Bridging-dewetting

In addition to having a lower surface energy than the foaming medium, defoamers must he insoluble in that medium, but also readily dispersible in it. There are five basic processes involved in the rupture of foam films by defoamers (7) entering, spreading, bridging, dewetting, and rupture. 

Figure 19.13 Different mechanisms by which particles can affect droplet dynamics (a) steric hindrance from particles at the interface, (b) steric hindrance from particles in the bulk phase, (c) particle-induced droplet bridging, (d) bridging-dewetting, (e) multiwall carbon nanotubes at the droplet interface. Baudouin et al. [181]. Reproduced with permission of Elsevier, (f) Clay platelets at the droplet interface. Hong et al. [179]. Reproduced with permission of Elsevier.

FIG. 1 Formation of asymmetric oil-water-air films (shaded areas) in two of the possible mechanisms of foam destruction by oil drops or lenses bridging-stretching (a-c-d) and (b-c-d) [11,12] bridging-dewetting (a-c-e) and (b-c-e) [2-6],... 

H. Schubert, Kapillaritat in porosen Systemen, Springer (1982). (Emphasis on capillary bridges in porous systems, static and dynamic systems, wetting and dewetting. The book was primarily intended for engineers.)... 

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

Figure 31. Suggested bridging mechanism of antifoaming in foam film by oil (90). The oil drop first enters one of the film surfaces and forms a lens. On further thinning of the film the lens enters the opposite film surface and an oil bridge is formed. The bridge is unstable because the capillary forces dewet the film from the bridge and the film ruptures. The arrows in the film indicate the direction of capillary forces.

Figure 32 Suggested mechanism of foam film bridging by solid particle (92—94). In part a, the hydrophobic particle bridges the film, the capillary pressure dewets the particle, and the film ruptures. In b, hydrophilic particle s capillary pressure acts in opposite direction and tends to increase the film thickness. The arrows in the film indicate the direction of capillary forces.

Hydrophobic antifoams, the second group, attract surfactant from the foam bubbles and lead to film, rupture dispersion of hydrophobed silica particles in silicone oil is believed to occur following this mechanism [49]. Another hypothetical mechanism for a silicone defoamer is proposed by Dippenaar [50], Garrett [51], and Berg [52], in which the hydrophobic particles bridge the foam lamella, dewetting occurs at the surface of the particle, and, consequently, the foam breaks. 

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