Stability of thin liquid films

Physikaliscbes Institut, Universitdt Freiburg, Hermann Herder Str. 3, 79104 Freiburg, Germany 

Nanoscale Liquid interfaces Wetting, Patterning, and Force Microscopy at the Molecular Scale Edited by Thierry Ondar uhu and Jean-Pierre Aime Copyright (c) 2013 Pan Stanford Publishing Pte. Ltd. 

Instabilities of thin liquid films have been investigated for many decades. The general contours of the mechanics and thermodynamics of thin-film instability have emerged during the last 50 years. On the theoretical side, Sheludko and Vrij  

It is not alwa clear what kind of interactions are responsible for the instability. One reason for ambiguities is that similar 

we aim to present a comprehensive picture of the film self-destruction process, including the initial amplification and subsequent dewetting, in order to provide a detailed understanding the various stages of thin-film breakup and morphology evolution. In particular, we consider fundamental and unifying scientific aspects of thin liquid films on solid substrates. Some of the central questions that will be treated are  

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Liquid films which form between approaching drops or bubbles are important structural elements of dispersed systems. The stability of these films controls the dispersion stability because the drops or bubbles cannot coalesce until the intervening film ruptures. The drainage and stability of thin liquid films attracted the attention of scientists already centuries ago [5,6]. 

Details about the theory of stability of thin liquid films, including foam films, can be found in some monographs [3-6]. However, the literature reflecting the theory of black foam films is rather poor. For this reason it will be granted special attention here. The new theoretical and experimental results accumulated during the recent years have brought nearer... 

Effect of Surfactant Type and Concentration. Surfactant concentration and type is of great importance for the stability of thin liquid films and for emulsion stability. Type and concentration of surfactants are responsible for the degree of lowering the interfacial tension and for the viscoelastic properties of droplet surface, as well as for the film thickness between two droplets. 

Chapters 10 and 11 are dedicated to nex concepts of flotation and microflotation. As a main act of these processes the formation and stability of thin liquid films are discussed in Appendix 2D. 

Fig. 8.13. Schematic representation of conditions for the stability of thin liquid films. Thermal fluctuations of the free liquid-air interface are (a) diminished and (b) amplified, resulting in stable film or decomposition of the film in liquid drops and dry patches, respectively. Left schematic representation of the disjoining pressure middle uniform film and a sketch of a capillary wave with a disjoining pressure indicating the sign and the magnitude right resulting film profile.

Foams and emulsions are achieved due to adsorption of foam stabilizing agents like surfactants at the interface between the dispersed and continuous phases. The foam stability is often related to the stability of thin liquid films formed between two air bubbles. All considered foam films are stabilized by ionic surfactant. 

Charwat A.F., Kelly R.E. and Gazley C. (1972). The flow and stability of thin liquid films on a rotating disk. J. Fluid Mech., 53, part2,227-255. 

Measuring the Forces and Stability of Thin-Liquid Films... 

Bisberink, C.G.J. The influence of spreading particles on the stability of thin liquid films, PhD thesis. University of Wageningen, the Netherlands, 1997. 

The authors review the theoretical analysis of the hydrodynamic stability of fluid interfaces under nonequilibrium conditions performed by themselves and their coworkers during the last ten years. They give the basic equations they use as well as the associate boundary conditions and the constraints considered. For a single interface (planar or spherical) these constraints are a Fickean diffusion of a surface-active solute on either side of the interface with a linear or an erfian profile of concentration, sorption processes at the interface, surface chemical reactions and electrical or electrochemical constraints for charged interfaces. General stability criteria are given for each case considered and the predictions obtained are compared with experimental data. The last section is devoted to the stability of thin liquid films (aqueous or lipidic films). 

The stability of thin liquid films on solid surfaces is a major topic both in fundamental and in applied science. In flotation, the aqueous film formed between a particle and an approaching bubble determines the interaction and thus the efficiency of the process [695, 753, 813, 814]. Polymer films preserve, isolate, or decorate materials [815-820]. Thin metal films in microelectronics serve as electrodes [821-823]. When such films dewet, a solid surface complex pattern is formed (Figure 7.15) [824]. 

Much of the interest in the way in which small particles and oil droplets, separately or in combination, affect the stability of thin liquid films has arisen as a result of the use of formulations containing oil and particles as aqueous antifoams. Unwanted foaming in many industrial systems and processes is an important problem [1],... 

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