Mechanism of bubble coalescence induced by surfactant covered antifoam particles.

Mechanism of inter-bubble coalescence by an aqueous fatty alcohol particle suspension antifoam containing a nonionic surfactant has been investigated. By observing visually two colliding air bubbles in a liquid pool in the presence of the antifoam, a four-step mechanism is identified. The role of the surfactant in the antifoam is, for the first time, proposed. A surface tension gradient due to the local surfactant concentration difference enables a surfactant laden hydrophobic particle located on bubble surface to move from the periphery of a liquid film between two colliding air bubbles to their region of contact. Drop volume tensiometry and macroscopic foam column experiments are used to further prove this observation. Subsequently, the particle bridges and dewets the bubbles resulting in film rupture. The rate of drainage of the liquid film depends on the particle hydrophobicity, which necessitates complete surfactant desorption from particle surface. This is corroborated experimentally by Wilhelmy plate tensiometry. In addition, cryo-scanning electron and atomic force microscopy are used to determine the particle shape and the force for its entry into the bubble.