Drainage, of foams

Nikolov AD, Wasan DT, Denkov ND, Kralchevsky PA, Ivanov IB (1990) Drainage of foam films in the presence of nonionic micelles. Prog Colloid Polym Sci 82 87-98... 

To understand drainage we have to discuss the pressure inside the liquid films. At the contact line between liquid films, a channel is formed. This is called the Plateau border. Due to the small bending radius (rP in Fig. 12.18), there is a significant Laplace pressure difference between the inside of the compartment and the liquid phase. The pressure inside the liquid is significantly smaller than in the gas phase. As a result, liquid is sucked from the planar films into the Plateau s border. This is an important effect for the drainage of foams because the Plateau borders act as channels. Hydrodynamic flow in the planar films is a slow process [574], It is for this reason that viscosity has a drastic influence on the evolution of a foam. Once the liquid has reached a Plateau border the flow becomes much more efficient. The liquid then flows downwards driven by gravitation. 

Fig. 5.10 shows the change of qexp /qtb ratio with decrease in border radius during drainage of foams with CBF and NBF. The size of border radius at which a total immobility of border surfaces is achieved is determined by extrapolation of the qtxp (rmin) dependence to < cxP/< th = 1. It is found that qexf> = qA for a foam with CBF when r, 3.5 Jim. 

Rate of drainage of foams from surfactant solutions with various viscosities (the experimental conditions are given in Fig. 5.17)... 

Drainage of foam containing oil drops is discussed in Section 7.11. 

Under the action of an outer driving force, the flnid particles approach each other. The hydro-dynamic interaction is stronger at the front zones and leads to a weak deformation of the interfaces in this front region. In this case, the nsnal hydrodynamic capillary number, Ca = V[VJa, which is a small parameter for nondeformable surfaces, should be modified to read Ca = y VJiJch, where the distance, h, between the interfaces is taken into account. The shape of the gap between two drops for different characteristic times was calcnlated numerically by many authors. " Experimental investigation of these effects for symmetric and asynunetric drainage of foam films were carried out by Joye et In some special cases, the deformation of the fluid particle can be... 

Figure 2.1. (a) Froth structure occurring during formation and drainage of foam in a column, (b) plateau borders, where tht capillary pressure sucks liquid into the borders... 

By using the thin-film balance developed by Schedud-luko and Exerowa (6), the drainage of foam films has been extensively studied. The apparatus employed is shown in Figure 2.16. 

Figure 2.16. The thin-film balance method used for evaluating the stability and drainage of foam films (a) schematic representation of the geometry of films in a foam, which occurs in the measuring cell of the Scheludko-Exerowa system (b) schematic of the set-up used for studying microscopic thin aqueous films (c) a typical interferogram of photocurrent versus time of drainage for the thinning process (adapted from ref. (6)), with permission from Elsevier Science...

FIGURE 1.6 Axisymmetric drainage of foam film with immobile air-water surfaces, (a) Dimple and barrier ring that form, (b) Defining maximum dimple size where barrier ring is absent and where film is formed in a cylindrical Scheludko cell (references [13,39] see Section 2.3.1). Radius of curvature, of Plateau border air-water surface is approximated by radius, R, of cell—is radius of curvature of air-water surface of the dimple. (Reprinted with permission from Joye, J. et al., Langmuir, 8, 3083. Copyright 1992 American Chemical Society.)... 

Observation of the drainage of foam films containing drops of these alcohols using a Scheludko cell revealed that the oil drops were swept out of the films without causing film rupture. Indeed, the drainage behavior and stability of the films were apparently totally unaffected by the presence of the oils despite a reduction in equilibrium surface tension. Arnaudov et al. [7] then concluded that these alcohols must cause foam collapse by rupturing Plateau borders rather than foam films as described in Section 4.5.3. 

This effect was even earlier described for the drainage of foam and emulsion films. Drainage of such liquid films could in many cases be described by effectively using a no-slip boundary condition [718]. Such a no-slip boundary condition is attributed to gradients in surfactant concentration caused by the flow, which resisted further flow at the surfaces [719-721]. 

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