Rupture of thin liquid films

Another approach to the rupture of thin liquid films, proposed by Tsekov and Radoev [84,85], is based on stochastic modeling of this critical transition. Autocorrelation functions for steady state [84] and for thinning [85] liquid films were obtained. A method for calculation of the lifetime At and hcr of films was introduced. It accounts for the effect of the spatial correlation of waves. The existence of non-correlated subdomains leads to decrease in At and increase in hcr as a result of the increase in the possibility for film rupture. Coupling of dynamics of surface waves and rate of drainage v leading to stabilisation of thinning films has also been accounted for [86,87]. 

Ruckenstein, E. and Jain, R. K., Spontaneous rupture of thin liquid films, Chem. Soc. London, Faraday Trans. II, Vol. 70, pp. 132-147, 1974. 

After the theory of Vrij (1966) surface waves play an important role. The critical thickness for the rupture of thin liquid films derived from the behaviour of surface waves is much smaller than the equilibrium thickness. Fig. 3.17. shows the thinning of a film due to surface waves generated by disturbances with squeesing modes. 

Aveyard and Clint [85] have analyzed the consequences of the presence of a significant line tension on the behavior of lenses for the role of liquid drops in the rupture of thin liquid films and hence in foam breaking. They note, for example, that Equation 4.35 indicates that a sufficiently high positive value for x could mean that the equilibrium entry coefficient becomes negative for small drops in the case... 

Drainage, interstitial forces, and rupture of thin liquid films are the topic of this chapter. We start by introducing the concept of the disjoining pressure. Then, we describe the drainage of liquid in thin films. Here, we distinguish between vertical films, in which gravitation dictates the direction of the flow, and horizontal films. After introducing thin film balance as the main device to measure forces across liquid... 

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]. 

In concentrated emulsions and foams the thin liquid films that separate the droplets or bubbles from each other are very important in determining the overall stability of the dispersion. In order to be able to withstand deformations without rupturing, a thin liquid film must be somewhat elastic. The surface chemical explanation for thin film elasticity comes from Marangoni and Gibbs (see Ref. [199]). When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded... 

The special properties of thin liquid films, in particular of foam films, involve studying various colloid-chemical aspects, such as kinetics of thinning and rupture of films, transition from CBF to NBF, isotherms of disjoining pressure, thermodynamic (equilibrium) properties, determination of the electrical parameters of surfactant adsorption layer at the liquid/gas... 

Study of processes leading to rupture of foam films can serve to establish the reasons for their stability. The nature of the unstable state of thin liquid films is a theoretical problem of major importance (it has been under discussion for the past half a century), since film instability causes the instability of some disperse systems. On the other hand, the rupture of unstable films can be used as a model in the study of various flotation processes. The unstable state of thin liquid films is a topic of contemporary interest and is often considered along with the processes of spreading of thin liquid films on a solid substrate (wetting films). Thermodynamic and kinetic mechanisms of instability should be clearly distinguished so that the reasons for instability of thin liquid films could be found. Instability of bilayer films requires a special treatment, presented in Section 3.4.4. 

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... 

Finally, we consider the hydrodynamic theory of thin liquid film rupture. The stability of the liquid films to a great extent is ensured by the property of the adsorbed surfactant to damp the thermally excited fluctuation capillary waves representing peristaltic variations in the film thickness [6]. In addition to the theory of stability of free foam and emulsion films, we consider also the drainage and stability of wetting films, which find application in various coating technologies [7]. 

The effects of particles on foam stability are usually discussed in terms of individual particles because a single particle can rupture a thin liquid film. Emulsion stabilization by particles alone presumably involves close-packed monolayers aroimd the emulsion drops. However, less than close-packed layers can, in principle, have important effects on emulsion stability in systems stabilized by smfactants, and this area warrants further study. 

The speed of wetting has been measured by running a tape of material that is wetted either downward through the liquid-air interface, or upward through the interface. For a polyester tape and a glycerol-water mixture, a wetting speed of about 20 cm/sec and a dewetting speed of about 0.6 cm/sec has been reported [37]. Conversely, the time of rupture of thin films can be important (see Ref. 38). 

The basic mechanism of dryout almost invariably involves the rupture of a residual thin liquid film, either as a microlayer underneath the bubbles or as a thin annular layer in a high-quality burnout scenario. Bankoff (1994), in his brief review of significant progress in understanding the behavior of such thin films, discussed some significant questions that still remain to be answered. 

Fig. 30. Different types of the momentary morphologies which are typically observed during wetting (a-c) and dewetting (d-f) events on a solid flat substrate, a droplet, b spherical cap with a precursor film.c thin film (eventually with a multilayer structure), d thin liquid film, e ruptured film with rims at the dewetting front,f droplets...

In addition to film drainage, the stability of a foam depends on the ability of the liquid films to resist excessive local thinning and rupture which may occur as a result of various random disturbances. A number of factors may be involved with varying degrees of importance, depending on the nature of the particular foam in question. 

A device using an alternative principle to that of the jet and ultrasonic nebulizers has been described but has not been adopted to any extent. The Babington nebulizer, shown in Fig. 7, uses a principle that was first devised for fuel atomization [143]. Liquid (for the purposes of this discussion, a drug solution) is supplied to the outer surface of a hollow sphere. A thin film forms over the entire surface of the sphere. Compressed air supplied to the interior of the sphere expands through a small rectangular orifice at the top of the dome. Fine liquid particles form as escaping air ruptures a portion of the liquid film... 

The gas bubbles in food foams are separated by sheets of the continuous phase, composed of two films of proteins adsorbed on the interface between a pair of gas bubbles, with a thin layer of liquid in between. The volume of the gas bubbles may make up 99% of the total foam volume. The contents of protein in foamed products are 0.1-10% and of the order of 1 mg/m2 interface. The system is stabilized by lowering the gas-liquid interfacial tension and formation of rupture-resistant, elastic protein film surrounding the bubbles, as well as by the viscosity of the liquid phase. The foams, if not fixed by heat setting of the protein network, may be destabilized by drainage of the liquid from the intersheet space, due to gravity, pressure, or evaporation, by diffusion of the gas from the smaller to the larger bubbles, or by coalescence of the bubbles resulting from rupture of the protein films. 

Fig. 9.2. A sketch of the three consecutive stages of the binary coalescence process. Two bubbles are approaching each other. The bubble surfaces deform and a thin liquid film is created between them. The liquid drains thinning the film, and hydrodynamic instabilities cause film rupture. The final result of binary bubble coalescence is a new larger bubble.

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