Droplets antifoaming

Based on this low surface tension feature and the commonly observed insolubiUty of defoamers, two related antifoam mechanisms have been introduced (29) (/) The agent dispersed in the form of fine drops enters the Hquid film between bubbles and spreads as a duplex film. The tensions created by this Spreading lead to the mpture of the original Hquid film. (2) A droplet of the agent enters the Hquid film between bubbles, but rather than spreading produces a mixed monolayer on the surface. This monolayer, if of less coherence than the original film-stabilizing monolayer, causes destabilization of the film. 

In antifoaming, the process by which a droplet or particle of antifoaming agent enters the gas-liquid interface and displaces some of the original liquid from the interface. The liquid is usually an aqueous phase, so the process is sometimes referred to as de-wetting . 

It can be considered from the scheme that one has to distinguish between the foam kinetics, i.e. the rate of generation of foam under well defined conditions (air input and mechanical treatment) and the stability and lifetime of a foam once generated. The foam kinetics is also sometimes termed foamability in the literature. These quantities can be related to interfacial parameters such as dynamic surface tension, i.e. the non-equilibrium surface tension of a newly generated surface, interfacial rheology, dynamic surface elasticity and interfacial potential. In the case of the presence of oily droplets (e.g. an antifoam, a... 

In the real polydisperse foam along with coalescence there always acts another process of internal collapse. This is the diffusion decrease in the specific surface which is accompanied by structural rearrangement, i.e. shift of knots and borders, and change in their orientation. This leads to the origination of various local disturbances (Act, Apa, AC, etc.). These local disturbances along with the rupture of individual films cause destruction either of other films and borders or of local volumes or of the whole foam (see Sections 6.5 and 6.6). Finally, various external factors can affect the foam (pressure drop, applied to the liquid phase reduced pressure of the liquid vapour above the foam, leading to evaporation the effect of antifoam droplets a-particle irradiation vibration, etc.). 

Garrett came to the conclusion that most important for the synergy action of an oil-particle antifoam seems to be the ability of the particles to facilitate the appearance of oil droplets into the air/water surface. However, the sizes of the antifoam oil/particle composites should be sufficiently small to ensure a high probability of presence in a given foam film, but not so small to slow down the film drainage and suppress antifoaming effect. It order to possess such properties the particles should be hydrophobic but not completely wetted by the oil. The contact angle 9ow at the oil/water interface should satisfy the condition [20]... 

Figure 7.45 A schematic drawing of the antifoaming mechanism of antifoam droplets (i) entering o foam lamella, (ii) bridging-de-wetting and (iii) rupturing the foam wall.

Antifoam droplets are seen in Fig. 7.45 entering the foam lamella, bridging between the surfaces, dewetting and then mpturing the foam wall. 

Droplets and Oil Lenses Which Cause Antifoaming and Detbaming... 

According to Ross [36] and Robinson and Woods [37], AFAs may affect foam in two different ways (1) The antifoam agent is dispersed into very small droplets which penetrate into the foam lamellae and form a duplex film. This film spreads on the lamellae. It bursts because of the strain caused by the extension of the duplex film. (2) The antifoam agent penetrates into the lamellae and forms a mixed monolayer on the lamellae which has less cohesion than the lamellae-stabilizing protein film in the absence of antifoam. 

For cost reasons and for the ease of incorporation, mineral oil defoamers can be emulsified in water to give an oil-in-water (0/W) emulsion defoamer The droplet size plays an important role in their efficacy and persistency l One of the difficulties with this approach concerns preparation of a stable emulsion which when diluted becomes unstable in order to drive the active antifoam ingredient to the air/water interface and not remain in the bulk of... 

Solubilised antifoamers such as tributyl phosphate and methyl isobutyl carbinol, when added to surfactant solutions such as sodium dodecyl sulphate and sodium oleate, reduce foam formation [27]. In cases where the oils exceed the solubility limit, the emulsifier droplets of oil can greatly influence the antifoam action. It has been claimed [27] that the oil solubilised in the micelle causes a weak defoaming action. Mixed micelle formation with extremely low concentrations of surfactant may explain the actions of insoluble fatty acid esters, alkyl phosphate esters and alkyl amines. 

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