ABSTRACT
A number of investigations about the breakdown of foams by dispersed insoluble oils [1-6], hydrophobic solid particles [7-15], and oils and particles in combina tion [16-18] as antifoams have been reported. The mixed-type antifoams are very effective; thus, they are widely employed in many industries. Several theories have been presented in the literature on the mechanisms of defoaming by these three types of antifoams [17,18]. The oil lens-bridge theory was suggested by Garrett [19] and Frye and Berg [9]. The oil drop first enters one of the foamfilm 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 de wet the film from the bridge, and the film ruptures. The theory of foam film bridging by a solid particle was suggested by Garrett [14]. When the hydrophobic particle bridges with the film, the capillary pressure dewets the particle, and the film ruptures. On the contrary, the hydrophilic parti cle’s capillary pressure acts in the opposite direction and tends to increase the film thickness. The mechanism for the mixed-type anitifoam was suggested by Wasan and co-workers [18,20]. They proposed that the antifoaming efficiency strongly depended on the stability of the pseudoemulsion film. The oil drop con taining solid particles collects and gets trapped in the thinning Plateau border. The pseudoemulsion film breaks, and a drop enters and forms a solid plus oil lens. This lens gets trapped in a later stage of thinning and then bridges the film at the Plateau border and the bridge ruptures. In these terms, it is qualitatively explained that the hydrophobic solid particles destabilized the pseudoemulsion film.
