Role in foam stability

The results of the experiments discussed and the conclusions drawn indicate that foam films play an extremely important role in foam stability. However, there is a number of other factors, such as rheological, that also affect foam stability, though in most cases the stability can be explained when the properties of foam films are taken into account. In this sense the correlation foam film/foam should be regarded as a general trend. 

If the pseudoemulsion film is stable, the larger drops formed from the unstable emulsion get trapped faster in the Plateau borders and stabilize the foam. On the other hand, if the pseudoemulsion film is unstable, the larger drops can bridge easier and break the foam lamellae faster (Figure 36). Thus, the unstable emulsion film also plays an important role in foam stability in the presence of emulsified oil. 

Adsorbed surfactant layer, role in foam stability, 407-408 Adsorbent, 488 Adsorption... 

An important role in foams stability can play liquid viscosity between the films. This is due to the feet that the rate of the liquid drainage is slowed down with viscosity increasing. However, at present time it is generally accepted that the surface viscosity can not be the determining factor for the foams stability, although the correlation between these properties exists. 

Secondary minimum flocculation is considered to play an important role in the stability of certain emulsions and foams. 

The main trends of the study of surface forces in foam films are briefly outlined here and the results obtained will be successively discussed in the next Sections. Furthermore, some earlier considerations will be commented, for instance, the first quantitative experimental verification of the DLVO-theory with the aid of foam films, since these results form the base of the further achievements in measurement and interpretation of surface forces and their role in the stability of foam films. 

The experiments indicated that foam films rupture at pressures lower than nmax is not due to occasional reasons. Critical pressure pcr was observed with different types of films (common foam, CBF and NBF) stabilised with various kinds of surfactants [171,303]. Similar effect has been observed by Black and Herrington [261] who studied films stabilised with three anion-active surfactants. However, details on the critical pressure of film rupture will not be discussed here, since a satisfactory theoretical explanation of this effect has not been proposed so far. There are some hypothesis on the matter. Nevertheless, this parameter has been successfully employed in clarifying the role of foam films in foam stability (see Chapter 7). No doubt, this parameter provides information about the stability of the different types of foam films and is awaiting its qualitative interpretation. 

We will consider only some more interesting aspects of stratification of microscopic horizontal foam films, giving an example of the role of stratified films in foam stability (see Chapter 7). 

The examples given with the two representatives of non-ionic surfactants, NP20 and Cio(EO)4 clearly indicate that the isoelectric state at the solution/air interface leads to foam film rupture and, respectively, to decrease in foam stability. This fact supports the idea about the role of foam films in the stability of foams. On the other hand, it provides an opportunity to regulate foam stability. 

The results presented below from the study of the behaviour of steady-state foams allow to estimate the role of foam films in foam stability. Two types of steady-state foam have been studied 1) wet steady-state foams from aqueous solutions of low surface active surfactants, e.g. normal alcohols [96] and 2) dry steady-state foams [121] from aqueous solutions of micellar surfactants, e.g. NaDoS, in the presence of electrolyte at different concentrations (ensuring different types of foam films). The device employed in this study represents a glass column (of inner diameter 3.4 cm) with a sintered glass filter as a bottom [94-96,121]. The gas volume passing through the column was measured by a rheometer. The total gas volume both in the foam and in the solution was measured when a steady-state was reached, i.e. when the system volume ceases changing. Usually the total gas volume V c as well as the gas rate vc were measured. 

Our objective in this study is to elucidate the complex phenomena occurring during the process of three phase foam thinning, to identify the interaction mechanisms between the oil droplets, the thinning foam film and the Plateau-Gibbs borders and the role of surface and interfacial tension gradients in foam stability, and to examine the implications upon crude oil displacement by foam in pourous media. 

