On the Role of Hydrophobic Particles and Surfactants in Defoaming

The role of hydrophobic particles in mineral oil-based defoamers was investigated by fluorescence labeling and microscopy. Defoamer emulsion droplets in water adhere to the air/water interface to become lenses that nucleate bubble coalescence. Fluorescent labels were covalently bonded to silica, and confocal laser scanning microscopy showed that the hydrophobic silica particles concentrate in the oil/water interface near the three-phase contact line. Furthermore, surfactants added to defoamers to facilitate emulsification are required for the transport of the silica to the water/oil interface. Removal of excess silicone oil from the emulsification process lowered defoamer performance, suggesting that very small amounts of free silicone oil have a role in the defoaming mechanism.