Coalescence, in foam

Mechanisms of Single-Foam Film Stability. Soap bubbles and soap films have been the focus of scientific interest since the days of Hooke and Newton (2—9). The stability and structure of foams are determined primarily by the relative rate of coalescence of the dispersed gas bubbles (10). The process of coalescence in foams is controlled by the thinning and rupture of the foam films separating the air bubbles. Experimental observations suggest that the lifetime (stability) of foam films is determined primarily by the thinning time rather than by the rupture time. Hence, if the approaching bubbles have equal size, the process of coalescence can be split into three stages ... 

There appear to be two stages in the collapse of emulsions flocculation, in which some clustering of emulsion droplets takes place, and coalescence, in which the number of distinct droplets decreases (see Refs. 31-33). Coalescence rates very likely depend primarily on the film-film surface chemical repulsion and on the degree of irreversibility of film desorption, as discussed. However, if emulsions are centrifuged, a compressed polyhedral structure similar to that of foams results [32-34]—see Section XIV-8—and coalescence may now take on mechanisms more related to those operative in the thinning of foams. 

Finding F Either Eq. (22-45) or Eq. (22-46) can be used to find the surface excess indirectly from experimental measurements. To assure a close approach to operation as a single theoretical stage, coalescence in the rising foam should be minimized by maintaining a proper gas rate and a low foam height [Brunner and Lemhch, Ind. Eng. Chem. Fundam. 2, 297 (1963)]. These precautions apply particularly with Eq. (22-45). 

The drainage theory breaks down for columns with tortuous cross section, large slugs of gas, or heavy coalescence in the rising foam. 

Tikhomirov VK (1975) Foams, theory and practice of their forming and coalescence (in Russian). Khimia Publ, Moscow... 

As drops of this dispersed phase collect near the separation interface, they will flocculate into a closely packed mass which can best be described by the term liquid-liquid foam. Each drop is surrounded by a thin film of the continuous phase. The film between two adjacent drops can rupture and the two combine by coalescence in the foam layer. Only those drops near the general phase boundary can coalesce into the general drop phase layer. The residence time in the flocculation zone can be many minutes, and considerable mass transfer may occur there. 

The proteins in the liquid film should 1) be soluble in the aqueous phase, 2) readily concentrate at the liquid-air interface, and 3) denature to form cohesive layers possessing sufficient viscosity and mechanical strength to prevent rupture and coalescence of the droplet. That is, the polypeptides of the denatured proteins in the liquid film should exhibit a balance between their ability to associate and form a film and their ability to dissociate, resulting in foam instability. 

Along with all the advantages, the refined blend structure is more sensitive towards the foaming parameters as foaming temperature and time. The higher number of cells and the elastomeric PB block promote rapid cell coalescence and collapse at elevated foaming temperatures and times. These phenomena can result in an increase in foam density and open-celled or inhomogeneous foam structures. 

Figure 5.16 Illustration of creaming, aggregation, and coalescence in an emulsion, foam or suspension.

The rate of foam drainage is determined not only by the hydrodynamic characteristics of the foam (border shape and size, liquid phase viscosity, pressure gradient, mobility of the Iiquid/air interface, etc.) but also by the rate of internal foam (foam films and borders) collapse and the breakdown of the foam column. The decrease in the average foam dispersity (respectively the volume) leads the liberation of excess liquid which delays the establishment of hydrostatic equilibrium. However, liquid drainage causes an increase in the capillary and disjoining pressure, both of which accelerate further bubble coalescence and foam column breakdown. 

The decrease in foam dispersity results from both bubble coalescence and diffusion bubble expansion. So, depending on the surfactant kind and the time elapsed after foam formation, one of these processes can have a prevailing effect on the rate of foam collapse. 

Foam column decay is also caused by gas diffusion from the upper layer bubbles into the ambient space and by surface coalescence, i.e. rupture of the surface films. The decrease in foam volume can be achieved layer by layer (each internal layer starts decaying only after the... 

The relation between the decrease in foam column height (volume) and the increase in the average bubble size has been treated in [41] at the assumption that 1) the decrease in the number of bubbles did not alter the foam volume 2) the decrease in volume (height) occurred layer-by-layer, starting from the top, so that the destruction of neighbouring bubbles in various layers was impossible 3) the probability for bubble disappearance in the foam bulk and at the surface resulting from coalescence and diffusion, was equal 4) the average bubble size on the... 

The same is the result when the number of EO-groups in the system containing oxyethylated octylphenol solution is reduced. Here again stable foams are only formed when SAD < 0. Regardless the way of destruction an instant coalescence of foams and emulsions is observed in the micellar phase range. 

In wet steady-state foams the bubble coalescence in the foam volume can be neglected. Then, it can be assumed that film rupture only occurs at the upper foam layers. In the other foam layers the average number of bubbles remains constant, the volume of the liquid outflow is equal to the liquid volume entering with the bubbles. 

Princen and Kiss [13] have studied the rheological properties of emulsions in which the effects of drainage, coalescence and diffusion transfer is much less expressed than in foams. A concentric-cylinder viscometer was used. The slip was estimated by rheograms in the t vs. 0) lx form (t is the stress measured on the inner cylinder wall, co is the angular velocity of the outer cylinder). The Xq values obtained are conform well with those from Eq. (8.24) but Xo (y) function is not a Bingham one, i.e. does not obey Eq. (8.11) at rj = const. 

In a third paper by the Bernard and Holm group, visual studies (in a sand-packed capillary tube, 0.25 mm in diameter) and gas tracer measurements were also used to elucidate flow mechanisms ( ). Bubbles were observed to break into smaller bubbles at the exits of constrictions between sand grains (see Capillary Snap-Off, below), and bubbles tended to coalesce in pore spaces as they entered constrictions (see Coalescence, below). It was concluded that liquid moved through the film network between bubbles, that gas moved by a dynamic process of the breakage and formation of films (lamellae) between bubbles, that there were no continuous gas path, and that flow rates were a function of the number and strength of the aqueous films between the bubbles. As in the previous studies (it is important to note), flow measurements were made at low pressures with a steady-state method. Thus, the dispersions studied were true foams (dispersions of a gaseous phase in a liquid phase), and the experimental technique avoided long-lived transient effects, which are produced by nonsteady-state flow and are extremely difficult to interpret. 

Khatib, Z. I., Hirasaki, G. J., and Falls, A. H., "Effects of Capillary Pressure on Coalescence and Phase Mobilities in Foams Flowing Through Porous Media", paper SPE 15442 presented at the 1986 SPE Fall Meeting, New Orleans, October 5-8, 1986. 

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