Film lamella rupture

In pure water, rupture takes place when the film is 100-400 nm thick. In surfactant solutions, rupture takes place when the film is 5-15 nm thick. The critical rupture thickness of thin films, he, can be calculated from the following semi-empirical equation  

---

When the lamella between two droplets thins and breaks, the droplets on either side coalesce into a single, larger droplet (41,72). Continuation of this backward" process eventually leads to the disappearance of the dispersion, if it is not balanced by the forward" mechanisms of snap-off and division. Lamellae are thermodynamically metastable, and there are many mechanisms by which static and moving thin films can rupture. These mechanisms also depend on the molecular packing in the film and, thus, on the surfactant structure and locations of the dispersed and dispersing phases in the phase diagram. The stability and rupture of thin films is described in greater detail in Chapter 7. 

Many poorly foaming liquids with thick film lamella are easily mptured, for example pure water and ethanol films (with thickness between 110 and 453 run). Under these conditions, rupture occurs by growth of disturbances which may lead to thinner sections [17]. Rupture can also be caused by the spontaneous nucleation of vapour bubbles (forming gas cavities) in the structured liquid lamella [18]. An alternative explanation for the rupture of relatively thick aqueous films containing a low level of surfactants is the hydrophobic attractive interaction between the surfaces that may be caused by bubble cavities [19, 20]. 

The extent and rate of drainage of surplus solution from the interior of the lamellae is one of the important factors determining foam stability, since drainage causes thinning of the film, and when the film reaches a critical thickness (50-100 A), the film may rupture spontaneously. Drainage of the film occurs under two influences gravity and pressure difference. 

In the foregoing, all instances of foam lamella rupture (types B and C foams) appeared to result from the imbibition (after emulsification) of oil droplets into the foam lamellae. Together with oil spreading (e.g., Kuhlman s observations) and pseudoemulsion film thinning (e.g., Manlowe and Radke s observations), the emulsification and imbibition brings forward a third possible mechanism of foam sensitivity to oil, each of which has been observed in microvisual experiments. These will be described further. 

Type C. If both and S are positive, then the oil will be drawn through and then spread as a film along the lamellae surfaces. This behavior leads to lamellae ruptures. 

Thinning of the pseudoemulsion films to the point of allowing entering (oil penetration of the gas—aqueous interface). This step may be sufficient for lamella ruptures. 

Pseudoemulsion film thinning probably comes into independent prominence when the system is quite dynamic and where the foam lamellae are thin. Pseudoemulsion film thinning, causing entering and subsequent lamella ruptures, is more likely to be a dominant mechanism when continuous gas injection into surfactant flooded media at residual oil saturation is practiced. 

When two emulsion drops or foam bubbles approach each other, they hydrodynamically interact which generally results in the formation of a dimple [10,11]. After the dimple moves out, a thick lamella with parallel interfaces forms. If the continuous phase (i.e., the film phase) contains only surface active components at relatively low concentrations (not more than a few times their critical micellar concentration), the thick lamella thins on continually (see Fig. 6, left side). During continuous thinning, the film generally reaches a critical thickness where it either ruptures or black spots appear in it and then, by the expansion of these black spots, it transforms into a very thin film, which is either a common black (10-30 nm) or a Newton black film (5-10 nm). The thickness of the common black film depends on the capillary pressure and salt concentration [8]. This film drainage mechanism has been studied by several researchers [8,10-12] and it has been found that the classical DLVO theory of dispersion stability [13,14] can be qualitatively applied to it by taking into account the electrostatic, van der Waals and steric interactions between the film interfaces [8]. 

The two surfactant films must fuse, forming a neck with direct contact between the dispersed liquid in the two droplets. Clearly, if the two surfactant films are poorly developed the lamella will rupture easily, while a fully saturated film could resist the fusion process. FIuc(nations of the surfactant surface density can trigger the fusion process. Temporarily bare spots attract each other and can break the lamella locally. 

In pure liquids, gas bubbles will rise up and separate, more or less according to Stokes law. When two or more bubbles come together coalescence occurs very rapidly, without detectable flattening of the interface between them, i.e., there is no thin-film persistence. It is the adsorption of surfactant, at the gas-liquid interface, that promotes thin-film stability between the bubbles and lends a certain persistence to the foam structure. Here, when two bubbles of gas approach, the liquid film thins down to a persistent lamella instead of rupturing at the point of closest approach. In carefully controlled environments, it has been possible to make surfactant-stabilized, static, bubbles, and films with lifetimes on the order of months to years [45],... 

Foam is a disperse system with a high surface area, and consequently foams tend to collapse spontaneously. Ordinarily, three-dimensional foams of surfactant solutes persist for a matter of hours in closed vessels. Gas slowly diffuses from the small bubbles to the large ones (since the pressure and hence thermodynamic activity of the gas within the bubbles is inversely proportional to bubble radius). Diffusion of gas leads to a rearrangement of the foam stmctures and this is often sufficient to rupture the thin lamellae in a well-drained film. 

---