a bundle of cylindrical capillaries, and on incomplete thermodynamic account of pore scale processes. For example, liquid adsorption due to surface forces and flow in thin films are often ignored. In this study we provide a review of recent progress in modeling liquid retention and interfacial configurations in variably saturated porous media and application of pore scale hydrodynamic considerations for prediction of hydraulic conductivity of unsaturated porous media. 

This chapter adonpts a complete review of the various mechanisms proposed for the action of antifoams over the past seven or so decades. It is a feature of this subject that some proposed mechanisms, although plausible, have been speculative. Thus, unequivocal experimental evidence has sometimes been lacking. Indeed, the full theoretical implications of proposed mechanisms have also often not been fully developed. In the main, aU of this derives from the extreme complexity of the relevant phenomena. As we have seen, foam is itself extranely complex, consisting of (usually) polydisperse gas bubbles separated by draining films. These films exhibit complicated hydrodynamics involving the distinct rheology of air-liquid surfaces and, for thin films, colloidal interaction forces. The nature of the foam film collapse processes that are intrinsic to foam are stUl imperfectly understood. 

In addition to measuring the equilibrium thickness of thin films, the method is widely used to analyze film stability and drainage [810, 811, 813]. In many practical applications, a system is far away from equilibrium and highly dynamic. One example is a flotation cell in which particles and bubbles are mixed. The attachment of a particle to a bubble is limited by the hydrodynamic interaction rather than equilibrium surface forces [695]. When a particle and a bubble approach each other, the liquid in between needs to have time to flow out of the closing gap [728]. This process of film drainage is also studied with the thin film balance. 

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