ABSTRACT
Effective antifoams for aqueous systems usually consist of mixtures of hydropho bic oils and hydrophobic particles [1]. Here, the particles adhere to oil-water interfaces and rupture the aqueous films between oil droplets and air bubbles. The oil droplets then emerge into the air-water surface. The behavior of the oil droplet in that surface is determined by the spreading coefficient S defined as
(1) where Yaw and y0w are respectively the air-water and oil-water surface tensions and Yao is the air-oil surface tension. If S < 0, the oil droplet will form oil lenses. In turn, these may bridge aqueous foam films. Such bridging lenses are necessarily unstable provided the condition
is satisfied, where В is the bridging coefficient [1-3]. When this inequality is satisfied, the bridging oil droplet must cause a capillary pressure in the foam film near the droplet leading to enhanced drainage rates and, according to Frye and Berg [4], to film collapse by “ film pinch-off.” The process is shown schemati cally in Figure 1. Here, Ө is the angle formed between the air-water and o ilwater interfaces at the three-phase contact line. If В > 0, then it is easy to show that Ө > n/2 [1 ].