The final filtration step is not meant to remove significant amounts of particles or to reduce turbidity. For economic reasons, there should not be many particles left from the first filtration step when entering into the second (final) filtration step. Only if this condition is maintained the costs for the secondary filtration can be kept low. Also, the filtration should only remove microorganisms, and not retain other useful components of beer, i.e., those proteins that have a role in foam formation and stability. On the other hand, bacteria, which should be separated from beer during final filtration, typically have sizes down to 0.5 p,m. This small difference in size between the desirable ingredients and those particles that should be removed, such as bacteria, shows that the selection of the filtration technique and media needs to be done very carefully. 

Frothing is an imwanted effect of surface active water soluble pol5nners. D5mamic surface properties of the solution-air interface due to the presence of the polymer play an important role in foam formation and stability. The surface tension decrease due to adsorbed polymer plays a lesser role (see for example Ref. [84]). 

Figure 45 summarizes our current understanding of the possible foam—oil interactions and highlights the role of single-foam lamellae, pseudoemulsion, and emulsion films in foam stability in aqueous foam systems containing solubilized or emulsified oils. Further research is warranted to... 

Abstract. The stability of suspensions/emulsions is under consideration. Traditionally consideration of colloidal systems is based on inclusion only Van-der-Waals (or dispersion) and electrostatic components, which is refereed to as DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. It is shown that not only DLVO components but also other types of the inter-particle forces may play an important role in the stability and colloidal systems. Those contributions are due to hydrodynamic interactions, hydration and hydrophobic forces, steric and depletion forced, oscillatory structural forces. The hydrodynamic and colloidal interactions between drops and bubbles emulsions and foams are even more complex (as compared to that of suspensions of solid particles) due to the fluidity and deformability of those colloidal objects. The latter two features and thin film formation between the colliding particles have a great impact on the hydrodynamic interactions, the magnitude of the disjoining pressure and on the dynamic and thermodynamic stability of such colloidal systems. 

Films of PMMA/copolymer blends nsed by Siripurapu et al. [77] were foamed by a saturation stage nnder pressnres from 6.89 to 34.5 MPa and temperatures between 40 and 120°C, followed by a qnick depressurization stage under isothermal conditions to promote the pressure-indnced phase separation. They observed that the copolymers play a role in the stabilization of the growing pores, but only a clear effect on the pore nucleation was found for PMMA blends with 2wt% of a low-molecular-weight PMMA-b-PFMA BCP foamed at 40°C and 34.5 MPa, leading to pore sizes of approximately 300 nm and pore densities of approximately 10 pores/cm. No information about the porosities obtained is provided, but according to the published parameters of the porous structure with low porosities can be expected. 

Another mechanism important in foam stability is the Gibbs-Marangoni effect, and this plays a role in preventing catastrophic thinning of the fluid films and subsequent bubble collapse. Consider two adjacent air bubbles in a foam, divided by a fluid film coated with surfactant molecules. As the bubbles grow, the dividing film will increase in area and become stretched. This means that the distribution of surfactant molecules... 

The secondary minimum that results due to the often weakly bounded floes (loose aggregates) deserves more discussion. For very small particles (radius less than about 10 nm) the secondary minimum is not deep enough to get flocculation. If the particles are larger, flocculation in the secondary minimum may cause observable effects. Secondary minimum flocculation is considered to play an important role in the stability of certain emulsions and foams as well as several colloidal systems containing odd-shaped particles like iron oxide and tobacco mosaic vims. 

Other experiments performed by Bergeron [34] on air foams stabilized with ionic surfactants reveal that the so-called Gibbs or dilatational elasticity e may play an important role in the coalescence process. The Gibbs elasticity measures the variation of surface tension yi t associated to the variation of the surfactant surface concentration F ... 

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

The wetting properties of the particles play a crucial role in flotation. We have already discussed the equilibrium position of a particle in the water-air interface (Section 7.2.2). The higher the contact angle the more stably a particle is attached to the bubble (Eq. 7.19) and the more likely it will be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficiency. For other minerals surfactants are used to improve the separation. These are called collectors, which adsorb selectively on the mineral and render its surface hydrophobic. Activators support the collectors. Depressants reduce the collector s effect. Frothing agents increase the stability of the foam. 

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